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Title: The Evolution Theory, Vol. 2 of 2
Author: Weismann, August
Language: English
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THE
EVOLUTION THEORY
VOLUME II
THE
EVOLUTION THEORY
BY
DR. AUGUST WEISMANN
PROFESSOR OF ZOOLOGY IN THE UNIVERSITY OF FREIBURG IN BREISGAU
TRANSLATED WITH THE AUTHOR'S CO-OPERATION
BY
J. ARTHUR THOMSON
REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN
AND
MARGARET R. THOMSON
ILLUSTRATED
IN TWO VOLUMES
VOL. II
LONDON
EDWARD ARNOLD
41 & 43 MADDOX STREET, BOND STREET, W.
1904
_All rights reserved_
CONTENTS
LECTURE PAGE
XX. REGENERATION 1
XXI. REGENERATION (_continued_) 23
XXII. SHARE OF THE PARENTS IN THE BUILDING UP OF THE
OFFSPRING 37
XXIII. EXAMINATION OF THE HYPOTHESIS OF THE TRANSMISSIBILITY
OF FUNCTIONAL MODIFICATIONS 62
XXIV. OBJECTIONS TO THE THESIS THAT FUNCTIONAL MODIFICATIONS
ARE NOT TRANSMITTED 80
XXV. GERMINAL SELECTION 113
XXVI. GERMINAL SELECTION (_continued_) 136
XXVII. THE BIOGENETIC LAW 159
XXVIII. THE GENERAL SIGNIFICANCE OF AMPHIMIXIS 192
XXIX. THE GENERAL SIGNIFICANCE OF AMPHIMIXIS (_continued_) 210
XXX. IN-BREEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION,
AND THEIR CONSEQUENCES 238
XXXI. THE INFLUENCES OF ENVIRONMENT 265
XXXII. INFLUENCE OF ISOLATION ON THE FORMATION OF
SPECIES 280
XXXIII. ORIGIN OF THE SPECIFIC TYPE 299
XXXIV. ORIGIN OF THE SPECIFIC TYPE (_continued_) 330
XXXV. THE ORIGIN AND THE EXTINCTION OF SPECIES 346
XXXVI. SPONTANEOUS GENERATION AND EVOLUTION: CONCLUSION 364
INDEX 397
LIST OF ILLUSTRATIONS
FIGURE PAGE
35 _B_ (repeated). _Hydra viridis_, the Green Freshwater Polyp. 4
96. A Planarian cut transversely into nine pieces 6
97. A Planarian which has been divided into two by a
longitudinal cut 14
98. The leg of a Crab, adapted for self-mutilation or
autotomy 17
99. Regeneration of the lens in a Newt's eye 21
100. Regeneration of Planarians 25
101. A Starfish arm 27
76 (repeated). Diagram of the maturation divisions of the ovum 39
82 (repeated). Fertilization in the Lily 59
91 (repeated). Hind-leg of a Grasshopper 83
102. Brush and comb on the leg of a Bee 84
103. Claw on the leg of a 'Beach-fly' 85
104. Digging leg of the Mole-cricket 86
105. Ovary of a fertile Queen-Ant and ovaries of a Worker 91
106. Three Workers of the same species of Indian Ant 97
107 _A_, _B_. Larva of a Caddis-fly 105
107 _C_. Leptocephalus stage of an American Eel 133
108. Nauplius larva of one of the lower Crustaceans 161
109 _A_, _B_. Metamorphosis of one of the higher Crustacea,
a Shrimp 162
109 _C_. Second Zoæa stage 163
109 _D_, _E_. Mysis-stage and fully-formed Shrimp 164
70 (repeated). Daphnella 166
110. The largest of the Daphnids (_Leptodora hyalina_), with
summer ova beneath the shell 166
111. Nauplius larva from the winter egg of _Leptodora hyalina_ 167
112. Development of the parasitic Crustacean _Sacculina
carcini_ 168, 242
113. The two sexes of the parasitic Crustacean _Chondracanthus
gibbosus_ 170
114. Zoæa-larva of a Crab 171
115. Caterpillar of the Humming-bird Hawk-moth _Macroglossa
stellatarum_ 178
3 (repeated). Full-grown caterpillar of the Eyed Hawk-moth 178
4 (repeated). Full-grown caterpillar of the Eyed Hawk-moth 179
8 (repeated). Caterpillars of the Buckthorn Hawk-moth 179
116. Development of the eye-spots in the caterpillar of the
Elephant Hawk-moth _Chærocampa elpenor_ 180
117. Caterpillar of the Bed-straw Hawk-moth _Deilephila galii_ 181
118. Two stages in the life-history of the Spurge Hawk-moth
_Deilephila euphorbiæ_ 182
119. Caterpillar of the Poplar Hawk-moth _Smerinthus populi_ 184
120. _A_, Symmetrical, and _B_, asymmetrical curve of frequency 207
121. Life-cycle of _Coccidium lithobii_ 214
122. Conjugation of a Coccidium (_Adelea ovata_) 216
123. Conjugation of _Coccidium proprium_ 218
79 (repeated). The two maturation divisions of the 'drone eggs' 236
124. Alternation of generations in a Gall-wasp 245
125. The two kinds of galls formed by the species 246
126. Ovipositor and ovum of the two generations of the same
species of Gall-wasp 247
127. Life-cycle of the Vine-pest (_Phylloxera vastatrix_) 249
128. Heterostylism 254
38 (repeated). A fragment of a Lichen 261
129. Aberration of _Arctia caja_, produced by low temperature 276
130. Skeleton of a Greenland Whale, with the contour of the body 313
131. Peridineæ: species of _Ceratium_ 325
LECTURE XX
REGENERATION
Budding and division--Every theory of regeneration in
the meantime only provisional, a mere 'portmanteau
theory'--Regeneration not a primary character--Volvox--Hydra--Vital
affinities--Planarians--Heteromorphoses--Enemies of
Hydroid-colonies--Regeneration in Plants--In Amphibians--In
Earthworms--Different degrees of regenerative capacity according
to the liability of the part to injury--Different results of
longitudinal halving in Earthworms and in Planarians--Regeneration
in Birds--The disappearance of the power of regeneration is very
slow--Morgan's experiments on Hermit-crabs--Autotomy in Crustaceans
and Insects--Regeneration of the lens in Triton.
We have endeavoured to explain the handing on of the complement
of heritable qualities from one generation to another as due to a
continuity of the germ-plasm, and we assumed that the germ-cells never
arise except from cells in the 'germ-track'; that is, from cells which
are equipped, from the fertilized egg-cell onwards, with a complete
sample of slumbering germ-plasm, and are thereby enabled to become
germ-cells, and, subsequently, new individuals, in which the aggregate
of inherited primary constituents implied in the germ-plasm can again
attain to development.
We have now to consider other cases of inheritance in relation to the
same problem--the origin of their hereditary equipment.
We know, of course, that new individuals may arise apart from
germ-cells, that, in many of the lower animals and in plants, they may
arise by budding and fission.
For both these cases the germ-plasm theory will suffice, with a
somewhat modified form of the same assumption which we made in regard
to the formation of germ-cells. The origin of a new individual by
budding seems often, indeed, to proceed from any set of somatic cells
in the mother animal; but somatic cells, if they contain solely the
determinants controlling themselves, cannot possibly give rise to a
complete new individual, since this presupposes the presence of _all_
the determinants of the species. But as these determinants cannot be
formed _de novo_, the budding cells must contain in addition to the
usual controlling somatic determinants, idioplasm in a latent, inactive
state, which only becomes active under certain internal or external
influences, and then gives rise to the formation of a bud. The source
of this accessory idioplasm must, however, be looked for only in the
egg-cell.
In plants this bud-idioplasm must be complete germ-plasm, because
the budding starts only from one kind of cell, the cambium-cells;
but in animals in which--as it seems--it always proceeds from at
least two different kinds of cells--those of the ectoderm and those
of the endoderm--the matter is more complex. In this case these two
kinds of cells will contain as bud-idioplasm two different groups of
determinants, which mutually complete each other and form perfect
germ-plasm, and only the co-operation of these two sets will give rise
to the formation of a bud. I will not, however, go further into detail
in regard to these relations, for the theory can do nothing more here
than formulate what has been observed; it is hardly in a position to
help us to a better understanding of the facts.
The case is not much clearer in regard to the processes which lead to
the replacing of lost parts. The manifold phenomena of regeneration
can also be brought into harmony with the theory, if we attribute to
those cells from which the replacing or entire reconstruction of the
lost part arises an 'accessory-idioplasm,' which, at least, contains
the determinants indispensable to the building up of the part. It is
possible that the assumed accessory idioplasm frequently contains
a much larger complex of determinants, and that it depends on the
liberating stimuli which, and how many of these, will become active.
If we take a survey of regenerative phenomena in the animal kingdom,
it strikes us at once that the capacity is very different in different
species, extraordinarily great in some and very slight in others.
In general it is greater in lower animals than in higher, but,
nevertheless, the degree of differentiation cannot be the only factor
that determines the capacity for regeneration. That unicellular
organisms can completely replace lost parts, that even a piece of an
infusorian can reconstruct the whole animal if only the piece contain
a part of the nucleus, we have already seen when discussing the
significance of the nuclear substance. In this case the nucleus must
contain the complete germ-plasm, that is, the collective determinants
of the species, and these induce the reconstruction of the lost part,
though they do so in a way that is still entirely obscure to us. In the
meantime, our interpretation will not carry us further, either here or
in regard to any other order of vital phenomena. To go further would be
little short of propounding a causal theory of life itself; it would
mean having a complete and real 'explanation' of what 'life' is. As yet
no one has been able to claim this position. We can see the different
stages through which every organism passes, and that they arise one
out of the other; we can even penetrate down to the succession of those
delicate and marvellously complex processes which effect nuclear and
cell-division; but we are still far from being able to deduce, except
quite empirically, from the present state of a cell what the succeeding
one will be, that is, from being able to understand the succession of
events as a necessary nexus which could be predicted. How a biophor
comes to develop from itself the phenomena of life is quite unknown to
us; we know neither the interaction of the ultimate material particles
nor the forces which bring it about; we cannot tell what moves the
hordes of different kinds of biophors to range themselves together in
a particular order, what molecular displacements and variations arise
from this, or what influence the external world has, and so forth.
We see only the visible outcome of an endless number of invisible
movements--growth, division, multiplication, reconstruction, and
differentiation.
As long as we are so far from an understanding of life no theory of
regeneration can be anything more than a 'portmanteau theory,' as
Delage once expressed himself in relation to the whole theory of
inheritance, a theory which is like a portmanteau in that one can
only take out of it what has previously been put in. If we wish to
explain the renewal of the aboral band of cilia in a Stentor, we
first pack our trunk, in this case the nucleus of the Infusorian,
with the determinants of the ciliated region, and then think of these
as being liberated by the stimulus of wounding, and being brought to
and arranged in the proper place by unknown forces to reconstruct the
ciliary region in some unknown way. No one could be more clearly aware
than I am that this is not an exhaustive causal explanation of the
process itself. Nevertheless, it is not quite without value, inasmuch
as it allows us at least to bring the facts together in rational
order--in this case the dependence of the faculty of regeneration on
the presence of nuclear substance--under a formula which we can use
provisionally, that is, with which we can raise new questions. As soon
as we ascend higher in the series of organisms the theory gains a
greater value, for, while we leave altogether out of account any answer
to the ultimate question, and thus renounce for the present the attempt
to find out how the determinants set to work to call to life the parts
which they control, we are brought face to face with other, in a sense,
preliminary questions which we _can_ solve, and the solution of which
seems to me at least not entirely without value.
The first of these questions runs thus: Is the power of regeneration a
fundamental, primary character of every living being in the sense that
it is present everywhere in equal strength, independently of external
conditions, and thus is an inevitable outcome of the primary characters
of the living substance? Or is it, though primaeval in its beginnings,
a phenomenon of adaptation, which depends on a special mechanism, and
does not occur everywhere in equal extent and potency?
We have already become acquainted with some facts which must incline us
to the latter view. The globular Alga-colonies of _Volvox_ (Fig. 63)
consist of two kinds of cells, of which only one kind, the reproductive
cells, possess the power of reproducing the whole, the others, the
flagellate, or, as we called them, somatic cells, being only able to
produce their like, but never the whole.
New investigations which have been carried out by Dr. Otto Hübner in my
Institute have placed these facts beyond doubt. We may conclude that,
in this case, a disintegration of the germ-plasm has taken place during
ontogeny, by means of differential cell-division, so that only the
reproductive cells receive the complete germ-plasm, while the somatic
cells receive only the determinants necessary to their own specific
differentiation, the somatic determinants.
In this case regeneration and reproduction coincide; there is no
regeneration except the origin of a new individual from a reproductive
cell.
[Illustration: FIG. 35 _B_ (repeated). _Hydra viridis_, the Green
Freshwater Polyp. Section through the body-wall, somewhere in the
direction of _ov_ in Fig. 35 _A_. _Eiz_, the ovum lying in the ectoderm
(_ect_), and including zoochlorellæ (_schl_) which have immigrated
from the endoderm (_ent_) through the supporting lamella (_st_). After
Hamann.]
Let us now ascend to the lowest of the Metazoa, for instance, the
freshwater polyp, Hydra (Fig. 35 _A_), and we find a high degree of
regenerative capacity in the restricted sense, for, in addition to
the power of producing germ-cells, that is, cells which, when two
combine in amphimixis, give rise again to a new animal, almost any
part of the polyp can regrow a whole animal. Not only has Hydra been
cut in from two to twenty different pieces, but it has even been
chopped up into innumerable fragments, and yet each of these, under
favourable circumstances, was able to grow again into a complete
animal. Nevertheless, we are not justified in concluding that every
cell possesses the power of reproducing the whole. If, with the help
of a bristle, we turn one of these polyps outside in like the finger
of a glove, and then prevent it turning right again by sticking the
bristle transversely through it, it does not live, but soon dies,
obviously because the cells of the two layers of the body, ectoderm
and endoderm, cannot mutually replace each other, and cannot mutually
produce each other. The inner layer, now turned outwards, cannot resist
the influence of the water, and the outer layer, now turned inwards,
cannot effect digestion; in short, one cannot be transformed into the
other, and we must therefore conclude that both are specialized, that
they no longer contain the complete germ-plasm, but only the specific
determinants of ectoderm and endoderm respectively.
The animal's high regenerative capacity must therefore depend on the
fact that certain cells of the ectoderm are equipped with the complete
determinant-complex of the ectoderm, in the form of an inactive
accessory idioplasm, which is excited to regenerative activity by
the stimulus of wounding, and that, in the same way, the cells of
the endoderm are equipped with the whole determinant-complex of the
endoderm. It need not be decided whether all or only many of the
cells, perhaps the younger ones, are thus adapted for regeneration;
in any case a great many of them must be distributed throughout the
whole body, with perhaps the exception of the tentacles, which are by
themselves unable to reproduce the whole animal. When the animal is
mutilated, the cells of both layers, equipped with their respective
determinant-aggregates, co-operate in reproducing the whole from a part.
It is true that even with these assumptions we only reach the threshold
of a real explanation. For, given that all the determinants of the
species must be present in a fragment, we are not in a position to show
how these set about reconstructing the animal in its integrity, and the
most that we can say is, that it must depend on the specific kind of
stimulus to which each of the cells is exposed through its direct and
more remote environment, which determinants are to be first liberated,
and therefore which parts are to be reconstructed.
That there are at work regulative forces, such as we were already
compelled to assume in regard to the division and regeneration of
unicellular organisms, as to the nature of which we cannot yet make
any definite statement, but which we may call 'polarities,' or, as I
prefer to say, 'affinities,' is shown by countless experiments which
have been made, particularly with the freshwater polyp. Thus Rand cut
off the anterior end of the polyp with its circle of tentacles, and the
excised disk of living substance lengthened in a transverse direction,
so that half the tentacles came to lie to the right, the other half to
the left, while the body developed between these two groups, so that
they became further and further separated from each other, till finally
the original transverse axis of the animal became the longitudinal
axis. One group of tentacles survived and surrounded the new mouth,
while the other at the opposite aboral pole, the new foot, died off.
This total change of structure in the polyp, as to the arrangement of
its main parts, points to unknown forces, which cannot depend on the
determinants as such, but on the vital characters of the living parts,
and on the interactions of these with one another.
[Illustration: FIG. 96. A Planarian cut transversely into nine pieces.
The regeneration of seven of these into entire animals is shown. After
Morgan.]
The same holds true of all the lower Metazoa that have highly developed
regenerative capacity, not only of polyps, but of worms such as the
Planarians. Through the experiments of Loeb, Morgan, Voigt, Bickford,
and others, we know that these animals respond to almost every
mutilation by complete reconstruction, that they may, for instance, as
is indicated in Fig. 96, be cut transversely into nine or ten pieces
with the result that each of these pieces grows again to a whole
animal, unless external influences are unfavourable and prevent it.
Something similar happens if the head be cut off a Tubularia-polyp, it
forms a new head with proboscis and tentacles. It does so, at least,
if the stalk of the polyp be left in the normal position; but if it
be stuck into the sand in the reverse position a head arises at the
end which is uppermost, where the roots arose previously, and the
previous head-end now sends out roots. By suspending a beheaded stalk
horizontally in the water a head can be caused to develop at each end
of the stalk, so that we must assume that every part of the polyp is,
under some circumstances, capable of developing a head, and that it
must be 'circumstances'--in this case gravity, contact with earth or
with water, and the mutual influence of the parts of the animal upon
each other--which decide what is to be produced. Loeb, who was the
first to observe this form of regeneration, called it heteromorphosis,
to express the fact that particular parts of the animal might be
produced at quite different places from those originally intended for
them.
It would certainly be erroneous to range these cases of heteromorphosis
against the determinant theory, but they certainly do not afford any
special evidence of its validity as an interpretation, for all that we
can say here again is that all, or at least many, cells of the animal
must contain the full determinant-complex of the ectoderm, and others
those of the endoderm, and that particular groups of determinants
become active when they are affected by certain external or internal
liberating stimuli. In regard to such animals the theory is hardly more
convincing than the rival theory, that the faculty of regeneration is a
general property of living substance, which does not attain to equally
full expression everywhere, because it is met by ever-increasing
difficulties involved in the increasing complexity of structure. The
validity of the theory only begins to be seen when we deal with cases
where it is demonstrable that every part cannot bring forth every
other, where the power of regeneration is limited, and occurs only in
definite parts in a definite degree, and can only start from particular
parts. Here the assumption of a general and primary regenerative
capacity fails. Any one who insists, as O. Hertwig does, that the
idioplasm in all cells of the body is the same, can always plead that,
in the cases in which regeneration does not occur, the fault lies,
not in the regenerative capacity, but in the absence of the adequate
liberating stimuli, and at first sight it does seem as if this position
were unassailable. We shall find, however, that there are facts which
make Hertwig's interpretation quite untenable.
My own view is that the regenerative capacity is not something
primary, but rather an adaptation to the organism's susceptibility to
injury, that is, a power which occurs in organisms in varying degrees,
proportionate to the degree and frequency of their liability to injury.
Regeneration prevents the injured animal from perishing, or from
living on in a mutilated state, and in this lies an advantage for the
maintenance of the species, which is the greater the more frequently
injuries occur in the species, and the more they menace its life
directly or indirectly. A certain degree of regenerative capacity is
thus indispensable to all multicellular animals, even to the highest
among them. We ourselves, for instance, could not escape the numerous
dangers of infection by bacilli and other micro-organisms if our
protective outer skin did not possess the faculty of regeneration,
at least so far that it can close up a wound and fill up with
cicatrice-tissue a place where a piece of skin has been excised.
Obviously, then, the mechanism which evokes regeneration must have
been preserved in some degree and in some parts at every stage of the
phyletic development, and must have been strengthened or weakened
according to the needs of the relevant organism, being concentrated
in certain parts which were much exposed to injury and withdrawn from
other rarely threatened parts. Thus the great diversity which we can
now observe in the strength and localization of the regenerative
capacity has been brought about. But all this can only be regarded as
adaptation.
I should like to submit a few examples to show that the regenerative
capacity is by no means uniformly distributed, and that, as far as we
can see, it is greater or less in correspondence with the needs of the
animal, both in regard to the whole and to particular parts.
It must first be pointed out that those lower Metazoa, like the
Hydroid polyps in particular, which are endowed with such a high and
general power of regeneration, do actually require this for their
safety; they are not only soft, easily injured and torn, but they
are most severely decimated by many enemies. In the beginning of May
I found on the walls of the harbour at Marseilles whole forests of
polyp-stocks of the genera _Campanularia_, _Gonothyræa_, and _Obelia_,
all large and splendidly developed, with thousands of individual polyps
and medusoids, but in a very short time the great majority of the
polyps were eaten up by little spectre-shrimps (Caprellids) and other
crustaceans, worms, and numerous other enemies, and towards the end of
May it was no longer possible to find a fine well-grown colony. It must
therefore be of decisive importance for these species if the stems and
branches, which are spared because protected by horny tubes, possess
the faculty of transforming their simple soft parts into polyp-heads,
or of giving off buds which become polyps, or even of growing a new
stock from the twigs which have been half-eaten and bitten loose from
the stock and have fallen to the ground. If, finally, a torn-off
polyp-stalk (of _Tubularia_) falls to the ground with the wrong side
up, the end which is now the lower will send out roots, and the end
now uppermost will give off a new head. This also appears to us
adaptive, and does not surprise us, since we have been long accustomed
to recognize that what is adapted to an end will realize this if it be
possible at all. Think again of the innumerable adaptations in colour
and form which we discussed in the earlier lectures. I hope later to
be able to show in more detail how it comes to pass that necessity
gives rise to adaptation. In regard to the case of the polyps, we can
understand that, as far as a high degree of regeneration and budding
was possible in these animals at all, it could not but be developed.
Regeneration and budding complete each other in this case, for the
former brings about in the individual 'person' what the latter does
in the colony, namely, a _Restitutio in integrum_. It is readily
intelligible that the former was not difficult to establish where the
latter--the capacity of budding--was already in existence.
It seems at first sight very striking that the higher plants, which all
depend upon budding, and which form plant-colonies (corms) in the same
sense as the polyps form animal-colonies, only possess the faculty of
true regeneration in a very low degree, although they are extremely
liable to injury.
We see from this that the two capacities are not co-extensive, that
germ-plasm may be contained in numerous cells of the body in a latent
state, and yet that regeneration of each and every detailed defect
may not be possible. This is the case in the higher plants in regard
to most of their parts. A leaf in which a hole has been cut does not
close the hole with new cell-material; a fern frond from which some
of the pinnules have been cut off does not grow new ones, but remains
mutilated. Even leaves which, if laid on damp earth, readily give off
buds which grow to new plants, as the Begonias do, do not replace a
piece cut out of the leaf; they are not at all adapted to regeneration.
From the standpoint of utility this is readily intelligible. It was,
so to speak, not worth Nature's while to make such adaptations in the
case of leaves or blossoms, partly because these are very transient
structures, and partly because they are rapidly and easily replaceable
by the development of others of the same kind. Moreover, the leaf in
which we have cut a hole continues to function, but the polyp whose
mouth and tentacles we have cut off could no longer take nourishment
unless it were adapted for regeneration. But that this adaptation
_could_ have been made in the case of plants is proved by the
root-tips which are formed anew when they are injured, and the closing
of wounds on the stem by a 'callus.'
I shall return to plants when we are dealing with the mechanism
of regeneration, but I must now direct more attention to animals,
inquiring further into the question as to whether the faculty of
regeneration is correlated with the degree of liability to injury to
which the animal is exposed, and with the biological importance of
the injured part, for this must be the case if regeneration be really
regulated by adaptation.
Hardly any other vertebrate has attained such celebrity on account of
its high regenerative capacity as the water-newt, species of the genus
_Triton_. It can regrow not only its tail, but the legs and their parts
if they are cut off. Spallanzani saw the legs grow six times, after
he had cut them off six times. In the blind newt (_Proteus_) of the
Krainer caves, a near relative of the common newt, the leg regenerated
only after a year and a half, although the animal stands on a lower
stage of organization than the newt, and thus should rather replace
lost parts more easily. But Proteus lives sheltered from danger in
dark, still caves, while Triton is exposed to numerous enemies which
bite off pieces from its tail or legs; and the legs are its chief means
of locomotion, without which it would have difficulty in procuring
food. It is different with the elongated eel-like newt of the marshes
of South Carolina, _Siren lacertina_. This animal moves by wriggling
its very muscular trunk, after the manner of an eel, and in consequence
of the disuse of its hind legs it has almost completely lost them. Even
the fore-legs have become small and weak, and possess only two toes,
and these do not regrow if they are bitten off, or only do so very
slowly.
Earthworms are exposed to much persecution; not only birds, such as
blackbirds and some woodpeckers, but, above all, the moles prey upon
them, and Dahl has shown that moles often lay up stores of worms in
winter which they have half crippled by a bite, while even Réaumur
knew that moles frequently only half devoured earthworms. It was
thus an obvious advantage to earthworms that a part of the animal
should be able to regrow a whole, and accordingly we find a fairly
well-developed regenerative capacity among them. But it varies greatly
in the different species, and it would be interesting if we knew the
conditions of life well enough to be able to decide whether the faculty
of regeneration rises and falls in proportion to the dangers to which
the species is exposed. Unfortunately we are far from this as yet; we
only know that, in the common earthworms of the genera _Lumbricus_ and
_Allolobophora_, the faculty of regeneration is still very limited,
for at most two worms, and sometimes only one, can develop from an
animal cut into two pieces. Cutting into a greater number of pieces
does not yield a larger number of worms, but usually only one, and
often none at all.
This corresponds to the behaviour of their enemies, which may often
bite off a piece or tear it away when the worm attempts to escape, but
never cut it up into pieces. The regenerative capacity is more highly
developed in the genus _Allurus_, more highly still in the worms of the
genus _Criodrilus_ which lives in the mud at the bottom of lakes, and
most highly of all in the genus _Lumbriculus_ which lives at the bottom
of small ponds. Long ago Bonnet cut up a specimen of _Lumbriculus_ into
twenty-six pieces, of about two millimetres in length, and he observed
most of these grow to complete worms again. His experiments have often
been repeated in recent times, and have been extended and made more
precise in many ways. Von Bülow was able to get whole animals from
pieces consisting of from four to five somatic segments, and with eight
or nine segments he almost invariably succeeded. A _Lumbriculus_ which
he had cut into fourteen pieces, one of which only measured 3.5 mm. in
length, gave rise to thirteen complete worms with head and tail; only
one piece perished.
These worms have little enemies with sharp jaws which may gnaw at them
behind or before but cannot swallow them whole. Lyonet, famous for his
analytic dissection of the wood-caterpillar (_Cossus ligniperda_),
observed when he was feeding the larvæ of dragon-flies with these
Lumbriculid worms that 'the anterior end of some whose posterior end
had been gnawed away by the larvæ continued to live on the ground.' We
can thus understand why a high power of regeneration is of use to these
worms, and at the same time why it is advantageous to them to contract
so that they break in pieces on very slight irritation, but to this we
shall refer again.
The very diverse potency of the faculty of regeneration in animals
belonging to the same small group, and nearly, if not quite, at the
same level of organization, seems to show clearly that we have here to
do with adaptation to different conditions of life, although we cannot
demonstrate this in detail. It would certainly be erroneous to regard
the conditions of life as uniform, since the worms in question not
only live in different places--in the earth, in mud, or in water--and
are thus exposed to different enemies, and since they may also be
quite different in regard to size and speed, in means of defence, and
possibly also of defiance, as is indeed in some measure demonstrable.
We meet with the same thing in a group of still smaller worms, Rösel's
'water-snakelets,' species of the genus _Nais_. These, too, behave in a
variety of ways in the matter of regeneration, for while many species,
such as _Nais proboscidea_ and _Nais serpentina_ will, if cut into
two or three pieces, become two or three worms respectively, Bonnet
expressly mentions an unnamed species of _Nais_ which does not bear
cutting up at all, and even dies if its head be cut off.
Thus neither the degree of organization nor the relationship alone
determines the strength of the regenerative capacity. And as nearly
related species may behave quite differently in this respect, so also
do the different parts of one and the same animal; and here, too, the
strength of the capacity seems to depend on the more frequent or rarer
injury of the relevant part and on its importance in the maintenance of
life. Let us take a few examples.
Parts which, in the natural life of the animal, are never injured, show
in many cases no power of regeneration. This is so in regard to the
internal parts of the newt, whose regenerative capacity is otherwise
so high. I cut half or nearly the whole of a lung away from newts
anæsthetized with ether; the wound closed, _but no renewal of the organ
took place_. The same thing happened when a piece of the spermatic duct
or of the oviduct was cut away. It is true that the kidney enlarges in
higher animals when a piece has been cut out, by the proliferation of
the remaining tissues, but that is a mere physiological substitution,
evoked by the increased functional stimulus, due to the accumulation in
the blood of the constituents of the urine. Such substitution depends
on the growth of parts already existing, and it occurs in man when one
kidney is removed, for the other, as is well known, may then grow to
double its normal size. This is mere hypertrophy of the part that is
left, it is not regeneration in the morphological sense, and it is not
comparable to the re-formation of a cut-off leg in the salamander, or
of a head in the worm, where the growth is not a mere increase of the
remaining stump, but a new formation. It would be regeneration if a
new kidney developed from the remnants of the kidney-tissue, or, in
the liver, if new lobes grew in place of those which were cut off. But
neither of these things happens, and, as far as I am aware, nothing of
the kind has ever been observed, nothing more than new formation of
liver-cells through increase of existing ones; that, however, is not
regeneration in the morphological sense.
I have referred to the slight power of regeneration possessed by the
blind Proteus in regard to its legs or tail, and I connected this with
the absence of enemies in its thinly peopled cave-habitat. But the same
animal can regenerate its gills when these are bitten off, and this is
probably associated with the habit that Proteus has, in common with
other newts with external gills, of nibbling at its neighbour's gills.
Thus, the power of regenerating the gills was retained even when the
animals migrated to the quiet caves of Krain, and were thus secured
from the attacks of other enemies.
In lizards, a leg which has been cut off does not grow again, but an
amputated tail does, and this has quite a definite biological reason,
since the active little animal will seldom be caught by the foot by any
pursuer, but may easily be caught by the tail, which is far behind.
Thus the tail is adapted not only for regeneration, but also for
'autotomy' that is, for breaking off easily when it is caught hold of.
We have already seen that some segmented worms have a very high
regenerative capacity; yet every part cannot produce every other, and
while, in _Lumbriculus_, any piece of from five to nine segments is
able to grow a new head or tail, neither ten nor twenty nor all the
segments together, if they are _halved longitudinally_, can reproduce
the other half, and the cause of this inability does not lie in the
fact that the animal is thereby hindered from taking food, for even the
transversely cut pieces do not feed until they have grown a new head
and tail. The reason must lie in the fact that the primary constituents
for this kind of regeneration are wanting, and they are so because a
longitudinal splitting of this cylindrical and relatively thin animal
never occurs under natural conditions, and thus could not be provided
against by Nature[1].
[1] Morgan maintains that this statement is incorrect, and that
_Lumbriculus_ is capable of lateral regeneration. But if we look into
the matter more closely we find that all he says is, that small gaps
made by cutting a piece out of one side are filled up again, while the
cut pieces perish. If the whole animal be halved, according to Morgan,
both halves die, or if a 'very long piece' be cut out of one side, not
only this piece dies, but also 'the remaining piece.' There is thus, as
I have said, an essential difference between the regenerative capacity
of _Lumbriculus_ and that of _Planaria_.
That regeneration of this kind could have been arranged for if it had
been useful we learn from the Planarians among the flat worms, in which
every piece cut out of the body, large or very small, from the middle,
from the left side, or from the right side of the animal, grows into
a complete Planarian. The animal can be halved longitudinally, as
in Fig. 97, and each half will grow to a whole. This again is quite
intelligible from the biological point of view, for these flat, soft,
and easily torn animals are exposed to all sorts of injuries, and are,
in point of fact, frequently mutilated by enemies which are unable to
swallow them whole. Von Graaf not infrequently found examples of marine
Planarians (_Macrostomum_) which lacked 'a part of the posterior end
or the whole tail region as far as the food-canal,' and of species of
_Monotus_ he found 'very often' in May specimens with the posterior
end split or broken off. Probably the persecutors of these flat-worms
are some species of Crustacean, but, at any rate, so much is proved,
that the Planarians have abundant opportunities of making use of their
faculty of regeneration, and that the species gains an advantage from
it in respect to its preservation.
[Illustration: FIG. 97. _A_, a Planarian, which has been divided into
two by a longitudinal cut. Each half can grow into an entire animal.
_B_, the left half at the beginning of the regenerative process. _C_,
the same completed. After Morgan.]
In contrast to this, worms which live within other animals, and
are thus secure from mutilation, such as the familiar round-worms
(_Nematoda_), have no power of regeneration at all, and do not survive
either longitudinal or transverse division.
Until recently birds were regarded as possessing a very low degree of
regenerative capacity, and, as a matter of fact, they cannot replace
a leg or a wing wholly or in part; but, what is otherwise unheard of
among higher vertebrates, they can renew the whole anterior portion of
the skeleton of the face, the bill, and can indeed almost reconstruct
it with new bones and horny parts. Von Kennel communicated a case of
this kind in regard to a stork, and for a long time this remained an
isolated case, but a few years ago Bordage showed that, in the cocks
which are used in the Island of Bourbon for the favourite sport of
cock-fighting, the bill is regularly renewed when it has been broken
off or shattered. Quite recently Barfurth gave an account of a case
of complete renewal of a broken bill in a parrot. Yet it should not
astonish us that the bill in birds has such a high regenerative
power, for of all parts in a bird it is the one that is most readily
injured; with it the bird defends itself against its enemies and its
rivals, masters its prey, and tears it to pieces, pecks holes in trees
(woodpecker), or climbs (parrot), or digs and burrows in the ground,
or builds its nest, and so on. That the faculty of regeneration could
be developed to so high a degree in relation to this particular part
of the body, while the rest of the very important but rarely injured
parts do not possess it at all, again points to the conclusion that the
faculty of regeneration has an adaptive character.
It does not affect matters to discover cases in which we cannot
recognize this relation between the regenerative capacity of a part
and its importance or its liability to injury. Such instances do not
lessen the convincingness of the positive cases, since we do not know
the exact conditions which may lead to the increase of regenerative
capacity in a part, and, above all, since we do not know the rate at
which such an increase may take place. If adaptation in general depends
upon processes of selection, these processes must also be able to give
rise to an increase in the power of regeneration. On the other hand, it
by no means follows that the disappearance of a faculty of regeneration
which was once present in a part, but which has become superfluous
in the course of time, must take place immediately through natural
selection. For it is the very essence of natural selection that it only
furthers what is useful, and only removes what is injurious; over what
is indifferent it has no power at all. Thus it follows that the faculty
of regeneration, when it has once been present in a part, cannot be
set aside by natural selection (personal selection), for it is in no
way injurious to its possessor. If it gradually decreases and becomes
extinct notwithstanding this, when it is of no further use, as seems to
be to some extent the case in regard to the legs and tail of the blind
Proteus, that must depend on other processes, on those which generally
bring about the gradual disappearance of disused parts or capacities.
We shall attempt to probe to the roots of these processes later on;
for the present let it suffice us to know that, according to our
experience, they go on with exceeding slowness, and that it has taken
whole geological periods to eliminate the legs of the snake-ancestors
so completely as has been done from the structure of most of our modern
snakes, while the Proteus which migrated into the caves of Krain as far
back as the Cretaceous period is indeed blind, but still retains its
eyes under the skin, though in a degenerate condition.
Since the degeneration of disused parts and capacities goes on so
slowly it need not surprise us that we meet many parts which still
possess regenerative capacity, although they are protected from
injury. Thus Morgan found that, in the hermit-crab, the limbs which
are protected within the mollusc shell were quite as ready to regrow
as those which are actually used for walking, and thus are exposed to
possibility of attack, but this proves nothing against the conclusion
we drew from the facts cited above, according to which the faculty of
regeneration comes under the law of adaptation. For the disappearance
of this faculty must take place very much more _slowly than its
growth_. For instance, the development of the tail-fin of the whale has
long been an accomplished fact, while the hind-legs of this colossal
mammal, which were rendered useless by the development of the tail-fin,
still lie concealed in a rudimentary state within the muscles of the
trunk. Yet these limbs must have lost their significance for the animal
exactly at the time that the tail-fin became more powerful. Thus the
retrogression must have taken place more slowly than the progressive
transformation.
It is clear, then, that the faculty of regeneration is not a primary
character of living beings occurring uniformly in all species of
equally high organization and in all parts of an animal in the same
degree; it is a power which occurs in animals of equal complexity in as
varying degrees as in their parts, and which is manifestly regulated by
adaptation. Between parts with the faculty of regeneration and parts
without it there must be an essential difference; there must be present
in the former something that is wanting in the latter, and, according
to our theory, this is the equipment with regeneration-determinants,
that is, with the determinants of the parts which are to be
reconstructed.
If this be really so it should be capable of proof, at least in so
far that we should be able to establish that the power of completing
or re-forming a damaged or lost part is a limited one, localized in
certain parts and cell-layers. This can be actually proved, as may
be seen from numerous cases in which the faculty of regeneration is
associated with autotomy, that is, with the power of breaking off or
dropping off a part of the body. Even in worms we find this power,
as we mentioned before in speaking of the high regenerative capacity
of _Lumbriculus_. This worm reproduces in summer by what is called
'schizogony,' that is, by breaking into two, three, or more pieces, and
it does not seem to require a very strong stimulus, such as pressure
of the end of the worm by the jaws of an insect larva, to start this
rupture; it often follows from quite insignificant friction on the
ground. Certainly the power of regeneration is so great in this animal
that it is out of the question to talk of localizing the primary
constituents of regeneration; almost every broken surface is capable of
regeneration.
But this localization is well illustrated in Insects and Crustaceans,
which possess the power of self-amputation in their appendages,
especially in their legs. As far back as 1826 MacCullock observed
this remarkable power in crabs, and described the mechanism on which
it depends. When the leg is irritated, for instance when it is pinched
at the tip and held fast, it breaks off at a particular place. This
line of breakage lies in the middle of the short second joint (Fig.
98, _A_ and _B_, _s_), just between the insertions of the muscles
(_me_, _mf_, _m_) which extend from this line towards the extremity of
the limb and in the opposite direction towards the body-wall. Between
these muscle-attachments the external skeleton is thin and brittle, and
forms a suture, _s_, which breaks through when the animal contracts
the muscles of the leg convulsively, and thus presses the lower
protuberance (_a_) against a projection (_b_) of the first upper joint.
Crabs require to make a very considerable muscular exertion before they
can throw off the limb, and therefore they can only do it when they are
in full vigour.
[Illustration: FIG. 98. The leg of a Crab, adapted for self-mutilation
or autotomy. _A_, the first three joints of the limb, _I_, _II_, _III_.
_s_, the suture, that is, a thin area on the second joint which is
predisposed to breakage. _mf_, flexor muscle, _me_, extensor muscle,
both inserted at the suture. _B_, the entire leg with its six joints
and with the suture (_s_). Slightly enlarged. After MacCullock.]
We have here a quite definite structural adaptation of the parts to a
danger which often recurs--that of falling entirely into the power of
an enemy which has seized the leg. By a sudden violent throwing-off of
the leg the crab escapes from this danger. Quite similar adaptations
are found among certain insects, such as the walking-stick insects
or Phasmids, in which the mechanism is much the same, and lies at an
almost exactly corresponding place, namely, at the line where the
second and third joints of the leg, the 'trochanter' and the 'femur'
meet. In this case the advantage of the arrangement is not merely that
the animals are thus enabled to escape from enemies; it is useful in
another connexion, for a knowledge of which we have to thank Bordage.
This naturalist observed that the Phasmids not infrequently perished at
one of their numerous moultings, by remaining partially fixed in the
discarded husk. Of 100 Phasmids nine died in this way, twenty-two got
free with the loss of one or more legs, and only sixty-nine survived
the moult without any loss at all.
That the moulting or ecdysis of insects is often hazardous may be
observed in our own country, and it is familiar to every one who has
reared caterpillars. These, too, often fail to get clear of their
'cast' cuticle, and they perish unless artificial aid is given to them.
I have never observed any autotomy in them, but in the Phasmids it
seems to be a much-used 'device' and is therefore of great importance
in the persistence of the species.
Limbs which are thus thrown off by autotomy regenerate again from
the place at which they broke off, that is from the 'suture.' It
had been noticed even by the earlier observers (e.g. Goodsir) that
there was a jelly-like mass of cells within the joint, and that the
development of the new limb started from this. It might be supposed
that the regeneration-primordium is present in the rest of the leg
also, but that is not the case, for the animal responds to the tearing
off of one joint or of a smaller number than to the suture, not by
regenerating the torn part directly, but by amputating the whole of
the leg up to the suture, and then from this the regeneration of the
whole leg takes place. In the Phasmids the case is similar, but with
the difference that regeneration is possible from three places, from
the tarsal joints, from the lower third of the tibia, and finally,
from the suture between the femur and the trochanter. There is thus
a regeneration-primordium (_Anlage_) at the beginning of the tarsal
joints, another in the tibia, and a third in the 'suture' and the first
must be equipped, as we should express it, with the determinants of
the five tarsal joints, the second with those for the lower end of the
tibia as well, and the third with all the determinants of the whole
leg, from the 'suture' downwards.
In any case, regeneration is here associated with definite localized
pieces of tissue, and is not a general character of all the cells of
the leg, and, as it obviously runs parallel at the same time with
another adaptation--that of autotomy--there can be no doubt that it too
is dominated by the principle of selection, and that it can not only
be increased, but that it can be concentrated at particular places and
removed from others. But this is only possible if it be bound up with
material particles which may be present in or absent from a tissue, and
which are therefore a supplement to the ordinary essential constituents
of the living cells, although they do not themselves belong to the
essential organization.
I might cite many more examples of localization of regenerative
capacity, but will confine myself to one other, which seems to me
particularly instructive, because it was first interpreted as an
indication of the existence of an adaptive principle in the organism,
a principle which always creates what is useful. I refer to the
regeneration of the lens in the newt's larva.
G. Wolff, an obstinate opponent of the theory of selection, attempted
to solve the same problem as I had before me in my experiments on the
regeneration of the internal organs of newts, that is, he tried to
answer the question whether organs which are never exposed to injury or
to complete removal in the conditions of natural life, and which could
not therefore have been influenced in this direction by the processes
of selection, are nevertheless capable of regeneration. He extirpated
the lens from the eye of Triton larvæ, and saw that in a short time it
was formed anew, and from this he concluded that there was here 'a new
adaptiveness appearing for the first time,' and that therefore adaptive
forces must be dominant within the organism. The current theory of
the 'mechanical' origin of vital adjustments seemed to some to be
shaken by this, and the proclamation of the old 'vital force' seemed
imminent. And in truth, if the body were really able to replace, after
artificial injury, parts which are never liable to injury in natural
conditions, and to do so in a most beautiful and appropriate manner,
then there would be nothing for it but at least to regard the faculty
of regeneration as a primary power of living creatures, and to think of
the organism as like a crystal, which invariably completes itself if it
be damaged in any part. But we have to ask whether this is really the
case.
What makes the regeneration of the lens seem particularly surprising
is the fact that in the fully formed animal it must arise in a manner
different from that in which it develops in the embryo, that is, it
must be formed from different cell-material. In the embryo it arises
by the proliferation and invagination of the epidermic layer of
cells to meet the so-called 'primary' optic vesicle growing out from
the brain--a mode of development which cannot of course be repeated
under the altered conditions in the fully developed animal. The
reconstruction of the organ must therefore take place in a different
way, and if the organism were really able, the very first time the lens
was removed, to react in a manner so perfectly adapted to the end,
and so to inspire certain cells, which had till then had a different
function, that they could put together a lens of flawless beauty and
transparency, we should have reason to suspect that nearly all our
previous conceptions were erroneous, and to fall back upon a belief in
a _spiritus rector_ in the organism.
But the excision of the lens in these experiments was not by any means
an unprecedented occurrence! It is true enough that newts in their
pools are not liable to an operation for cataract, but it does not
follow that the lens is never liable to injury, and could not therefore
be adapted for regeneration. It can be bitten out along with the rest
of the eye by water-beetles or other enemies, and as far back as the
time of Bonnet and Blumenbach (1781) it was known that the eye of the
newt would renew itself if it were cut out, given that a small portion
of the bulb was left. But if this were removed the possibility of
regeneration was at an end. Thus, before the first artificial excision
of the lens, a regeneration-mechanism must have existed, by means of
which the eye with its lens was reconstructed, and this depends on the
characters of the cells of the eye itself--it is localized in the eye,
and without the presence of a piece of eye-tissue no regeneration can
take place. Is it then so especially remarkable that the lens should
be renewed when it is artificially removed without the rest of the
eye? The mechanism for its renewal is there, and is roused to activity
whether the lens alone or other parts of the eye also be removed.
We do not need, therefore, to assume the existence of a purposeful
or adaptive force; it is more to the point to inquire where the
regeneration-mechanism which suggests this inference is to be found.
A definite answer to this is given in a detailed experimental work
recently published by Fischel. It corroborates what Wolff had already
found, that the substance of the new lens develops from cells which
cover the posterior surface of the iris, that is, from cells of the
retinal layer of the eye. First, the margin of the pupil begins to
react to the stimulus of the injury (extraction of the lens); its cells
enlarge, become clear, while previously they were filled with dark
pigment, and finally they proliferate. They thus form a cell-vesicle
similar to the ectoderm-vesicle from which the lens arises in the
embryo, and into this the already mentioned retina-cells from the
posterior wall of the iris grow, elongate, and arrange themselves to
form the so-called 'lens-fibres,' on whose form, arrangement, and
transparency the function of the lens depends. This is marvellous
enough, but not more marvellous than that a whole foot should grow
on the cut stump of a newt's leg, or that a whole eye should arise
from a residual fragment. Here, again, we do not know the processes
which cause the arrangement of the cells and their often manifold
locally-conditioned differentiations, in short, we do not know the
_essential nature of regeneration_. But, in the meantime, we can
endeavour to find out which cell-groups regeneration is bound up with
in particular cases, so as to know where the vital particles, the
'determinants,' which condition regeneration, are placed by nature.
[Illustration: FIG. 99. Regeneration of the lens in the Newt's eye.
_A_, section through the iris (_J_); from its margin and posterior
(retinal) surface the primordium of a new lens (_L_) has developed
after the artificial removal of the old one. _B_, section through the
eye after duplicated regeneration of the lens (_L_) from two areas of
the iris. _Gl_, vitreous humour. _J_, iris. _C_, cornea. _R_, retina.
After Fischel.]
In this case there can be no doubt on that point: they are the cells
on the posterior wall and the margin of the iris. And it is certainly
not the _absence_ of the lens which gives rise to its renewal, as would
necessarily be the case if it were due to the dominance of an adaptive
force. If the lens, instead of being excised, be simply pressed back
into the vitreous humour occupying the cavity of the eye, a new lens is
developed all the same from the irritated margin of the pupil. And if
by chance this margin has been irritated in two places while extraction
of the lens was being performed, then two small lenses will develop
(Fig. 99, _B_). Indeed, several may begin to develop at the posterior
wall of the iris, although they do not attain to full development;
mechanical irritation of any part of this cell-layer is responded to by
the formation of lenses. This surely disposes of the 'mystical nimbus'
which would dazzle us with a new force of life, always creating what
is appropriate. We have before us an adaptation to the liability of
newts' eyes to injury, which, like all adaptations, is only relatively
perfect, since under the usual conditions of eye injury it gives
rise to a usable lens, but under unusual conditions to unsuitable
structures. It is exactly the same as in the case of animal instincts,
which are all 'calculated' for the _ordinary_ conditions of life, but,
under unusual conditions, may operate in a manner quite unsuited to the
necessary end. The ant-lion has the instinct to bore backwards into the
sand, and he makes the same backward-pressing movements when placed
on a glass plate into which he cannot force the tip of the abdomen.
The same is true of the mole-cricket, which makes its usual digging
movements with the forelegs even on a plate of glass. The wall-bee
roofs over her cell when she has laid an egg in it, but she does so
even if the egg be taken out beforehand, or if a hole be made in the
bottom of the cell, so that the honey which is to serve the larva for
food when it emerges from the egg runs out (Fabre). Her instinct is
calculated for filling the cell _once_ with honey, and _once_ laying
an egg in it, because such disturbances as we may cause artificially
do not occur or occur very rarely in natural conditions. There are
countless facts of this kind, for every instinct and every adaptation
can, in certain circumstances, go astray and become inappropriate. This
should be considered by those who still persist in opposing the theory
of selection, for herein lies one of the most convincing proofs of its
correctness. Adaptations can only arise in reference to the majority of
occurrences, and variations which are only useful in an individual case
must, according to the principle, disappear again. Adaptation always
means the establishment of what is appropriate in an average number of
cases.
Therefore the inappropriate reaction of the margin of the iris to an
artificial double stimulus affords additional reason for regarding
regeneration as an adaptive phenomenon. If it were the outcome of
an adaptive force it could never be inappropriate; and if it were
the operation of a general and primary power of the organism it
would be exhibited by the nearly-related frog as well as by the
newt. But, in the frog, extraction of the lens gives rise only to a
sac-like proliferation of the cells of the iris margin, which form no
transparent lens, but an opaque cluster of cells, which destroys vision
altogether. It appears, therefore, that the frog no longer requires the
power its ancestors possessed of regenerating a lost lens.
LECTURE XXI
REGENERATION (_continued_)
Phyletic origin of the regenerative capacity--The liberating stimuli
of regeneration--Production of extra heads and tails in Planarians
(Voigt)--Regeneration in the Starfish--Atavistic regeneration in
Insects and Crustaceans--Progressive regeneration--Regeneration has
its roots in the differentiation of organisms--The nuclear substance
of unicellular organisms is the first organ for regeneration--The
ultimate roots of regeneration.
In the previous lecture we have considered many different forms of
regeneration, and have recognized them as adaptive phenomena; we have
now to inquire how such regeneration-adaptations have arisen, and this
is a very difficult question even in general, while in particular
cases it is often quite unanswerable at present. In regard to the
case last discussed, the regeneration of the lens in the eye of
Triton, our hypotheses would require to reach back to the time of the
primitive vertebrates with an unpaired eye, for the lens of the paired
vertebrate eye, from Mammals down to the lowest Fishes, does not arise
in embryonic development from the retinal cells, but always from the
corneal epithelium, as the elaborate researches of Rabl have recently
shown. It is true that the unpaired parietal eye of some reptiles forms
its lens from the cells of the retinal layer, but it would be difficult
to demonstrate the possibility of a genetic connexion between it and
paired eyes, and in the meantime we must refrain from elaborating a
hypothesis as to the origin of the marvellous faculty the retinal cells
possess of transforming themselves into lens-fibres.
But it is easier to form some sort of picture of the origin and
adaptation of the faculty of regeneration in general.
We saw that the power of regenerating a part can be localized, and
that it does not belong to all the cells of the body, but only
to some of them, and we have to ask how and by what steps it has
been imparted to these. The faculty depends on the possession of a
regeneration-primordium (_Anlage_), and this again, in our mode of
expression, consists of a definite complex of determinants, and as
determinants are the products of an evolution, and thus are vital units
which have arisen historically, they can nowhere suddenly originate
anew in a species, but must be derived directly or indirectly from the
sole basis which, in each species, forms the starting-point of the
individual--that is to say, in the Metazoa, from the germ-plasm of the
ovum. From it the determinant-complex of every regeneration-rudiment
mast in the ultimate instance be derived.
We may think of the matter thus: all the determinants of the germ-plasm
vary, grow slowly or quickly, and in certain circumstances may be
doubled. In this way there arise what we may call 'supernumerary'
determinants, which are not required in the primary building up of
the body from the ovum, and which may remain in an inactive state in
the nuclei of certain cells, ready to become active under certain
circumstances and to produce anew the part which they control. Such
regeneration-idioplasm will at first come to lie in the younger
cells of the determinate organ, but it is conceivable that under the
influence of selection it may be gradually shifted to other cells of
a later developmental origin, or, conversely, to others in a less
external position, so that, for instance, the regeneration-rudiment for
the finger of a newt may be contained not merely in the cells of the
hand, but in those of the fore-arm or even of the upper arm.
But all such segregation of determinant-groups cannot have taken place,
as we might perhaps be inclined to think, at the periphery in the organ
itself during its development; it must take place in the germ-plasm of
the ovum, for otherwise it could not be transmissible, and could not be
directed and modified by the processes of selection, as is actually the
case, as I shall show in more detail later on.
I have already pointed out the importance of the rôle played by
liberating stimuli in regeneration, and not only of extra-organismal
stimuli, such as gravity, but above all of intra-organismal stimuli
that is, the influences exerted in a mysterious manner by other parts
of the animal on the parts which are in process of regeneration. It
is a great merit of the modern tendency in evolution theory that it
has demonstrated the importance of such internal influences. Although
we are still far from being able to define the manner in which these
influences operate, we may say so much, that it depends essentially on
the nature and extent of the loss which parts are reproduced by the
regenerating cells, and, also, on the position and direction of the
injured surface from which the regeneration starts. The influences,
still quite beyond our comprehension, which are exerted on the
regenerating part by the uninjured parts constitute the liberating
stimuli, which evoke the activity of one or other of the determinants
contained in the regeneration-idioplasm.
[Illustration: FIG. 100. Regeneration of Planarians. _A_, an animal
divided into three parts by two oblique cuts. _B_, the fragments(_a_,
_b_, _c_) in process of regeneration. _C_, an animal with various
oblique incisions in the margin of the body, which have induced the new
formation of heads (_k_), of tails (_s_), and pharynx (_ph_). _A_ and
_B_ after Morgan; _C_ after Walter Voigt.]
Walter Voigt has shown, by a series of most interesting experiments,
that it is possible not only to cause the development of a new head
in Planarians by cutting them, in which case a tail may grow from the
anterior portion and a head from the posterior portion, but it is
also possible in an intact animal, that is, one with both head and
tail, to cause the production of a second head, or a second tail, or
both at once, at any part of the body margin at will, according to
the direction of the cut. If the margin of the body be cut obliquely
forwards (Fig. 100, _A_) a supernumerary tail arises (_C_, _s_), if
it be cut obliquely backwards a supernumerary head arises (_C_, _k_),
and in this way several heads and several tails may be produced in
the same animal. It is obvious, then, that the interaction, in the
first place, of the cells of the cut surface, but probably also of the
deeper-lying cells, decides which determinants are to come into action,
those of the head or those of the tail, but both must be present at
every part of the cut. How far below the cut surface the cells take
part in this determination we cannot make out, but that it cannot be
due to the co-operation of all parts is clear in this case at least,
since the animal still possesses its original head and tail. The extra
heads and tails thus produced prove, at any rate, that there can be no
question here of the expression of an adaptive principle, a _spiritus
rector_, or a vital force, which always creates what is good, but that
it is rather a purely mechanical process, which takes its course quite
independently of what is useful or disadvantageous, and that it must
take this course according to the given regeneration-mechanism and the
stimulus supplied in the special case. It cannot be supposed that these
supernumerary heads and tails are purposeful, but who would expect an
adaptive reaction from the animal in a case like this, since cuts of
the kind which we make artificially, and _must keep open artificially_
if the deformities are to develop, hardly occur in nature, and, if they
did occur, would very quickly close up again? Adaptations can only
develop in response to conditions which occur and recur in a majority
of cases, and when they have a useful, that is, species-preserving
result. The adaptiveness of the organism is blind, it does not see the
individual case, it only takes into account the cases in the mass,
and acts as it must after the mechanism has once been evolved. The
case is the same as that of 'aberrant' or mistaken instincts, whose
origin by means of selection is the more clearly proved, since we must
recognize such an instinct as a pure mechanism and not as the outcome
of purposeful forces.
In the regeneration of Planarians we must think of the
regeneration-idioplasm as containing the full complex of the
collective determinants of the three germinal layers, and possibly
we must add to this cells with the complete germ-plasm for giving
rise to the reproductive cells. But when the amputated tail of the
newt is regenerated, or its leg, or the arm of a starfish, or the
bill of a bird, we have no ground for assuming that the cells, from
which regeneration starts, contain the whole germ-plasm, since the
determinants of the replaceable parts suffice to explain the facts.
We must even dispute the possibility of the presence of the whole
germ-plasm in this case, because the faculty of regeneration of the
relevant cells is really no longer a general one, but is limited to
the reproduction of a particular part. This is seen in the fact that,
in the starfish, whose high regenerative capacity is well known, the
central disk of the body may indeed give rise to new arms[2]; but
an excised arm, to which no part of the disk adheres, is in most
starfishes unable to give rise to the body. Thus the arm does not
contain in its cells the determinants of the disk, but the latter
contains those of the arm. We are not surprised that the amputated
tail of the salamander does not reproduce the whole animal, but this
can only be because the impelling forces to the regeneration of the
whole animal are wanting, that is, that the cut surface only contains
the determinants of the tail and not the complete germ-plasm. It might
be objected here that the tail-piece is too small to give rise to the
whole body, but in _Planaria_ it is only very diminutive heads and
tails which grow from the artificial incisions, and the same is true
of starfishes when only a single arm and a small piece of the disk
have been left. Notwithstanding the small amount of living substance
at their disposal, and although they are at first unable to take
nourishment, they send out very small new arms (Fig. 101), close up the
wounded surface, and, after reconstruction of the mouth and stomach,
begin to feed anew. The new arms may then grow to the normal size.
[2] I see now that there are contradictory statements in regard to this
case. Possibly these depend on the different behaviour of different
species, and this on the varying frequency of mutilation. Starfishes
which live on the shore between the rocks, for instance on the movable
stones of a breakwater, are very frequently mutilated; in some places
it is rare to find a specimen without traces of former wounds. H. D.
King counted among 1,914 specimens of _Asterias vulgaris_ 206 in the
act of regenerating a part, that is, 10.76 per cent. In the case of
the starfishes from deep water this cause of injury does not of course
exist.
[Illustration: FIG. 101. A starfish arm, growing four new arms; the
so-called 'comet-form.' After Haeckel.]
We must therefore assume that, in many cases, the
regeneration-primordium consists of cells which only contain a definite
complex of determinants in the form of latent regeneration-idioplasm,
as, for instance, certain cells of the tail of Triton contain the
determinants of the tail, certain cells of its leg the determinants of
the leg, and so on. In many cases we can speak even more precisely,
and determine from which cells the nerve-centres, from which the
muscles, and from which the missing section of the food-canal will be
formed, as was recently shown by Franz von Wagner in regard to the worm
_Lumbriculus_, whose regenerative capacity is so extraordinarily high.
We must then attribute to each of the relevant cells an equipment of
regeneration-idioplasm, which includes only the relevant complex of
determinants.
I need not here go further into detail, but I should still like to
show that, in reality, as I assumed in regard to the regenerative
capacity of a part, the root of the regeneration-idioplasm lies
in the germ-plasm, that it is present there as an independent
determinant-group, and, like every other bodily rudiment (_Anlage_),
must be handed on from generation to generation. This assumption is
necessary, as has been already indicated, on the ground that the
faculty of regeneration is hereditary, and hereditarily variable, on
the same ground, therefore, as that on which the whole determinant
theory is based. The regeneration-determinants must be contained _as
such_ in the germ-plasm, otherwise a twofold phyletic development could
not have occurred, as it actually has, in many parts. The tail of the
lizard is adapted for autotomy; it breaks off when it is held by the
tip, and this depends on a special adaptation of the vertebræ, which
are very brittle in a definite plane from the seventh onwards. This is
thus a very effective adaptation to persecution by enemies. The tail
which has been seized remains with the pursuer, but the lizard itself
escapes, and the tail grows again. But this regeneration does not take
place in the same way as in the embryo; no new vertebræ are formed,
but only a 'cartilaginous-tube,' a new structure, a substitute for the
vertebral column; the spinal cord with its nerves is not regenerated
either, and the arrangement of the scales is somewhat different.
This last point, in particular, indicates that the determinants of
the regeneration-rudiment may pursue an independent phylogenetic path
of their own, for this scale arrangement of the regenerated tail
is an atavistic one, that is, it corresponds to a more primitive
mode of scale arrangement in these Saurians. We know quite a number
of cases similar to this. It not infrequently happens that cut-off
parts regenerate, but that they do so not in the modern form, but in
one that is in all probability phyletically older. Thus the legs of
various Orthoptera, as of the cockroaches and grasshoppers, regenerate
readily, but with a tarsus composed of four joints instead of five[3],
and the long-fingered claws of a shrimp (_Atyoida potimirim_) is
replaced by the older short-fingered type of claw, while in the
Axolotl an atavistic five-fingered hand grows instead of the amputated
four-fingered one.
[3] New investigations, specially directed to this point, by
R. Godelmann, have shown that 'in the great majority of cases'
the regenerated legs of a Phasmid (_Bacillus rossii_) exhibit a
four-jointed tarsus; but the regeneration of five joints also occurs,
though only after autotomy, and only in seven out of fifty cases
(_Archiv für Entwicklungsmechanik_, Bd. xii, Heft 2, July 1901).
The regeneration-rudiment in this species seems to be in process of
advancing slowly to the five-jointed type.
This last case shows that it is not merely a lesser power of growth
that accounts for the difference between the regenerated part and
the original, for here more is regenerated than was previously
present. There remains nothing for it but the assumption that the
regeneration-determinants have remained at a lower phyletic level,
while the determinants which direct embryogenesis have varied, and
either developed further or retrogressed. It is easy to understand that
the regeneration-rudiment must vary phyletically much more slowly
than the parts which evolved in the ordinary way and much more slowly
than the determinants of these parts, for natural selection means a
selection of the fittest, and the speed with which the establishment
of a variation is attained depends, _ceteris paribus_, on the number
of individuals that are exposed to selection with respect to the
varying part. If in a species of a million living at the same time
nine-tenths perish by accident, there will remain only 100,000 from
which to select the 1,000 which we will assume constitute the normal
number of the species. The more of these 100,000 which possess the
useful variation the higher will be the percentage of the normally
surviving 1,000 possessing it, and the more rapidly will the useful
variation increase. But when it is a question of the variation of
the regeneration-primordium, the selection will take place not among
all the 100,000 individuals which chance has spared, but only among
those of them which have lost a limb by accident, and thus are in
a position to regenerate it more or less completely. If we assume
that this takes place in 10 per cent. of cases, then selection for
the improvement of the regeneration-apparatus will only take place
among 1,000 individuals, and thus the process of modification of the
regeneration-primordium must go on very much more slowly than that of
the limb itself.
I do not see how the opponents of the germ-plasm theory can explain
these facts at all, for the appeal to external influences is here
entirely futile, and that to internal liberating stimuli does not
suffice, since these must be different after a part has been cut
off from what they were when the limb developed normally, and also
different from those which prevailed at the normal origin of the
limb in ancestral forms. The four-jointed tarsus of the ancestors
of our cockroaches did not arise as a result of amputation. We
cannot therefore avoid referring the processes of regeneration to
particular 'regeneration-determinants,' which are contained in the
germ-plasm and are handed on in ontogeny with the other determinants
from cell-division to cell-division, till ultimately they reach the
cells which are to respond, or may have to respond, to the stimulus
of injury by some expression of their regenerative capacity. As these
determinants, as has been shown, can often only be very slightly
subject to the influence of selection processes, they will, in many
respects, lag behind in the phyletic development, and will tend to
belong to an ancestral type of the relevant part. They will often
remain for a long time at this ancestral level, and they will always
adapt themselves to new requirements more slowly than the parts which
arise in the normal way, and the determinants representing these in
the germ. But the regeneration-determinants _are_ variable, and,
indeed, are so hereditarily, and independently of the structure of the
normal parts. They thus follow their own path of phyletic development,
and this one fact is enough to secure a preference for the germ-plasm
theory above others that have hitherto been suggested. None of these
has even attempted an explanation of this fact; the tendency has rather
been to call it in question. This, however, can be done at most only in
regard to the explanation of the regenerations as atavistic, certainly
not in regard to the progressive variations of the regenerated part,
such as have been established by Leydig and Fraisse in regard to the
lizard's tail. It may be doubted whether the most primitive insects had
only four tarsal joints, but there is no disputing the kainogenetic
deviation of the lizard's-tail.
I have interpreted the regenerative capacity as secondary and acquired,
not as a primary power of all living substance, and I should like to
substantiate this in another way.
Let us go back to the simplest organism conceivable, which must have
represented the beginning of life on our earth, and we see that this
need not have possessed any special power of regeneration, because, for
an organism without differentiation of parts, growth is equivalent to
regeneration. But growth is the direct outcome of one of the primary
characters of the living substance, the capacity of assimilation.
This cannot be an adaptive phenomenon, nor can it have arisen through
selection, because selection presupposes reproduction, and reproduction
is only a periodic form of growth; but growth follows directly from
assimilation. The fundamental characters of the living substance, above
all the dissimilation and assimilation which condition metabolism, must
have been in existence from the first when living substance arose,
and must depend on its unique chemico-physical composition. But the
faculty of regeneration could only be acquired when organisms became
qualitatively differentiated, so that each part was no longer like
every other part or like the whole. As soon as this stage was reached
the faculty of regeneration would necessarily be developed, if further
multiplication was to take place. For when each fragment could no
longer become a whole by simply growing, some arrangement had to be
made by which each fragment should receive, in the form of primary
constituents, what it lacked to make up the whole. We do not know the
first beginning of this adaptation, but, in its further development,
it appears in the form of 'nuclear substance,' enclosed in the nucleus
of the cell, and, as is well known, it is now to be found in all
unicellular organisms. That the nucleus there precedes regeneration
in the sense that without a piece of it the cell-soma is not able to
complete itself alone, we have already seen, and the explanation of
this fact has always seemed to me to be that invisibly minute vital
units relating to the regeneration of the injured part leave the
nucleus and evoke the development of the missing parts by laws and
forces still unknown to us. Loeb has recently claimed that the nucleus
is the cell's organ of oxidation; but if that be true it would still
not exclude the possibility that the nucleus is also and primarily
a storehouse of the material bearers of the primary constituents
of a species. It must be regarded as such when we call to mind the
phenomena of amphimixis in its twofold aspects as conjugation and as
fertilization, and its obvious outcome among higher organisms where it
implies the mingling of the parental qualities.
Thus the 'nuclear substance' of unicellular organisms is for us the
first demonstrable organ of regeneration, and first of all for normal
regeneration, which takes place at every reproduction, for instance,
of an Infusorian. For we have already seen that, in the transverse
division of a trumpet animalcule (_Stentor_), the anterior part must
develop the posterior half anew, while the posterior half must develop
the much more complex anterior half, with mouth region and spiral
bands of cilia. But as soon as the arrangement for normal reproduction
was elaborated, as soon as the nucleus was present, as a depôt of
'primary constituents,' this implied the possibility of regeneration
in exceptional cases, that is, after injury. The mechanism was already
there, and it came into operation as soon as a part of the animal was
missing.
It is in the first nucleus, therefore, that we have to look for
the source of all regenerative capacity, both in unicellular and
multicellular organisms. But with the origin of the latter a limitation
took place, either quite at the beginning or a little later, for each
nucleus of the cell-colony no longer contained the whole complex of
'primary constituents' or determinants of the species, but, in many
cases, only the reproductive cell possessed them. As soon as this began
to develop into a whole by cell-division the determinant-complex was
segregated. Thus the first cell-colonies with two kinds of cells arose,
as we have seen in the case of _Volvox_--the reproductive cells with a
complete equipment for regeneration in their nucleus, and the somatic
cells with a limited equipment for regeneration in their nuclei. The
somatic cell could no longer give rise anew to the whole organism, but
could only reproduce itself or its like.
But as many of the lower Metazoa and Metaphyta possess _the power of
budding_, that is, are able not only to produce a new individual
from definite cells--the reproductive cells--with or without sexual
differentiation, but from other cell-groups also, these must contain
the whole complex of determinants appertaining to the reconstruction
of the organism, and we have to ask how this is reconcilable with the
differentiation of a multicellular organism, whose different kinds of
cells depend, according to our interpretation, on the fact that they
are controlled by different determinants.
Obviously, there is only one way out of this difficulty, and it is the
one we have already indicated, that although the diffuse regenerative
capacity which we have just alluded to occurs in species which exhibit
gemmation, this does not exclude the control of a cell by a specific
determinant; other determinants may be contained in the cell, in a
state, however, in which they do not affect it, that is, in an inactive
or latent state.
Thus we arrive in this way also at our earlier assumption that an
inactive accessory-idioplasm is given to all, or at least to many
cell-generations. Only among plants must this necessarily be complete
germ-plasm, and among the lower plant-forms, as in _Caulerpa_ among
the Algæ, in _Marchantia_ among Liverworts, it must be assumed to
be present in nearly all the cells, according to the experiments
in regeneration made by Reinke and Vöchting. But in multicellular
animals which develop from two different germinal layers equipped
with a different complex of determinants budding arises from a
combination of at least two different kinds of cells, and we must
only ascribe to each of these its own peculiar determinant-complex as
regeneration-idioplasm. Higher plants show us that well-marked power
of budding is not necessarily associated with a high regenerative
capacity, the histologically specialized cells among them will contain
no inactive germ-plasm, because they do not need it. But in animals the
power of budding is probably always combined with high regenerative
capacity, as is shown by the Polyps and Medusoids above all, and in a
different way by the Ctenophores, which exhibit no budding and at the
same time a very slight regenerative capacity, although they possess
an organization scarcely higher than that of the Hydromedusæ. In the
Ctenophores each of the first segmentation-cells, when artificially
separated, yields only a half-embryo, and we may conclude from this
that it contains no complete germ-plasm in an inactive state, or at
least very little, and certainly not a sufficient quantity to make it
readily regenerative.
Undoubtedly, however, the regenerative capacity occurs apart from the
capacity for budding, yet this in no way contradicts the theory. As
we have seen, a high regenerative capacity is to be found among many
animals which occur only as 'persons' and not as colonies or stocks,
but only in those which are readily liable to injury, and only in
the manner conditioned by their injury. In the higher Metazoa the
regenerative power becomes more and more limited, and in the Mammals it
sinks to a mere closing up of wounds.
If we take a survey of the assumptions we have been compelled to
make from the standpoint of the theory to explain the development of
germ-cells, budding, and regeneration, it would seem as if it were
contradictory to assume that, on the one hand, complete germ-plasm
should be given to certain cell-series as inactive accessory idioplasm,
and, on the other, that very numerous cells, at least in the lower
Metazoa, should have received the idioplasm of budding, and still
more numerous cells that of regeneration. But it is obvious that
among the lower Metazoa the idioplasm of budding and the idioplasm of
regeneration are equivalent; the same idioplasm, which, when liberated
by stimuli unknown to us, co-operates from two or three germinal layers
in the formation of a bud, effects, in response to the known stimulus
of injury, the regeneration of the mutilated part. But germ-cells
can never arise in the Metazoa from the partial budding-idioplasm or
regeneration-idioplasm, because this is not complete germ-plasm, and
because it can only give rise to budding or regeneration through the
co-operation of two or more kinds of cells, while germ-cells always
originate from _one_ cell and never arise from the fusion of cells.
Germ-cells can thus only arise from the cells of the germ-track, and
in no other way, no matter whether the germ-track lie in the ectoderm,
as in the Hydromedusæ, or in the endoderm, as in true jellyfishes
(Acalephæ) and the Ctenophores, or in the mesoderm, as in many higher
groups of animals. It is only apparently that these cells belong to
one particular layer, for in reality they are unique in kind, and they
are simply assisted in their development by one or other cell-layer,
from which they not infrequently emancipate themselves, as happens
so notably in the Hydromedusæ. As we have already said, it is only
among plants that we must think of budding as arising from cells which
contain complete germ-plasm, for here there are no 'germinal layers'
corresponding to those of animal development, and the cells of 'the
growing point' must be equipped with the complete germ-plasm. The
plant, like the Hydroid stock and the Siphonophore colony, is saved
from death, in spite of the frequent loss of its members, mainly by
the fact that it is capable of producing, at almost any part above the
ground, buds which develop into new shoots, with leaves and the like.
This makes a power of regeneration on the part of the individual leaves
and flower-parts superfluous, but at the same time it implies that an
enormous number of cells must be distributed over the whole surface
of the plant, each of which can in certain circumstances become the
starting-point of a bud. That is to say, each must contain, in a latent
state, the complete germ-plasm which is necessary for the production of
an entire plant.
We must therefore assume that, in the higher colony-forming plants,
germ-plasm is contained in a great many cells, perhaps in all which
are not histologically differentiated, and sometimes even in those
which are so, as, for instance, in the leaves of Begonias. I suppose,
therefore, that in the higher plants the process of development implies
a segregation of the determinant-complexes of the germ-plasm, but that
this takes place at a late stage, and that in a much higher degree than
among animals the individual or the 'person' carries with it germ-plasm
in a latent state. To this must be attributed the fact that the plant
is not only able to make good its losses in twigs and branches by
sending out new shoots, but that cuttings, that is, detached shoots,
are also able to take root, and in general to give rise to what is
necessary to complete themselves according to the position of the part
in question. In the ontogeny of animals, too, we must assume that it
requires a liberating stimulus to rouse the determinants to activity,
that this stimulus is to be sought for in the influence exercised by
the constitution of the cell on the idioplasm contained within it,
and that this constitution in its turn is subject to influences from
external conditions, including the cell-soma itself. We may therefore
suppose that, among plants also, the germ-plasm latent in numerous
cells only becomes active in whole or in part according to the
influences exerted on it by the state of the cell at the moment; but
this varies with external circumstances, according to whether the cell
is exposed to light or lies under ground, according as it is influenced
by gravity, by moisture, chemical stimuli, and so on.
It might be objected to this that it would be simpler not to assume
a segregation of the germ-plasm into determinant-complexes at all in
order to explain the process of development, but rather to credit each
cell with a complete equipment of germ-plasm from the beginning to the
end of the ontogeny, and to attribute the differences in the cells,
which condition the structure of the plant and its differentiation,
solely to the different influences, external and internal, to which
the cell is exposed, and which rouse some determinants to activity at
one part and others at another. Perhaps the botanists would be more
readily reconciled to this idea, but it seems to me that there are two
points which tell against the possibility of its being correct. In the
first place, it is far from being established that every cell in the
higher plants is capable of giving rise, under favourable conditions,
to a whole new plant; every tree and every higher plant has a multitude
of cells in its leaves, its flowers, and so on, which cannot do this,
which are in fact differentiated in one particular direction, that is,
they contain only one kind of determinants, like the histologically
differentiated cells of the tissues of the human body. Secondly,
there are other organisms besides plants, and a theory of development
cannot be based on the phenomena to be observed among plants alone,
any more than a theory of heredity can. There are obvious differences
in the processes of life among plants as contrasted with those among
animals, but it is improbable that there is any thoroughly fundamental
difference. It is, however, indubitable that the cells forming the
tissues of higher animals, the nerve, muscle, and glandular cells,
are really differentiated in one direction, and are quite incapable,
under any circumstances whatever, of growing into an entire organism,
and even from this alone we might conclude that they contain only one
primordium or determinant. Are we then to assume that the vascular
cells, epidermis-cells, wood-cells, and so on, of the higher plants,
which are also differentiated in one direction, do nevertheless contain
the complete germ-plasm? I do not see any ground for such an assumption.
To conclude what can be said on the subject of regeneration we must
return to the question of an ultimate explanation of this marvellous
phenomenon. I have declined to attempt any explanation at all, because
I do not consider it possible to give a sufficient one as yet, but I
should like at least to give an indication as to the direction in which
we must look for it.
We assumed that there is a regeneration-idioplasm, and therefore that
there are 'primary constituents' at certain positions in the body,
but how does it happen that these are able to build up the lost parts
in the proper situation and detail? A theoretical formula might well
be thought out, according to which the determinants of successive
parts would become active successively, and would thus liberate one
another in an appropriate order of sequence, but there would not be
much gained by this, especially as what we already know in regard to
the regrowth of the legs and toes in Triton does not harmonize with
such an assumption. It appears to me more important--though even here
we must still be very vague as to details--to recognize that, in all
vital units, there are forces at work which we do not yet know clearly,
which bind the parts of each unit to one another in a particular order
and relation. We were obliged to assume such forces even in regard
to the lowest units, the biophors, since otherwise they could not be
capable of multiplication by division, on which all organic growth
depends, unless we are to assume, as Nägeli did, a continual _generatio
æquivoca_ of the specific kinds of biophors (his 'micellæ'). But we
shall see later, when we come to speak of spontaneous generation, that
we cannot acquiesce in such an assumption. If, then, we cannot conceive
of a power of division arising from within and depending solely on
growth by means of assimilation, without such attractive and repellent
forces or 'vital affinities' the internal parts would necessarily fall
into disorder at every division. It seems to me therefore that such
'affinities' must be operative at _all stages_ in the life of the vital
units, not only in biophors, but also in the cell, and in the 'person'
as well as in determinant and id. It is true that 'persons' no longer
generally possess the power of multiplying by division, but in plants
and lower animals many do possess it; and the power of giving rise
anew to certain parts is obviously a part of that power of doubling
the whole by division. The ultimate roots of regeneration, then, must
lie in these 'affinities' between the parts, which preside over their
arrangement and are able to maintain it and to give rise to it anew.
In this respect the organism appears to us like a crystal whose broken
points always complete themselves again from the mother-lye after
the same system of crystallization, obviously in this case too as a
result of certain internal directive forces, polarities, which here
again we are unable precisely to define. But the difference between
the organism and the crystal does not--as people have been hitherto
inclined to believe--lie only in the fact that the crystal requires the
mother-lye to complete itself, while the vital unit itself procures the
material for its further growth; it lies also in the fact that such
regeneration is not possible in every organism and at every place, but
that _special_ 'primary constituents' are necessary, without which the
relevant part cannot arise. The indispensableness of these primary
constituents, the determinants, seems to me to depend on the fact
that the new structure cannot be built up simply by procuring organic
material, but _that specially hewn stones, different in every case,
are necessary_, which can only be supplied in virtue of an historical
transmission, or, to abandon the metaphor, because the vital units of
which the organ is to be reconstructed possess a specific character
and have a long history behind them; thus they can only arise from
such vital units as have been handed on through generations, that is,
from the determinants. But these primary constituents are given to the
different forms of life in very varying degrees and in very unequal
distribution, and as far as we can see according to their suitability
to an end.
LECTURE XXII
SHARE OF THE PARENTS IN THE BUILDING UP OF THE OFFSPRING
The ids are 'ancestral plasms'--The reducing division brings
about a diversity of germ-plasm in the germ-cells--Bolles
Lee's 'Neotaxis' even in the primordial germ-cells--Häcker's
observations on the persistent distinctness of the maternal and
paternal chromosomes--Identical twins--The individuality is
determined at fertilization--Unequal share of the ids in the
determination of the offspring--Preponderance of one parent in the
composition of the offspring--Certain ids of the ancestors remain
unchanged in the germ-plasm of the descendants--Struggle of the
Biophors--Alternation of the hereditary sequences in the parts
of the child--Reversion--Datura-hybrids--Zebra-striping in the
horse--Three-toed horses--New experiments in hybridization among
plants by Correns and De Vries--Xenia.
As far as the phenomena of regeneration and budding are concerned, we
have not been able to do much more than bring them under a formula,
which harmonizes with the germ-plasm theory. But the case is different
with the actual phenomena of inheritance in the restricted sense, for
instance, with regard to the transmission of individual peculiarities
_from parent to child_. Here the theory really increases our insight
and lets us penetrate deeper into the causes of the phenomena; it is
here no longer a mere 'portmanteau-theory.'
We are well aware, especially from observation on ourselves, that is,
on Man, that the children of a pair often resemble one another but
are never alike, and that one child frequently resembles one parent,
another the other, while a third may exhibit a mingling of both
parents. How does this come about? Since the germinal substance of both
parents is derived from that of the ovum, from which they themselves
have arisen--and must therefore be the same in all the germ-cells
to which they give rise--new determinants cannot be added, and old
ones cannot be dropped out, and variation of the determinants, the
possibility of which is granted, would still not directly bring about
the familiar mingling of resemblances to the two parents, but would at
most give rise to something new and strange.
Here the theory helps to elucidate matters. We found ourselves obliged
to assume that the germ-plasm is composed of ids, that is, of
equivalent portions of germ-plasm, each of which contains all the kinds
of determinants appertaining to the building up of an individual, but
each of these kinds in a particular individual form. I have already
called these ids 'ancestral plasms,' and the term is appropriate, in
so far that in every fertilization an equal number of ids from the
father and from the mother are united in the ovum, so that the child
is built up of the ids of his two nearest 'ancestors.' But as the ids
of the parents are derived from those of the grandparents, and these
again from those of the great-grandparents, the ids are in truth the
idioplasm of the ancestors.
The expression, however, has been very frequently misunderstood, as
if it were intended to mean that the ids retained _unchanged_ for all
time the character of their respective ancestors, and I have even
been credited with supposing that our own ids still consist of the
determinant-complexes of our fish-like or even Amœba-like ancestors.
But in reality no id exactly or completely corresponds to the type,
that is, to the whole being of any one of the ancestors in whose
germ-plasm it was formerly contained, for each of the ancestors had
many ids in his germ-plasm, and his entire constitution was not
determined by any one of these alone, but by the co-operation of them
all. The individual arising from a germ-cell must necessarily be the
result of all the ids which make up his germ-plasm, but undoubtedly the
share taken by some of them may be much stronger than that taken by
others. It is also clear that, if we leave out of account any possible
variation on the part of the ids, each of them belongs, not to one
ancestor only, but to a whole series of ancestors, and must have taken
part in their development, so that it is not the idioplasm of any
particular ancestor, but only ancestral plasm in the general sense. In
this sense we may quite well retain the designation, 'ancestral plasm,'
for the id.
Thus, according to our view, the germ-plasm consists of ids, each of
which contains all the determinants of the whole ontogeny, but usually
in individually different quality.
Returning for a moment to the processes by which the reduction of the
chromosomes, that is, of the nuclear rods of germ-plasm in the ovum
and sperm-cell is brought about, we recall the fact that this happens
at the last two divisions of the germ-cell, the so-called 'maturing
divisions.' In these the nuclear substance, as we have seen, is
divided between the two daughter-nuclei in a manner quite different
from the usual one, for a longitudinal splitting of the rods, bands,
or spheres in the equatorial plane of the nucleus does not take
place, but half the number of rods move into the right and half into
the left daughter-nucleus without previous division, so that in each
daughter-nucleus the number of rods is reduced to half (Fig. 76).
[Illustration: FIG. 76. Diagram of the maturation divisions of the
ovum. _A_, primitive germ-cell. _B_, mother-egg-cell, which has grown
and has doubled the number of its chromosomes. _C_, first maturation
division. _D_, immediately thereafter; _Rk_ 1, the first directive cell
or polar body. _E_, the second maturation spindle has been formed; the
first polar body has divided into two (2 and 3); the four chromosomes
remaining in the ovum lie in the second directive spindle. _F_,
immediately after the second maturation division; 1, the mature ovum;
2, 3, and 4, the three polar cells, each of these four cells containing
two chromosomes.]
Although the distribution of the rods in this manner takes place twice
in succession, the normal number is not, as we have already seen,
reduced to a quarter, because, long before the occurrence of the first
maturing division, a duplication of the rods by means of longitudinal
division had taken place, and thus the first division differs from an
ordinary division in that the splitting of the rods does not take place
during the process of dividing but long beforehand. Only the second
maturing division differs from all other nuclear divisions known to
us, since it is not associated with any splitting of the rods at all,
but conveys half of the existing rods into each daughter-nucleus. It
is the time reducing division, through which the number of the rods is
reduced to one half[4].
[4] Recent investigations have shown that the reduction of the
chromosomes does not always take place exactly in accordance with the
scheme here indicated, but that it differs from it in many cases. But
as investigations on this point are as yet by no means complete, I need
not go into the question further; the ultimate result is the same in
any case.
This numerical reduction must, however, have other consequences; it
must make the germ-cells of the same individual qualitatively unlike,
that is, in relation to their value in inheritance. Let us assume only
four chromosomes of the rod-form ('idants') as the nuclear elements
of a species, two of which, _A_ and _B_, come from the mother, and
other two, _C_ and _D_, from the father, the last maturing division
may, as far as we can see, result either in removing the combination
_A_ and _B_ from _C_ and _D_, or _A_ and _C_ from _B_ and _D_, or _A_
and _D_ from _B_ and _C_; there is thus a possibility of one of six
different combinations of rods in any one germ-cell. What is the same
thing, six different kinds of germ-cells differing in their hereditary
primary constituents may _be developed in the same_ individual. As this
new combination, or, as we may call it, neotaxis of the germ-plasm
elements, takes place in female as well as in male individuals, there
is a possibility that, in fertilization, 6 × 6 = 36 individuals with
different primary constituents may arise from the germ-cells of the
same two parents. Of course the number of possible combinations
increases very considerably in proportion to the normal number of
rods, for with eight of these it comes up to 70, and with sixteen to
12,870; the number of individuals differing in their inherited primary
constituents would thus be enormous, for each of the 70 or of the
12,870 different hereditary minglings of the ovum could combine in
amphimixis with 70 or 12,870 different sperm-cells, so that 70 × 70
and 12,870 × 12,870 offspring individually different in their primary
constituents might arise from the same two parents. In Man there are
said to be sixteen nuclear rods; so that in his case the last-mentioned
number of parental hereditary minglings might occur. This may seem a
disproportionately high number as compared with the small number of
children of a human pair, but we must not judge from the case of Man
alone, and in plants and animals, which we have already discussed, the
number of descendants is very much larger, and is often enormous. We
saw what significance this apparent extravagance on the part of nature
has, for without it adaptation to changed conditions of life would
not be possible, since, if only so many were born as could attain to
reproduction, no selection of the fittest could take place. The same
would be the case if all the young of a species were alike, and even if
all the descendants of a single pair were alike, effective selection
would be excluded, since only as many individualities could be selected
as there were pairs of parents. It is easy to understand that selection
works more effectively the larger the number of descendants of a
species and the more they differ from each other. The chance that the
best possible combination of characters will occur is thereby increased.
Although we cannot calculate how many individuals of different
combinations of characters natural selection requires to work upon in
order to direct the evolution of the species[5], we can understand
that only as large a choice as possible can secure that the best
possible adaptations of all parts and organs are brought about and
maintained. Precisely in the fact that in every generation such an
enormous superfluity of individuals is produced lies the possibility
of such intensive processes of selection as must continually take
place, if the adaptation of all parts is to be explained. For if among
the thousands of descendants of a fertile species each hundred were
alike among themselves, these hundreds would have, as far as natural
selection was concerned, only the value of a single variant. But such
an all-round adaptation as actually exists in the structure of species
requires as many variants as possible; it requires that each individual
should be a _peculiar complex of hereditary characters_; that is, that
all the fertilized germ-cells of a pair should possess an individually
well-marked character.
[5] For this reason I have left the number of id-combinations given
above unaltered, though, according to the most recent researches into
the processes of maturation, they are probably too high, since every
conceivable combination does not actually occur. We are here concerned
less with the exact number than with the principle.
The justification of this postulate becomes all the clearer if we take
into consideration the male germ-cells as well as the female. Let us
think of the enormous number of sperm-cells which are produced by many
animals, and indeed by the highest of them--an almost incalculably
large number which certainly goes far beyond millions. Let us assume
that in Man there may be 12,870 million spermatozoa, then, with
sixteen ids, and with an equally frequent occurrence of all possible
combinations of germ-plasm--there would be 12,870--there would be a
million of each type containing identical germ-plasm. The danger that
several ova would be fertilized by identical sperm-cells would be by no
means small.
It cannot, therefore, surprise us that other means have been employed
by Nature to secure re-groupings of the ids. The simplest means would
be, if before each division of the primitive germ-cells the nuclear
rods were to divide, and if the split halves were irregularly
intermingled, then at the formation of the next nuclear spindle an
entirely new arrangement of the halves would result. But in animals, at
least, this is certainly not the case; the processes of reduction are
restricted to the maturing divisions.
Years ago Ischikawa observed that, in the conjugation of _Noctiluca_,
the nuclei of the two animals become closely apposed, but that they do
not fuse, although they behave like a single nucleus in the division
which follows. In this case _paternal and maternal nuclear substance
remain separate_ (Fig. 83, vol. i. p. 317). The same phenomenon has
since been repeatedly observed in many-celled animals, first by Häcker,
then by Rückert in the Copepods, and afterwards by Conklin in the eggs
of a Gastropod (_Crepidula_). But all these observations referred only
to the earlier stages of ovum-segmentation up to twenty-nine cells,
and it could not be affirmed that the distinctiveness of the paternal
and maternal chromosomes lasted till farther on into the ontogeny.
Professor Häcker now informs me, however, that he has been able to
trace this separateness in a Copepod (_Canthocamptus_) not only from
the beginning of segmentation on to the primitive genital cell, but
also through the divisions of this up to the mother-egg-cell[6].
Thus we may now assume that the paternal and maternal hereditary
bodies remain distinct, not only for a time, but throughout the whole
development, a fact which confirms our assumption of the independence
of the nuclear rods, notwithstanding their apparent breaking-up in
the nuclear reticulum of the 'resting' nucleus. This new knowledge
throws fresh light in another direction; it proves to us that the
remarkable and complicated processes which go on in the nucleus
during the maturing divisions have really the significance which I
long ago ascribed to them[7], that of effecting the maximum diversity
of intermingling of the paternal and maternal hereditary elements.
For Häcker has shown that during the second maturation-division
the paternal and maternal chromosomes are no longer united each
in a special group, but occur scattered about in the nucleus, and
subsequently come together again to form two differently combined
groups.
[6] Since this was written Häcker has published his results. See
_Anatom. Anzeiger_ xv (1902), p. 440.
[7] See my essay, _Amphimixis_, Jena, 1891.
If this were not so, if the maternal and paternal chromosomes remained
separate, then the reducing division would cause only one of these
groups to reach each of the germ-cells, and thus each mature ovum
or sperm-cell would contain either only paternal or only maternal
hereditary bodies. But this would make a reversion to more than three
generations back impossible, and as such reversions undoubtedly occur,
we must conclude that manifold new combinations of the paternal and
maternal chromosomes take place. This obviously happens during the
maturation-divisions, at least in the Metazoa.
The more numerous the rods or the free individual ids in a species
are, the more numerous are the possible combinations. Whether all the
mathematically possible combinations actually occur is a different
question, which I should not like to answer in the affirmative just
yet; but in any case the _actual_ number of combinations in a species
with many nuclear elements will be greater than in one with few, and
in this respect those species in which the ids occur as independent
granules will have an advantage over those in which they are combined
into rods or bands (idants). These latter, however, afford us a better
possibility of deducing the new combinations of the ids, although the
idants themselves are not outwardly distinguishable from each other.
I must refrain from going into these highly interesting processes in
more detail just now. So much is certain, that Nature makes use of
_various means_ to bring about the re-combination, and at the same time
the reduction of the ids during the two 'reducing divisions.' This is
proved by the fact recently established by Montgomery, that in many
animal groups reduction results from the _first_ maturing division.
Whether it operates at this stage with rings, bands, double rods,
X-shaped structures, groups of four (tetrads), and so on, all this
serves the same end, the more or less thoroughgoing re-arrangement of
the hereditary vital units. I am convinced that new investigations
into these processes, if they were undertaken from this point of view,
would lead to very important results[8]. It would be important to find
out how great the variations are which thus arise, for it is very
probable that they differ in degree in the different animal-groups.
Even the combination of the ids into rods (idants) indicates that some
species may be more conservative than others in maintaining their
id-combinations, and that there will be among them a greater tenacity
in the hereditary combinations of characters (i.e. of the 'type' of
the parents). If we should succeed in penetrating more deeply into
these processes we should probably also understand why in certain human
families the hereditary characters are transmitted more purely and more
tenaciously than those of other families with which they have mingled,
and so on. It may well be that the persistence of character is due
to the fact that ids which have once combined into rods hold firmly
together, for it seems to me in no way impossible that individual
differences should occur even in these most delicate processes.
[8] Since this was written for the first edition observations of this
process have been considerably increased, and discussions as to the
exact interpretation of these are in full tide; we are surrounded by
a wealth of new observations, facts, and explanations, without having
attained to a consistent and unified theory. Several naturalists, such
as Boveri, Häcker, Wilson, and others, have attempted interpretations,
but these are in many points contradictory to one another. It is
therefore impossible to enter into the question in detail here;
further light from new observations must be awaited. So much we may
say, however, that it is not chance alone which presides over the
re-arrangement of the chromosomes during the reducing divisions;
_affinities_ play a part also; there are stronger or weaker attractions
between the chromosomes, which aid in determining their relative
position to one another.
But let us leave these more intimate questions out of account
altogether, and turn our attention to the more obvious and less
delicate phenomena, and we find that the re-arrangement of ids
(Neotaxis) which we have just discussed affords a simple explanation
of the generally observed phenomenon of the differences between
individuals! Each individual is different from every other, not in
the case of Man alone, but in all species in which we can judge of
differences, and this is true not only of descendants of different
parents, but even of those of the same parents.
Of course the differences between two brothers or two sisters do not
depend entirely on the hereditary basis, but in part also on external
conditions which have affected them from embryonic development onwards.
Let us suppose that of two brothers who have sprung from identical
germ-cells one becomes a sailor, the other a tailor; it would not
surprise us to find them very different in their fiftieth year, one
weather-beaten and tanned, the other pale; one muscular, straight,
and vigorous, the other weakly and of bent carriage. The same primary
constituents develop differently according to the conditions to which
they are exposed. But the two brothers will still resemble each other
in the features of the face, colour of hair, form of eyes, stature and
proportion of limbs, perhaps even in a birthmark, more than any other
human beings of their own or any other family, and this resemblance
will depend upon the identity of the hereditary primary constituents,
on the similar id-combination of the germ-plasm.
Man himself affords a particularly good example in favour of this
interpretation in the case of so-called 'identical twins.' It is well
known that there are two kinds of twins, those that are not strikingly
alike, and often very different, and those that are alike to the extent
of being mistakable for one another. Among the latter the resemblance
may go so far that the parents find it necessary to mark the children
by some outward sign, so that they may not be continually confused.
We have now every reason to believe that twins of the former kind
are derived from two different ova, and that those of the latter kind
arise from a single ovum, which, after fertilization, has divided into
two ova. This not infrequently occurs in fishes and other animals,
and we can bring it about artificially in a number of species by
experimentally separating the two first blastomeres.
We have here, then, a case of absolute identity of the germ-plasm in
two individuals, for the id-combination of the two ova derived from the
same process of fertilization must be exactly the same. That in such a
case, notwithstanding the inevitable differences of external influences
to which the twins are exposed from intra-uterine life onwards, such a
high degree of resemblance should arise is a fact of great theoretical
importance. From the basis of the germ-plasm theory we can very well
understand it, for, according to the theory, only precisely similar
combinations of ids can give rise to identical individuals.
But we learn more than this from the occurrence of identical twins.
They prove above all that the whole future individual is determined at
fertilization, or, to express it theoretically, that the id-composition
of the germ-plasm is decisive for the whole ontogeny. It might have
been supposed that the combination of ids could change again during
development, and that a greater multiplication of some than of others
might take place at certain stages of development, or through certain
chance external influences. It might have been thought that there was a
struggle among the ids in the sense that some of them were suppressed
and set aside. All such suppositions break down in face of the fact of
identical twins, which teaches us that identical germ-plasm evokes an
ontogeny which runs its course as regularly as two chronometers, which
are constructed and regulated alike.
But when I say that a struggle of the ids, in the sense of a material
setting aside of some of them, cannot take place, I by no means intend
to maintain that the influence which each individual id exerts on the
course of development may not be disproportionate to that exerted by
others, and, under some circumstances, very disproportionate indeed.
I must refrain from entering into this subject in detail now, but I
should like to give at least an indication of what I mean.
If the germ-plasm consists of ids, these ids collectively must
determine the structure, the whole individuality--let us say, briefly,
the 'type' of the offspring; it is the resultant of all the different
impelling forces which are contained in the different ids. If these
were all equally strong, and all operating in the same direction,
they would necessarily all have the same share in the resultant of
development, the 'type' of the child. But this is not the case.
Numerous experiments on the hybridization of two species of plant have
taught us that the descendants of such hybridization usually maintain
a medium between the ancestral species; but it is not always the case,
for in many hybrids the character of one species, whether paternal or
maternal, preponderates in the young plant.
We recognize the same thing still more clearly in Man, whose children
by no means always maintain a medium between the characters of the two
parents, but frequently resemble one--the father or the mother--much
more strongly than the other.
How can this fact be theoretically explained? Must we ascribe to the
ids of the father or of the mother a greater determining power? Without
excluding such an assumption as on _a priori_ grounds inadmissible,
I am inclined to believe that we do not require it to explain _this_
phenomenon. For, if we take our stand simply on the fact of the
preponderance of one parent, it follows directly from this that not
all the ids control the type of the child, let the cause of the
non-co-operation of some of them be what it may. But if in this case
only a portion of the ids contained in the germ-plasm controls the
type, this combination of ids suffices to make the child resemble
one parent, the father, for instance, and consequently half the
number of ids is sufficient in some circumstances to determine the
child--taking for granted that the one-sidedness of the inheritance is
complete, which never actually happens. But the half number of ids can
only suffice if it includes the same combinations of ids which have
determined the type in the case of the father; as soon as one or more
ids of this particular combination are replaced by others the paternal
germ-plasm alone is not enough to call forth complete resemblance in
the child.
But, at the reduction, a change of arrangement of ids takes place,
and a new combination arises, and thus each germ-cell receives its
particular group of ids. It may thus happen that, in one particular
sperm-cell, exactly the same group of ids is contained as that which
determined the type of the father, and that the same is true of a
particular egg-cell in regard to the type of the mother. Let us now
assume that a sperm-cell and an egg-cell meet, which contain both those
groups of ids which had determined the type of the father and of the
mother; if the determining power of the maternal and paternal ids were
equal a child would result which would maintain the medium between
father and mother.
As is well known this does happen not infrequently, although it is
difficult or impossible to demonstrate it precisely. In plant-hybrids
proof is easier, and it has been established that by far the greater
number of hybrids maintain a medium between the characters of the two
ancestral species. This proves that our assumption of equal strength
of the ids of both species must be correct in general, for we know
definitely in this case, as I shall show later, that the paternal
and the maternal ids are equivalent as regards the characters of the
species. This is the case, for instance, with the hybrids between the
two species of tobacco-plant, _Nicotiana rustica_ and _N. paniculata_,
which were reared by Kölreuter as far back as the eighteenth century,
and which then, as now, maintained a fairly exact medium between the
two ancestral species, and did so in all the individuals. Both species
thus strive to stamp their own character on the young plant, and in
both the hereditary power is equally great; in both it is contained
in the same number of ids, that is, in the half, for both kinds
of sex-cell have undergone reducing division. We have here, then,
strict proof that the half number of ids suffices to reproduce in the
offspring the type of the species, or, more generally, of the parents.
If we apply these results to the inheritance of individual differences
in Man, we may say, that those germ-cells, to which at the reducing
division the same combination of ids has been handed on as that which
already determined the type of the parent, will endeavour to impress
this image again on the child. If a female cell of this kind combine
with a male which likewise contains the facies-combination of the
parent, in this case the father, the same thing will happen which we
described in the case of the plant-hybrids, that is, a medium form
between the type of the two parents will arise.
Not infrequently, however, there is a marked preponderance of the one
parent in the type of the child, and we have to inquire whether the
theory gives us any help with regard to such a case.
One might be inclined to assume a difference in the determining power
of the paternal and maternal ids, but if we cannot show to what extent
and for what reason this power may be different such an assumption
remains rather an evasion than an explanation. Moreover, it would not
always apply to the conditions in Man, for if, for instance, the ids
of a particular mother were in general stronger than those of the
father, all the children of the pair in question would necessarily
take after the mother; but it happens not infrequently that one child
resembles the father preponderantly, and another the mother. Moreover,
the ids pass continually from the male to the female individual, and
conversely, by virtue of the continuity of the germ-plasm, so that the
idea that sex can have anything to do with the relative strength of the
ids is altogether erroneous.
But, as I have already said, unilateral inheritance occurs even in
the mingling of species-characters, and most clearly in the case of
plant-hybrids. Thus, for instance, hybrids between the two species of
pink, _Dianthus barbatus_ and _Dianthus deltoides_, resemble the latter
species much more closely than the former, and the hybrid between the
two species of foxglove which are wild in Germany, _Digitalis purpurea_
and _Digitalis lutea_, is much more like the latter than like the
former.
It might be reasonably asked whether, in these crossings, the normal
number of ids in one species is not greater than in the other. We
know that, among animals at least, differences in the normal number
of chromosomes occur even in very nearly related species. It is not
impossible that this, in many cases, is really the cause of the
diversity of transmitting power in different species. Nevertheless, we
cannot rest satisfied with this, for, in the first place, this cause
could not apply to the apparent unilateral inheritance from one parent
in Man, since the normal number of ids, as far as we know, is strictly
maintained in the same species, and second, this would not explain
certain phenomena of inheritance in plant-hybrids.
It happens not only frequently, but usually, that the different parts
of the hybrid take after one or other parent in different degree, and
this is the case also with children. In the hybrid between the two
species of tobacco-plant, _Nicotiana rustica_ and _N. paniculata_,
which I have already given as an example of a medium form between
the two parents, such diversities occur regularly in all the hybrid
individuals. Thus the corolla-tube of the hybrid is nearer _N.
paniculata_ in regard to its length, but nearer _N. rustica_ in regard
to its breadth. Many hybrids suggest one parent-form in the leaves, the
other in the blossoms. In the same way in the child the form of eye
may be that of the father, the colour of the iris that of the mother,
the nose maternal, the mouth paternal--in short, the preponderance in
heredity swings hither and thither from part to part, from organ to
organ, from character to character, and this is even the rule though
the oscillations may not always be apparent and are often invisible.
If we think of the proposition we arrived at earlier, and which was
proved chiefly by the case of identical twins, that the facies or
'type' of the descendant is determined at fertilization, we may be
inclined to regard such an oscillation of the hereditary tendencies
as almost impossible, for it means that, with the given mingling of
parental germ-plasms, the potency of inheritance from the two parents
in every part of the offspring is determined once for all in advance.
But the case of identical twins corroborates these oscillations, for
in them, too, the father predominates in one part, the mother in
another, and it proves, at the same time, that these oscillations do
not depend on any chances whatever in development, but that they are
exactly predetermined in the mingling of the hereditary substances in
the germ-plasm of the fertilized ovum, and are strictly adhered to
throughout development.
This fact can only be explained thus: the primary constituents of
the different parts and characters of the body are contained in the
parental germ-plasm in varying degrees of hereditary or transmissive
strength, and this can be understood very well from our point of view
without putting anything new into the 'portmanteau' of our theory
(Delage).
But I must digress a little in order to make this plain.
When, in speaking of plant-hybrids, I said that the collective ids
of the germ-plasm of a species must be equivalent in regard to the
characters of the species, I did not speak quite precisely; in the
majority of ids, in many cases in an overwhelming majority, this must
be the case, but not actually in all, at least not on the assumption
we make that the transformation of species is accomplished under the
control of natural selection.
Let us recall what we have already established in regard to the
evolutionary power of natural selection, namely, that the changes which
it controls can never transcend the range of their utility, and it will
be clear to us that, of the many ids which make up the germ-plasm of
the species, only so many will be modified as are necessary to evoke
the character which has varied. Just as the protective resemblance
of an insect to a leaf may be raised to a very high pitch, but can
never become perfect, because an imperfect resemblance is already
sufficient to deceive the persecutor, and the selective process comes
to a standstill because individuals which possessed a still greater
resemblance to a leaf would be no better protected from destruction
than the others, so in the modification of a species the whole of the
ids need not at once be modified, if a majority is sufficient to stamp
the great majority of individuals with the desired variation. But it
may happen that, at the reduction of ids during the development of the
germ-cells, an id-combination with wholly or almost wholly unchanged
ids may come together in one germ-cell, and if another sperm-cell of
this kind meets with an egg-cell similarly constituted, an individual
of the old species must arise. But this must--on our assumption--be
at a disadvantage as compared with the transformed individuals in the
struggle for existence, and will perish in it, and therefore the number
of unmodified ids in the germ-plasm of the species will gradually
diminish. It is obvious, however, that this will take place very
slowly, as we may conclude from the phenomena of reversion, of which I
shall have to speak later on.
But what is true of the ids is true also of their constituent parts,
the determinants, and that--if I mistake not--is fundamental in the
interpretation of the alternation of hereditary succession in the parts
of the child.
According to our theory, the ids do not collectively exert a
controlling influence on the cells, not even on the germ-cells, whose
histological differentiation into spermatic or egg-cells can only
depend on control through specific sex-cell determinants. It is the
different determinants of the ids that control; transformations of
the species will, it is true, depend on transformations of the ids,
but this need not necessarily consist in a variation of _all_ the
determinants of the id. If, for instance, two species of butterfly,
_Lycæna agestis_ in Germany and _Lycæna artaxerxes_ in Scotland, only
differ from each other in that the black spot in the middle of the wing
in _L. agestis_ is milk-white in _L. artaxerxes_, no other determinants
in the id of the germ-plasm can be different except those which control
this particular spot. In a majority of the ids in _L. artaxerxes_ the
determinants of this spot must have been modified, let us say, to the
production of 'milk-white.' This majority will increase very slowly if
the white colour has no pronounced advantage for the persistence of
the species, but it will increase gradually, as we have already seen,
though extremely slowly, through the elimination of those individuals
whose germ-plasm at the reducing division has chanced to receive a
majority of ids with the old, unmodified determinants, and which have
therefore reverted to the ancestral form. This will happen whenever the
new character has any use, however small, in maintaining the species.
But in most modifications of species quite a number of parts and
characters have undergone variation either simultaneously or in
rapid succession; in many cases nearly all the details of structure,
and therefore almost all the determinants of the germ-plasm, must
have varied. We must not, however, assume that all the equivalent
determinants, for instance, all the determinants _K_ in all the ids,
have varied[9], and above all we must not take for granted that
the determinants of different characters or parts of the body, for
instance, the determinants _L_, _M_, or _N_, must all undergo variation
in an equal number of ids. It will depend on two factors whether a new
character is implicit, in the form of varied determinants, in a small
or in a very large majority of ids: first, on the relative age of the
character, and, secondly, on its value in relation to the persistence
of the species. The more important a character is for the species the
more frequently is it decisive for the life or death of the individual,
and the more sharply will individuals not possessing it be eliminated,
and the more rapidly, therefore, will those whose germ-plasm still
contains a majority of the unvaried determinants of this character
tend to disappear. In this way these determinants will tend to sink
down from generation to generation to an ever smaller minority in the
germ-plasm of those that survive.
[9] By equivalent or homologous determinants I mean the determinants of
different ids which determine _similar parts_, e.g. the scales of that
wing-spot in _Lycæna agestis_ which is alluded to above, and to which
we must refer again in more detail.
Thus in the ids of any species which has been in some way
transformed--and that is as much as to say, in every species--the
equivalent or homologous determinants are modified in a very varied
percentage. A very modern and at the same time not very important
character _K´_ will only be contained in a small majority of ids,
while in the remainder the original homologous ancestral determinant
_K_ is contained; an older but not very much more important character
_M´_ must have its determinants in a larger majority in the ids, while
a character _V´_ of decisive importance for the preservation of the
species, if it has been in existence long enough, must be represented
in almost all the ids, so that the homologous unvaried determinants of
the ancestral species _V_ can only have persisted in an id here and
there.
If this argument be correct, many phenomena of inheritance become
intelligible, especially the variability in the expression of the
inheritance in the parts of the offspring, which is more or less
rigidly predetermined at fertilization. For the germ-plasm thus
contains in advance every kind of determinant in diverse _nuances_,
and in definite numerical proportions. In a plant _N´_, for instance,
_Ba´_ may be the determinant of the modern leaf-form, and may occur
in twenty-two out of twenty-four ids of the germ-plasm, while the two
remaining ids still contain unmodified the old leaf-form determinants
_Ba_, which the ancestral form _N_ possessed. But the flower of _N´_
may be of still more recent origin, and contain the modern flower
determinants _Bl´_ only in sixteen out of twenty-four ids, while in the
other eight the old flower-determinant _Bl_ of the ancestral form has
persisted. Let us now suppose that another nearly related species _P´_
has, conversely, a recently changed leaf-form but a very ancient form
of flower, so that the former is represented only in sixteen ids by the
determinants of the leaf _ba´_ and the latter in twenty-two ids by the
flower-determinants _bl´_: it is obvious that when the two species are
crossed, notwithstanding the equal number of ids in the germ-plasm, the
leaves of the hybrid will resemble more closely those of the ancestral
form _N_, and the flowers those of the form _P_; it is even conceivable
that in such a case the numerically preponderating leaf-determinants
_N_, and the equally preponderating flower-determinants of _P_ may form
a close phalanx, so to speak, against the much less numerous homologous
determinants of the other species, and that against this power working
in a definite direction the others can make no headway and are simply
condemned to inaction.
How we may or can picture this as occurring is a question which of
course admits only of being answered very hypothetically, and it leads
us, moreover, into the region of the fundamental phenomena of life,
with the interpretation of which we are not here concerned. For the
present we have assumed that life is a chemico-physical phenomenon, and
we have postponed the deeper explanation of it to the remote future,
that we may confine ourselves in the meantime to the solution of the
problem of inheritance on the basis of the forces resident in the vital
elements. But we may, nevertheless, make the supposition that a kind of
struggle between the different kinds of biophors may take place within
the cell, if the homologous determinants of all the ids for the control
of the cell have entered into it.
In many cases this struggle will be decided by the numerical
preponderance of one kind of determinant over the other, but it is
certainly conceivable that dynamic differences may also have something
to do with it.
Let us, however, abstain from trying to penetrate further into the
obscurity of these processes, and let us content ourselves with
establishing that the preponderance of one parent in some or many
parts of the child may be almost if not quite complete, and that this
compels us to assume that the hereditary substance of the other parent
is in such cases rendered inoperative--for we know it is present--since
the ids of both parents all go through the whole ontogeny, and are
contained in every somatic cell.
Upon this struggle between homologous determinants depends the
possibility of the entire suppression or inhibition of the influence
of one parent, the whole diversity in the mingling of paternal and
maternal character in the body of the offspring. It is in this that we
must seek the explanation of the fact that not only whole bodily parts
of the child, such as arms, legs, the nature of the skin, the form of
the skull, may take after sometimes the father, sometimes the mother
wholly or predominantly, but that the small separate subdivisions of
a complex organ may sometimes turn out more maternal, sometimes more
paternal. Thus intelligence from the mother and will from the father,
musical talent from the father and a talent for drawing from the
mother, may be inherited by the same child. I do not doubt that genius
depends in great part on a happy combination of such mental endowments
of the ancestors in one child. Of course something more is necessary,
namely, the strengthening of certain of these hereditary endowments,
but of this we shall speak later.
It is, however, not only the immediate ancestors, that is to say,
the parents, that have to be taken account of in this mingling of
hereditary contributions, but also those more remote. Not a few
characters in the child do not occur in either parent, but were present
in the grandparents, and their reappearance is called 'atavism' or
'reversion.' Let us consider this phenomenon in more detail, and try
to find out whether and how far it can be interpreted by means of our
theory.
The simplest and clearest cases are again found among plant-hybrids.
It may happen, for instance, that a hybrid between two species, when
dusted with its own pollen, gives rise to descendants, some of which
resemble only one of the ancestral forms: thus we have reversion to
one of the grandparents. The explanation of this lies in the different
modes in which the reducing divisions are effected; if they take place
in such a manner that all the paternal ids of the hybrid are separated
from the maternal ids, then the result is germ-cells which are like
those of the grandparents, that is, those of the parent species, and
these, if they happen to combine in amphimixis, must give rise to a
pure seedling of one or other of the two ancestral species. This case
occurs less rarely than was formerly supposed, and than it could do if
absolutely free combination of the idants took place at reduction. If
combination were quite unrestricted, all other possible combinations
would be likely to occur as frequently as these. But recent experiments
have shown that, in many plant-hybrids, the germ-cells of the hybrids
which are fertilized by their own pollen are either purely paternal or
purely maternal. There cannot, therefore, be free combination of the
idants at the reducing divisions; the idants of the two parent-forms
separate from one another and do not combine. It is doubtful, however,
whether the same thing occurs within a race, for instance, in the case
of reproduction within a human race.
In Man reversion to a grandparent occurs not infrequently, and we
may explain it thus: the id-group which controlled the type of the
grandfather was also contained in the germ-cell which gave rise to
the existence of the father, but it did not dominate the type in
that case because a more powerful id-group was opposed to it in the
germ-cell of the grandmother. When, later on, at the reducing divisions
of the germ-cells of the father, this id-group again arrived in the
sperm-cells of the father, it would predominately control the type of
the child, that is, of the third generation, provided that the egg-cell
with which it combined contained a weaker id-group.
In the case of ordinary plant-hybrids what are designated reversions
can only be called so in a wide sense, for the ancestral characters
are contained _visibly_ in the parent, although mingled with those
of the other parent. In human families, however, there are undoubted
cases in which one or more characters of the grandparent reappear in
the child which were not in any visible way expressed in the parent,
and must therefore have been contained in the parent's germ-plasm in
a latent state. And there are both in animals and plants reversions
to ancestors lying much further back, to characters and groups of
characters which have not been visible for many generations, and the
occurrence of these can only be explained on the assumption that
certain groups of ancestral determinants have been carried on in the
germ-plasm in too small a number to be able to give rise ordinarily
to the relevant character. Such isolated determinants may, however,
in certain circumstances be strengthened by the amphimixis of two
germ-cells both containing small groups of them, and thus augmented
they may gain a controlling influence. In this case the chances of the
reducing divisions have a part to play, since they bring together the
old unvaried ancestral determinants which, as we have seen, may persist
in the germ-plasm of any species through a long series of generations.
This would, of course, only suffice to bring about a reversion if the
determinants of the ancestral species were still contained in the
germ-plasm in comparative abundance. If this is no longer the case,
something more is necessary, and that is the relative weakness of the
more modern determinants.
If two white-flowered species of thorn-apple, _Datura ferox_ and
_Datura lævis_, be crossed, there arises a hybrid with bluish violet
flowers and brown stalks instead of green. This was interpreted by
Darwin as a reversion to violet-flowering ancestral species on both
sides, for there are even now a great number of species of _Datura_
with violet flowers and brown stalks. When the two white forms are
crossed the reversion takes place every time, not merely in some cases,
and we may conclude from this that in both these species there is still
such a strong admixture of the same unvaried ancestral ids that they
always excel the ids of the two modern species crossed, in strength
though certainly not in number. And this superiority must again depend
on the fact that similar determinants of the same part are cumulative
in their effect, while dissimilars are not.
For this reason reversions to remote ancestors occur readily when
species and breeds are crossed, while they are rare in the normal
inbreeding of a species. The reversions of the breeds of pigeon to
their wild ancestral form, the slaty-blue rock-pigeon, never result, as
Darwin showed, and as we have already noticed, from pure breeding of
one race, but only when two or more breeds are repeatedly crossed with
one another. Even then it occurs by no means always, but only now and
again. The germ-plasm of the breeds must therefore still contain ids
of the rock-pigeon, but in a small number, varying from individual to
individual. If by fortunate reducing divisions and the meeting together
of a sperm-cell rich in ancestral ids with a similarly endowed egg-cell
the number of ancestral ids be raised to such a point that it exceeds
the number of modern-breed ids contained in each one of the conjugating
germ-cells, the ancestral ids control the development and reversion
occurs, for the ancestral ids together have a cumulative effect while
the ids of the two parent breeds are different and therefore, as far
as they are so, cannot be co-operative in their influence. But it must
be understood that they need not be different as far as _all_ their
determinants are concerned, but usually only as regards some groups,
and thus it happens that reversion does not occur in regard to all, but
only in regard to particular characters--thus in the _Datura_ hybrids,
chiefly in regard to the colour of the flowers and the stem, and in
the hybrids between different breeds of pigeon mainly in regard to the
colour and marking of the plumage.
The reversions of the horse and ass to striped ancestors, which Darwin
has made famous, go much further back into the ancestral history of the
species, for while we know the ancestral form of the domestic pigeon
in the still living rock-dove (_Columba livia_), the ancestral form
common to the horse and the ass is extinct, and we can only suppose
that it was striped like a zebra, because such striping occasionally
occurs in pure horses and pure asses, at least in their youth, although
now only on the legs, and because this striping is often more marked
in the hybrid between the horse and the ass, the mule. In Italy, where
one sees hundreds of mules, the striping is not exactly frequent,
but it may occur in about two per cent., while in America it is said
to be much more frequent. The germ-plasm of the horse and the ass
must therefore contain, in varying numbers, ids whose skin-colour
determinants represent in part still unmodified ancestral characters.
When two germ-cells chance to meet in fertilization, both of which have
received, through a favourable reducing division, a relatively large
number of such ids, a relative majority of these in the fertilized
ovum is opposed to the dissimilar and therefore mutually neutralizing
homologous determinants of horse and ass, and reversion to the
ancestral form occurs.
These cases of reversion are enough to show us that the old unmodified
ancestral determinants may persist in the germ-plasm through long
series of generations. But an even deeper glimpse into the dim ancient
history of our modern species of horses is afforded by the occurrence
of three-toed horses, references to a small number of which the
palæontologist Marsh was able to discover in literature, and one of
which he was able to observe in life. Julius Caesar possessed a horse
whose three-toed feet represented a reversion to the horses of Tertiary
times, _Mesohippus_, _Miohippus_, and _Protohippus_ or _Hipparion_; for
all these genera possessed, in addition to the strong middle toe, two
weaker and shorter lateral toes.
In the germ-plasm of our modern horses there must still persist in
certain ids the determinants of the ancestral foot, which, after a long
succession of favourable reducing divisions combined with favourable
chances of fertilization, may come to be in a majority, and may thus be
able to induce a reappearance of a character which has long been hidden
under the surface of the present-day type of the species.
I do not propose to enter further on the discussion of the phenomena
of inheritance. A more detailed investigation of the phenomena of
reversion is to be found in '_The Germ-plasm_' published ten years
ago; and the discussion could not be resumed here without a critical
consideration of a relatively large series of newly acquired facts
not always harmonizing, and, as yet, not even fully available. The
year 1900 has given us the investigations of three botanists, De
Vries, Correns, and Tschermak, who have sought by experiments in
hybridization between different sorts of peas, beans, maize, and other
plants, to throw light on the phenomena of inheritance, and thus on
the actual processes which occur in the germ-plasm at the reducing
division. This led to the discovery that similar experiments had been
published as far back as 1866 by the Abbot of Brünn, Gregor Mendel, and
that these had been formulated as a law which is now called Mendel's
law. Correns showed, however, that this law, though correct in certain
cases, did not by any means hold good in all, and we must thus postpone
the working of this new material into our theory until a very much
wider basis of facts has been supplied by the botanists. There is less
to be hoped for from the zoologists in regard to this problem owing
to the almost insuperable difficulties in the way of a long series
of experiments in hybridization in animals. I myself have repeatedly
attempted experiments in this direction, and have always had to abandon
them, either because the crossing succeeded too rarely, or because the
hybrids did not reproduce among themselves, or did so defectively, or
because the distinguishing characters of the crossed breeds proved
insufficiently tenacious or diagnostic. But it would be a fine task for
zoological gardens to undertake such experiments from the point of view
of the germ-plasm theory, and their success would afford material for
the criticism of the theory, the more valuable because it is apparent
from the experiments on plants that the processes of heredity are
manifold, and are far from being uniform in different domains[10].
[10] Castle and Allen have recently published the results of
experiments in crossing white mice with grey, and these confirm
Mendel's Law.
I have assumed for my theory that the reducing division took place
according to the laws of chance, and that thus every combination of ids
occurred with equal frequency. This assumption seems to be confirmed,
by the experiments of the botanists I have mentioned, only in so far
that in the crossing of hybrids with one another every combination
of distinctive characters occurred with equal frequency. But, on the
other hand, the splitting of the germ-plasm at the reducing division
seems, as I said before, in many cases to take place in such a way that
the id-groups of the two parents are discretely separated from one
another; this was so in the stocks, peas, beans, and other hybrids.
But even if this were always the case in these, we could hardly infer
that it must be the same everywhere; we should rather expect that the
relationship of the two parents and their ids would bring into play the
finer attractions and repulsions between the ids of the germ-plasm,
and would thus determine their arrangement and grouping. Further
investigations may clear up this point; in the meantime we can only
say that already--even among hybrids--many deviations from Mendel's Law
have been established, for instance, by Bateson and Saunders (1902).
Before I conclude this lecture I should like to refer briefly to a
phenomenon which Darwin was acquainted with and sought to explain
through his theory of Pangenesis, but which at a later date was
regarded as not sufficiently authenticated to justify any attempt
at a theoretical explanation, since it seemed to contradict all our
conceptions of hereditary substance and its operations. I refer to the
phenomenon to which the botanists have given the pretty name of Xenia
(guest-gifts), and which consists in the fact that in crosses of two
different plants the characters of the male may appear not only in the
young plant but even in the seed, so that a transference of paternal
characters seems to take place from the pollen-tube to the mother, to
the 'tissue of the maternal ovary.' In heads of yellow-grained maize
(_Zea_) it is said that, after dusting with pollen from a blue-seeded
variety, blue seeds appear among the yellow, and similar observations
on other cultivated plants have been on record for more than half a
century. Thus dusting the stigma of green varieties of grape with the
pollen of a dark blue kind is said frequently to give rise to dark blue
fruits.
Darwin accepted these observations as correct, and endeavoured
to explain them as due to a migration of his 'gemmules' from the
fertilized ovum into the surrounding tissue of the mother-plant. His
explanation was not correct, we can say with confidence now, but he
was right so far, for the phenomena of Xenia do occur; they are not
illusory as most modern botanists seem inclined to believe. I myself
was at first inclined to wait for further facts in proof that the
phenomena of Xenia really occurred before attempting to bring them
into harmony with my theory, and this will not be found fault with
when it is remembered that these cases of Xenia seem to stand in
direct contradiction to the fundamental postulates of the germ-plasm
theory. For this depends essentially on a definite stable structure
of the germ-substance, which lies within the nucleus in the form of
chromosomes, and which cannot pass from one cell to another in any
other way than by cell-division and division of the nucleus; how then
could it pass from the fertilized ovum to the cells of the endosperm
which do not derive their origin from it at all, but from other cells
of the embryo-sac? In point of fact some of my opponents have cited
Xenia as an actual refutation of my theory.
[Illustration: FIG. 82. Fertilization in the Lily (_Lilium martagon_),
after Guignard. _A_, the embryo-sac before fertilization; _sy_,
synergidæ; _eiz_, ovum; _op_ and _up_, upper and lower 'polar nuclei';
_ap_, antipodal cells. _B_, the upper part of the embryo-sac,
into which the pollen-tube (_pschl_) has penetrated with the male
sex-nucleus (♂_k_) and its centrosphere; below that is the ovum-nucleus
(♀_k_) with its (also doubled) centrosphere (_csph_). _C_, remains of
the pollen-tube (_pschl_); the two sex-nuclei are closely apposed.
Highly magnified.]
That cases of Xenia really do occur is now established by the
comprehensive and at the same time exceedingly careful experiments
recently made by C. Correns with _Zea Mais_; it is only necessary to
look through the beautiful figures with which his work is adorned to be
convinced that heads of maize whose blossoms have been dusted with the
pollen of a different kind produce more or less numerous seeds of the
paternal kind, usually mingled with those of the maternal. Thus heads
of the variety _Zea alba_ resulting from fertilization with _Z. cyanea_
exhibited a majority of white grains, but among them a smaller number
of blue; and the converse experiment, of dusting _Z. cyanea_ with
the pollen of _Z. alba_, yielded heads in which a minority of white
grains appears among a majority of blue. But it is always only the
nutritive layer surrounding the embryo--the endosperm--which exhibits
the character of the paternal species, and even the capsule surrounding
the seed shows nothing of it, but is purely maternal. Thus the heads
of different species with pale-yellow capsule, when dusted with the
pollen of _Z. rubra_, never have red seeds like those of _Z. rubra_,
but always seeds with a pale-yellow skin, while, in the converse
experiment, dusting of the red-skinned species _Z. rubra_ with pollen
from _Z. vulgata_, all the seeds are red, like those of the maternal
species, and the influence of the paternal species only shows when the
strong red skin has been removed, so that the intense yellow colour of
the endosperm, which in the pure maternal species is white, is exposed
to view. Thus the mysterious influence of the pollen never goes beyond
the endosperm, and the riddle of this influence is solved in the most
unexpected manner, indeed was solved even before Correns had securely
established the genuine occurrence of Xenia. The explanation is due
to recent disclosures in regard to the processes of fertilization in
flowering plants.
It had long been known that the pollen-tube contains not merely one
generative nucleus but two, which arise from one by division. But what
had till recently remained unknown was that not only one of these
penetrated into the embryo-sac to enter into amphimixis with the
egg-cell, but that the other also makes its way in, and there fuses
with the two nuclei which had long been designated the upper and lower
polar-nuclei (Fig. 82, op. cit.). Nawaschin and Guignard demonstrated
that these two nuclei fuse _with the second male_ nucleus; thus two
acts of fertilization are accomplished in the embryo-sac, and one of
these gives rise to the embryo, while the second becomes nothing less
than the endosperm, the nutritive layer which surrounds the embryo,
whose origin from the polar nuclei had been previously recognized.
Thus the riddle of Xenia is essentially solved. We understand how
paternal primary constituents may find their way into the endosperm,
and indeed must do so regularly; we understand also how the paternal
influence never goes beyond the endosperm. The riddle is thus not only
solved, but at the same time the view which assumes a fixed germ-plasm,
and believes it to lie in the nuclear substance of the germ-cells,
receives further confirmation, if it should need any, for if facts
which are apparently contradictory to a theory can be naturally
brought into harmony with it, this affords a stronger argument for the
correctness of the theory than the power of explaining the facts which
were used in building it up.
There is much more to be said in regard to Xenia, and I am sure
that much that is of interest will be brought to light by deeper
investigation; theoretical difficulties will still have to be overcome,
and I have already pointed out one of these in my '_Germ-plasm_,' but I
must here rest satisfied with what has been already said.
We have now passed in review and attempted to fit into the theory a
sufficiently large number of the phenomena of heredity for the purpose
of these lectures. Although, as is natural, much of this must remain
hypothetical, we may accept the following series of propositions as
well founded: there is a hereditary substance, the germ-plasm; it is
contained in very minimal quantity in the germ-cells, and there in
the chromosomes of the nucleus; it consists of primary constituents
or determinants, which in their diverse arrangement beside or upon
one another form an extremely complex structure, the id. Ids and
determinants are living vital unities. Each nucleus contains several,
often many, ids, and the number of ids varies with the species and is
constant for each. The ids of the germ-plasm of each species have had
a historical development, and are derived from the germ-plasm of the
preceding lineage of species; therefore ids can never arise anew but
only through multiplication of already existing ids.
And now, equipped with this knowledge, let us return to the point
from which we started, and inquire whether the Lamarckian principle
of evolution, the inheritance of functional modifications, must be
accepted or rejected.
LECTURE XXIII
EXAMINATION OF THE HYPOTHESIS OF THE TRANSMISSIBILITY OF FUNCTIONAL
MODIFICATIONS
Darwin's Pangenesis--Alleged proofs of functional
inheritance--Mutilations not transmissible--Brown-Séquard's
experiments on Epilepsy in guinea-pigs--Confusion of infection of the
germ with inheritance, Pebrine, Syphilis, and Alcoholism--Does the
interpretation of the facts require the assumption of the transmission
of functional modifications?--Origin of instincts--The untaught
pointer--Vom Rath's and Morgan's views--Attachment of the dog to his
master--Fearlessness of sea-birds and seals on lonely islands--Flies
and butterflies--Instincts exercised only once in the course of a
lifetime.
As I have already said in an earlier lecture, Darwin adhered to
Lamarck's assumption of the transmission of functional adaptations,
and perhaps the easiest way to make clear the theoretical difficulties
which stand in the way of such an assumption is to show how Darwin
sought to present this principle as theoretically conceivable and
possible.
Darwin was the first to think out a theory of heredity which was worthy
of the name of theory, for it was not merely an idea hastily suggested,
but an attempt, though only in outline, at elaborating a definite
hypothesis. His theory of 'Pangenesis' assumes that cells give rise to
special gemmules which are infinitesimally minute, and of which each
cell brings forth countless hosts in the course of its existence. Each
of these gemmules can give rise to a cell similar to the one in which
it was itself produced, but it cannot do this at all times, but only
under definite circumstances, namely, when it reaches 'those cells
which precede in order of development' those that it has to give rise
to. Darwin calls this the 'elective affinity' of each gemmule for
this particular kind of cell. Thus, from the beginning of development
there arises in every cell a host of gemmules, each of which virtually
represents a specific cell. These gemmules, however, do not remain
where they originated, but migrate from their place of origin into the
blood-stream, and are carried by it in myriads to all parts of the
body. Thus they reach also the ovaries and testes and the germ-cells
lying within these, penetrate into them, and there accumulate, so that
the germ-cells, in the course of life, come to contain gemmules from
all the kinds of cells which have appeared in the organism, and, at
the same time, all the variations which any part may have undergone,
whether due to external or internal influences, or through use and
disuse.
In this manner Darwin sought to attribute to the germ-cells the power
of giving rise, in the course of their development, to the same
variations as the individual had acquired during its lifetime in
consequence of external conditions or functional influences.
I abstain from analysing the assumptions here made; their improbability
and their contradictions to established facts are so great that it
is not necessary to emphasize them; the theory shows plainly that it
is necessary to have recourse to very improbable assumptions, if an
attempt is to be made to find a theoretical basis for the transmission
of acquired (somatogenic) characters. Even when Darwin formulated
his theory of Pangenesis his assumptions were hardly reconcileable
with what was known of cell-multiplication; now they are above all
irreconcileable with the fact that the germ-substance never arises
anew, but is always derived from the preceding generation--that is,
with the continuity of the germ-plasm.
If we were now to try to think out a theoretical justification we
should require to assume that the conditions of all the parts of
the body at every moment, or at least at every period of life, were
reflected in the corresponding primary constituents of the germ-plasm
and thus in the germ-cells. But, as these primary constituents are
quite different from the parts themselves, they would require to vary
in quite a different way from that in which the finished parts had
varied; which is very like supposing that an English telegram to China
is there received in the Chinese language.
In spite of this almost insuperable theoretical obstacle, various
authors have worked out the idea that the nervous system, which
connects all parts of the body with the brain and thus also with each
other, communicates these conditions to the reproductive organs, and
that thus variations may arise in the germ-cells corresponding to those
which have taken place in remote parts of the body.
Even supposing it were proved that every germ-cell in ovary or testis
was associated with a nerve-fibre, what could be transmitted to it by
the nerves, except a stronger or weaker nerve-current? There is no such
thing as _qualitative_ differences in the current; how then could the
primary constituents of the germ be influenced by the nerve-current,
either individually or in groups, in harmony with the organs and parts
of the body corresponding to them, much less be caused to vary in a
similar manner? Or are we to imagine that a particular nerve-path leads
to every one of the countless primary constituents? Or does it make
matters more intelligible if we assume that the germ-plasm is without
primary constituents, and suppose that, after each functional variation
of a part, telegraphic notice is sent to the germ-plasm by way of the
brain as to how it has to alter its 'physico-chemical constitution,'
so that the descendants may receive some benefit from the acquired
improvement?
I am not of the number of those who believe that we already know all,
or at least nearly all, that is essential, but am rather convinced that
whole regions of phenomena are still sealed to us, and I consider it
probable that the nervous system in particular is not yet exhaustively
known to us, either in regard to its functioning or in regard to its
finest structural architecture, although I gratefully recognize the
advances in this domain that the last decades have brought about. In
any case, such assumptions as I have just indicated, or similar ones,
seem to me quite too improbable to furnish any foothold for progress.
Yet we must always remain conscious that we cannot decide as to the
possibility or impossibility of any biological process whatever from
a purely theoretical standpoint, because we can only guess at, not
discern, the fundamental nature of biological processes. At the close
of this lecture I shall return to the question of the theoretical
conceivability of an inheritance of functional adaptations; but first
of all we must consider the facts and be guided by them alone. If they
prove, or even make it seem probable, that such inheritance exists,
then it must be possible, and our task is no longer to deny it, but to
find out how it can come about.
Let us therefore investigate the question whether an inheritance
of acquired characters, that is, in the first place, of functional
adaptations, is demonstrable from experience. We shall speak later on
of the effect of climatic and similar influences in causing variation;
the case in regard to them is quite different, because they undoubtedly
affect not only the parts of the body but the germ-cells as well.
When we inquire into the facts which have been brought forward by
the modern adherents of the Lamarckian principle as proofs of the
inheritance of acquired characters in this restricted sense, we shall
find that none of them can withstand criticism.
First, there are the numerous reputed cases of the inheritance of
mutilations and losses of whole parts of the body.
It is not without interest to note here how opinion in regard to this
point has altered in the course of the debate.
At the beginning of the discussion they were all brought forward as
evidence of undoubted value for the Lamarckian principle.
At the Naturalists' Congress in Wiesbaden in 1887, kittens with only
stumps of tails were exhibited, and they were said to have inherited
this peculiarity from their mother, whose tail, it was asserted, had
been accidentally amputated. The newspapers reported that the case
excited great interest, and biologists of the standing of Rudolf
Virchow declared it to be noteworthy, and regarded it as a proof, if
all the details of it were correct. From many sides similar cases were
brought forward, intended to prove that the amputation of the tail
in cats and dogs could give rise to hereditary degeneration of this
part; even students' fencing-scars were said to have been occasionally
transmitted to their sons (happily not to the daughters); a mutilated
or torn ear-lobe in the mother was said to have given rise to deformity
of the ear in a son; an injury to a father's eye was said to have
caused complete degeneration of the eyes in his children; and deformity
of a father's thumb, due to frostbite, was said to have produced
misshapen thumbs in the children and grandchildren. A multitude of
cases of this kind are to be found in the older textbooks of physiology
by Burdach, and above all by Blumenbach, and the majority have no
more than an anecdotal value, for they are not only related without
any adequate guarantee, but even without the details indispensable to
criticism.
As far back as the eighteenth century the great philosopher Kant, and
in our own day the anatomist Wilhelm His, gave their verdict decidedly
against such allegations, and absolutely denied any inheritance of
mutilations; and now, after a decade or more of lively debate over
the pros and cons, combined with detailed anatomical investigations,
careful testing of individual cases, and experiment, we are in a
position to give a decided negative and say _there is no inheritance of
mutilations_.
Let me briefly explain how this result has been reached.
In the first place, the assertion that congenital stump-tails in dogs
and cats depended on inherited mutilation proved to be unfounded. In
none of the cases of stump-tails brought forward could it even be
proved that the tail of the relevant parent had been torn or cut off,
much less that the occurrence, in parents or grandparents, of short
tails from internal causes was excluded. At the same time anatomical
investigation of such stump-tails as occur in cats in the Isle of Man,
and in many Japanese cats, and are frequently found in the most diverse
breeds of dogs, showed that these had, in their structure, nothing
in common with the remains of a tail that had been cut off, but were
spontaneous degenerations of the whole tail, and are thus deformed
tails, not shortened ones (Bonnet).
Experiments on mice also showed that the cutting off of the tail,
even when performed on both parents, does not bring about the
slightest diminution in the length of tail in the descendants. I
have myself instituted experiments of this kind, and carried them
out through twenty-two successive generations, without any positive
result. Corroborative results of these experiments on mice have
been communicated by Ritzema Bos and, independently, by Rosenthal,
and a corresponding series of experiments on rats, which these two
investigators carried out, yielded the same negative results.
When we remember that all the cases which have been brought forward in
support of an inheritance of mutilations refer to a _single_ injury
to one parent, while, in the experiments, the same mutilation was
inflicted on both parents through numerous generations, we must regard
these experiments as a proof that all earlier statements were based
either on a fallacy or on fortuitous coincidence. This conclusion is
confirmed by all that we know otherwise of the effects of oft-repeated
mutilations, as for instance the well-known mutilations and distortions
which many peoples have practised for long, sometimes inconceivably
long, ages on their children, especially circumcision, the breaking
of the incisors, the boring of holes in lip, ear, or nose, and so
forth. No child of any of these races has ever been brought into the
world with one of these marks: they have to be re-impressed on every
generation.
The experience of breeders agrees with this, and they therefore, as
Wilckens remarks, have long regarded the non-inheritance of mutilations
as an established fact. Thus there are breeds of sheep in which, for
purely practical reasons, the tails have been curtailed quite regularly
for about a century (Kühn); but no sheep with a stump-tail has ever
been born in this breed. This is all the more important because there
are other breeds of sheep (fat-rumped sheep) in which the lack of
the tail is a breed character; it is thus not the case that there is
anything in the intrinsic nature of the tail of the sheep to prevent
it becoming rudimentary. The artificially rounded ear of fox-terriers,
too, though cut for generations, never occurs hereditarily. Mr. Postans
of Eastbourne informs me that the cocks which are to be used for
cock-fighting are docked of their combs and wattles beforehand, and
that this had been done for at least a century, but that no fighting
cock without comb and wattles has been reared. In the same way various
breeds of dog, such as the spaniels, have had their tails cut to half
their length regularly and in both sexes for more than a century, yet
in this case there is no hereditary diminution of the length of tail.
Deformed stump-tails do indeed occur in most breeds of dog, but, as I
said before, their anatomical character is quite different from that of
artificially shortened tails, moreover they may occur in breeds whose
tails have not suffered from the fashion of docking, as, for instance,
in the Dachshund.
We may therefore affirm that an inheritance of artificially produced
defects and mutilations is quite unproved, and in no way bears out the
supposed inheritance of functional changes.
This is now admitted by the great majority of the adherents of the
Lamarckian principle, and we may now regard this kind of 'proof' as
disposed of.
In addition to the above, various sets of facts have been brought
forward as proofs, and in particular the much discussed experiments of
Brown-Séquard on guinea-pigs, from which it was inferred that epilepsy
artificially induced could be transmitted. But these experiments do
not really prove anything in regard to the question at issue, because
epileptic-like convulsions may have very various causes, and these
are, for the most part, quite unknown. Since artificial epilepsy
can be induced in guinea-pigs by the most diverse injuries to the
central or peripheral parts of the nervous system, this of itself
points to the fact that it is not a question of the mere lesion of
anatomical structure, I mean, of the breaking of the continuity of
a definite part, and of its transmission. The result would, in any
case, differ according to whether certain centres of the brain, or
half the spinal cord, or the main nerve-trunks were cut through. There
must, therefore, be something more needed to produce the appearance of
epilepsy--some morbid process which may arise at different parts of
the nervous system, and be continued from them to the brain-centres.
This is corroborated by the fact that it takes at least fourteen days,
and often from six to eight weeks, for epilepsy to develop after the
operation, and that in many cases it does not develop at all. I have
made the suggestion that, during or after the operation, some kind of
pathogenic micro-organism might easily reach the wounded parts, and
there excite inflammation, which may extend centripetally to the brain.
Similar processes have been observed in connexion with lymph-vessels,
and why should they not occur in connexion with nerves?
It has been objected to this that the guinea-pig's epilepsy may be
produced by blows on the skull, and also by a destructive compression
of the _nervus ischiadicus_ through the skin, and that in both cases
the epilepsy may reappear in the following generation; and this, it
is supposed, shows that the intrusion of microbes is excluded. If
this were so beyond a doubt, and if we could exclude the possibility
that there were previously various microbes within the body, which
could only penetrate into the nervous substance after the cutting
or destruction of the neurilemma, nothing would be gained that
would in any way support the Lamarckian principle. One could only
say: Certain injuries to the nervous system give rise secondarily
in guinea-pigs to morbid phenomena like epilepsy, and all sorts of
functional disturbances of the nervous system often appear in the next
generation, including in rare cases even the phenomena of epileptic
convulsions. That this is a case of the transmission of an acquired
anatomical modification brought about by the injury is not only
unproved, but is decidedly negatived, for the injuries themselves are
never transmitted. Thus what is transmitted must be quite different
from what was acquired, for no one has ever detected in the offspring
the lesion of the nerve-trunk which was cut through in the parent, or
any other result except the disease to which the original injury gives
rise. Moreover, the inheritance of these morbid phenomena has been
again brought into dispute quite recently owing to the investigations
of such experts in nervous diseases as Sommer and Binswanger, and the
correctness of Brown-Séquard's results, which have dragged through the
literature of the subject for so long, has been emphatically denied[11].
[11] See H. E. Ziegler's report in _Zool. Centralblatt_, 1900, Nos. 12
and 13.
Clearly formulated problems, like that of the inheritance of acquired
characters, should not be confused by bringing into them phenomena
whose causes are quite unknown. What do we know of the real causes
of those central brain-irritations which give rise to the phenomena
of epilepsy? It is certain enough that there are diseases which are
acquired and are yet 'inherited,' but that has nothing to do with
the Lamarckian principle, because it is a question of _infection_
of the germ, not of a definite variation in the constitution of the
germ. We know this with certainty in regard to the so-called Pebrine,
the silkworm disease which wrought such devastation in its time; the
germs of the pebrine organism have been demonstrated _in the egg_ of
the silk-moth; they multiply, not at once but later, in the young
caterpillar, and it is the half-grown caterpillar, or even the moth,
that succumbs to the disease.
Whether in this case also the disease germs are transmitted through
the male sex-cells is not proved, as far as I am aware, but that this
can happen is shown by the transmission of syphilis from father to
child. That in this case, also, the exciting cause of the disease is a
micro-organism cannot be doubted, although it has not yet been proved.
Thus even the minute spermatozoon of Man can contain microbes, and
transmit them to the germ of a new individual.
This discussion of scientific questions ought not to be brought down
to the level of a play upon words, by bringing forward cases like the
above as evidence for the inheritance of 'acquired characters,' as was
done, for instance, by M. Nussbaum, who cited as a proof of this the
migration of the alga-cells which live in the endoderm of the green
freshwater Hydra into the ovum, which is originally colourless, and
originates in the ectoderm of the animal (Fig. 35_B_, p. 169, vol.
i). It seems to me better to make a precise distinction between the
transmission of extraneous micro-organisms through the germ-cells and
the handing on of the germ-plasm with the characters inherent in its
structure. Only the latter is inheritance in the strict scientific
sense, the former is infection of the germ.
Still less than the cases of inherited traumatic epilepsy can the
morbid constitution of the children of drunkards be regarded as a
proof of the inheritance of somatogenic characters, though this has
often been maintained. I will not lay any stress on the fact that the
allegation itself is, according to the most competent observers, such
as Dr. Thomas Morton[12], far from being established. But even if it
were quite certain that the numerous diseases of the nervous system,
amounting sometimes to mania, which are frequently observed in the
children of drunkards, were really _caused_ by the drinking of the
parents, it ought not to be overlooked that we have here to do not with
the hereditary transmission of somatic variations, but of variations
_directly_ induced in the germ-plasm of the reproductive cells, for
these are exposed to the influence of the alcohol circulating in the
blood, just as any other part of the body is. That by this means
variations in the germ-plasm can be brought about, and that these
may lead to morbid conditions in the children cannot be denied, and
ought not on _a priori_ grounds to be called in question. For we are
acquainted with many other influences--climatic, for instance--which
directly affect and cause variation in the germ-plasm. Whether this
is so in the case of drunkenness, and in what manner it comes about,
whether through direct action of the alcohol, or through infection of
the germ with some microbe, we must leave to the future to decide; the
whole question is out of place here; it can in no way help us to clear
up the problem with which we are now occupied.
[12] Morton, 'The Problem of Heredity in Reference to Inebriety,'
Proceed. Soc. for the Study of Inebriety, No. 42, Nov. 1894.
But even if there were not a trace of proof of the transmissibility of
functional modifications, that alone would not justify us in concluding
that the transmission is impossible, for many things may happen that we
are not in a position to prove at present. If it could be shown that
there was a whole group of phenomena that could not be explained in any
other way than on the hypothesis of such inheritance, then we should
be obliged to assume that it really occurred, although it was not
demonstrable, and, indeed, not even theoretically conceivable. This is
the standpoint of the adherents of the Lamarckian principle at present.
They say there are a great number of transformations which are simply
and easily explained, if we regard them as the effects of inherited
use or disuse, but which admit only of a strained explanation, and
sometimes of none at all, on the basis of natural selection, and these
are not a few isolated cases, but whole categories of them.
I will submit a few of these, and show at the same time why I cannot
regard them as convincing, even if it be the case that we are not at
present in a position to explain them without the aid of the Lamarckian
principle. But let me hasten to add that it is my belief that we can do
this, although certainly not without first giving a somewhat extended
application to the principle of selection.
It has often been maintained that the existence of animal instincts is
in itself enough to prove that the Lamarckian principle is operative.
In one of the earlier lectures I showed that at least the greater
number of instincts must have originated in purely reflex actions,
and therefore, like these actions themselves, can only be explained
through natural selection. A reflex action, such as coughing, sneezing,
shutting of the eyelids, and so on, differs from an instinctive
action in the lesser complexity and shorter duration of the series
of movements liberated by a sense-impression, and also in that it
does not require to enter into consciousness at all; but no very
precise boundary can be drawn between the two, and, in any case, both
depend, as we have already seen, on a quite analogous anatomical
basis. It is only a difference in degree whether, at the sight of a
rapidly approaching object, the muscles of the eyelids contract, and
by shutting the lids, protect the eye, or whether the fly, which we
intend to seize with our hand, is impelled by the sight of the rapidly
approaching shadow of the hand to fly quickly up. The action of the fly
may be regarded as reflex, or equally well as instinctive. But there
is also only a difference in degree, not in kind, between this simple
action and the complex and protracted behaviour of a mason-bee, the
sight of whose colony impels her to fly out and fetch clay, with it
gradually to build a neat cell, to fill this with honey, to lay an egg
in it, and finally to furnish the cell with a roof of clay. Since all
reflex mechanisms, and all the natural instincts of animals, contribute
to the maintenance of the species, and are therefore useful, their
origins must be referable to natural selection, and we have only to ask
whether they must be referred to it always, and to it alone.
It cannot be doubted that, in Man, and in the higher animals voluntary
actions which are often repeated gradually acquire the character
of instinctive actions. The individual movements pertaining to the
particular action are no longer each guided by the will, but a single
exercise of will is enough to liberate the whole complex action, such
as writing, speaking, walking, or the playing of a whole piece of
music; frequently the will-impulse may be absent altogether, and the
action be set going simply by an adequate external stimulus, as in
the case of sleep-walking, which is observed in fatigued children and
soldiers, and in somnambulists. The external stimulus is transmitted
to the proper group of muscles as unfailingly as in the case of true
instincts, and this happens not only in regard to actions which,
like walking, are essential to the life of the species, but also in
regard to those which have arisen from chance habits or exercises.
Often a short practice is sufficient to make an action in this sense
instinctive, and the complexity of the instinct-mechanism gained by
such practice is often astounding. Under some circumstances a person
may play a piece on the piano from the score, and yet be thinking
intently of other things, and be quite unconscious of what is played.
In the same way it may happen that a person dominated by violent
emotion, when trying to free himself from it by reading, may read a
whole page, line by line, without understanding in the least what has
been read. In the last case it is not directly demonstrable that the
reader has made all the complex delicate eye-movements which would be
liberated by the sight of the words, but in the case of playing, the
listeners can perceive that the piece is correctly played, and thus
that the stimulus exercised by each note on the retina of the eye
is translated into the complex muscular movement of arm and finger,
corresponding both to the pitch and the duration of the note, and to
the simultaneousness of several notes.
In all these cases it is probably not always quite new paths which
are established in the brain, but use is made of particular tracks
in the innumerable nerve-paths already existing in the nerve-cells
(neurons) which are 'more thoroughly trodden' by practice, so that
the distribution of the nerve-current takes place more easily along
them than along others[13]. This much-used metaphor does not indicate
the actual structural changes which have taken place, but it serves
at least to indicate that we have to do with material changes in the
ultimate living elements of the nerve-substance (nerve-biophors)
whether these changes be in position or in quality. Now, if such
brain-structures and mechanisms acquired through exercise in the
individual life could be transmitted, new instincts would certainly
arise in this way, and many naturalists hold this view still.
[13] This, however, is by no means intended to cast doubt on the
possibility that quite new paths may arise during the individual life,
as is made probable by the recent investigations of Apáthy, Bethe, and
others.
If the inheritance of acquired characters had already been proved in
other ways, we could not refuse to admit that it might play a part in
the higher animals in the modification and new formation of instincts.
We should then have to admit that habits can be inherited, and that
instincts actually are or may be, as they have often been said to be,
inherited habits. But to make the converse conclusion, and to infer
from the result of the brain-exercise in the individual life and its
similarity to inborn instincts that the latter also depend on inherited
exercise, and that there must therefore be inheritance of acquired
characters, is hardly admissible.
It might be all very well if there were no other explanation! But as
instincts depend on material brain-mechanisms which are variable, like
every other part of the body, and as, furthermore, they are essential
to the existence of the species, and, down to the minutest detail, are
adapted to the circumstances of life, there is no obstacle in the way
of referring their origin and transformation to processes of selection.
It has been asserted that the results of training, for instance in
dogs, can be inherited, since the untaught young pointer points at
the game, and the young sheep-dog runs round and barks at the flock
of sheep without biting them. It is, however, often forgotten that,
not only have these breeds arisen under the influence of artificial
selection by Man, but that they are even now strictly selected. My
colleague and friend, Dr. Otto vom Rath, who unhappily died all too
soon for Science, and who was not only a capable investigator, but
an experienced sportsman, told me that huntsmen distinguish very
carefully between the better and the inferior young in a litter, and
that by no means every whelp of a pair of pointers can be used for
hunting game-birds. Lloyd Morgan points out the same thing, and he is
undoubtedly a competent judge in the domain of instinct; he confirms
the statement that the pointer 'often points at the quarry, it may be a
lark's nest, without instruction,' but he says at the same time, that
the power is inborn in very varying degrees, and that, in his opinion,
selection undoubtedly plays a part.
It must not, therefore, be believed that the habit of the pointer
depends on training; it is only strengthened in each individual by
training, but it depends on an innate predisposition to creep up
to the game, and is thus a form of the hunting instinct. Man has
taken advantage of this, and has increased it, but has certainly not
ingrafted it into the breed by whipping. And something similar will be
found to be true in all cases of so-called inheritance of the effects
of training. It must not be forgotten what astounding results can
be achieved in the individual by training. The elephant is the best
example of this, for it only exceptionally breeds in captivity, and
all the thousands of 'domesticated' elephants in India are tamed wild
elephants. Yet they are as gentle and docile as the horse, which has
been domesticated for thousands of years; they perform all kinds of
tasks with the greatest patience and carefulness, in many cases without
being under constant superintendence. They are indeed animals of great
intelligence; they understand what is required of them, and they
accommodate themselves readily to new conditions of life.
The attachment of the dog to its master and to Man generally has
often been cited as a proof of the origin of a new instinct by the
inheritance of acquired habitude; but the dog is a sociable animal
even in a wild state, and by living in co-operative association with
Man it has transferred its sociable affections to him. We find exactly
the same thing in the elephant which has been caught wild and tamed.
It is particularly emphasized by those who have accompanied animal
transports in Africa that the young elephants are wild and malicious
towards the blacks who teased and maltreated them, but complaisant and
harmless towards the whites who treated them kindly. The attachment of
elephants to their keepers and to every one who shows them kindness is
familiar enough; it does not depend on a newly acquired impulse, but
on the sociable impulse inherent in the species, which, in the wild
state, causes them to live in fairly large companies, and on their
inoffensive, timid, and, we may almost say, affectionate disposition.
Of course it is easy enough to give an imaginative theoretical
interpretation of the origin of a new instinct from a newly acquired
habit. We have often heard that sailors have found the birds in distant
uninhabited islands quite free from fear; they let themselves be struck
down with cudgels without attempting to escape. The extermination of
the Dodo three centuries ago is a well-known example of this. Chun, in
his magnificent work on the German Deep Sea Expedition of 1898, has
recently communicated numerous interesting examples of the indifference
of birds towards Man when they have not learned what his presence
means: thus the sea-birds of Kerguelen, penguins, cormorants, gulls,
'kelp-pigeons' (_Chionis_), and others, behaved towards Man very much
like the tame geese of our poultry yards. Even enormous mammals like
the 'sea-elephant,' a seal with a proboscis-like prolongation of the
nose, neither attempted to escape nor showed any hostility to man, but
quietly let itself be caught. Similar tales were told by Steller in
1799, after he had been obliged to pass a winter with his sailors on an
island in the Behring Straits. The numerous gigantic sea-cows (_Rhytina
stelleri_) which lived there were so confiding that they allowed the
boat to come quite up to them, and the sailors were able to kill many
of them from time to time, using their flesh for food. But towards the
end of the winter the animals began to be shy, and, in the following
winter, when other sailors to the polar regions endeavoured to hunt
them too, it was very difficult to secure them; they had recognized
man as an enemy, and fled from him when they saw him from afar. Thus
the same individuals which had earlier carelessly allowed man to come
up to them now avoided him as an enemy. _This was not instinct, it was
a behaviour controlled by the will and founded on experience._ But it
would soon become 'instinctive' if the meeting with the enemy were
often repeated, just like the winding-up of a watch, which is often
done at a wrong time, for instance, on changing clothes during the day,
and thus without reflection. It is quite easy to conceive that if the
material brain-adaptation which causes flight without reflection at
the sight of man were transmissible, the flight-instinct might become
a congenital instinct in the species in question. But this assumption
is unfounded; for, as is shown by the case of the sea-cow, we do not
require it where the animal is of sufficient intelligence to perform by
its own discernment the action necessary to its existence. The action
may thus become 'instinctive' through exercise and imitation in the
_individual_ life, without however attaining to transmissibility.
But in many cases this is not enough, namely, in all cases in which the
degree of intelligence is not sufficiently high, or where the flight
movement must follow so rapidly that it would be too late if it had to
be regulated by the will, as, for instance, the shutting of the lids
when the eye is threatened, or the flight of the fly or the butterfly
when an enemy approaches. Both fly and butterfly would be lost in every
case if they had voluntarily to set the flight-movement going after
they became conscious of danger. If they had first of all to find
out from whom danger threatened no individual would escape an early
death, and the species would die out. But they possess an instinct
which impels them to fly up with lightning speed, and in an opposite
direction, whenever they have a visual perception of the rapid approach
of any object of whatever nature. For this reason they are difficult
to catch. I once watched the play of a cat, ordinarily very clever at
catching, as she attempted to seize a peacock-butterfly (_Vanessa Io_),
which settled several times on the ground in front of her. Quietly and
slowly she crept within springing distance, but even during the spring
the butterfly flew up just before her nose and escaped every time, and
the cat gave it up after three attempts.
In this case the beginning of the action cannot lie in a voluntary
action, for the insect cannot know what it means to be caught and
killed, and the same is true of innumerable still lower animal forms,
the hermit-crabs and the Serpulids, which withdraw with lightning
speed into their houses, and so forth. It seems to me important
theoretically, that the same action can be liberated at one time by
the will, at another by the inborn instinct-mechanism. In both cases
quite similar association-changes in the nerve-centres must lie at
the root of the animal's action, but in the first case these are
developed only in the course of the individual life by exercise, while
in the second they are inborn. In the former, they are confined to the
individual, and must be acquired in each generation by imitation of
older individuals (tradition) and by inference from experience, in the
latter they are inherited as a stable character of the species.
It has been maintained by many that the origin of instincts
through processes of selection is not conceivable, because it is
improbable that the appropriate variations in the nervous system,
which are necessary for the selective establishment of the relevant
brain-mechanism, should occur fortuitously. But this is an objection
directed against the principle of selection itself, and one which
points, I think, to an incompleteness in it, as it was understood by
Darwin and Wallace. The same objection can be made to every adaptation
of an organ through natural selection; it is always doubtful whether
the useful variations will present themselves, as long as they are
due solely to chance, as the discoverers of the selective principle
assumed. We shall attempt later to fill up this gap in the theory,
but, in the meantime, I should like to point out that the process
of selection offers the only possible explanation of the origin of
instincts, since their origin through modifications of voluntary
actions into instinctive actions, with subsequent transmission of the
instinct-mechanism due to exercise in the individual life, has been
shown to be untenable.
If any one is still unconvinced of this, I can only refer to the cases
we have already discussed of instincts which are only exercised once
in a lifetime, since, in these, the only factor that can transform
a voluntary action into an instinctive one is absent, namely, the
frequent repetition of the action. In this case, if any explanation
is to be attempted at all, it can only be through natural selection,
and as we have assumed once for all that our world does admit of
explanation, we may say, _these instincts have arisen through natural
selection_.
Even though it may be difficult to think out in detail the process of
the gradual origin of such an instinctive activity, exercised once in
a lifetime, such as that, for instance, which impels the caterpillar
to spin its intricate cocoon, which it makes only once, without ever
having seen one, and thus without being able to imitate the actions
which produce it, we must not push aside the only conceivable solution
of the problem on that account, for then we should have to renounce
all hope of a scientific interpretation of the phenomenon. We may
ask, however, whether there is not something lacking in our present
conception of natural selection, and how it comes about that useful
variations always crop up and are able to increase.
But if we must explain, through natural selection, such complex series
of actions as are necessary to the making of the cocoon of the silkworm
or of the Saturnia moth (_Saturnia carpini_), what reason have we
for not referring other instincts also to selection, even if they be
repeated several times, or often, in the course of a lifetime? It is
illogical to drag in any other factor, if this one, which has been
proved to operate, is sufficient for an explanation.
Thus, as far as instincts are concerned, there is no necessity to
make the assumption of an inheritance of functional changes, any more
than there is in regard to any purely morphological modifications. As
the instincts only exercised once show us that even very complicated
impulses may arise without any inheritance of habit, that is, without
inheritance of functional modification, so there are among purely
morphological characters not a few which, though effective, are purely
passive, which are of use to the organism only through their existence,
and not through any real activity, so that they cannot be referred
to exercise, and therefore cannot be due to the transmission of the
results of exercise. And, if this be the case, then transformations
of the most diverse parts may take place without the inheritance of
acquired characters, that is, of functional modifications, and there is
no reason for dragging in an unproved mode of inheritance to explain
a process which can quite well be explained without it. For if any
part whatever can be transformed solely through natural selection,
why, since there is general variability of all parts, should this be
confined to the passive organs alone, when the active ones are equally
variable, and equally important in the struggle for existence?
There are, indeed, many of these passive parts among animals; I need
only recall the coloration of animals, the whole set of skeletal parts,
so diversely formed, of the Arthropods, the legs, wings, antennæ,
spines, hairs, claws, and so on, none of which can be changed by the
inherited results of exercise, because they are no longer capable of
modification by exercise; they are ready before they are used; they
come into use only after they have been hardened by exposure to the
air, and are no longer plastic; they are at most capable of being
used up or mutilated. Finally, even so convinced an advocate of the
Lamarckian principle as Herbert Spencer has stated that among plants
the great majority of characters and distinctive features cannot be
explained by it, but only through the principle of selection; all the
diverse protective arrangements of individual parts, like thorns,
bristles, hairs, the felt-hairs of certain leaves, the shells of nuts,
the fat and oil in seeds, the varied arrangements for the dispersal
of seeds, and so on, all operate by their presence alone, not through
any real activity which causes them to vary, and the results of which
might be transmitted. An acacia covered all over with thorns seldom
requires to use its weapons even once, and if a hungry ruminant does
prick itself on the thorns it is only a few of these which are thus
'exercised,' the rest remain untouched.
But since all these parts have originated notwithstanding their
passivity, there must be a principle which evokes them in relation to
the necessities of the conditions of life, and this can only be natural
selection, that is, the self-regulation of variations in reference to
utility. And if there is this principle, we require no other to explain
what is already explained.
I can quite well understand, however, that many naturalists, and
especially palæontologists, find it difficult to accept this
conclusion. If we think only of those parts that actively function,
and thus change by reason of their function, being strengthened by use
and weakened and diminished in size by disuse, and if, further, we
follow these parts through the history of whole geological epochs, we
may certainly get the impression that the exercise of the parts has
directly caused their phyletic evolution. The direction prescribed by
utility in the course of the individual life and in the phylogeny is
the same, and the intra-selection, that is, the selection of tissues
within the individual animal, leads towards the same improvements
as the selection of 'persons.' Thus it appears as if the phyletic
variations followed those of the individual life, while in reality the
reverse is true; the changes arising from variations in the _germ are
primary, and they determine the course of phylogenesis_, while the
tissue-selection in the individual life only elaborates and improves,
according to the demands made upon it, the material afforded by the
primordial equipment of the germ.
The American palæontologist, Osborn, cites the case of the horse's
feet as an example in support of his view that modification brought
about by use in the individual life must be transmitted in order that
the phyletic transformations may be brought about, but this example
is perhaps the best that could be chosen to prove the contrary. He
supposes that, in every young horse, the means of locomotion are
improved at every step, so to speak, through the contact with the
ground, and I am quite willing to admit that this is so. But that only
proves that, even now, an elaboration and improvement of the equipment
which the germ affords is indispensable, as it has been at all times
and in all animals, and thus that, notwithstanding the enormous number
of generations which our modern horse has behind it, the functional
acquirements of the individual have not yet been impressed upon the
germ. Why not? Because the horse becomes perfect without this, and
there was no reason why personal selection should perfect the primary
constituents of the germ still further, since the finishing touch of
perfection through use is readily afforded by the conditions of each
individual life.
Moreover, when Osborn, Cope, and other palæontologists emphasize that,
in phyletic evolutionary series, _definite paths of evolutionary_
change are adhered to, and are not deviated from either to right or to
left, they are undoubtedly right, but the conclusion which they draw is
not justifiable, whether they assume with Nägeli that there is a power
of development, a principle of perfecting, or whether, as Osborn does,
they assume the transmission of the modifications brought about through
use in the individual life. There remains a third possibility, that
the quiet and constant evolution in a definite direction is guided by
selection, and as, in passively useful parts, that principle alone is
admissible, I see no justification for assuming it to be inoperative
in regard to those which are actively functional. All these variations
which have led up, for instance, to the modern form of the horse's foot
are useful; if they were not, they could not have been produced either
by use or by disuse in the individual life.
At the same time, here again, we are justified in inquiring whether the
assumption of 'chance' germinal variations, which we have hitherto made
with Darwin and Wallace, affords a sufficient basis for selection.
Osborn says very neatly in this connexion, 'We see with Weismann and
Galton the element of chance; but the dice appear to be loaded, and in
the long run turn "sixes" up. Here arises the question: What loads the
dice?'
Until recently we might have answered, 'external conditions'; it is
they that load the dice one-sidedly, and condition that the same
straight path of phylogenesis is adhered to, and exactly the same
direction of variations is preferred and maintained. It has to be
asked, however, whether this answer, which is certainly not absolutely
incorrect, is sufficient by itself, whether the dice are not falsified
and one-sidedly loaded in another sense, so that they always throw a
preponderating number of the useful variations. We shall attempt very
soon to solve this problem, but in the meantime I must refer to another
argument in favour of assuming the Lamarckian principle, perhaps
the most important and it may be thought the most difficult of all
to refute, the so-called co-adaptation of the parts of an organism,
that is, the fitting together of many individual organs for a common
purposeful functioning.
LECTURE XXIV
OBJECTIONS TO THE THESIS THAT FUNCTIONAL MODIFICATIONS ARE NOT
TRANSMITTED
Giant stag as an example of co-adaptation or 'harmonious
adaptation'--This occurs even in passively functioning parts--Skeleton
of Arthropods--Stridulating organ of ants and crickets--Limbs
of the mole-cricket--Wing-venation--Colorations which form
mimetic pictures--Harmonious adaptations in worker-bees and
ants--Degeneration of their wings and ovaries--The quality of food
acts as a liberating stimulus--Vom Rath's case of drones fed with
royal food--Transition-forms between females and workers--Wasmann's
explanation of these--The Amazon ants--Two kinds of workers--Appendix:
Zehnder on the case of ants--On the skeleton of Arthropods--Hering's
interpretation of Ehrlich's Ricin experiments--Hering's position in
regard to the transmission of functional modifications.
It was Herbert Spencer, the English philosopher, who first brought
the argument of co-adaptation into the field against my view of the
non-inheritance of functionally acquired modifications. He pointed
out that many, if not, indeed, most modifications of bodily parts, to
be effective, implied further changes, often very numerous, in other
parts, and these latter must therefore have changed _simultaneously_
with the part which was being changed under the control of natural
selection; this, however, is only conceivable as due to an inheritance
of the changes caused by use, since a simultaneous alteration of so
many parts through natural selection would be impossible. If, for
instance, the antlers of our modern stag were to grow to the size of
those of the Giant Stag of the Irish peat-bogs, which measured over
ten feet across from tip to tip, this would mean--as has already been
shown--a simultaneous thickening of the skull, and to bear the heavy
burden, a strengthening of the _ligamentum nuchæ_, of the muscles of
the neck and back, of the bones of the legs and their muscles, and,
finally, of all the nerves supplying the muscles; and how could all
this happen simultaneously with, and in exact proportion to the growth
of the antlers, if it depended--as natural selection assumes--on chance
variations of all these parts? What if the appropriately favourable
variation in one of these organs did not occur? A harmonious variation
of all the parts--bones, muscles, nerves, ligaments--which unite in a
common activity, is an inadmissible assumption, because, in many cases,
such co-operating groups of organs have in the course of evolution
developed in opposite directions. In the giraffe, for instance, the
fore-legs are longer than the hind-legs, which is the reverse of what
obtains in the majority of ruminants; in the kangaroo the hind-legs,
on the contrary, have developed to a disproportionate size, while
the fore-legs have degenerated into relatively small grasping arms.
Co-operating parts, like the fore and hind limbs, may thus follow
opposite paths of evolution; their variations need not always be
directed to the same end.
The difficulty presented by these so-called co-adaptations or
harmonious correlations cannot be denied, and we must also admit
that, if the results of exercise were inherited, the explanation of
the phenomenon would, in many cases--but not, indeed, in all--be
easy, because the adaptation of the secondarily varying parts in each
individual life would correspond exactly to the altered function of
the part, and would be transmitted to the descendants, and in them
would again be subject to such a degree of variation, according to the
principle of histonal selection, as might be conditioned by the further
progress of the primary variation. The simplicity of the explanation
is striking, if only it were at the same time correct! But there are
whole series of facts, or rather of groups of facts, which prove that
the causes of co-adaptation do not lie in the inheritance of functional
modifications, and this must be recognized, even though we may not
yet be in a position to state the causes of co-adaptation, and to say
whether natural selection suffices to explain it or not.
I must first point out that co-adaptations occur not only in _actively,
but also in passively functioning parts_. Very numerous instructive
examples are to be found among the Arthropods, whose whole skeleton
belongs to this category. It has been objected that this is not wholly
passive, but that, like the bones of vertebrates, it is stimulated by
the contraction of the muscles and incited to functional reaction,
and that it thickens at places where strong muscles are inserted, and
becomes or remains thin where it is not exposed to any strain from
the muscles. But this is not the case, for the chitinous skeleton
can only offer resistance to the muscular contractions when it is no
longer soft, as it is immediately after it is secreted. As soon as it
has become hard, it can no longer be altered, and can at most be worn
away externally by long use. The proof of this lies in the necessity
for moulting, which is indispensable to all Arthropods as long as they
continue to grow, but does not occur later. Every one who has followed
the growth of an insect or a crustacean knows well that the moultings
or ecdyses are often accompanied by great changes, and hardly ever
occur without some slight changes in the form of the body, especially
of the limbs, with their teeth, bristles, spines, and so on. These
new or transformed parts are formed before the throwing-off of the
old chitinous shell, and under its protection, and they are brought
about by an elaboration or transformation of the living soft matrix
of the skeleton, the hypodermis, which consists of cells, and is the
true skin. They must thus have arisen in the ancestors of our modern
Arthropods in the same way, that is, not by a gradual modification
_during_ use, but by a slight sudden transformation before use. The
steps in the transformation may have been very small, a bristle may
have become a little longer in the second stage of life than it was
in the first, or instead of five bristles a particular spot may bear
six in the second or third stage of life; but the variations in the
phyletic development must always be caused by germ-variations which
effect from within the variation in the relevant stage of development.
But the part which has varied can only function after it has become
firm and immodifiable.
If these circumstances be kept clearly in mind, they furnish a quite
overwhelming mass of proof against the views of the Lamarckians.
Furthermore, it is not even true that the thickest parts of the
external skeleton are those at which the muscles are inserted. The
wing-covers of beetles offer the best proof to the contrary, for
there are no muscles at all in them, yet they are, in many species,
the hardest and thickest part of the whole chitinous coat of mail.
The reason is not far to seek; they protect the wings and the soft
skin of the back, which lies concealed beneath them, and the muscles
are inserted in this!--a relation which can be explained only by its
suitability to the end, and not as due to any direct effect.
When we remember the origin--which we have just described--of the
external skeleton from the soft layer of cells underneath it, the
thickness of the chitinous skeleton, which is very different at
different places in the same animal, but always adapted to its end,
furnishes a case of co-adaptation in parts which have a purely passive
function. The thickened part cannot be due to the insertion of a
muscle, but it is always there in advance, from internal causes, so
that the muscle finds sufficient resistance. Close to it there may lie,
perhaps, the edge of a segment, and at this spot the chitinous skeleton
becomes almost suddenly thinned to a joint membrane capable of being
bent or folded, not _because_ there was no pull from the muscles at
this spot, but in order that the two segments may be connected movably.
Thus, nowhere in the whole body of the Arthropod can the adaptation
of the skeleton, in regard to thickness and power of resistance, be
regulated by function itself, but only by processes of selection
which imparted to each spot the thickness it required, in order to be
effective in its function, whether that be offering resistance to the
strain of the muscles, or giving suppleness to a joint, or affording
the necessary hardness for biting the prey, or for boring into wood or
earth, or merely for protecting the animal from external injuries.
There are, however, many individual functions of the Arthropods the
exercise of which depends on the simultaneous change of several
skeletal parts; as, for instance, many of the 'singing' or vocal
apparatuses in insects. In quite recent times such vocal organs have
been discovered in ants, in which they consist of a small striated
region on the surface of the third abdominal segment, and a sharp ridge
on the segment in front; the latter is rubbed against the former by the
movements of the two segments. Quite a similar 'stridulating organ' has
long been known in the bee-ant (_Mutilla_), and the whistling sound
produced by it is easily heard by our ears; moreover August Forel has
heard it in the large wood-ant (_Camponotus ligniperdus_), and has
described it as an 'alarm-signal,' which the animals give each other
on the approach of danger--an observation which has recently been
confirmed by Wasmann and extended by Robert Wroughton in regard to
Indian ants. All these arrangements for producing sound depend always
on two organs, of which one resembles the bow, the other the strings
of a violin; the one is of no value without the other, and they must
therefore have developed simultaneously, yet they cannot have arisen
through use, and the inheritance of the results of use, because they
are both dead chitinous parts, which are never strengthened by rubbing
against each other with the movements of the abdomen, but are rather
worn away.
[Illustration: FIG. 91 (repeated). Hind-leg of a Grasshopper
(_Stenobothrus protorma_), after Graber. _fe_, femur. _ti_, tibia.
_ta_, tarsal joints. _schr_, the stridulating ridge.]
The same is true of the chirping organs of grasshoppers, beetles,
and crickets; in all cases they consist of two different parts,
which together produce a sound, and which therefore must have arisen
simultaneously, and the origin of which cannot be referred to the
inheritance of the results of exercise, but rather to selection. It is
thus possible that co-adaptation of at least two parts may take place
even when the hypothetical Lamarckian principle is altogether excluded.
When I say that we have here a case of two parts adapted to each
other, that is, strictly speaking, understating the case, for, in the
crickets and locusts, for instance, there is a whole series of peg-like
chitinous papillæ (Fig. 86), the so-called 'bridge,' each of which
must have arisen by itself through variation of the corresponding
spot of skin. At least I can see no ground for the assumption that
the chitinous surfaces on which the 'bridge' is now placed would
necessarily, from internal reasons, have varied precisely in the line
of the bridge as it has done.
[Illustration: FIG. 102. Brush and comb on the leg of a Bee (_Nomada_).
_tib_, end of the tibia. _t^1_, first tarsal joint with the brush and
its comb (_tak_). Between these and the tibial spine (_tisp_) with its
lappet (_L_) the cross-section of an antenna (_At_) is indicated. Drawn
from a preparation by Dr. Petrunkewitsch.]
Instructive examples of the co-adaptation of several parts to a common
action in organs which are not subject to the Lamarckian principle
are afforded by the diverse arrangements for cleaning the antennæ the
bearers of the smelling-organ which are so important to the life of
insects (Fig. 102). Here even the adaptation of an indented area on
the tibia of the anterior leg to the cylindrical form of the antenna
which passes through it, is sometimes so striking (Fig. 102, _tak_)
that it might be thought that it must have arisen through a gradual
wearing out; yet this is impossible, since we have to do with hard
dead chitinous surfaces, and moreover not with a solid mass, like a
hone, which is worn down by the knife, but with a hollow, thin-walled
tube. In ants, bees, and ichneumon-flies this minute, semi-circular
indentation contains small, pointed, triangular saw-teeth, closely
set like those of a comb (_tak_), and the apparatus is made usable
by the fact that a firm spine (_tisp_), fused to the end of the
tibia, overhangs the notch and presses the antenna towards it. In
many species this spine is double, or it is furnished with a thin
comb or lappet (Fig. 102, _L_), or with rows of teeth, or with short
bristles; in short, it may be equipped in the most different ways. Not
infrequently, as in wasps of the genera _Sphex_, _Scolia_, _Ammophila_,
the spine itself is also bent in a semicircle on the surface directed
towards the notch, and this may be effected in very different ways,
either by a bending of the whole thickness of the spine, or by the
presence of a comb which is concave on its inner surface. I should
never come to an end if I were to enumerate all the remarkable details
which may be found in the two main parts of this apparatus, and which
show very clearly how essential a co-operation of the two is in
fulfilling the function of cleaning the antennæ. This fitting together
of the two main parts cannot have been brought about in accordance with
the Lamarckian principle; the adaptation must therefore have come about
in some other way.
The same thing is shown by the legs and other appendages of insects and
crustaceans, which are adapted for the most diverse functions, and the
individual sections of which must be correlated if the function is to
be possible. Let us consider only the claw structures in crustaceans
and scorpions. Here, too, it seems as if the outgrowth of the last
joint of the leg, which functions as the arm of the claw, must have
arisen as a direct effect of use, through the pressure of an object
held fast by the last joint, the movable half of the claw. Frequently,
moreover, tooth-like protuberances occur on the fixed blade of the
claw (Fig. 103). But how could these have arisen as a direct effect
of pressure, since they are always preformed during the soft state of
the appendage _before use_, and are only made use of after it is fully
hardened. The soft crustaceans, the so-called 'butter-crabs' which
have just cast their shells, creep carefully away and avoid using
their limbs until they have become hard again. Here, too, we have the
co-adaptation of two parts which vary independently, and which cannot
be affected by the Lamarckian principle.
[Illustration: FIG. 103. Claw (_Sch_) on the leg of a 'Beach-fly,' an
Amphipod Crustacean (_Orchestia_). _I_, _II_, the two first joints.
_uA_, the lower blade of the claw, a non-mobile prolongation of the
penultimate joint. _oA_, the upper blade of the claw, the movable
last joint; the tubercles and indentations of the two blades fit one
another. After F. Müller.]
But the appendages furnish more complex examples of mutual adaptation.
Thus the individual sections of the anterior leg of the mole-cricket
(_Gryllotalpa_) have varied greatly, yet quite differently, and the
whole together forms a most effective digging-tool. With it the animal
digs out the earth before it to right and to left, and to do this
it makes with both legs simultaneous outward movements, which are
otherwise quite unusual among insects, and does so with such strength
that Rösel von Rosenhof saw two bodies each weighing three pounds
pushed away in this manner. In this case four chief parts of the leg
(Fig. 104), the coxa (_cox_), the femur (_fe_), the tibia (_tib_),
and the tarsi (_tars_) are so adapted to each other in form, joints,
thickness of skeleton, and size, that they cannot have varied otherwise
than in relation to each other, but each piece has done so in an
individual manner. Most remarkable of all is the short broad tibia,
equipped with four large, hard teeth, which has to perform the digging
in the ground after the manner of a spade, while the disproportionately
thin and weak tarsal joints, the last of which bears two perfectly
straight spines instead of claws, are directed upwards, and do not
touch the ground, being no longer used for walking. Rösel supposed,
probably correctly, that they are used for cleaning the spade when it
becomes clogged up with earth, since the animal cannot clean it with
its mouth. These quite unusually formed parts of the limb cannot have
become what they are as the direct results of use, because, for one
thing, it would have been not their broad surfaces, but their narrow
edges, which would most easily cut through the earth, that would have
been directed outwards. The peculiar curving, first concave, then
convex, of the outer surface of the digging foot is exactly what is
best adapted for cutting into the earth and for the pushing aside which
follows, but it is not what it would have become if the chitin-wall had
yielded to the pressure of the earth and adapted itself to it. But,
as we are again dealing with the chitinous skeleton, there can be no
question of the direct effect of use, and, it seems to me, it must be
admitted that here we have a case of co-adaptation of at least seven
different parts, which have varied independently of each other, without
any assistance from the Lamarckian principle.
[Illustration: FIG. 104. Digging leg of the Mole-cricket
(_Gryllotalpa_). _cox_, coxa attaching the limb to the thorax. _fe_,
the short broad femur. _tib_, the tibia forming a broad spade with six
large sharp teeth. _tars_, the tarsal joints, which are turned upwards
and cannot be used in locomotion. After Rösel.]
But much more complicated cases than this might be cited, if we were in
a position to estimate exactly the functional value of the individual
parts of the wing-venation in the different insects, for it is well
known that this venation serves the systematist as a basis for the
definition of genera, especially in Lepidoptera and Hymenoptera. That
is to say, it varies from genus to genus in a characteristic manner,
obviously corresponding to the differences in the wing-form, and in
the flight itself. But, unfortunately, we are still far from being
able to make more than quite general hypotheses as to the meaning of
the lengthening and strengthening, or conversely, the degeneration or
elimination, of this or that vein. From extreme cases, however, as for
instance the rich venation in good fliers with large wings, and the
scanty venation in poor fliers with small wings, we learn at least so
much, that the degree and even the manner of venation bears a definite
relation to the function of the wing, and this we might have assumed.
But these wing-veins, in as far as they serve as a support for the
weak wing-membrane, are purely chitinous structures, skeletal parts
which are not even renewed from time to time like the skeletal parts
of the leg and many other parts of the insect. As they are laid down
at first in the pupa as soft strings of cells, so they remain, and
they only begin to be used when they are completely hardened. _They
can therefore never have been caused to vary through use in the course
of the phyletic development of species and genera_, and the Lamarckian
principle can have no part in their transformations. But if they follow
the most subtle changes, which we cannot precisely demonstrate, of the
whole wing-surface and in the mode of flight, as a man is followed by
his shadow, there must be some other principle which adapts the organ
to its function, and which is able continually to adapt the large
number of individual wing-veins in the manner most advantageous for the
general function. Here, therefore, we have a state of matters exactly
corresponding to that obtaining in the transformation of actively
functioning parts which form a system with common co-operative action,
as, for instance, in the case we first discussed, that of the stag's
antlers.
Other even more complicated examples of harmonious adaptation of
passively functioning parts are afforded by the markings of animals,
such as those of the butterfly's wing. The colours have only a passive
rôle, whether they be due to pigments alone, or to structure, or to
both combined. When the coloration of a surface undergoes adaptive
variation, this cannot be due to any action of the colour, but must
depend on adaptation through selection. Yet it is well known that there
are many butterfly-wings whose surfaces exhibit different colours and
different shades of colour on their different parts, and that in such
a way that together they form a picture, that of a leaf, a piece of
bark, a stone overgrown with lichen, an eye, and so on. In such a case
the individual colour-spots stand in a particular, indirect relation
to each other; although they are independent of each other in their
variation, they are not indifferent and due to chance, for together
they produce a common picture; this is harmonious adaptation of many
parts, where the Lamarckian principle is absolutely excluded.
It may, perhaps, be objected that this mimetic picture does not
arise all at once, but very slowly in the course of long series of
generations, and, indeed, of species. This must of course be so; the
simple beginnings are complicated and perfected through the course
of long ages. This is implied in the principle of selection as we
understand it. But does any one suppose that the gigantic antlers of
the giant-stag were developed in a few generations? In this case, too,
must not numerous races have succeeded each other before the primitive
antlers attained this enormous size? If this must be assumed there was
abundance of time for the adaptation, through _germinal_ variations, of
the secondarily varying parts, the muscles, tendons, nerves, and bones,
for all these parts function actively, and can without difficulty meet,
in the individual life, the increased claims made upon them by a slight
increase in the size of the antlers. For the certain and indubitable
consequence of exercise, of increased use, is the strengthening of the
functioning parts.
Thus the appropriate germinal variation of the secondarily varying
parts may be delayed for a little without the individual being any
the less effective, or being obliged to succumb in the struggle for
existence. I do not, however, assert for a moment that the whole
explanation of the phenomena of co-adaptation is included in this;
on the contrary, I hope soon to be able to show that we may in such
cases assume a preponderance of variational tendencies in a favourable
direction, and that there is thus an indirect connexion between
the utility of a variation and its actual occurrence. In the first
place, however, I must refer to the other group of facts which I have
indicated, which show, likewise, that the simultaneous co-adaptation
of different parts may arise in certain circumstances, although the
Lamarckian principle be excluded. These are the facts presented to us
by the sterile forms of those insects, which, like bees, termites, and
ants, live together in large societies.
Ants and bees are of special interest to us in this connexion, because
they have long been carefully watched by a number of distinguished
naturalists, and most of their vital functions have been precisely
studied. Ever since the days of 'Old Peter Huber' in Geneva there have
again and again been excellent observers who have devoted almost the
whole of their life-work and talents to the more complete study of
these wonderful animals. These insects are of interest to us here,
because, in the course of the social life, a type of individual has
arisen which diverges in structure in many parts of the body from
both the male and the female, although it is sterile and does not
reproduce, or does so in so few instances that the fact is of no moment
in considering the origin of the present bodily structure. As is well
known, these neuters, or better, workers, are, among ants and bees,
females which differ from the true females not only in their smaller
size and their infertility, but in many other points as well. Among
ants, for instance, they are absolutely wingless, and at the same time
they have a much smaller and differently formed thorax and a larger
head. But the most striking point is the difference in their instincts,
for while the females, concerned only with reproduction, pair and
lay eggs, it is the workers who feed and clean the helpless emerging
larvæ, and put them in places of safety, who carry the pupæ into the
warm sunshine, and afterwards back again to the sheltered nest, who
make this nest itself, and keep it in order, after having collected or
prepared the material for it; it is they alone who defend the colony
against the attacks of enemies, who undertake predatory expeditions,
attacking the nests of other ants, and engaging in obstinate combats
with them.
How can all these peculiarities have arisen, since the workers do not
reproduce, or do so only exceptionally, and, in any case, are incapable
of pairing, and therefore--among bees at least--only produce male
offspring? Obviously it cannot have been through the transmission of
the effects of use and disuse, since they leave no offspring to which
anything could be transmitted.
Herbert Spencer has attempted to maintain the position that the
characters of the workers of to-day already existed in the pre-social
state, that is, before the ants began to form colonies, and that,
therefore, they have not been newly evolved but only preserved. But,
even if this be conceded in regard to the care of the brood and the
building instinct, so much remains that could not have existed at that
stage, that the problem of the origin of these new characters remains
unsolved. The wings, for instance, among ants, can only have been lost
when females appeared which did not reproduce, for the pairing of ants
is associated with a nuptial flight high in the air. The wings are not
merely absent in the workers, they do not even develop in the pupæ;
they are, as Dewitz showed, present even now in the larva in the form
of imaginal disks, but from the pupa-stage onwards they degenerate, and
the segments of the thorax to which they are attached likewise appear
small and modified. A variation of the germ-plasm must therefore have
taken place, and to this is due the fact that the wing-primordia no
longer develop, and that the thorax has a different development from
what it had at the time when the animals were still fertile.
It has indeed been said that there is no need for assuming a variation
of the germ-plasm, since the degeneration of the wing might be produced
by inferior nourishment. This opinion is based on the fact that, among
bees, the workers do actually arise from female larvæ which have
received a meagre diet poor in nitrogenous elements, while the same
female larvæ supplied with an abundant diet rich in nitrogen develop
into queens.
But even though we may assume that there is a similar difference in the
mode of feeding among most ants, because the workers are considerably
smaller than the fertile females, it would be quite erroneous to
conclude that the difference between the two types rests solely on the
effect of differences in diet. The elimination of an individual organ
has never yet been determined by bad and scanty nourishment; it is the
whole animal with all its parts that degenerates and becomes small and
weakly. Often as caterpillars of different species have been placed
on starvation diet, whether for experimental purposes or to procure
very small butterflies, it has never yet happened that a single organ,
such as antenna, leg, or wing, has thereby been eliminated or caused
to degenerate. I have myself instituted many such experiments with the
maggots of the blue-bottle fly, by supplying them from their earliest
youth with just as little food as possible without actually starving
them to death, yet never have these larvæ given rise to flies in which
the wings were absent or rudimentary.
Nor did these starved flies ever exhibit degenerate ovaries; they
were always completely developed and equipped with the full number
of ovarian-tubes. It was to decide this particular point that
these experiments were instituted, for my opponents maintained
that degeneration of the ovaries was a direct result of inferior
nourishment. But that is not the case. Special investigations in regard
to ants, undertaken at my request by Miss Elizabeth Bickford, showed
that the anatomical results reached by earlier investigators, like
Adlerz and Lespès, in regard to the degeneration of the ovaries in
workers, were absolutely correct, and that the 'degeneration' consists
not merely in the fact that the ovarian-tubes and ovum-primordia remain
small, but also in a diminution of the _number_ of ovarian-tubes (Fig.
105); the workers have always fewer ovarian-tubes than the females of
the same species, and--what is of especial importance--the reduction
in the number of ovarian-tubes has been effected to a different extent
in different species of ants. In the red wood-ant (_Formica rufa_) the
workers still possess from twelve to sixteen ovarian-tubes; in the
meadow-ant (_Formica pratensis_) only eight, six, or four; in _Lasius
fuliginosus_ there are usually only two (one on either side); and in
the little turf-ant (_Tetramorium cæspitum_) there are none at all. We
have here, therefore, a phylogenetic process of degeneration, which
has reached different degrees in the different species, and has only
been completed in one (_Tetramorium_). The case stands as I previously
stated it: 'The elimination of a typical organ is not an ontogenetic
process, but a phylogenetic one,' it depends not upon 'the mere
influences of nutrition which affect the development of the individual,
but always on variations in the germ-plasm, which, to all appearance,
can only come about in the course of a long series of generations'[14].
[14] _Aeussere Einflüsse als Entwickelungsreize_, Jena, 1894.
[Illustration: FIG. 105. Ovary of a fertile Queen-Ant and ovaries of
a Worker. _Od_, oviduct. _A_, one ovary of _Myrmica lævinodis_ with
many ovarian-tubes, in each of which there is an almost ripe egg (_Ei_)
and a younger egg (_Ei´_). _B_, the ovaries of a Worker of _Lasius
fuliginosus_; each ovary has only one ovarian-tube, and no ripening
egg-cells. After Elizabeth Bickford.]
Against this proposition an observation by O. vom Rath has been
cited. According to it, three drone larvæ which had been accidentally
fed by the workers with royal food exhibited striking retrogressive
peculiarities in their sexual organs. The testes contained only
immature sperms (just before emergence from the pupa), and the
copulatory organ was entirely wanting. That a certain degree of fatty
degeneration of the testes should be caused by the 'unusual fattening'
is not surprising, but it seems to me very questionable whether the
absence of the copulatory organ can be referred to the abnormal
diet; it ought to be definitely decided, by the investigation of
numerous cases, whether some abnormal peculiarity in the constitution
of the germ-plasm in these eggs was not the true cause. Hitherto,
unfortunately, I have not been able to procure the fresh material
necessary to decide this point[15]. From all this it must be evident
that we are not justified in regarding either the absence of wings or
the degeneration of the ovaries as a direct result of the inferior
nourishment supplied to the workers in the larval state: but should
any one still have doubt on this point I may mention that, among our
indigenous ants, there are two species in which the workers are just as
large as the fertile females, and that in tropical America a species
(_Myrmecocystus megalocola_) occurs in which the workers are larger
than the true females; this must mean that they have received more food
than the females, though perhaps not the same mixture of food.
[15] Since completing my manuscript I note that the point was settled
three years ago, when Koshewnikow had the opportunity of investigating
drone-pupæ which were abnormally reared in royal cells, and therefore
fed with royal food. He found their sexual organs perfectly normal, and
agrees with me that the abnormalities in Vom Rath's case must have been
due to some other cause. (See the report by Von Adelung on the Russian
paper in _Zool. Centralblatt_, Sept. 10, 1901.)
From all the facts we have discussed we can confidently conclude that
the differences in structure, which distinguish the workers from the
true females, do not depend upon the influence, in the individual
lifetime, of a poorer diet, but upon variation in the primary
constituents of the germ; we must conceive of the germ-plasm of ants as
containing, in addition to male and female ids, special ids of workers,
in which the determinants of wing and ovary are degenerate in some
degree, while the determinants of other parts, such as the brain, are
more highly developed. The manner of feeding, however, and perhaps the
mingling with the food of a special secretion of the salivary glands,
acts as a stimulus which determines whether one kind of id or another
is to be liberated, that is, to become active and to enter on the path
of development.
A proof of this view is to be found, it seems to me, in the existence
of transition forms between workers and true females, which was first
brought to general knowledge by Forel. Perhaps it would be better to
call these 'mongrel forms' for their various parts do not maintain a
medium between the two types, but many parts follow the type of the
worker, and others that of the true female. Thus Forel twice found a
nest of the red wood-ant which contained a large number of these mixed
forms, all of which possessed the small head and large curved thorax of
the queen, but otherwise resembled the workers in size and appearance,
and also in the degeneration of the ovaries. Many of them were very
small, only 5 mm. in length, and had probably received very little
food, and, according to the theory of direct influence, these should
have been pure workers. That they possessed the head and thorax of a
queen is a proof that the characters of both forms of individual were
present in the germ-plasm as primary constituents, or indeed entire
ids. In normal circumstances only one kind of these ids would have
become active, either the worker-id or the queen-id, but in abnormal
circumstances they might both be liberated to activity simultaneously,
and then they would stamp one part of the body with the character of
a queen, another with that of a worker. Forel observed one of these
nests in two successive years, and both times found the mixed forms
in large numbers[16]. In the second year he found a great number of
newly-emerged individuals of this type. I have already inferred from
this observation that the mixed forms were probably in both years
the offspring of the same mother, and this may well have been the
case. My further conclusion, that the mixed forms must be due to some
abnormality in the constitution of the germ-plasm of the maternal eggs,
no longer appears to me so convincing as it did formerly, because, in
the interval, we have learnt, through that indefatigable investigator
of ants, Pater Wasmann, that there is another possible explanation of
these mixed forms; it, too, is based upon a hypothesis, but it is so
interesting that I must briefly outline it to you.
[16] There are different kinds of 'mixed forms' among ants, which may
owe their origin to a variety of conditions, as Forel, Wasmann, and
Emery have shown in detail.
Like Forel and myself, Pater Wasmann had supposed that the reason of
this kind of mixed form (the so-called pseudogynous worker) lay in an
abnormality of the constitution of the germ-plasm, but he now regards
it as the result of a change in the mode of rearing instituted by the
workers with respect to the constitutionally female or queen larvæ,
because there was a scarcity of workers. The hypothesis sounds very
daring, but it is well founded, at least in so far that there really
is a reason why a scarcity of females must occur at certain times in
some colonies of ants, and this might certainly determine the workers
in charge of the larvæ to feed females with worker food, so as to rear
them to render the necessary assistance.
This reason lies in the occasional presence of a parasitic beetle,
_Lomechusa strumosa_, whose larvæ, curiously enough, are cared for and
fed by the ants as though they were their own, and in return they eat
up the larvæ of the ants, often destroying them in large numbers.
Wasmann informs us that the parasitic larvæ grow up just at the time at
which the ants are rearing their workers, and it is these, therefore,
which fall victims to the Lomechusa-larvæ, and the result is that a
scarcity of young workers must soon make itself felt. The workers
seek to make this good by rearing as workers all the larvæ previously
destined for queens. But this only succeeds partially, because the
development towards true females has already begun; thus mixed forms
arise.
This explanation would be rather in the air if we did not know that,
among bees, such changes in the manner of rearing are by no means
uncommon. Indeed they occur regularly when the queen of a hive perishes
and no more 'female' eggs are in store; young worker larvæ are then fed
with royal food, and these develop into queens. There can thus be no
doubt that these insects have it in their power to liberate to activity
either the female ids or the worker ids by a specific mode of feeding,
and there is nothing contrary to reason in admitting the possibility
of an alternation of this influence in the course of development, for
something analogous occurs in regard to secondary sexual characters,
as, for instance, the appearance of male decorative colours in ducks
that have become sterile.
But this change in the mode of rearing bee-larvæ gives rise to pure
queens and not to mixed forms, and we must therefore regard it as
undecided whether Wasmann's explanation is correct in this case, and
whether an abnormality in the constitution of the germ-plasm may not be
the true cause of this or other kinds of mixed forms among ants. In any
case the 'Lomechusa hypothesis' rests upon the assumption of different
kinds of ids in the germ-plasm, as Pater Wasmann expressly states, and
the differences between the worker and queen-ants have their cause in
this, and not directly in the kind of larval food.
If there were not different ids corresponding to the different kinds
of individuals in the germ-plasm a kind of polymorphism might indeed
have arisen in the colony through differences in nutrition, but it
could not have been of the kind we now see--that is, a sharply defined
differentiation of persons, in adaptation to their different functions.
This presupposes elements in the germ which can vary slowly and
consistently in a definite direction without causing any change in the
rest of the germ.
This state of affairs gives to the phyletic evolution of the workers
a great theoretical significance, for it proves that positive as
well as negative variations of the most diverse parts of the body,
that simultaneous and correlative variations of many parts, can take
place in the course of the phylogeny, without the co-operation of
the Lamarckian factor. I have not hitherto laid any special emphasis
upon the degree of differences occurring between workers and queens;
but I must now add that this may far exceed the degree that we are
familiar with in our common indigenous ants, both in regard to
instinct and to bodily form. Even in the red Amazon ant of Western
Switzerland, _Polyergus rufescens_, we find quite a new instinct[17],
that of carrying off the pupæ of other species of ants, not to devour,
but to introduce them to their own nest and thus secure 'slaves.'
For these workers of a strange species, which emerge in a strange
nest, naturally regard the place of their birth as their home, and
do there what instinct impels them, and what they would have done in
the nest of their parents: they feed the larvæ, fetch food, collect
building material, and so on. The domestic activity of the workers
of the master-species thus becomes superfluous, and they have ceased
to exercise it, and have now entirely lost the power of caring for
their brood, searching for food, and keeping up the nest. They have
even forgotten how to take food themselves, because they are always
fed by the 'slaves.' Forel informs us--and I have myself repeated the
experiment--that _Polyergus_ workers, which are shut up with a drop
of honey on the floor of their prison, will leave it, their favourite
food, untouched, and finally starve, unless one of their 'slaves' be
shut up with them. As soon as this happens, and the slave perceives the
honey, it partakes of it, and then the 'mistress' comes and strokes the
'slave' with her antennæ to signify her desires, whereupon the 'slave'
proceeds to feed her from its own crop.
[17] 'New' in this sense, that the instinct is not exhibited by most
worker-ants, that it did not occur in the primaeval ancestors of modern
ants. It is, however, exhibited by a number of modern forms, and even
by some German species.
But while the _Polyergus_ workers have forgotten their domestic habits,
and have even ceased to be able to recognize their food, remarkable
changes have taken place in their jaws; these have lost the blunt teeth
on the inner margin, which, in other species, serve for masticating
the food, for seizing building material, and for other domestic
occupations, and have become sharp weapons, bent in the form of a
sabre, very well suited for piercing the head of an enemy, but also
well adapted for carrying off the pupæ, because they can seize them
without doing them any injury.
No one will doubt that the predatory expeditions of the Amazon ants,
and the slave-making habit, can only have developed after the habit
of living in large companies had long existed, and this case proves
that variations of instinct, as well as of bodily structure, can
take place even after the workers have long been sterile. The case
is the more instructive that it _seems_ as if it were due to the
transmission of a newly acquired and inherited habit of life, while
in point of fact these Amazon-workers can transmit nothing, because
they bear no offspring. But if old instincts can be lost, and new ones
acquired, when all possibility of inheritance is excluded, we see that
Nature has no need of the Lamarckian factor of modification for her
transformations and new adaptations.
If we wish to understand clearly that, in these changes, we have
to do not merely with the alteration of a single part, but of many
parts which all work together, we have only to think of the still
more striking physical modifications which have taken place in many
tropical ants, and which have led to a dimorphism of the workers.
In many species, certainly, the only difference is in size, so that
one can distinguish between large workers and small, and the former
are sometimes five times as big as the latter. But even in the South
European _Pheidole megalocephala_, which is abundant in Italy, the
larger workers are also different in structure from the smaller, for
they have an enormous head with powerful jaws. They are usually known
as 'soldiers,' and are entrusted with the defence of the colony.
Emery directly observed in regard to _Colobopsis truncata_, an ant
which lives in the trunks of trees, that the soldiers, with their
enormous heads, occupied all the entrances to the nest, ready to seize
any intruder with their powerful jaws. In the Sauba ant (_Œcodoma
cephalotes_) Bates described three different types of worker, differing
in size, and although he was not able to determine with certainty what
the particular function of each was, there can be no doubt that they
have special offices, and that the differences in their structure are
adaptations to the differences in their functions. The same is true of
the Indian ant, _Pheidologeton diversus_, depicted in Fig. 106, whose
three forms of workers I owe to the kindness of Professor August Forel.
If the increase in the size of the head and jaws must bring with it an
increase in the thickness of the skeleton of these parts, as well as
a strengthening of the musculature of the head, it follows that the
strain on the body must be greater, just as in the case of the increase
in the weight of the stag's antlers, so that the skeleton of the
thorax must likewise have become thicker and heavier, the muscles and
nerves of the legs stronger, the articulations of the joints capable
of greater resistance; in short, a whole series of variations of other
parts must have taken place simultaneously, if the primary variation
was to be of use, and not to lead to the destruction of its possessor.
Here again we have a proof that the co-adaptation of many parts can
take place without any intervention of the Lamarckian principle, and
that there must be some other factor which brings this about.
[Illustration: FIG. 106. Three workers of the same species of Indian
Ant (_Pheidologeton diversus_), drawn from specimens supplied by
Prof. August Forel. _A_, the largest, _B_, the intermediate, _C_, the
smallest form.]
Where, then, shall we look for this other factor, if not in the
processes of selection, in the selecting of the most suitable
variations among all those which occur? We are confronted with the
alternative of either working out a sufficient explanation with this
factor, or of giving up the attempt at explanation altogether. Yet the
application of the principle of selection in relation to the neuters
of colony-forming insects is by no means simple, for, as the workers
are sterile, a modification of them through processes of breeding
cannot begin directly with themselves. The workers which exhibit the
most suitable variations cannot be selected for breeding, but only
their parents, the sexual animals, and these according to whether
they produce better workers or worse. This is how Darwin looked at
the matter, and his view receives support from one peculiarity in
the composition of these animal colonies, whose significance becomes
apparent in relation to this problem. It has long been known that
in a bee-hive there are from 10,000 to 20,000 workers, but only one
true female, the so-called queen, and the meaning of this remarkable
arrangement probably is, that the adaptation of the workers through
natural selection becomes much more easily possible, _since the whole
number are the children of a single pair_. It is not the individual
workers, but the whole colony, that is, the whole progeny of the
queen, which is selected, according to the greater or less degree of
effectiveness displayed by the workers. Strictly speaking, it is the
single queen that is selected in relation to her power of producing
superior or inferior workers. A colony whose queen was unsatisfactory
in this respect could not hold its own in the struggle for existence,
and only the best colonies and the best hives would survive, that
is, through their descendants. If the hive contained a hundred
queens instead of a single queen the process of selection would be
much more complex and less clear, and it is even quite conceivable
that the production of specially modified workers, adapted to their
functions, or of two or three different kinds of workers, would not
have been possible at all. For it would not have helped much if one
out of a hundred females had produced workers of better structure;
only a majority of such females could give the colony any advantage as
compared with other colonies.
It has not been definitely established whether, among ants, a single
female is in all cases the founder of the whole colony, but it is
certain that there are only a few females. In the tropical Termites we
know that the ovaries of the female attain to such a colossal size that
one female must certainly suffice for the necessities of the largest
colonies. Grassi has shown, indeed, that, as far as the South European
Termites are concerned, not only are there several females present,
but that even the workers frequently reproduce; but the Termites in
general are inhabitants of warm countries, and the few European species
probably hardly represent the original composition of these animal
colonies. But of the tropical species, which have as yet not been
sufficiently studied, we know at least the extraordinary dimensions
of the body, and the corresponding fertility of the queens, and we
conclude from this that only a few can be present in each termitary[18].
[18] Ingwe Sjostedt has recently established in Africa that it is
usually a single queen and a single king that found a termitary
(_Schwed. Akad. Abh._ Bd. 34, 1902).
Now that we have discussed all these facts it will not be out of place
to summarize the results, in as far as they have any relation to the
acceptance or rejection of the theory of the inheritance of acquired
characters.
No _direct_ proof of such transmission could be found; on the contrary,
it has been shown that all that has hitherto been advanced as such will
not stand the test of close examination; an inheritance of wounds and
mutilations does not exist, the transmission of traumatically induced
epilepsy is not only doubtful as regards its causes, but cannot even
be considered as the transmission of a particular morphological lesion.
We may regard as _indirect_ proofs such facts as can only be explained
on the assumption of this mode of inheritance, and in this connexion
our opponents have cited especially the correspondence between
modifications acquired through use in the individual lifetime,
and worked out through histonal selection, with the phyletic
transformations of the same parts. But it has been shown that a number
of parts which do not function actively at all, but only passively, and
thus cannot be caused to change through use, like the hard skeletal
parts of the Arthropods, vary phyletically in the same certain and
direct course as those which function actively, so that we have every
ground for assuming that there are other factors operating in the
transformation of the active as well as of the passive parts. Finally,
we discussed the last and strongest argument which has been put forward
in favour of the Lamarckian principle, that of co-adaptation, that is,
the simultaneous adaptation of many parts co-operating in a common
action, and we were able to controvert this altogether by showing
that exactly similar phenomena of co-adaptation occur in systems
of passively functioning parts, and further, that they occur also
among the workers of ants and bees, that is, in animals which do not
reproduce, and which, therefore, cannot transmit the acquired results
of exercise during their life.
We therefore reject--and are compelled to reject--the Lamarckian
principle, not only on the ground that it cannot be proved correct, but
also because the phenomena, to explain which it is used, occur also
under circumstances which absolutely exclude any possibility of the
co-operation of this principle.
_Supplementary note on the Transmissibility of Acquired Functional
Modifications._
I cannot conclude this section without some reference to the utterances
of some naturalists who have quite recently attempted to represent the
inheritance of functional modifications as a conceivable and even a
necessary assumption.
I may name first Ludwig Zehnder, a physicist who has wandered into the
domain of biology. In regard to the very facts which I have adduced
as evidence _against_ the existence of such inheritance, he has
endeavoured to show how we might conceive of them as having by this
very means arisen[19].
[19] Zehnder, _Die Entstehung des Lebens_, Freiburg-i.-Br., 1899.
He deals with the case of ants, that is to say, with the
differentiation of the sterile workers into several castes, in the
following interesting manner.
The task of the workers is to procure all the food necessary for all
the individuals of the colony in quantity and quality corresponding
to the demand; failing this the whole colony would perish. Now the
different persons of the colony need different food, according to their
constitution and their functions. Soldiers, for instance, are more
powerful than ordinary workers, since they are adapted for fighting,
and they therefore require a different kind of food from the weaker
workers who are adapted only for other duties. Since the soldiers have
evolved from the latter by selection, what we may, for the sake of
brevity, call the soldier-food in the common stores of the ants would
be drawn upon more lavishly than before, and would therefore disappear
more quickly, and, whenever this occurred, those workers which had
already brought in this kind of food would be impelled to bring more
and more to satisfy the demand for it. But in order to do this they
would require to exert themselves more, and would therefore require a
larger quantity of food--not of course soldier-food, but the particular
kind which their particular qualities demanded. Probably this second
importation of food was undertaken by a second kind of worker, for,
according to Zehnder, each worker does not carry all the kinds of food;
they are divided into legions, each of which has its particular task of
food-collecting to fulfil.
In the end the storehouse of the ant-colony must contain a provision
in which the different kinds of food are in exact proportion to the
necessities of the different kinds of persons in the colony. It must
alter in its composition again as soon as, in the course of time, one
or other kind of person acquires new characters, for these presuppose a
new kind of diet.
But how are these acquired characters to be transmitted since neither
soldiers nor workers reproduce? Zehnder answers this by pointing out
that the sexual animals eat _all_ the kinds of nourishment which are
accumulated in the stores, that is to say, all the different kinds of
food exactly in the proportion in which they have been imported--the
proportion in which the different kinds of persons are represented in
the colony. Thus the kinds of nourishment which caused the appearance
of the newly acquired characters in the non-sexual animals also
reached the sexual animals and their sex-cells, and there gave rise to
substances which evoke the relevant qualities in their descendants, for
instance, in the soldiers, or in the still more modified workers, and
so on; and thus we have an 'inheritance of acquired characters.'
This is certainly ingeniously and cleverly thought out, and it reads
even better and more smoothly in the original than in my brief summary,
but it will hardly be regarded as a refutation of my position; the
hypotheses are all too daring for that. We have no knowledge that
particular modifications in form can be produced and conditioned by
particular kinds of food, and, indeed, the contrary has been proved,
namely, that the two or three different castes of polymorphic species
have precisely similar diet. I need only recall the six forms of female
in _Papilio merope_, of which at least three have been obtained from
the same set of eggs, and by feeding with the same plant.
It is true that there are ants which lay in stores of nourishment,
but these consist, for the most part, of one kind of seeds, or of
honey, not of different substances, and we have no knowledge that
the different persons use different food, or even that there is any
diversity in the mode of feeding the helpless larvæ. The feeding in
some species takes place from mouth to mouth, and therefore cannot be
precisely investigated, and we can only suppose from analogy with bees
that the larvæ of the males and females frequently receive not only
more abundant, but qualitatively different food. They are fed from the
crop unless the food consists of the pith of a tree in which the larvæ
are imbedded, as Dahl informs us is the case with some tropical ants of
the Bismarck Archipelago.
But even if we assume that the soldiers take different food from the
ordinary workers, and different again from that of the sexual animals,
is it by virtue of the quality of their food that they have become
what they are? Have our breeds of pigeons or hens been produced by
different diet, or do we know anything in the whole range of animal
life of such a parallelism between food and bodily structure as Zehnder
here assumes? And if, in reality, let us say, the breeds of pigeon had
arisen through specific dieting, and we were to feed one pair with the
specific food-stuffs of three different breeds, would the descendants
of this pair exhibit the form of these three breeds? Or would they
exhibit them in precisely the proportion in which the food-stuffs had
been mixed? It seems to me that Zehnder's assumptions diverge so far
from what we are accustomed to regard as solid ground in biology that
they hardly require consideration, and yet he not only uses them for
the explanation of the case of the ants, but bases upon them the whole
of his theory of the inheritance of acquired characters.
He considers that the results of use (that is, increased function)
are generally transmitted, because the increase in the organ which
is functioning more strongly changes the composition of the blood,
by withdrawing from it in a greater degree the specific substances
which the organ in question--a muscle, for instance--requires for its
activity. All parts of the animal are thereby affected and modified,
but especially those smallest vital units or 'fistellæ' (corresponding
to biophors) which preside over digestion, and of which there are
several sorts. Among them those work most arduously which have to
produce the specific substances which serve for the nutrition of the
muscles with increased function, because these are needed in larger
quantities. This kind of digestive 'fistella' therefore multiplies,
while other kinds, whose products are not required and therefore not
used up, cease to be so active, diminish in number, and in course of
time disappear. In this way the composition of the blood is altered,
and with it to a greater or less degree all the characters of the whole
organism. Of course the reproductive cells are also under the influence
of this change in the composition of the blood, because the different
nutritive substances are accumulated within them in an altered
proportion corresponding to the changed composition of the blood, the
nutritive substances for the muscles with increased function being
contained in it in a larger quantity, and thus the greater development
of the muscle will repeat itself in the progeny, that is to say, _the
acquired character is transmitted_.
It is obvious that this is precisely the same line of argument as that
used in reference to the origin of the worker and soldier ants. The
different kinds of 'digestive fistellæ' correspond to the different
food-carrying workers, and the blood to the assumed storehouse
from which soldiers and workers select the food suitable for their
respective needs, while the sex-cells in the one case, the sexual
animals in the other, partake of all kinds exactly in the proportion
in which they are stored, and thus the organ which functions most
vigorously must be stronger in the offspring.
How the minute quantity of nutritive material contained in the ovum,
still less in the sperm, is to effect the strengthening of the
particular muscles in the descendants is not stated; moreover, such
minimal quantities of food must soon be exhausted, and cannot possibly
increase. It would seem as if the muscles could not even begin by
being stronger, much less that they should remain so, if they were
not exercised equally vigorously by the descendants. If the specific
nutritive stuffs were 'fistellæ,' that is to say, were living units
capable of multiplication, one could understand it. But there can,
of course, be no possibility of a production of living units through
digestion; that can only give rise to digested substances. Or if the
alteration in the composition of the blood produced in the determinant
system of the germ-plasm just those variations requisite to bring
about a strengthening of the muscular system, it would remain to be
shown how this could happen, for the gist of the problem lies in this.
For muscles do not lie in the germ-plasm as miniature models of the
subsequent muscular system, and even if they did, would not all the
muscles, and not merely those which were no longer exercised, decrease
hereditarily when a particular group, like the muscles of the ear in
man, degenerates? Zehnder replies to this with the hypothesis that
the muscles are not all chemically alike, but that each possesses a
particular chemical formula, though they may all be very similar, and
that, therefore, the nutritive materials required by each must be
slightly different. In that case there would require to be, in the
ovum and sperm of man, in order that functional modifications might be
transmitted, as many special nutritive substances as there are muscles,
and, in addition to these, innumerable hosts of other kinds of specific
nutritive substances for all the other parts of the body, since all of
them can be strengthened by exercise and weakened by disuse. And even
if we suppose that all these millions of specific nutritive substances
are accommodated within the germ-cells, as Zehnder's theory requires,
they could not perform what Zehnder ascribes to them, for, as we have
already said, they cannot multiply in the manner of living units, and
so control the growing organism. The different specific nutritive
materials contained in the blood are just as powerless to perform the
task ascribed to them by Zehnder as the specific kinds of food in the
hypothetical storehouse of the ants are to give rise to the different
persons of the ant-colony.
Zehnder also attempts to overthrow the arguments against the Lamarckian
principle which I based on the skeleton of Arthropods.
It does not seem to him probable that the chitinous coat of mail can
be an absolutely dead structure, and he supposes that very delicate
nerve-fibrils penetrate into all its most minute parts, and so are
stimulated by 'every pressure and every strain' exerted on the
chitinous skeleton. They 'work' when they are stimulated, and in
doing so they use up 'their specific food-stuffs.' At places which
are frequently stimulated the corresponding nerves develop more than
elsewhere. The necessary specific food-stuffs for these particular
nerves therefore increase proportionately within the body, and also
in the reproductive cells. Accordingly, in the germ-cells there is an
increase of the aforesaid nervous substances, which in the offspring
become associated with the relevant part of the chitinous covering, and
induce in development the secretion of chitin at this part. At this
particular spot, then, the chitin will be specially thick.
This clearly implies that each particular part of the skin has its
specific nutritive substances, necessitated by the nerves which
traverse it! Thus there must be as many nerve-nutritive substances
as there are skin-nerves, specific chemical combinations for every
part of the body which is capable of heritable variation. This is so
extraordinarily improbable that I need say nothing more about it. If
the Lamarckian principle requires this kind of hypothesis to bolster it
up, it is undoubtedly doomed.
If we disregard altogether the positive aspect of Zehnder's hypothesis,
and assume that the skin-nerves are really stimulated through the
chitin by every strain and pressure to which a spot of skin is exposed,
and that they cause a correspondingly greater secretion of chitin,
which would then, according to the Lamarckian principle, be hereditary,
does this harmonize with what actually occurs in the development of
the skeleton as we know it in the case of Insects and Crustaceans? Not
at all! Can we suppose that the carapace of a crab or the enormously
hard wing-covers of a water-beetle are exposed to a continual pounding
and pressing and pushing? Exactly the contrary is the case. Every
assailant takes care not to grasp the animal where it is so well
protected, and seeks out the most vulnerable parts for its attacks. It
may be answered that, while this is certainly the case now, the animals
were badly protected when the ancestral forms were evolving. But that
they could not have become hard by dint of being frequently bitten or
otherwise wounded should be obvious from the fact that the whole of the
wing-covers and the whole of the carapace is uniformly covered with
thick chitin, while each wound would only stimulate particular spots;
and we should also have to admit that, since these parts of the skin
which are now so well protected are no longer seized and stimulated,
they would long ago have become thin again, according to the principle
of the degeneration of parts no longer used, or, in this case, no
longer stimulated. But there is no need for wasting time over such
quibbles, since there is a fact which absolutely contradicts Zehnder's
hypothesis. I mean the degeneration of the chitinous skeleton in those
Crustaceans and Insects which protect the abdomen within a shelter
like the hermit-crabs, the caddis-flies (Phryganidæ), (Fig. 107) and
the sack-carrying caterpillars of the Psychidæ among Lepidoptera. The
hermit-crabs, as is well known, squeeze their abdomen into a usually
spirally-coiled Gasteropod shell, and they always choose houses which
are wide enough to conceal the whole body up to the hard claws when
necessity arises. In this case there is surely a continual pressure on
the abdomen, which, being soft, must be squeezed very tightly every
time the animal retreats into its shell. One of my opponents has
described the disappearance of the tough integumentary skeleton from
the abdomen of these animals as an inherited result of this pressure,
and another regards it as the inherited result of the degeneration
of the muscles in this part of the body. But, according to Zehnder,
this continuous pressure, and the frequent rubbing up and down of
the abdomen on the inner surface of the Gasteropod's shell, would
undoubtedly have a stimulating effect on the skin-nerves, and would
therefore bring about a thickening of the chitinous cuticle. In regard
to the larval Phryganidæ and Psychidæ, the case would be the same,
though perhaps hardly to the same degree, for while these larvæ make
their own houses, and will therefore at least make them big enough to
begin with, the pressure and friction must increase with the growth of
the animal.
[Illustration: FIG. 107. Larva of a Caddis-fly, after Rösel. _A_,
removed from its case, showing the hooks (_h_) which attach it thereto,
and the whitish abdomen, covered only by a thin cuticle. _B_, the same
larva, moving about with its case.]
If the regulation of the strength of the integumentary skeleton be
referred to selection, we see at once why carapace and wing-covers
should be of equal thickness throughout their whole extent, and why
they do not disappear, although they do not function actively, and
are less stimulated than any other parts of the skeleton; and we also
understand why the abdomen of hermit-crabs and of larval Phryganidæ
and Psychidæ has become soft, whether it be exposed to pressure or
friction in a greater or a less degree. It no longer requires to be
hard, because it is protected by the house, and in the case of the
Pagurids it must not be hard because it could not then be readily
squeezed into the hard-walled and narrow recesses of the Gasteropod
shell; in this case there has therefore been positive selection. I have
not yet referred to the fact that the chitinous covering is certainly
not living, though it is not exactly dead; it is a secretion of the
epidermic cells, not a tissue, and we cannot suppose that there are
any nerve-endings in it. It is almost superfluous to say that the fact
that the skin is cast is in itself enough to make such an assumption
untenable, for the whole of the assumed delicate nervous network would
be shed at every moult and torn away from the nerves which lead to it.
As far as my knowledge goes, nothing of this kind occurs anywhere in
the whole range of the animal kingdom.
Even if we assume, for the benefit of Zehnder's hypothesis, that
although there are no nerves in the chitin itself yet irritations
affecting the chitinous coat may be transmitted through it to the
delicate nerve-endings lying beneath it, this should take place in a
greater degree at the thin places of the skeleton than _at the thick
parts_! But this interpretation is again fallacious, for we see that
the tactile organs of Arthropods always break through the chitinous
cuticle and protrude beyond it in the form of setæ.
Of the many other opponents of my views in regard to the
transmissibility of acquired functional modifications, I need only deal
in detail with Oscar Hertwig.
He seeks for direct proofs of an inheritance of acquired characters,
and believes that he has found these in the hereditary transmission of
acquired immunity from certain diseases. He reminds us of Ehrlich's
well-known experiments on mice with ricin and abrin.
Even small doses of these two poisons kill mice, but they are tolerated
in very minute doses, and if their administration be continued for some
time in such minute doses, the animals gradually acquire a high degree
of insensitiveness to these poisons; they become immune to ricin and
abrin.
This immunity is transmitted from mother to young, but it only lasts
for a short time, about six to eight weeks after birth. Yet this is
regarded by Hertwig as an illustration of the transmission of an
acquired character, as an acquired modification of the cells of the
body, for he explains the immunity on the assumption that all the cells
of the body undergo a particular variation due to the influence of the
poison, and are thus, to a certain extent, modified in their nature,
and that the ovum also undergoes this variation and transmits it to the
young animal. The immunization might certainly come under the category
of functional modifications, and it might be thought that we have here
a case of transmission of such an acquired character.
Against this, however, we have to put the fact that _the acquired
immunity is not transmitted from the father to the descendants_.
Hertwig attempts to explain this by saying that the short duration
of the experiments has only allowed the poison to affect the
cell-substance (cytoplasm) and not the nucleus, that is, the hereditary
substance of the sperm-cells, an assumption which has little
probability considering the intimate nutritive relations between the
cell-nucleus and the cytoplasm. I should be rather inclined to conclude
from the difference in the transmitting power of the sperm and of the
ovum, that this 'inheritance of immunity' does not depend, as Hertwig
supposes, on a modification of the cells to 'ricin immunity,' but, as
Ehrlich and the bacteriologists believe, on the production of so-called
'anti-toxins,' and that these anti-toxins are handed on to the embryo
not by the ovum itself, but by the interchange of blood between mother
and offspring which lasts throughout the whole embryonic period. It is
then self-evident why no transmission of immunity through the father
occurs.
But it would lead me too far were I to attempt to refute all the
attempts that have been made to interpret individual cases as due to
the inheritance of acquired characters. I should, however, like to say
something as to the theoretical possibility of such an assumption.
When we try to conceive how experiences and their consequences can
be entailed, how new acquirements of the 'personal part can have
representative effects on the germinal part,' we find ourselves
confronted with almost, if not entirely, insuperable difficulties.
How could it happen that the constant exercise of memory throughout
a lifetime, as, for instance, in the case of an actor, could
influence the germ-cells in such a way that in the offspring the same
brain-cells which preside over memory will likewise be more highly
developed--that is, capable of greater functional activity? We know
what Zehnder's answer to such a question would be; he would make the
blood the intermediary between the brain-cells and the germ-cells, but
we have seen that specific food-stuffs for each specific cell-group
cannot be assumed, and that, even if they could, they would not meet
the necessities of the case. Yet every one who does not regard the
germ-plasm as composed of determinants is constrained to make some such
assumption. But if we take our stand upon the theory of determinants,
it would be necessary to a transmission of acquired strength of
memory that the states of these brain-cells should be communicated by
the telegraphic path of the nerve-cells to the germ-cells, and should
there modify only the determinants of the brain-cells, and should do
so in such a way that, in the subsequent development of an embryo
from the germ-cell, the corresponding brain-cells should turn out to
be capable of increased functional activity. But as the determinants
are not miniature brain-cells, but only groups of biophors of unknown
constitution, and are assuredly different from those cells; as they
are not 'seed-grains' of the brain-cells, but only living germ-units
which, in co-operation with the rest exercise a decisive influence on
the memory-cells of the brain, I can only compare the assumption of the
transmission of the results of memory-exercise to the telegraphing of a
poem, which is handed in in German, but at the place of arrival appears
on the paper translated into Chinese.
Nevertheless, as I have said before, I do not disagree with those who
say, with Oscar Hertwig, that the impossibility of forming a conception
of the physiological nexus involved in the assumed transmission does
not _ipso facto_ constrain us to conclude that the transmission
does not occur. I cannot, however, agree with Hertwig that the case
is exactly the same as in the 'converse process,' that is, 'in the
development of the given invisible primary constituents in the
inheritance of the cell into the visible characters of the personal
part.' Certainly no one can state with any definiteness how the germ
goes to work, so that from it there arises an eye or a brain with its
millionfold intricacies of nerve-paths, but although the process cannot
be understood in detail, it can in principle, and this is just what is
impossible in regard to the communication of functional modifications
to the germ. Moreover, in addition to this, there is the very important
difference that, in the one case, we know with certainty that the
process actually takes place, although we cannot understand its
mechanical sequence in detail, while in the other we cannot even prove
that the supposed process is a real one at all. From the fact that we
are unable to form clear conceptions of a hypothetical process, we are
not justified, it seems to me, in assuming it to be real, even though
we are aware of many other processes in nature which we are unable to
understand.
Nor does Hertwig take up this position, for he is at pains to show
the mechanical possibility of the process of inheritance which he
assumes, and he bases this upon the suggestions made by Hering in his
famous work _Ueber das Gedächtniss als eine allgemeine Function der
organisirten Materie_ [_On memory as a general function of organized
matter_], 1870. As this essay probably contains the best that can be
said in favour of a transmission of functional modifications, and as
it also includes some indisputable truths, we may consider it in some
detail.
Hering is undoubtedly right in regarding 'the phenomena of
consciousness as functions of the material changes of organic
substance, and conversely.' That is, he believes that every sensation,
every perception, every act of will arises from material changes in
the relevant nerve-substances. But we know that 'whole groups of
impressions, which our brain has received through the sense-organs, are
stored up in it, as if resting, and below the margin of consciousness,
to be reproduced when occasion arises, in correct order of space and
time, and with such vividness, that we may be deceived into regarding
as a present reality what has long ceased to be present.' There
must therefore remain in the nerve-substance a 'material impact,' a
modification of the molecular or atomic structure, which enables it 'to
ring out to-day the note that it gave forth yesterday if only it be
rightly struck.'
Hering attributes a similar power of memory and reproduction to
the germ-substance; he believes that he is justified in making the
assumption that acquired characters can be inherited, although he
admits that it 'appears to him puzzling in the highest degree'
how characters which developed in the most diverse organs of the
mother-being can exert any influence on the germ. That he may be able
to assume this he points to the interconnexion of all organs by means
of the nervous system; it is this that makes it possible that 'the
fate of one reverberates in the other, and that, when excitement takes
place at any point, some echo of it, however dull, penetrates to the
remotest parts.' To the delicate-winged communication by means of the
nervous system, which unites all parts among themselves, must be added
the general communication by means of the circulation of the fluids of
the body. According to Hering's view, the germ experiences, in some
degree, in itself all that befalls the rest of the organs and parts
of the organism, and these experiences stamp themselves more or less
upon its substance, just as sense-impressions or perceptions stamp
themselves upon the nerve-substance of the brain, and these experiences
are reproduced during the development of the germ, just as the brain
brings memory-pictures back to consciousness. He says, 'If something in
the mother-organism has so changed its nature, through long habit or
exercise repeated a thousand times, that the germ-cell resting in it is
also penetrated by it in however weakened a fashion, when the latter
begins a new existence, expands, and increases to a new being whose
individual parts are still itself and flesh of its flesh, it reproduces
what it experienced as part of a great whole. This is just as wonderful
as when an old man suddenly remembers his earliest childhood, but it is
not more wonderful than this.'
But I think it is more wonderful. There exist demonstrably in the
brain thousands upon thousands of nerve-elements, whose activity
is a definite and limited one, because each particular visual
impression, for instance, only excites to activity certain definite
nerve-elements, and can leave memory-pictures in these alone. According
to my conception of it, the germ-plasm is quite as complex in its
composition, and does not consist of homogeneous elements, but of
innumerable different kinds, which are not related to the parts of the
complete organism indiscriminately, but only to particular parts. But
is it allowable to assume that there are invisible nerve-connexions,
not only to every germ-cell, but also within the germ-plasm, to
every determinant, like the nerve-paths which lead from the eye
to the nerve-cells in the optic-area of the brain? For if it were
otherwise, how could we conceive of the modification of an organ--as,
for instance, the ear-muscles in Man--communicating itself to the
precise determinants of these muscles in the germ-plasm? I have often
been met with the reproach that my conception of the composition of
the germ-plasm is much too complex--but the complexity of Hering's
suggestion seems to me to go a long way beyond mine.
Hering's ideas, which are not only ingenious but very stimulating,
might be accepted as the first indication of an understanding of the
assumed inheritance of functional modifications, if it could be proved
that such inheritance is a fact; but, as we have seen, that is not
the case. The assumption might be permitted, perhaps, if it could be
shown that certain groups of phenomena left no other possibility of
explanation open except this assumption, but that also, as far as I
can see, is not the case. Of course, others hold a different opinion,
but chiefly because they have rejected without much reflection the
sole explanation which presents itself for numerous phenomena--I
mean the processes which we are about to study under the name of
'germinal selection.' But, in any case, Hering's ideas seem to me very
valuable, because they make it apparent that, however much we know of
the organism, we only know it in a general way, and that numberless
delicate processes go on in it which leave no trace for our microscope,
and that we can only recognize the final results of numerous invisible
and often, in their subtlety, also unimaginable factors. This ought to
be taken to heart, especially by all those who speak of simplicity in
reference to the germ-plasm. So much at least is certain: If there were
any inheritance of functional modifications, we should have another
proof that the germ-plasm is composed of determinants, for without them
there could be no possibility that the 'experiences' of an individual
organ would be transmitted to the germ in the way that the Lamarckian
principle implies. Something, and that something material, must be
modified in the germ-plasm if the vigorous use of a group of muscles,
or of a gland, or of a nerve-cell, is to be communicated to the germ,
and not to the whole germ-plasm, but only to so much of it as is
necessary to cause variation in the corresponding group of cells in the
child. It may perhaps be said that this still does not necessitate the
assumption of special determinants for these cell-groups, and that one
might, with Herbert Spencer, conceive of the germ-plasm as consisting
of homogeneous units which vary in the development in accordance with
the diverse regularly alternating influences to which it is exposed
from step to step, and that, therefore, in each of these units of very
complex structure only a single molecule, or perhaps only a single
atom, would need to vary in order that, in the course of development,
the resulting cell-group should appear in the rudiment in somewhat
altered strength.
But I do not believe that a chemical molecule, still less an atom,
is sufficient for this, for reasons which I have already stated--yet
we need not go into this now, but rather deduce the consequences of
this admission. It follows that the 'unit' is made up of numerous
'molecules' or 'atoms,' of which each, by dint of changes it has
undergone, causes particular parts of the body to vary in a definite
manner; in other words, we have here again a theory of determinants,
only they are on a much smaller scale, since each invisible little
vital particle or 'unit' contains all the determinants within itself,
while in my theory it is only the id, that is, the visible chromosome,
which includes the determinant complex. Such a theory would be far from
a simplification of mine, it would rather complicate it enormously,
and that without anything being gained. At most it would be made more
evident how inconceivably complex the nerve-paths must be which lead
from the part that has been modified by exercise to the germ-plasm,
and must also lead to all the innumerable 'molecules or atoms' of the
individual 'units.' But even on my theory of the composition of the
ids as aggregates of living determinants, such nervous transmission of
qualities would be a monstrosity which no one would accept, and I think
on this account that my argument as to the impossibility of conceiving
of the transmission of the modifications of the personal part to the
germinal part retains its force, notwithstanding Hering's interesting
analogy.
If the transmission of functional modifications were an indisputable
fact, I repeat, we should have to give in, and then we might regard the
'memory of organized material' as affording a hint of the possibility
of the unimaginable process. But as long as the occurrence of this
transmission cannot be proved either directly or indirectly, such
a vague possibility of explanation need not induce us to assume an
unproved process.
LECTURE XXV
GERMINAL SELECTION
On what does disappearance after disuse depend, if not on
the Lamarckian principle?--Panmixia--Romanes--Fluctuations
in the determinant-system of the germ-plasm due to unequal
nutrition--Persistence of germinal variations in a definite
direction--The disappearance of non-functioning parts--Preponderance
of minus germinal variations--Law of the retrogression of
useless parts--Variation in an upward direction--Artificial
selection--Influence of the multiplicity of ids and of sexual
reproduction--Personal selection depends on the removal of certain
id-variants--Range of influence of germinal selection--Self-regulation
of the germ-plasm, which is striving towards stability--Ascending
variation-tendencies may persist to excess--Origin of secondary sexual
characters--Significance of purely morphological characters--The
markings of butterflies.
Now that we have recognized that the assumption of a transmission of
functional modifications is not justifiable, let us discuss some of
the many phenomena to explain which many people believe the Lamarckian
principle to be indispensable, and let us inquire whether we are in a
position to give any other explanation of these. How has it come about
that the effects of use and disuse _appear_ to be inherited? Can we
find a sufficient explanation in the principle of selection, and in the
natural selection of Darwin and Wallace?
The answer to these two questions will be most quickly found if we
begin by seeking for an explanation of the disappearance of a part when
it ceases to be exercised.
That this cannot lie in the Lamarckian principle we have already learnt
from the fact that passively functioning parts, such as superfluous
wing-veins, also disappear, and that the loss of the wings and
degeneration of the ovaries has taken place in worker ants, which can
transmit nothing because they do not reproduce.
We might be inclined to regard this gradual disappearance and ultimate
elimination of a disused organ as a direct gain, on the ground that
the economy of material and space thus effected may be of decided
advantage to the individual animal and thereby also for the maintenance
of the species, and that those animals would have an advantage in the
struggle for existence in which the superfluous organ was reduced to
the smallest expression. But that would be far from supplying us with
a sufficient explanation of the phenomenon; the individual variations
in the size of an organ which is in process of degenerating are,
even in extreme cases, far too slight to have any selection-value,
and I cannot call to mind a single case in which the contrary could
be assumed with any degree of probability. What advantage can a newt
or a crustacean living in darkness derive from the fact that its eye
is smaller and more degenerate by one degree of variation than those
of its co-partners in the struggle for existence? Or, to use Herbert
Spencer's striking illustration, how could the balance between life and
death, in the case of a colossus like the Greenland whale, be turned
one way or another by the difference of a few inches in the length of
the hind-leg, as compared with his fellows, in whom the reduction of
the hind-limb may not have gone quite so far? Such a slight economy of
material is as nothing compared with the thousands of hundredweights
the animal weighs. As long as the limbs protrude beyond the surface
of the trunk they may prove an obstacle to rapid swimming, although
that could hardly make much difference, but as soon as the phyletic
evolution had proceeded so far that they were reduced to the extent of
sinking beneath the surface, they would no longer be a hindrance in
swimming, and their further reduction to their modern state of great
degeneration and absolute concealment within the flesh of the animal
cannot be referred even to negative selection.
Years ago I endeavoured to explain the degeneration of disused parts
in terms of a process which I called Panmixia. Natural selection not
only effects adaptations, it also maintains the organ at the pitch
of perfection it has reached by a continual elimination of those
individuals in which the organ in question is less perfect. The longer
this conservative process of selection continues, the greater must be
the constancy of the organ produced by it, and deviations from the
perfect organ will be of less and less frequent occurrence as time goes
on.
Now if this conservative action of natural selection secures the
maintenance of the parts and organs of a species at their maximum of
perfection, it follows that these will _fall below this maximum as soon
as the selection ceases to operate_. And it does cease as soon as an
organ ceases to be of use to its species, like the eye to the species
of crustacean which descends into the dark depths of our lakes, or to
the abyssal zones of the ocean, or into a subterranean cave-system.
In this case all selection of individuals ceases as far as the eye is
concerned; it has no importance in deciding survival in the struggle
for existence, because no individual is at a disadvantage through
its inferior eyes, for instance, by being in any way hindered in
procuring its food. Those with inferior organs of vision will, _ceteris
paribus_, produce as good offspring as those with better eyes, and the
consequence of this must be that there will be a general deterioration
of eyes, because the bad ones can be transmitted as well as the good,
and thus the selection of good eyes is made impossible.
The mixture thus arising may be compared to a fine wine to which a
litre of vinegar has been added; the whole cask is ruined because the
vinegar mingles with every drop of the wine. As deviations from the
normal are always occurring in every part of every species, and among
them some that lessen the value of the organ, rarely perhaps at first,
but after a time in every generation, a sinking of the organ from
the highest point of possible perfection becomes inevitable as soon
as the organ becomes superfluous. The functional uselessness of the
organ must go on increasing the longer it is disused, as will readily
be admitted if it be remembered that only the most perfect adaptation
of all the separate parts of an organ can maintain its functional
capacity, that all the parts of an organ are subject to variation, and
that every deviation from the optimum implies a further deterioration
of the whole. An eye, for instance, can no longer vary in the direction
of 'better' if it has already reached the highest possible point of
perfection; every further variation must deteriorate it.
Romanes gave expression to this idea, that the cessation of natural
selection alone must cause the degeneration of a part, a decade
before I did, but neither he nor the scientific world of his time
attached great importance to it, and it was forgotten again. This was
intelligible enough, for, at that time, the validity of the Lamarckian
principle had not been called in question, and therefore the need for
some other principle to explain the disappearance of disused parts had
not begun to be felt.
I found myself in quite a different position. As my doubts regarding
the Lamarckian principle grew greater and greater, I was obliged to
seek for some other factor in modification, which should be sufficient
to effect the degeneration of a disused part, and for a time I thought
I had found this in panmixia, that is, in the mingling of all together,
well and less well equipped alike. This factor does certainly operate,
but the more I thought over it the clearer it became to me that
there must be some other factor at work as well, for while panmixia
might explain the deterioration of an organ, it could not explain
its decrease in size, its gradual wearing away, and ultimate total
disappearance. Yet this is the path followed, slowly indeed, but quite
surely, by all organs which have become useless. If panmixia alone
guided the deterioration of the organ, and it was thus only chance
variations which were inherited through panmixia and gradually diffused
over the whole species, how could it come about that all the variations
were in the direction of smaller size? Yet this is obviously the case.
Why should no variations in the direction of larger size occur? And
if this were so, why should a useless organ not be maintained at its
original size, if it be admitted that an increase in size would be
prevented by natural selection? But this never occurs, and diminution
in size is so absolutely the rule that the idea of a 'vestigial or
rudimentary' organ suggests a 'small organ' almost more than an
'imperfect' one.
There must then be something else at work which causes the
minus-variations in a disused organ to preponderate persistently and
permanently over the plus-variations, and this something can lie
nowhere else than where the roots of all hereditary variations are
to be found--in the germ-plasm. This train of thought leads us to
the discovery of a process which we must call selection between the
elements of the germ-plasm, or, as I have named it shortly, _Germinal
Selection_.
If the substance of the germ-plasm is--as we assumed--composed of
heterogeneous living particles, which have dissimilar rôles in the
building up of the organism, there must of necessity be among them
a definite labile state of equilibrium, which cannot be disturbed
without modifying in some way the structure of the organism itself
which arises from the germ-plasm. But if our further view be correct,
that these individual and different living units of the germ-plasm are
'determinants,' that is, are the primary constituents of particular
parts of the organism, in the sense that these parts could not arise if
their determinants were absent from the germ-plasm, and that they would
be different if the determinants were differently composed, we can draw
far-reaching deductions.
It is true that we cannot learn _anything directly_ in regard to the
intimate structure of the germ-plasm, and even in regard to the vital
processes going on within it we can only guess a very little, but so
much we may say--that its living parts are nourished, and that they
multiply. But it follows from this that nourishment in a dissolved
state must penetrate between its vital particles, and that whether the
determinants grow, and at what rate they do so, depends mainly on the
amount of nourishment which reaches them. As long as the germ-cells
multiply by division the determinants have no other function but
to grow; a part of their substance undergoes oxidation and thereby
yields the supply of energy necessary to assimilation, that is, to the
formation of new living substance.
If each kind of determinant always secured the same quantity of
nourishment, all would grow in the same degree, that is, in exact
proportion to their power of assimilation. But we know that in less
minute conditions which we can observe more directly, there is
nowhere absolute equality, that all vital processes are subject to
fluctuations; any little obstacles in the current of the nutritive
fluid, or in its composition, may cause poorer nutrition of one
part, better of another. We may therefore assume that there are
similar irregularities and differences in the minute and unobservable
conditions of the germ-plasm likewise, and the result must be a slight
shifting of the position of equilibrium as regards size and strength in
the determinant system; for the less well-nourished determinants will
grow more slowly, will fail to attain to the size and strength of their
neighbours, and will multiply more slowly.
But the vigour of growth does not depend only on the influence of
nourishment; one cell grows quickly, another slowly in the same
nutritive fluid; it depends in great part on the cell's power
of assimilation. In the same way the assimilating power of the
determinants and their affinity for nourishment will vary with their
constitution, and a weaker determinant will remain smaller than a
stronger one, even when the stream of nourishment is the same.
It seems to me that it is upon the unequal nutrition of the
determinants conditioned by the chances of the food-supply that
individual hereditary variability ultimately depends. If, for instance,
the determinant _A_ receives poorer nourishment at a particular time
than the determinant _B_, it will grow more slowly, remain weaker, and
then, when the germ-cells develop into an animal, the part to which it
gives rise will be weaker than it usually is in other individuals.
These primary inequalities in the equipment of the determinants which
are caused by a passing inequality in the food-stream are, of course,
so slight that we are unable to observe their consequences. They must
persist for a considerable time before they become observable, but
they may persist for a long time, and their effect must then mount
up, because every diminution in the strength of the determinant also
signifies a lessened power of assimilation, and growth becomes slower
for the twofold reason that passive and active nutrition decrease
at the same time. In the less minute conditions observable in the
histological elements of the body we know that function strengthens
the organ, and that disuse weakens it, and we are justified in
applying this proposition also to these more intimate conditions and
minuter vital units. Thus, in the course of the multiplication of
the germ-cells, the less vigorously working determinant, _A_, will
gradually, but very slowly, become weaker, that is, of diminished
power of assimilation, presupposing of course that the intra-germinal
food-stream does not become stronger again at the same place--a
possibility to which I shall subsequently refer. But while one
determinant may be slowly becoming weaker, its neighbour, on the other
hand, may be varying on an ascending scale, just because the former is,
on account of its diminished power of assimilation, no longer able to
exhaust completely the food-stream which flows to it.
The determinants are thus in constant motion, here ascending, there
descending, and it is in these fluctuations of the equilibrium of the
determinant-system that I see the roots of all hereditary variation,
while in the fact that the variation-directions of particular
determinants must continue the same without limit as long as they meet
with no obstacle lies the possibility of the adaptation of the organism
to changing conditions, the increase and transformation of one part,
the degeneration and disappearance of another, in short, the processes
of natural selection. The reason why such variation movements must
continue until they meet some resistance is that every chance upward
or downward movement--due, that is, to mere passive fluctuation in the
food-supply, at the same time strengthens or weakens the determinant,
and makes it either more or less capable of attracting nourishment to
itself; in the former case an increasingly strong stream of food will
be directed towards it, in the latter more and more of the available
food-supply will be withdrawn from it by its neighbour-determinants
on all sides; in the former the determinant will go on increasing
in strength as long as it can go on attracting more nourishment, in
the latter it will continue to become weaker until it disappears
altogether. To the ascending progression, as is evident, there are
limits set, not only by the amount of food which can circulate through
the whole id, but also by the neighbour determinants, which will sooner
or later resist the withdrawal of nourishment from them; but for the
descending progression there are no limits except total disappearance,
and this is actually reached in all cases in which the determinants are
related to a part which has become useless. But both these movements,
the upward and the downward alike, are quite independent of natural
selection, i.e. of personal selection; they are processes of a unique
kind which run their course purely in accordance with intra-germinal
laws. Whether a determinant 'ascends' or 'descends' depends solely
upon the play of forces within the germ-plasm, not upon whether the
direction of the variation in question is useful or prejudicial, or
on whether the organ in question, the determinate, is of value or
otherwise. In this fact lies the great importance of this play of
forces within the germ-plasm, that it gives rise to variations quite
independently of the relations of the organism to the external world.
In many cases, of course, personal selection intervenes, but even then
it cannot directly effect the rising or falling of _the individual_
determinants--these are processes quite outside of its influence--but
it can, by eliminating the bearers of unfavourably varying
determinants, set a limit to further advance in such directions. This
we shall consider in more detail later on. Personal selection operates
by removing unfavourably varying individuals from the genealogical tree
of the species, but at the same time the determinants which are varying
unfavourably are also removed, and their variation is thus put a stop
to for all time.
I have called these processes which are ceaselessly going on within the
germ-plasm, Germinal Selection, because they are analogous to those
processes of selection which we already know in connexion with the
larger vital units, cells, cell-groups and persons. If the germ-plasm
be a system of determinants, then the same laws of struggle for
existence in regard to food and multiplication must hold sway among its
parts which hold sway between all systems of vital units--among the
biophors which form the protoplasm of the cell-body, among the cells of
a tissue, among the tissues of an organ, among the organs themselves,
as well as among the individuals of a species and between species which
compete with one another.
If this be the case, we have here ready to hand the explanation of
every heritable variation of a part, ascending and descending alike.
Let us consider for a little the latter category--that is, the
disappearance of functionless _or useless organs_. It is clear that,
from the moment in the life of a species that an organ, _N_, becomes
useless, natural selection withdraws her hand from it; individuals
with better or worse organs _N_ are now equally capable of life and
struggle, the state of panmixia is entered upon, and the organ _N_
of necessity falls somewhat below its previously attained degree of
perfection.
That this must be so will be admitted when it is remembered that each
organ of a species is only maintained at its highest level because
personal selection keeps ceaseless watch over it, and sets aside all
the less favourable variations by eliminating the individuals which
exhibit them. But this is no longer the case with a useless organ. When
a weaker variant of a disused organ arises through the intra-germinal
fluctuations of nutrition, this is transmitted to the descendants
just as well as the normally developed organ, and in the course of
generations will be inherited by a greater and greater number of
individuals, and must ultimately be inherited by all in some degree or
other. The objection has been urged from many sides that variations
upwards would be quite as likely to arise as those downwards, but
this is an error. Even if, at the beginning, the minus-variations
were rarer than the plus-variations, in the course of generations
the minus ones would preponderate because ascending variations of
disused organs are not indifferent for the organism but injurious to
it. Perhaps an increase in the size of the organ itself would do no
harm, but in that of its determinant it certainly would, because an
ascending determinant requires more nourishment than previously, and
withdraws it from its surroundings, and thus from the determinants in
its immediate neighbourhood; but these are those of functioning and
indispensable parts. Individuals in whose germ-plasm the determinants
of disused organs ascend, and thereby depress the determinants of
organs which are still active, are subject to personal selection,
and are eliminated. There thus remain only those with descending
determinants; in other words, the chance of variants in the direction
of weakness in useless determinants far outweighs that of variants in
the direction of increased strength; the latter will soon cease to
occur at all, for as soon as a determinant has fallen a little below
its normal level, it finds itself upon an inclined plane, along which
it glides very slowly but steadily downwards. This might be disputed if
it could be maintained that, at every stage of the descent, a change
of direction was possible. But this probably takes place rarely and
only in the case of individual ids, and will therefore not be permanent
because in general the stronger neighbour determinants will possess
themselves of the superfluous nourishment, and a lasting ascent will
thus be impossible to the weakened determinant. This is precisely what
I have called Germinal Selection. The determinant whose assimilating
power is weakened by ever so little is continually being robbed by
its neighbours of a part of the nourishment which flows towards it,
and must consequently become further weakened. As no more help will
be given to it by natural selection, since the organ is no longer of
any value to the species, the better among the weakened determinants
of _N_ are never selected out, and they must gradually give way in the
struggle with the neighbouring determinants which are necessary to the
species, becoming gradually weaker and ultimately disappearing.
This process can, of course, no more be proved mathematically than
any other biological processes. No one who is unwilling to accept
germinal selection can be compelled to do so, as he might be to accept
the Pythagorean propositions. It is not built up from beneath upon
axioms, but is an attempt at an explanation of a fact established by
observation--the disappearance of disused parts. But when once the
inheritance of functional modifications has been demonstrated to be a
fallacy, and when it has been shown that, even with the assumption of
such inheritance, the disappearance of parts which are only _passively_
useful, and of any parts whatever in sterile animal forms, remains
unexplained, he who rejects germinal selection must renounce all
attempt at explanation. It is the same as in the case of personal
selection. No one can demonstrate mathematically that any variation
possesses selection value, but whoever rejects personal selection gives
up hope of explaining adaptations, for these cannot be referred to
purely internal forces of development.
The total disappearance of a part which has become useless takes place
with exceeding slowness; the whales, which have existed as such since
the beginning of the tertiary period, have even now not completely lost
their hind-limbs, but carry them about with them as rudiments in the
muscular mass of the trunk, and the birds, which are even older, still
show in their embryonic primordia the five fingers of their reptilian
forefathers, although even their bird-ancestors of the Jurassic period,
if we may argue from _Archæopteryx_, had only three fingers like our
modern birds. A long series of similar examples might be given, and
modern embryology in particular has contributed much that, like this
example of birds' fingers, points to a certain orderliness in the
disappearance of the individual parts of an organ which has become
superfluous. Parts which, in the complete animal, have disappeared
without leaving a trace, appear again in each embryonic primordium, and
disappear in the course of the ontogeny. Speaking metaphorically, we
might express this on the basis of the determinant theory, by saying
that the determinants, as they become weaker, can only control an
increasingly short period of the whole ontogeny of the organ, so that
ultimately nothing more than its first beginning comes into existence.
But this is only a metaphor; we cannot tell what really happens as
long as we are ignorant of the physiological rôle of the determinants,
and even of the laws governing the degeneration of a useless organ.
In respect of the latter, much might still be achieved if comparative
anatomy and embryology were studied with this definite end in view, and
perhaps we should even be able to draw more definite conclusions in
regard to the composition and activity of the determinants in the germ.
In the meantime we must be content with the knowledge that, on the
determinant hypothesis, the disappearance of organs which have
become useless may be regarded as a process of intra-selection going
on between the 'primary constituents' (_Anlagen_) of the germ, and
depending on the same principle of the 'struggle of parts' which
William Roux introduced into science with such brilliant results. If a
struggle for food and space actually takes place, then every passive
weakening must lead to a permanent condition of weakness and a lasting
and irretrievable diminution in the size and strength of the primary
constituent concerned, unless personal selection intervenes, and
choosing out the strongest among these weakened primary constituents,
raises them again to their former level. But this never happens when
the organ has become useless.
This explains why not only parts with active function, like limbs,
muscles, tendons, nerves, and glands, disappear when they cease to
function, but also passive parts like the colouring of the external
surfaces of animals, the lifeless skeletal parts of Arthropods and the
exact adaptation of their thickness to the dwindling function, the
disappearance of superfluous wing-veins, and of the hard chitinous
covering of the abdomen when it is concealed in a protecting house,
as in the case of hermit-crabs, Phryganidæ, and Psychidæ. Here too we
find a sufficient explanation of the fact that parts which have become
functionless, such as the wings of ants, can disappear even in the case
of sterile workers.
* * * * *
The principle of germinal selection, however, can only be understood
in its full significance if we take the positive aspect also into
consideration. We had reached the conclusion that because of the
fluctuations of the food-supply one set of the homologous determinants
represented in the various ids may vary in a minus direction, and
another set in a plus direction, and that this direction will be
adhered to as long as no intra-germinal obstacles come in the way. As
long as this does not happen the determinant concerned will pursue the
path of variation it has once struck out, and indeed the tendency will
be strengthened, because every passive variation, upwards or downwards,
results in a strengthening or weakening of the determinant's power of
assimilation.
Let us take a case of positive variation of the determinants of an
organ _N_, which would be more useful to the species if it were
more highly developed than it had previously been. The variation in
an upward direction is at first purely passive, having arisen from
fluctuations in the food-supply, but it soon becomes active, since the
determinants that have become stronger will have a stronger affinity
for food and will attract more and more of the available supply. The
increased food-stream is thus maintained, and its gradual result is
such a strengthening of the determinants in the course of generations
of germ-cells, that the parts controlled by these determinants--the
determinates--must enter on a path of plus-variations. If to this
there be added personal selection, either natural or artificial, any
fluctuations of this primary constituent towards the minus side will be
effectually prevented, the direction of variation will remain positive,
and the continued intervention of personal selection may raise its
development to its possible maximum, that is, so far that further
development in the same direction would not make for greater fitness,
and personal selection must call a halt. This will always happen as
soon as further increase of the organ would be prejudicial to the
living power of the whole, and when the harmony of the bodily parts
would thereby be permanently disturbed.
That variation in an upward direction really can persist for a long
time is shown by artificial selection as practised by Man in regard
to his domesticated animals and cultivated plants. At first general
variability, or at least variability in many directions, sets in as
a result of the greatly altered conditions of life; the ordinary
fluctuations of the determinants are intensified by the greater
fluctuations in the nutritive stream, and it becomes possible for Man
consciously or unconsciously to select for breeding whatever he prefers
among the chance variations that arise in individual parts or in whole
complexes of parts, and he may thus give rise to a long-continued,
often apparently unlimited, augmentation of variations in the same
direction, although he cannot exercise _any direct_ influence upon
the germ-plasm or its determinants. When a determinant has assumed
a certain variation-direction it will follow it up of itself, and
selection can do nothing more than secure it a free course by setting
aside variations in other directions by means of the elimination of
those that exhibit them.
That artificial selection can cause the increase of a part has long
been established, but in what way this is possible, and how it can
be theoretically explained has hitherto been very obscure, for even
if we take the favourable case that both parents possess the desired
variation, it cannot be supposed that the characters of the parents
are, so to speak, added together in the child; all we can say is that
the probability that the children will also exhibit the character
in question--for instance, a long or crooked nose--becomes greater.
Certainly an increase of the character may result if in both parents
the determinants _K_ are present in excess as compared with the
heterodynamous determinants _K´_ and _K´´_, for in that case there
is an increased probability that, through reducing divisions and
amphimixis, there will again be a preponderance of the determinants
_K_ composing the germ-plasm of the child, and further, that these
determinants _K_ will dominate strongly as compared with the few
_K´_'s. It may thus happen that the long nose of the two parents
will give rise to a still longer nose in the child, or that parents
of considerable bodily size may have still bigger children, but such
increase would be confined to one generation, and would not lead to a
permanent increase of the character; permanent increase cannot depend
merely on the number of the determinants _K_ and on their supremacy
over their converse, the determinants _K´_; it must also depend on
their own variation, and this again can depend only on germinal
selection and not upon personal selection, although the former can be
materially assisted by the latter.
That inheritance from both parents is only a secondary consideration
in regard to the increase of a part by artificial selection is made
evident by the fact that _many secondary sexual characters_ have been
modified, although the breeder selected only in regard to one parent.
Nevertheless in this very domain the greatest results have been
achieved; witness the Japanese breed of cocks with tail-feathers six
feet long. This astonishing result has been reached by the strictest
selection of the cocks in which the feathers were a little longer
than those of other cocks, and the increase in the length of feathers
depended--according to our theory--simply on the fact that, by the
selection of the determinants which were already varying in the
direction of increased length, this process of increase was guarded
from interruption by chance unfavourable conditions of nutrition.
The continuance of variation in the upward direction in which it had
already started is not effected directly by personal selection, but
is so indirectly, for without this constant fresh intervention of
selection the increase would be apt to come to a standstill, or the
variation might even take a contrary direction. There are two other
factors operative to which we have not yet given sufficient attention.
They are, the multiplicity of the ids in every germ-plasm, and sexual
reproduction.
If--as we must assume--each germ-plasm is made up of several or many
ids, there must be several or many determinants of each part of the
organism, for each id contains potentially the whole organism, though
with some individuality of expression. The child is thus not determined
by the determinants of a single id, but by those of many ids, and the
variations of any part of the body do not depend on the variations of a
single determinant _X_, but on the co-operation of all the determinants
_X_ which are contained in the collective ids of the relevant
germ-plasm. Thus it is only when a majority of the determinants have
varied upwards or downwards that they dominate collectively the
development of the part _X´_ and cause it to be larger or smaller.
We have assumed passive fluctuations in nutrition to be the first
cause in individual variation, and it is obvious that the action of
this first cause of dissimilarity must be greatly restricted by the
multiplicity of the ids and the corresponding homologous determinants.
For although passive fluctuations in nutrition should occur continually
in the case of all determinants, this would not imply that they would
follow the same direction in all the determinants _X_ of all ids, for
some determinants _X_ might vary upwards, and others downwards, and
these might counteract each other in ontogeny; so that in many cases
the fluctuations of the individual determinants will not be felt in
their products at all. But since there are--as we shall see later--only
two directions of variation, upwards and downwards, plus and minus,
it must also sometimes happen that a majority take one direction, and
this affords the basis on which germinal selection can build further,
and on which it is materially supported by reducing division and the
subsequent amphimixis.
For reducing division removes half of the ids and thus of the
determinants from the mature germ-cell, and according as chance leaves
together or separates a majority of _X_-determinants varying in the
same direction, this particular germ-cell will contain the primary
constituents of a plus- or of a minus-variation of _X_, and it is
possible that the presence of a majority or a minority may be entirely
due to the reduction. The germ-plasm of the parent may contain,
for instance, the determinant _X_ in its twenty ids 12 times in
minus-variation form, 8 times in plus-variation form; and the reducing
division, according to our view, may separate these into two groups
of which one contains eight plus- and two minus-variations, the other
ten minus-variations, or the one six plus- and four minus-variations,
the other two plus- and eight minus-variations, and so on. Now every
germ-cell which contains a majority of plus- or minus-variations--and
this must be the case with most of them--may unite, if it attains to
amphimixis, with a germ-cell which also contains a majority of plus
or minus _X_-determinants, and if similar majorities let us say
plus--meet together, the plus-variation of _X_ must be all the more
sharply emphasized in the child.
Thus, although the individual determinants _X_ may not be incited to
further variation by their co-operation with others varying in the
same direction, the collective effect of the plus-determinants will
be greater, and adherence to the same direction of variation in the
following generation will be assured, for if in the germ-plasm of the
parent there be, for instance, sixteen out of twenty determinants
possessing the plus-variation, a minus-majority can no longer result
from reducing division.
It is upon this that the operation of natural selection, that is,
personal selection, must depend--that the germ-plasms in which the
favourable variation-direction is in the majority are selected for
breeding, for it is this and nothing else that natural selection does
when it selects the individuals which possess the preferred variations.
The ascending process is thus considerably advanced, because the
opposing determinants are more and more eliminated from the germ-plasm,
till the preferred variations of _X_ are left, and among these, as
ascent in the direction begun continues, the opposing variations
are again set aside by germinal selection, and so on. Reducing
divisions and amphimixis are thus powerful factors in furthering
the transformations of the forms of life, although they are not the
ultimate causes of these.
Now that we have made ourselves familiar with the idea of germinal
selection we shall attempt to gain clearness as to what it can do,
and how far the sphere of its influence extends, and, in particular,
whether it can effect lasting transformations of species without the
co-operation of personal selection, and what kind of variations we may
ascribe to it alone.
First, I must return for a moment to the question we have already
briefly discussed--whether the variation of a determinant upwards or
downwards must so continue without limit. We might be inclined to
think that the great constancy which many species exhibit was a plain
contradiction of this, for if every minute variation of a determinant
necessarily persisted without limit in the same direction, we should
expect to find all the parts of the organism in a state of continual
unrest, some varying upwards, some downwards, always ready to break the
type of the species. Must there not be some internal self-regulation
of the germ-plasm which makes it impossible that every variation
which crops up can persist unlimitedly? Must there not be some kind
of automatic control on the part of the germ-plasm, which is always
striving to re-establish the state of equilibrium that has once been
attained by the determinant system whenever it is disturbed?
It is difficult to give any confident answer to this question. We
cannot reach clearness on this point through our present knowledge
of the germ-plasm, because we possess no insight into its structure;
we can only draw conclusions as to the processes in the germ-plasm
from the observed phenomena of variation and inheritance. But two
facts stand in direct antithesis to one another, first, the high
power of adaptation possessed by all species, and the undoubted
occurrence of unrestricted persistence in a given direction of
variation, as seen in artificial selection, and in the disappearance
of parts which have ceased to function; and, secondly, the great
constancy of old-established species which do indeed always exhibit a
certain degree of individual variability, but without showing marked
deviations as a frequent occurrence or in all possible directions,
as they certainly would if every determinant favoured by a chance
increase in the nutritive stream necessarily and irresistibly went on
varying further in the same direction. Or can the constancy of such
species be maintained solely by means of personal selection, which
is continually setting aside all the determinants which rise above
the selection-value by eliminating their possessors? I was for long
satisfied that this was the true solution of the difficulty, and even
now I do not doubt that personal selection does, in point of fact,
maintain the constancy of the species at a certain level, but I do not
believe that this is sufficient, but rather that it is necessary to
recognize an equalizing influence due to germinal selection, and to
attribute to this a share in maintaining the constancy of a species
which has long been well adapted. I am led to this assumption chiefly
by the phenomena of variation in Man, for we find in him a thousand
kinds of minute hereditary individual variations, of which not one is
likely to attain to selection value. Of course the constant recurrence
of reducing divisions prevents any particular id which contains a
varying determinant from being inherited through many generations; for
so many ids are being continually removed from the genealogical tree
by the constant rejection of the half of all ids of every germ-plasm,
that only a small part of the ancestral id remains in the grandchild,
great-grandchild, and so on. Certainly some of the ids of the ancestors
compose the germ-plasm of the descendants, and if all the determinants
of one of these ids had begun to vary persistently upwards or downwards
in an ancestor, then all the determinants of the relative id in the
descendants would possess the variation in an intensified degree; and
however slowly the variation advanced it would attain selection-value
in some one or other of the descendants, and would thus break
the previously stable type of the most perfectly adapted species.
The descendant in question would then succumb in the struggle for
existence. But as the number of the determinants in the germ-plasm is
probably much greater than that of the descendants of one generation,
every descendant would in the course of time deviate unfavourably in
some one character from the type of the species, and then either all
the descendants would be eliminated or the type would become unstable.
But neither of these things happens, and there are undoubtedly
species which remain constant for long periods of time, therefore the
assumption must be false and every variation of a determinant does not
of necessity go on in the same direction without limit.
I therefore suppose that although slight variations are ceaselessly
taking place upwards or downwards in all determinants, even in constant
species, the majority of these turn again in the other direction before
they have attained to any important degree of increase, at least in
the germ-plasm of all species which have had a definitely established
equilibrium for thousands of generations. In such a germ-plasm, or
to speak more precisely, in the id of such a germ-plasm, marked
fluctuations in the nutritive stream will not be likely to occur as
long as the external conditions are unchanged, but slight fluctuations,
which will not be wanting even here, may often alternate and turn in
an opposite direction, and thus the upward movement of a determinant
may be transformed into a downward one. Every determinant is surrounded
by several others, and we can imagine that the regular nutritive
stream which we have assumed may be partially dammed up by a slight
enlargement of the determinant, and that this will drive the surplus
back again. But however we may picture these conditions, which are for
all time outside of the sphere of observation, the assumption of a
self-regulation of the germ-plasm, up to a certain degree, cannot be
regarded as inconceivable or unphysiological.
But there are limits to this self-regulation; as soon as the increase
or decrease of a determinant attains a certain degree, as soon as it
has got beyond the first slight deviation, it overcomes all obstacles,
and goes on increasing in the direction in which it has started.
This must happen even in the case of old and constant species, and
frequently enough to admit of an apparent capacity for adaptation in
all directions. Every part of a species can vary beyond the usual
individual fluctuations, and as this is possible only by means of
intra-germinal processes, we must assume that even in the case of
germ-plasms which have long remained in a state of stable equilibrium
there may occasionally be marked fluctuations in the nutritive stream,
and thus more than usually pronounced variations of the determinants
affected by it will occur. These yield the material for new adaptations
if they are in the direction of fitness, or they are eliminated either
by the chances of reducing division or by personal selection if new
adaptations are not required.
The old-established hereditary equilibrium of the germ-plasm must be
most easily disturbed when the species is in some way brought into new
conditions of existence, as, for instance, when plants or animals are
domesticated, and when in consequence, as we have already assumed, the
nutritive currents within the id gradually alter, quantitatively and
qualitatively; and on this account alone certain kinds of determinants
are favoured, while others are at a disadvantage. In this way there
arises the intensified general variability of domesticated animals
and cultivated plants which has been known since the time of Darwin.
Something analogous to this must occur in natural conditions, though
more slowly, when a species is subjected to a change of climatic
conditions, but we shall discuss this later on in more detail.
We have thus arrived at the idea that the slight variations of the
determinants may be counteracted whether they be directed upwards or
downwards, and that in the case of so-called constant species they
do frequently equalize themselves; but that more marked variations,
produced by more pronounced nutritive fluctuations, may in a sense go
on without limit, and then can only be restricted and controlled by
personal selection, that is, by the removal of the ids concerned from
the genealogical lineage of the species.
In one direction variation can be proved to go on without limit, and
that is downwards, as is proved by the fact of the disappearance of
_disused organs_, for here we have a variation-direction, which has
been followed to its utmost limit, and which is completely independent
of personal selection; it proceeds quite _uninterfered with_ by
personal selection, and is left entirely to itself. It is a significant
fact that the disappearance of the individual parts of a larger organ,
according to all the data that are as yet available, proceeds at a
very _unequal rate_, so that it evidently depends to a great extent on
chance whether a disused part begins to degenerate sooner or later.
Thus in one of the Crustaceans living in the darkness of the caves of
North America the optic lobes and optic nerves have disappeared, while
the retina of the eye, the lens, and the pigment have been retained,
and in others the reverse has taken place, and the nerve-centres
have persisted while the parts of the eye have been lost (Packard).
Variations of the relevant determinants towards the minus direction
may thus occur, sometimes sooner, sometimes later; but when once they
have started they proceed irresistibly, though with exceeding slowness.
But variation in an upward direction also, when it has once been set
a-going, may in many cases go on unchecked until limits are set to
it by personal selection, when the excess of the organ would disturb
the harmony of the parts, or in any other way lessen the individual's
chances of survival in the struggle for existence. This is proved
especially by the phenomena of artificial selection, for almost all
the parts of fowls and pigeons have been caused to vary to excess by
breeding, and must thus have been, so to speak, capable of unlimited
increase; and yet, as we have seen, personal selection cannot directly
cause progress in any direction of variation; it can only secure a
free course by excluding from breeding the bearers of variations
with an opposite tendency. The beards of hens, the tail-feathers of
the long-tailed domestic cocks, the long and short, straight and
curved bills of pigeons, the enormously long ruffled feathers of the
Jacobin, the multiplication of the tail-feathers in the fan-tail, and
innumerable other breed-characters of these playthings of the breeder,
prove that when variation-tendencies of any part are once present, that
is, when they have arisen through germinal selection, they apparently
go on unchecked until their further development would permanently and
irretrievably destroy the harmony of the parts. As soon as this is
threatened the breed loses its power of survival, and Darwin in his
time cited the case of many extremely short-billed breeds of pigeon,
which require the aid of the breeder before they can emerge from the
hard-shelled egg, because their short and soft bills no longer allow
them to break their way out. Here the correlation between the hardness
of the egg-shell and that of the pigeon's bill has been disturbed, and
the breed can now only be kept in existence by artificial aid.
There must be a possibility of something similar occurring in natural
conditions, and when it does the species concerned must die out. But in
the majority of cases the self-regulation which is afforded by personal
selection will be enough to force back an organ which is in the act
of increasing out of due proportion to within its proper limits. The
bearers of such excessively increased determinants succumb in the
struggle for existence, and the determinants are thus removed from the
genealogical lineage of the species.
Having now established the fact that determinants can continue their
direction of variation without limit because of internal, that is
intra-germinal, reasons, we have come nearer an understanding of
many secondary sexual characters, whose resemblance to the excessive
developments artificially produced in our domestic poultry is so
very striking. Here, too, we shall have to regard germinal selection
as the root of the variations of plumage and other distinguishing
characters, which have evolved by intra-germinal augmentation into
the magnificently coloured crests, tufts, and collars, into the long
or graduated, multiplied or erectile tail-feathers of the birds of
Paradise, pheasants, and humming-birds. The conception of sexual
selection formulated by Darwin will be so far modified, that we are no
longer compelled to regard every minute step in this cumulative process
as due to the selection of the males by the females. A preference of
the finest males may still take place, and is probably general, since
only thus could the distinguishing male characters become common
property, that is, be transmitted to all or the majority of the ids
of the germ-plasm, but the increase of the individual determinants
which are in the act of varying goes on in each individual id, quite
independently of this personal selection.
As it is not a single id with its determinant _a_ in ascending
variation that controls the organ _A_, but as it always requires
a majority of the ids _a_, this must be secured here by personal
selection just as it is in ordinary natural selection. If the
handsomest males are the successful competitors, then a majority of
the transformed ids _a´_ will be transmitted to a number of their
descendants, and the oftener this happens the larger will the majority
be, and the less becomes the danger that it will be dispersed again
by reducing division and amphimixis. Personal selection is thus in
no way rendered superfluous by germinal selection, only it does not
produce the augmentation of the distinguishing characters, but is
chiefly instrumental in fixing them in the germ-plasm; it collects,
so to speak, only the favourably varying ids, and, where complex
variations depending on the proper variation of many ids are concerned,
it combines these. How very great the influence of personal selection
may be in this case of secondary sexual characters we see clearly from
the soberly coloured mates of the brilliant males, for here natural
selection has been operative in conserving the coloration inherited
from remote ancestry.
But if the question be asked, how _the first majority_ of determinants
varying in the same direction is brought about, there are two
possibilities: first, by chance, and secondly, by influences which
cause particular determinants of all the ids to vary in almost exactly
the same manner. We shall find illustrations of the latter among
climatic varieties; but the cases of the first kind are the more
important, for they form the foundation and the starting-point for
processes of selection of a higher order, for personal selection. It
might seem perplexing that processes of such importance should depend
ultimately upon chance; but when we remember that there are only two
directions of variation, namely a plus direction or a minus direction,
we recognize that the chance of a majority in one direction or another
is much greater than that of absolute equilibrium between the two, and
there is therefore a very strong probability that in many individuals
of the species either the upward or the downward movement of a
determinant _A_ will preponderate.
Now as such variation movements, when they are of a certain strength,
increase automatically, we can easily see that they must gradually
attain to a level at which they acquire selection value, and how then,
by personal selection, the ids with favourably varying determinants may
be collected together.
Of course it is not possible to state positively the time at which
in individual cases a variation acquires a biological significance,
that is, selection value. We can only say in a general way that, as
soon as it attains this, personal selection either in a positive or a
negative sense _must_ intervene; an injurious variation tends to the
elimination of its possessor, a useful one increases the probability of
its survival.
There must, however, be for every variation a stage of development in
which it has as yet no decisive biological importance, and this stage
need not by any means be so insignificant that we cannot see it, or
can hardly do so: in other words, there are characters which have
arisen through germinal selection, which are of purely 'morphological
importance.'
It has often been disputed whether there can be any such thing
as 'purely morphological characters,' which are indifferent as
far as the existence of the species is concerned. This question
used to be an important one, because the sphere of operation, and
therefore the importance of the Darwin-Wallace selection--personal
selection--depends on the answer, since this mode of selection only
begins when a character has some biological importance. But as soon as
we take germinal selection into consideration the question loses its
importance, because we now know that every variation is indifferent
to begin with, but every one can, under favourable circumstances,
be increased to such a pitch that it attains biological importance,
and that personal selection then takes over the task of carrying it
on, either in a positive or a negative sense. We may therefore leave
this disputed point alone just now, for while germinal selection
seems still far from being generally recognized, we have to remember
that we are not at all in a position to judge with any certainty as
to the biological value of a character. What labour and painstaking
investigation it has cost to give a verdict as to this even in a
few instances! Innumerable characters appear indifferent, and are
nevertheless adaptations. Darwin in his day pointed out the need for
caution in this matter, referring to the case of animal coloration as
an example; very little attention had been directed to it for a long
time because it had been believed to be without significance. And how
many diverse kinds of characters among animals and plants, which had
likewise been regarded as 'purely morphological,' have on more careful
investigation shown themselves of very great biological importance.
I need only refer to the shape, position, hair-arrangement, colour,
and lustre of flowers, and their relation to cross-fertilization by
means of insects, or to the thickness and shape of the leaves of
tropical trees with their coating of wax and their gutter-like outlets
for carrying off the tropical rain which falls in terrible downpour
(Haberlandt, Schimper), or to the limp, perpendicular drooping of the
tufts of the young and tender leaves of the same trees, which also
secures protection from being battered and torn by the rain.
[Illustration: FIG. 107, _C._ Leptocephalus stage of an American Eel,
with seven pigment spots, of which three are on the left (_l_) and four
on the right (_r_) side. After Eigenmann.]
There are even characters the biological use of which is unknown to
us, but in regard to which we can affirm that they have a use. Thus
Eigenmann described the larva of an American eel, which differs from
other so-called 'Leptocephali' in that a row of seven black spots runs
along its side. Apparently all these lie upon the side turned towards
us, but in reality they are distributed on both sides, three lying on
the left and four on the right, and so arranged that they look like a
single row of spots at regular intervals, for the flat little fish is
absolutely transparent. The habits of this larva are not yet known, but
we may conclude that this appearance of a simple row of spots must have
some value for the animal, for such a significant asymmetry could not
have arisen for purely internal reasons (Fig. 107, _C_). It is possible
that the fish is thus made to resemble parts of some marine alga, and
that it is thereby protected from many enemies; that there is not a
complete row upon each side may depend upon the fact that the two rows
would be visible at the same time, and that they would blur each other
in the eyes of the swimming enemy, and so destroy the resemblance of
the picture to its unknown model.
But it cannot be denied that there are characters which have no special
biological significance. There are doubtless many such characters,
which stand beyond the threshold of good or bad, and which are
therefore not affected by personal selection; it is difficult and often
impossible to point these out with certainty. The shape of the human
nose and of the human ear, the colour of the hair and of the iris, may
be such indifferent characters whose peculiarities are to be referred
solely to germinal selection. On the other hand, I would not venture to
assert that the gay colouring and the complex markings on the wings of
our modern Lepidoptera are always and in all cases unimportant, even
when we cannot interpret their details either as protective, or as a
sign of nauseousness, or as mimetic. The usually very exact similarity
of the colour pattern in the individuals of each species seems to point
to the intervention of personal selection in some form or other, for in
what other way could such a large majority of variations in the same
direction have developed in the germ-plasm as this constancy of the
character indicates.
We know, of course, that the colours of butterflies and moths can be
caused to vary through external and especially climatic influences,
but this would only account for simple modifications of colour, and
not for the origin of the complex colour patterns that actually
occur. I therefore believe with Darwin that sexual selection has had
much to do with this by giving a slight preference to the variations
produced by spontaneous germinal selection, and thus preventing the
majority of varied ids once acquired from being scattered again, but
always collecting more of them, and so securing free play for the
increase of the new character through intra-germinal processes. In
this way have arisen not only the brilliance of our Lycænidæ and of
the large Morphidæ of South America, but also many of the coloured
spots, streaks, bands, eyes, and other components which have gradually
in the course of time evolved into the complex colour pattern of
many of our modern butterflies. I should like to remind any one who
doubts this of a fact which corroborates the view that personal
selection has co-operated in the production of these colours--I
refer to the inconspicuous colouring of the females of many of these
brilliant males--while in contradistinction to these cases there are
other species in which both sexes are alike brilliant, so that it is
impossible that mere spontaneous germinal selection can have determined
that the females, because of their femaleness, should vary in a
different manner from the males.
But while I believe that sexual selection in particular has had much to
do with producing the colours of Lepidoptera, the basis of all these
colour variations must still be looked for in germinal selection, and
we shall see later on how it is possible to think of the diversified
and often relatively abrupt transformations of marking as the resultant
of the co-operation of climatic influences with germinal selection.
Of course there must also be unimportant changes in butterfly-markings
which depend solely on the internal play of forces in the determinant
system, and to this must be referred the markings of many of the
'variable' species whose variations are mere fluctuations in the
details of marking, which have therefore caused much trouble to the
systematists. Truly unimportant variations will rarely or never
combine into a 'constant' form, and the fact that there are species
which are 'variable' in such a high degree is enough to make us refer
their variations to their lack of importance, for if they possessed
any biological value the less valuable among them would gradually be
removed by selection. Perhaps the variable species of certain moths
like _Arctia caja_, and especially _Arctia plantaginis_, the little
'bear' of the Alps and Apennines, must be reckoned among these.
But from the fact that there are such fluctuations in the markings
of Lepidoptera, it seems to me that we must conclude that species
which show a high degree of constancy in their markings have been
influenced by selection, or by climatic influences which turned the
play of forces within the determinant system in the same direction in
all individuals. All these considerations and conclusions are quite
sound and serviceable theoretically, but they are difficult to apply
to individual cases, and where this is attempted it must be with the
greatest caution, and, if possible, on a basis of investigations
specially undertaken for the purpose; for how should we know whether
a species which to-day is highly variable may not a geological
epoch later become a very constant one? We must in any case assume
that marked fluctuations of characters are associated with many
transformations.
LECTURE XXVI
GERMINAL SELECTION (_continued_)
Germinal selection, spontaneous and induced--Climatic
forms of _Polyommatus phlæas_--Deformities--Excessive
augmentation of variations--Can it lead to the elimination
of a species?--Saltatory variations, copper-beech, weeping
trees--Origin of sexual distinguishing characters--Formation
of breeds among domesticated animals--Degenerate jaws--Human
teeth--Short-sightedness--Milk-glands--Small hands and feet--Ascending
variation--Talents, intellect--Combination of mental endowments--The
ultimate roots of heritable variation--There are only plus- and
minus-variations--Relations of the determinants to their
determinates--The play of forces in the determinant system of the
id--Germinal selection inhibited by personal selection--Objection on
the score of the minuteness of the substance of the germ-plasm.
Hitherto we have derived the variations of the determinants of the
germ-plasm, upon which we based the process of germinal selection,
from _chance local_ fluctuations in nutrition, such as must occur in
an individual id, independently of the nutrition of the other ids of
the same germ-plasm. But there are doubtless also influences which set
up similar nutritive changes in _all ids_, and by which, therefore,
all homologous determinants, in as far as they are sensitive to the
nutritive change in question, are affected in the same manner. To
this category belong changes in the external conditions of life, and
particularly climatic changes. It is, then, germinal selection alone
which brings about the presence of a majority of ids with determinants
varying in the same direction, and personal selection has no part
in the transformation of the species. Many years ago I instituted
experiments with a small butterfly, _Pararga egeria_, and these
showed that a heightened temperature so influenced the pupæ of this
form that the butterflies emerged with a different and deeper yellow
ground-colour, similar to that of the long-known southern variety
_Meione_. More thoroughly decisive, however, were the experiments on
_Polyommatus phlæas_, the small 'fire-butterfly,' which were carried
on in the eighties by Merrifield in England and by myself almost at
the same time. I shall discuss these later in more detail, and will
only say here that this butterfly, whose range extends from Lapland
to Sicily, occurs in two forms, the southern distinguished by a
'dusting' of deep black from the northern, in which the wing-surfaces
are of a pure red-gold. The experiments showed that the southern
form can be artificially produced by warmth, and the interpretation
must be that the direct influence of higher temperature affects the
quality of the nutritive fluids in the germ-plasm, and thereby at
the same time the determinants of one or more kinds of wing-scales
are caused to vary in all the ids in the same direction, in such a
fashion that they give rise to black scales instead of the former
red-gold ones. It is thus certain that there are external influences
which cause particular determinants to vary in a particular manner.
I call this form of germinal variation 'induced' germinal selection,
and contrast it with 'spontaneous' selection, which is caused, not by
extra-germinal influences, but by the chances of the intra-germinal
nutritive conditions, and which will, therefore, not readily occur at
the same time in all the ids of a germ-plasm, and so will not give rise
to variation of the same kind in the homologous determinants of all the
ids.
The two processes must also be distinguished from each other in their
relation to personal selection, for induced germinal selection will
go on increasing until the maximum of variation corresponding to the
nature of the external influences and of the determinants concerned
is reached. Since _all_ the ids are equally affected and caused to
vary in the same way, personal selection has nothing to take hold of,
and the variation might go on intensifying even if it should become
biologically prejudicial. But it is quite otherwise with spontaneous
germinal selection, which has its roots not in all, but only in a
majority of the ids. Here the variation may go on increasing by
germinal selection alone, but only until it acquires a positive or
negative biological value, that is, until it becomes advantageous or
prejudicial to the life of the individual; then personal selection
intervenes and decides whether it is to go on increasing or not.
Spontaneous germinal selection can therefore only lead to the general
variation of a whole species when it is supplemented by some external
factor such as, especially, the utility of the variation.
This does not imply, however, that indifferent variations of large
amount could not arise through spontaneous germinal selection, but
they would remain confined to a small number of individuals, and
would sooner or later disappear again. The congenital deformities of
Man may in part fall under this category. If, for instance, certain
determinants are, by reason of specially favourable local nutritive
conditions, maintained for a long time in progressive variation, they
will become so strong that the part which they determine will turn
out excessive, perhaps double. Hereditary polydactylism in Man may
perhaps be explained on this principle, and I had already referred
it to the more rapid growth and duplication of certain determinants
of the germ even before formulating the idea of germinal selection.
In this I was at one with the pathologist Ernst Ziegler, who had
designated polydactylism as a germ-variation, and in contrast to
others had not interpreted it in an atavistic sense, as a reversion to
unknown six-fingered ancestors. All excessive or defective hereditary
malformations may be referred to germinal selection alone, that is, to
the long-continued progressive or regressive variation of particular
determinant-groups in a majority of ids.
The fact that, as far as our experience goes, superfluous fingers
are never inherited for more than five generations may be simply
explained, for there has been no reason for the intervention of
personal selection, either in the negative sense, for the six-fingered
state does not threaten life, nor in the positive, since it is not of
advantage. The deformity depends on spontaneous germinal variation,
which must have taken place in a majority of ids or it would not
have become manifest. But such a majority of 'polydactylous' ids is
liable to become scattered again in every new descendant, and to be
reduced again into a minority which can no longer make itself felt by
the chances of reducing division and the admixture of normal ids in
amphimixis. A polydactylous race of men could only arise through the
assistance of personal selection; in that case there would doubtless
be just as much chance of success in breeding a six-fingered race as
there was in breeding the crooked-legged Ancon sheep from a single ram
which was malformed in this manner. Without a gradual setting aside of
the germs with normal ids, that is, without personal selection, such
spontaneous deformities, and indeed all _spontaneous_ variations, must
fail of attaining to permanent mastery.
This must frequently be the case in free nature also, but we shall have
to investigate later on, in the section devoted to the formation of
species, whether external circumstances (inbreeding) may not also occur
which make it possible for spontaneous variations to become constant
breed-characters, even although they remain neither good nor bad, and
are thus not subject to the action of personal selection.
In general, however, amphigony with its reduction of the ids and its
constant mingling of strange ids will form the corrective to the
deviations which may arise through the processes of selection within
the id, and which lead to excessive or superfluous development of
certain structures, to a complete disturbance of the harmony of the
parts, and ultimately to the elimination of the species.
It must be admitted, however, that Emery was probably right when he
directed attention to the possibility of a 'conflict between germinal
and personal selection.' It is quite conceivable that in cases of
useful variations, that is, of adaptations, the processes of selection
within the germ-plasm may lead to excessive developments, which
personal selection cannot control, because, on account of their earlier
usefulness, they have in the course of a series of generations and
species become fixed not only in a majority of ids, but in almost all
the ids of the collective germ-plasm of the species. In this case a
reversal must be difficult and slow, for the gathering together of ids
with relatively weaker determinants can only take place slowly, and
it is questionable whether the species would survive long enough for
the slow process to take effect. But, apart from the question of time,
such a reduction of an excessive development would sometimes be quite
impossible, for the simple reason that there is nothing for personal
selection to take hold of.
Döderlein has pointed out that many characters go on increasing through
whole series of extinct species, and ultimately grow to such excess
that they bring about the destruction of the species, as, for instance,
the antlers of the giant stag or the sabre-like teeth of certain
carnivores in the diluvial period. I shall have to discuss this in more
detail in speaking of the extinction of species; it is enough to say
here that such long-continued augmentations in the same direction can
never be referred _solely_ to germinal selection, since it is hardly
conceivable that a species--much less a whole series of species--should
arise with injurious characters; they would have become extinct while
they were still in process of arising. Although we see that the
Irish stag, with his enormous antlers over ten feet across from tip
to tip, was heavily burdened, we are hardly justified in concluding
that the size and weight of the burden on his head tended to his
destruction from the first--for in that case the species would never
have developed at all--but it may well be that at some time or other
the life-conditions of the species altered in such a manner that the
heavy antlers became fatal to it. In this case the variation-direction
which had gained the mastery in all ids could no longer be sufficiently
held in check by personal selection, because the variations in the
contrary direction would be much too slight to attain to selective
value. Sudden, or at least rapidly occurring changes in the conditions
of life, such as the appearance of a powerful enemy, exclude all chance
of adaptation by the slow operation of personal selection.
If we look into the matter more carefully, we see that it is not
strictly true to say that germinal selection alone brings about the
extinction of a species by cumulative augmentation of structures which
are already excessive; _it is the incapacity of personal selection
to keep pace with the more rapid changes in the conditions of life
and to reduce excessive developments to any considerable extent in a
short time_. This would always be possible in a long time, for the
determinants of the excessive organ _E_ can never be equally strong in
all the ids; they always fluctuate about a mean, however high this mean
may be. Here again it must still be possible that reducing-divisions
and amphimixis may lead to the formation of majorities of ids with
weaker _E_-determinants, and if sufficient time be allowed, artificial
selection could, by consistently selecting the individuals with, let
us say, weaker antlers, give rise to a descending variation-movement.
There are no variation-movements which cannot be checked; every
direction can be reversed, but time and something to take hold of
must be granted. That was wanting in the case of the giant stag, for
it would not have been saved even if its antlers had at once become a
couple of feet shorter, and germinal selection can hardly make so much
difference as that.
Analogous to hereditary deformities, and of special interest in
connexion with the processes within the germ-plasm, are '_sports_'
variations of considerable magnitude which suddenly appear without our
being able to see any definite external reason for them. I have already
discussed these in detail in my _Germ-plasm_, and have shown how simply
these apparently capricious phenomena of heredity can be understood _in
principle_ from the standpoint of the germ-plasm theory.
The chances of the transmission of the saltatory variation will be
greater or less according to whether the variation of the relevant
determinants involves a bare majority of ids or a large majority, for
the more ids that have varied, the greater is the probability that the
majority will be maintained throughout the course of ensuing reducing
divisions and amphimixis, that is, that the seeds of the plant will
reproduce the variation, and will not revert to the ancestral form.
Although one of the most satisfactory results of the id-theory lies
precisely in the interpretation of these conditions, I do not wish
to enter into the matter here, but will refer to the details in my
_Germ-plasm_, published in 1894, which I consider valid still. At that
time I had not formulated the idea of germinal selection, but the
explanation of the occurrence of such sport-variations which I gave was
based upon the assumption of nutritive fluctuations in the germ-plasm,
which gave rise to variations in certain determinants. There was still
lacking the recognition that the direction of variation once taken must
be adhered to until resistance was met with, and that the determinants
stand in nutritive correlation with one another, so that changes in
one determinant must re-act upon the neighbouring ones, as I shall
explain more fully afterwards. I also showed from definite cases that
such sports, though they are sudden--'saltatory'--in their mode of
occurrence, are long being prepared for by intimate processes in the
germ-plasm. This 'invisible prelude' of variation depends on germinal
selection. When a wild plant is sown in garden-ground it does not
require to vary at once; several, even many, generations may succeed
each other which show no sports; suddenly, however, sports appear,
at first singly, then, perhaps, in considerable numbers. It is not,
however, by any means always the case that considerable numbers occur,
for some varieties of our garden flowers have arisen only once, and
then have been propagated by seed; and such saltatory sports in plants
which are raised from seed are usually constant in their seed, and if
they are fertilized with their own pollen they breed true--a proof that
the same variations must have taken place in the relevant determinants
in a large majority of ids.
In animals, it would appear, such saltatory variations occur much more
rarely than in plants; the case examined in detail by Darwin of the
'black-shouldered peacock' which suddenly appeared in a poultry-yard is
an example of this kind. Much more numerous, however, are the instances
among plants, and especially among plants which are under cultivation.
This indicates that we have here to do with the effect of external
conditions, of nutritive influences which cause the slow variation of
certain determinants, sometimes abetting and sometimes checking. As
soon as a majority of ids varied in this way comes to lie in a seed, a
sport springs up suddenly and apparently discontinuously--a plant with
differently coloured or shaped petals or leaves, with double flowers,
with degenerate stamens, or with some other distinguishing mark, and
these new characters persist if the variety is propagated without
inter-crossing.
But it happens sometimes, though more rarely, that not the whole
plant but individual shoots may exhibit the variation. To this class
belong the 'bud-variations' of our forest trees, the copper-beeches,
copper-oaks, and copper-hazels, the various fasciated varieties of oak,
beech, maple, and birch, and the 'weeping trees'; also the numerous
varieties of potato, plantain, and sugar-cane. It seems that only a
few of these breed true when reproduced from seed, or in other words,
they usually exhibit reversions to the ancestral form: on the other
hand, in the weeping oak for instance, nearly all the seedlings exhibit
the character of the new variety, though 'in varying degrees.' The
records as to the transmissibility of bud-variations through seed are
probably not all to be relied upon, and new investigations are much
to be desired, but the fact that in many cases they may be propagated
not only by means of layers and cuttings but by seed also, is most
important in our present discussion, for it proves that here too the
varied determinants must be contained in a majority of ids. As it
is only a single shoot that exhibits the saltatory variation, only
the germ-plasm which was contained in the cells of this one shoot
can have varied, and it must have done so in so many ids that the
variation prevailed and found expression. But that, in this case also,
the variation does not appear in all, but only in a small majority
of ids, is proved by the frequent reversion of bud-varieties to the
ancestral form. I have already reported a case of this kind shown to
me by Professor Strasburger in the Botanic Gardens in Bonn, where a
hornbeam with deeply indented 'oak-leaves' had one branch which bore
quite normal hornbeam leaves. In my own garden there is an oak shrub
of the 'fern-leaved' variety, whose branches bear some leaves of the
ordinary form; variegated maples with almost white leaves often exhibit
in individual branches a reversion to the fresh green leaves of the
ancestral form. We see from this that what is so energetically disputed
by many must in reality occur--namely, differential or non-equivalent
nuclear division--for otherwise it would be unintelligible how
the ids of the new variety, if they once attain a majority in the
tree, could give place in an individual branch to a majority of the
ancestral ids. Only differential nuclear division, in the manner of a
reducing division, can be the cause of this. Of course this implies
only a dissimilar or differential distribution of the ids between the
two daughter-nuclei, not a splitting up of the individual ids into
non-equivalents.
That in free Nature bud-variations left to themselves can ever become
permanent varieties is probably an unlikely assumption, because of the
inconstancy of their seeds which only breed true in rare cases; nor
is it likely that such variations as the copper-beech, the weeping
ash, and so on could hold their own in the struggle for existence
with the older species; but there is certainly nothing to prevent our
assuming that, in certain circumstances, saltatory variations, when
they have a germinal origin, may become persistent varieties and may
even lead to a splitting of the species. This may happen, for instance,
when the variations remain outside the limits of good and bad, and
thus are neither of advantage to the existence of the species nor a
drawback thereto. In the next chapter we shall discuss the influence of
isolation upon the formation of species, and it will be seen that in
certain conditions even indifferent variations may be preserved, and
that saltatory variations, as for instance in the evolution of species
of land-snails or butterflies, may have materially contributed to bring
this about.
I should like to emphasize still more the part played by saltatory
variations arising from germinal selection in the origin of secondary
sexual characters. As soon as personal selection, whether sexual
or ordinary, prefers as useful in any sense a saltatory variation,
it is not only preserved and becomes a character of a variety, but
it may increase, and we have to ask whether such sudden variations
are frequently of a useful kind, especially when not individual
characters alone, but whole combinations of them are implicated. If
we may judge from the sports of the flowers and the leaves of plants,
transformations useful to the species as a whole rarely occur suddenly,
that is, they occur only in a few out of very numerous sports; they
are much more frequently indifferent, although quite visible and often
conspicuous variations.
For this reason I am disposed to attribute to saltatory variations a
considerable share in the production of distinctive sexual characters.
From saltatory variations in flowers, fruits, and leaves we know that
these may be conspicuous enough even on their first appearance, and so
we are justified in finding in such variations the first beginnings of
many of the decorative distinguishing characters which occur in the
males of so many animals, especially butterflies and birds. As soon as
it is admitted that variations of considerable amount, which have been
slowly prepared in the germ-plasm by means of germinal selection, can
suddenly attain to expression, one of the objections against sexual
selection is disposed of, for conspicuous variations are necessary for
the operation of this kind of selection, since the changes in question
must attract the attention of the females if they are to be preferred.
Without such preference, even though it be not quite strict and
consistent, a long-continued augmentation of the decorative characters
is inconceivable.
But as intra-germinal disturbances of the position of equilibrium in
the determinant system is at the root of the saltatory variations of
our cultivated plants, it must also have played a large share in the
evolution of breeds among our domesticated animals, which is therefore
by no means wholly due to artificial selection operating upon the
variation of individual characters. In all breeds in the formation of
which the production of more than a single definite character was
concerned, as, for instance, in the broad-nosed breeds of dog--bull-dog
and pug-dog--we may refer the peculiar variation of many parts to
disturbances of the equilibrium of the determinant system, which bring
to light, not suddenly as in the case of saltatory variations, but
gradually and increasingly, the curious complex of characters. Darwin
referred such transformations of the whole animal facies, where a
single varying character is deliberately selected, to correlation, and
by this he understood the mutual influence of the parts of an animal
upon one another. Such correlation certainly exists, as we have already
seen in discussing histonal selection, but here we have rather to do
with the correlation of the parts of the germ-plasm, with the effects
of germinal selection, which, affected by the artificial selection of
particular characters, gradually brings about a more marked disturbance
in the whole determinant system.
In the evolution of our breeds of domesticated animals, germinal
selection in the negative sense must also have played a part--I mean
through the weakening and degeneration of individual determinants.
Only in this way, it seems to me, can we explain the tameness of our
domestic animals, dogs, cats, horses, &c., in which all the instincts
of wildness, fleeing from Man, the inclination to bite, and to attack,
have at least partly disappeared. It is, of course, very difficult to
estimate how much of this is to be ascribed to acquired habitude during
the individual lifetime. The case of the elephant might be cited in
evidence of tameness which arises in the individual lifetime, for all
tame elephants are caught wild, but it seems that captured young beasts
of prey, such as the fox, wolf, and wild cat, not to speak of lions and
tigers, never attain to the degree of tameness exhibited by many of our
domesticated dogs and cats. The very considerable differences in the
degree of tameness of dogs and cats go to show that the case is one of
instincts varying in different degree.
If this be so, then the instinct of wildness, if I may express myself
so for the sake of brevity, has degenerated in consequence of its
superfluity, and through the process of germinal selection, which
allowed the determinants of the brain-parts concerned to set out on a
path of downward variation upon which they met with no resistance on
the part of personal selection.
Herbert Spencer adduced against my position the case of the reduction
in the size of the jaws in many breeds of dog, especially in pugs and
other lap-dogs, which he regarded as evidence of the inheritance of
acquired characters. But this and analogous cases of the degeneration
of an organ during a long period in which the animal had been withdrawn
from the conditions of natural life is intelligible enough on the
assumption of persistent germinal selection aided by panmixia. The jaws
and teeth in these spoilt pets no longer require to be maintained at
the level of strength and sharpness essential to their ancestors which
depended on these characters, and so they fell below it, became smaller
and weaker, but could not disappear altogether, for the process of
degeneration was brought, or is being brought, to a standstill by the
intervention of personal selection.
Even the lower jaw in Man is declared by many authors to be degenerate.
Collins found that the lower jaw of the modern Englishman was one-ninth
smaller than that of the ancient Briton, and one-half smaller than that
of the Australians; Flower showed that we are a microdont race like
the Egyptians, while the Chinese, Indians, Malays, and Negroes are
mesodont, and the Andamanese, Melanese, Australians, and Tasmanians are
macrodont. This does not of itself imply that we exhibit a degeneration
of dentition, though this conclusion is hinted at by other facts,
such as the variability of the wisdom-teeth. It need not surprise us,
indeed, that a retrogressive variation tendency should have started
in this case, for, with higher culture and more refined methods of
eating, the claims which personal selection was obliged to make on the
dentition have been greatly diminished, and germinal selection would
thus intervene.
Every one knows how the quality of human teeth has deteriorated with
culture, and this not in the higher classes only, but even among the
peasantry, as Ammon has observed. The time is past when raw flesh
was a dainty, and when bad teeth meant poor nutrition, if not actual
starvation. Even nowadays famine plays a terrible and periodically
recurrent rôle as an eliminator among some negroid races.
Many other organs in man have been reduced from their former pitch of
perfection through culture, and some of them are still in process of
dwindling. When I formulated the idea of panmixia and applied it to
explain cases which had previously been referred to the inheritance of
the results of disuse, I regarded the short-sightedness of civilized
Man from this point of view. My opinion aroused lively opposition at
the time, especially on the part of oculists, who very emphatically
referred the phenomenon to the inheritance of acquired shortsight,
and indeed regarded it as a proof of the transmission of functional
modifications.
But, apart from the fact that the assumption of this mode of
inheritance must now be regarded not only as unproved, but as
contradicted by reliable data, panmixia, in conjunction with
the ceaseless fluctuations within the germ-plasm--germinal
selection--affords a better explanation than the other theory was ever
in a position to offer. At that time I pointed out that the survival
of the individual among civilized races had not for a very long time
depended on the perfection of his eyesight, as it does for instance
in the case of a hunting or warlike Indian, or of a beast of prey,
or of a herbivore persecuted by the beast of prey. And this is by no
means due solely to the invention of spectacles, but in a much greater
degree to the fact that every man no longer has to do everything, so
that numerous possibilities of gaining a livelihood remain open to the
less sharp-sighted; that is, the division of labour in human society
has made the survival of the short-sighted quite feasible. As soon as
this division of labour reached such a degree that the founding of a
family offered no greater difficulty to the short-sighted individual
than to one with normal sight, short-sightedness could no longer be
eliminated; and partly because of the mingling with normal sight, but
partly also because of the never-failing minus-fluctuations of the
germ-plasm determinants concerned, a variation in a downward direction
was bound to set in, and will continue until a limit is set to it by
personal selection. Meantime, we are obviously still in the midst of
the process of eye-deterioration; and the resistance to it is somewhat
inhibited in its operation, because although individuals with extremely
bad sight are for the most part hindered from gaining an independent
livelihood and having a family, this is certainly, thanks to our
mistaken humanity, not always the case. There are even instances of
marriage between two blind persons!
As yet, however, the deterioration of eyes has not advanced very far;
not nearly all families are affected by it, and even in Germany,
the land of the 'longest school form' and of the greatest number
of spectacle-wearers, short-sight is still usually acquired by
individuals, although there must frequently be a more or less marked
predisposition to it. It is a common objection to this view that in
England, France, and Italy the percentage of short-sighted individuals
is much lower, and, in point of fact, one sees far fewer people wearing
spectacles in those countries. This, however, does not prove that a
similar deterioration of eyes has not begun there also, for how could
the small inherited beginnings be detected if they were not accentuated
by the spoiling of the eyesight in the lifetime of the individual by
much reading of bad print, and by writing with bent head, as is still
too often the case in many German schools.
That our interpretation, through panmixia on a basis of germinal
selection, is the correct one, we infer also from the fact that
short-sightedness has been proved to be a frequent character even among
our domesticated animals, such as the dog and the horse. These animals
receive protection and maintenance from Man, and their survival and
reproduction no longer depend on the acuteness of their sight, and
thus the eye has fallen from its original perfection, just as in Man,
although in this case reading and writing play no part.
A whole series of similar slight deteriorations of individual organs
and systems of organs might be enumerated, all of which have appeared
in consequence of long and intensive culture in Man. All these must
depend upon germinal selection, on a gradually progressive weakening
of the determinant-groups concerned, under the conditions of panmixia,
that is, in the absence of positive selection.
To these must be added the deterioration of the mammary-glands and
breasts, and the inability to suckle the offspring which results
chiefly from this. Here we have a variational tendency which could not
appear in a people at a lower stage of culture, and it has not become
general in the lower classes of society among ourselves.
The muscular weakness of the higher classes is another case in point,
and all gymnastics and sports will be of no avail as long as a relative
weakness of the muscles is not a hindrance to gaining a livelihood, and
having a family. Even universal conscription will do nothing to check
this falling off of the bodily strength. Certainly military service
strengthens thousands, and hundreds of thousands of individuals, but it
does not prevent the weaklings from multiplying, and thus reproducing
the race-deterioration. But it would indeed be well if only those who
had gone through a term of military service were allowed to beget
children.
It is only among the peasantry, inasmuch as they really work and
do not merely look on as proprietors of the ground, that such a
deterioration of the general muscular strength could not become the
permanent variational tendency of the determinants concerned, because
among genuine peasants bodily strength is a condition of having and
supporting a family--at least on an average.
The diminution in the firmness and thickness of the bones in the
higher classes, and many another mark of civilization, must be looked
at from the point of view of panmixia and germinal selection; perhaps
also the smaller hands and feet which frequently occur along with a
more graceful general build in the higher ranks of European peoples.
It would certainly not be surprising if in families which usually
intermarry, and which in no way depend for their material subsistence
on the possession of large and powerful hands and feet or bones
generally, a downward variation of the relevant germ-determinants
should have developed, but this could never overstep a certain limit,
because it would then be prejudicial even in civilized life. That we
must be very careful not to regard large hands and feet as the direct
result of hard physical toil was brought home to me by an observation
of Strasburger's. He was particularly struck by the fact that the
peasants of the high Tatra (Carpathians) were distinguished by the
smallness of their hands and feet.
But while civilization has excited numerous downward variations in the
germ, it has, on the other hand, been the cause of numerous hereditary
improvements--variations in an upward direction. This opens up new
ground, for hitherto we have been confronted with the alternative of
either accepting the inheritance of acquired characters, and on this
basis referring the talents and mental endowments of civilized Man to
exercise continued throughout many generations, or of admitting an
increase of mental powers only in as far as they possess 'selection
value,' that is, as they may be decisive in the struggle for existence.
To these mental qualities belong cleverness and ingenuity in all
directions, courage, endurance, power of combination, inventive power,
with its roots in imagination and fertility of ideas, as well as desire
for achievement, and industry. Throughout the long history of human
civilization these mental qualities must have increased through the
struggle for existence, but how have the specific talents such as those
exhibited in music, painting, and mathematics come into existence?
And how have the moral virtues of civilized Man been evolved, and
particularly unselfishness? For it can hardly be maintained of any of
these endowments that they possess selection-value for the individual.
It is not my intention to discuss these questions in detail; they are
too many-sided and of too much importance to be treated of merely in
passing; moreover, I gave expression years ago to my views on this
subject by dealing with one example--the musical sense in Man. I do
not believe that the musical sense had its beginnings in Man, or that
it has materially increased since the days of primitive Man, but in
conjunction with the higher psychical life of civilized peoples its
expressions and applications have risen to a higher level. It is, so to
speak, an instrument which has been transmitted to us from our animal
ancestors, and on which we have learnt to play better the more our mind
has developed; it is an unintended 'accessory effect' of the extremely
fine and highly developed organs of hearing with their nerve-centres
which our animal ancestors acquired in the struggle for existence, and
which played a much more important rôle in the preservation of life in
their case than it does in ours. The musical sense may be compared to
the hand, which was developed even among the apes, but which civilized
Man in modern times no longer uses merely to perform its original
function, grasping, but also for many other purposes, such as writing
and playing the piano. And just as the hand did not originate through
the necessities of the piano, neither did the extremely delicate sense
of hearing of the higher animals develop for the sake of music, but
rather that they might recognize their enemies, friends, and prey, in
darkness and mist, in the forest, on the heath, and at great distances.
The case is probably the same with the rest of the special psychical
endowments or talents. I do not of course maintain that they, like
the musical sense, did not at some time play a rôle in the struggle
for existence and survival, and therefore could not increase, but
the increase was certainly not continuous, but much interrupted,
so that it would extend only to small groups of descendants, and
therefore could only contribute very slowly to the elevation of the
psychic capacities of a whole people. But in certain individuals
and families such augmentations would certainly take place through
germinal selection, and it seems to me probable that these would never
be wholly lost again, even if they appeared to be so, but would be
handed on, in id-minorities, through the chain of generations, and
would slightly raise the average of the talent in question, and might
even, under favourable circumstances, combine in the development of
a genius. We know how strongly hereditary such specific talents are;
let us suppose that the determinants of, say, the musical sense have,
by the intra-germinal chances of nutrition, been started on a path of
ascending variation; they will continue in this path until a halt is
called from some quarter or other. This can only happen if, in the
reducing division, or in amphimixis, the highly developed musical
determinants are wholly or partly eliminated, or are reduced to a
minority. As long as this does not happen the ascending variation will
go on, and then we may have the birth of a Mozart or of a Beethoven.
Personal selection will not interfere either in a positive or a
negative sense, since high development of the musical sense has no
effect either in advancing or retarding the struggle for existence;
the increase will therefore go on until the large majority of highly
developed musical determinants, which we must assume in the case of
a musical genius, is reduced, or even transformed into a minority,
through unfavourable reducing divisions of the germ-cells, and by
association with the germ-cells of less musical mates.
The fact that highly developed specific talents have never been known
to be inherited through more than seven generations is quite in keeping
with this view. But even this persistence has been observed only in
the case of musical talent, and the long continuance of the inherited
talent may well be due, as Francis Galton suggests in his famous
statistical investigations into the phenomena of inheritance, to the
fact that musical men do not readily choose wives who are absolutely
lacking in this talent. It would be easy to rear an exceedingly
highly gifted musical group of families within the German nation, if
we could secure that only the highly-gifted musically should unite
in marriage--that is, if personal selection could play its part. In
another more general domain of mental endowment a case of this kind
has been recorded, for Galton tells us of three highly gifted English
families which intermarried for ten generations, and in that time
scarcely produced a descendant who did not deserve to be called a
distinguished man in some direction or other.
Of course, such continued persistence, through a long series of
generations, of a high general mental level is more possible than the
transmission and increase of a specific talent, for in the former case
it is a question of a mixture of different high mental endowments,
of which not all need be developed in every individual, and yet the
individual need not fall to mediocrity if he possesses a combination
of other qualities. But in musical talent, on the other hand, the
falling from the height once attained takes place as soon as this one
character is no longer represented in a sufficiently strong majority
of determinants. Of course it would be a mistake to believe that the
talent of a Sebastian Bach or a Beethoven depended solely on the
highly developed musical sense; in them, as in all great artists, many
highly developed mental qualities must have combined with the musical
sense; a simpleton could never have written the Mass in B minor or the
Passion of St. Matthew even if he had possessed the musical genius of
Sebastian Bach. In this fact lies a further reason why genius is seldom
found at the same pitch in two successive generations; the combination
of mental characters always varies from father to son, and slight
displacements may give rise to very great differences in relation to
the manifestations of the specific talent. Under certain circumstances,
the weak development of a single trait of character, as, for instance,
power of action, or the excessive development of another, such as
indecision or desultoriness, may so nullify the existing favourable
combinations of mental characters, such as, let us say, musical sense,
inventive talent, depth of feeling, &c., that they bear no fruit
worth mentioning. And since as we have already seen, the different
mental qualities of the parents are to a certain extent separately
transmitted, that is, since they may appear in the children in the most
diverse combinations, we should rather be surprised that pronounced
talent in a specific direction can persist in a family for two and a
half centuries than that it should do so very rarely. For reducing
division is always combining the existing mental qualities anew, and
amphimixis is adding fresh ones to them.
Thus germinal selection, that is, the free, spontaneous, but definitely
directed variation of individual groups of determinants, is at the
root of those striking individual peculiarities which we call specific
talents; but it can attain to the highest level only rarely and in
isolated cases, because these talents are not favoured by personal
selection, and therefore the excessively highly developed determinants
upon which they depend may be dispersed in the course of generations;
they may sink to smaller majorities, or even to minorities, in which
case they will no longer manifest themselves in visible mental
qualities.
We deduced the process of germinal selection on the basis of the
assumption that the nutrition of all the parts and particles of
the body, therefore also of the determinants and biophors of the
germ-plasm, is subject to fluctuations. We regarded the resulting
variations of these last and smallest units of the germ-plasm as the
ultimate source of all hereditary variation, and therefore the basis of
all the transformations which the organic world has undergone in the
course of ages and is undergoing still.
We have still to inquire whether we can give any more precise account
of the nature of these units of the germ-plasm. If I mistake not,
we may say at least so much, that all variations are, in ultimate
instance, quantitative, and that they depend on the increase or
decrease of the vital particles, or their constituents, the molecules.
For this reason I have hitherto always spoken of only two directions
of variation--a plus or a minus direction from the average. What
appears to us a qualitative variation is, in reality, nothing more
than a greater or a less, a different mingling of the constituents
which make up a higher unit, an unequal increase or decrease of these
constituents, the lower units. We speak of the simple growth of a cell
when its mass increases without any alteration in its composition,
that is, when the proportion of the component parts and chemical
combinations remains unchanged; but the cell changes its _constitution_
when this proportion is disturbed, when, for instance, the red
pigment-granules which were formerly present but scarcely visible
increase so that the cell looks red. If there had previously been no
red granules present, they might have arisen through the breaking up
of certain other particles--of protoplasm, for instance, in the course
of metabolism, so that, among other substances, red granules of uric
acid or some other red stuff were produced. In this case also the
qualitative change would depend on an increase or decrease of certain
simpler molecules and atoms constituting the protoplasm-molecule. Thus,
in ultimate instance, all variations depend upon quantitative changes
of the constituents of which the varying part is composed.
It might be objected to this argument that chemistry has made us
acquainted with isomeric combinations whose qualitative differences
do not depend upon a different _number_ of the molecules composing
them, but upon their different arrangement; it might be supposed that
something similar would occur also in morphological relations. And, in
point of fact, this seems to be the case. We may, for instance, imagine
one hundred hairs as being at one time equally distributed on the back
of a beetle, and at another standing close together and forming a kind
of brush, but although this brush would be a new character of the
beetle, yet its development would depend upon quantitative differences,
namely, on the fact that the same skin-area, which in the first case
bore perhaps only one hair, had in the second case a hundred. The
quantity of hair cells has notably increased upon this small area. In
the same way the characteristic striping of the zebra depends not on
a qualitative change in the skin as a whole, but upon an increased
deposit of black pigment in particular cells of the skin, therefore
on a quantitative change. In relation to the whole animal it is a
qualitative variation, as contrasted, for instance, with the horse, but
in respect of the constituent parts which give rise to the qualitative
variation it is purely quantitative. The character of the whole edifice
is changed when the proportion of the stones of which it consists are
altered.
Thus the determinants of the germ may not only become larger or smaller
as a whole, but some kinds of the biophors of which they are made up
may increase more than others, under definite altered conditions, and
in that case the determinants themselves will vary qualitatively, so
that, from the changing numerical proportions of the different kinds of
biophors, a variation of the characters of the determinants can arise,
and consequently also qualitative variations of the organs controlled
by the determinants--the determinates. But, since nothing living can
be thought of as invariable, the biophors themselves may, on account
of nutritive fluctuations, grow unequally, and thereby vary in their
qualities. To follow this out in greater detail and attempt to guess
at the play of forces within the minutest life-complexes would at
present only be giving the rein to imagination, but in principle no
objection can be made to the assumption that every element of life down
to the very lowest and smallest can, by reason of inequalities in its
nutrition, be not only started on an ascending or descending movement
of uniform growth, but can also be caused to vary _qualitatively_,
that is, in its characters, because its component parts change their
proportions.
Of course we know nothing definite or precise with regard to the units
of the germ-plasm, and we cannot tell what is necessary in order
that a determinant shall determine a part of the developing body in
this way or in that; thus we have no definite idea of the relations
subsisting between the variations of the determinants and those of
their determinates, but we know at least so much, that hereditary
variation of a part is only possible when a corresponding particle
in the germ-plasm varies; and we may at least assume that these
correspond to each other so far, that a greater development of the one
implies a greater development of the other, and that a reversal of
these relations is impossible. If the determinant _X_ disappears from
the germ-plasm the determinate _X´_ disappears from the soma. It is
therefore justifiable to infer from the degree of development of an
organ the strength of its determinant, and to assume that plus- and
minus-variations in both are correspondingly large.
But in addition to the fluctuations in the equilibrium of the
germ-plasm which lie at the root of all hereditary variation, we have
to take into account something which we have already touched upon
briefly--the correlation of the determinants, the influencing of one
determinant by those round about it. I have spoken for the sake of
brevity of 'the determinant' of a part, although all the large and
more important parts must certainly be thought of as represented by
several or many, if not, indeed, by whole groups of determinants.
Although it is quite out of our power to follow the complex processes
of the mutual influences of the determinants upon each other, we can
say this at least, _that these influences must exist_, and we have
here a faint indication of what must occur in the case of spontaneous
variations within the germ-plasm. We must, in the first place, think
of the individual determinants as arranged in groups, so that, for
instance, the determinants of the right and left half of the body lie
together, and therefore are frequently affected together by influences
which cause variation, so that both vary in the same direction at
the same time. In point of fact, analogous deformities, such as
polydactylism of both right and left hands, and even of hands and feet
at once, do actually occur. That the right and left hands, the fore-
and hind-limbs, are represented in the germ by particular determinants,
may be inferred from their frequently different phyletic evolution into
different forms of hand and foot, e.g. into flipper and rudimentary
hind-leg in the whale, as well as from the cases of particulate
inheritance, which are rare, but which undoubtedly do occur, such
as when, in Man, there is a maternal blue eye on one side of the
head and a paternal brown eye on the other. But almost more striking
than the differences between these homologous or homotypic parts
are their points of resemblance, and these may probably be in part
referred to their disposition side by side and common history in the
germ-substance, although a far larger proportion of them are probably
due to their adaptation to similar functions, and are therefore to be
regarded as a phenomenon of convergence within the same organism.
We have already seen that the first increase in the growth of one
determinant means a withdrawal of nourishment, however slight, from its
neighbours; this can, of course, be equalized again if the claims on
the common nutritive stream from another quarter are at the same time
diminished; but it is possible that the claims from another quarter
may also be increased, and the withdrawal will then be more marked,
and the determinants being thus injured from two directions at once
will sink downwards with greater rapidity. But it is also conceivable
that the majority of determinants of a part may vary upwards, and,
by their combined increased power of assimilation, direct towards
themselves such a greatly increased stream of nourishment that the
whole organ--for instance, a particular feather in a bird--varies in an
upward direction, and becomes larger and larger, as we see in the case
of many decorative feathers; or that certain determinants vary only as
far as some of their biophors are concerned, and similarly for their
determinates, as when a group of scales on a butterfly's wing that had
previously been black turn out a brilliant blue. It can probably also
happen that such variations within the determinants are transmitted to
neighbouring determinants because the nutritive conditions which caused
the first to vary have extended to those about them. The increase of
brightly coloured spots in birds and butterflies gives us ground for
concluding that there are processes of this kind within the germ-plasm.
I will refrain from following this idea into greater detail, and
translating the observable relations and variations of the fully-formed
parts of the body into the language of the germ-plasm; but so much may
be taken as certain, that multitudinous inter-relations and influences
exist between the elements of the germ-plasm, and that one variation
brings another in its train, so that--usually at a very slow rate,
that is, in the course of generations and of species-forming, definite
variations occur from purely intra-germinal reasons--variations
which as far as they remain outside the limits of good or bad may of
themselves change the character of a species, but which when they are
seized upon by personal selection may, by sifting and combination of
the ids, be led on to still higher development.
If we consider further that the variation of a part must depend
not only on the quality of the external stimulus but also upon the
constitution, the reacting power of the part, we shall understand that
similar nutritive variations may cause two different determinants to
vary in different ways, and when we reflect that every nutritive change
must extend from the point from which it started with diminishing
strength in a particular direction, we have a further factor in the
variation of determinants and one which influences even similar
determinants differently.
Finally, if we remember that determinants of different constitution
will also extract different ingredients from the nutritive stream and
thus set up in it different kinds of chemical change, thus causing
an altered supply of nutritive substances to flow to the neighbour
determinants, we get some insight into a very complex and delicate
but perfectly definite set of processes, into a mechanism which we
can certainly only guess at, but whose results lie plainly before
us in the spontaneous variations of the organism. We understand in
principle the possibility of saltatory variation, as a more or less
widespread, more or less marked disturbance of the species-type in
this or that group of characters, and we may acknowledge that those
'kaleidoscopic variations' which Eimer supposed to be the sole basis
of the transformation of species, and which have been brought to the
foreground again quite recently by De Vries[20], are probably factors
in transmutation operative within a limited sphere.
[20] See end of chap. xxxiii.
But we must think of all these struggles and mutual influencings as
taking place on the smallest possible scale, so that it is only by
long summation that they can produce any visible effect, and we must
never forget the essential significance of the plurality of ids, for
these 'spontaneous' variations may take place in a different and quite
independent manner in each individual id. If this were not so no
intervention of personal selection would be possible, natural selection
would not exist, and the adaptation of the organism from the single
cell up to the whole would remain wholly unexplained. The whole crop of
spontaneous germ-variations, whenever it ceases to be 'indifferent,'
and becomes either 'good' or 'bad,' comes under the shears of personal
selection and under its almost sovereign sway.
On the other hand, the sudden first appearance of a saltatory variation
takes place quite independently of personal selection, depending on
similar variations in a number of ids, which remain latent until they
have by the process of reducing division which precedes amphimixis,
chanced to attain a majority. In sudden bud-variations we may perhaps
suppose that reducing division occurring in some still unverified
abnormal manner is the reason why the germinal variation suddenly makes
itself visible--a supposition previously suggested as the explanation
of the reversion of these sports.
The rarity of bud-variation is thus explained, while the greater
frequency of saltatory variations in plants propagated by seed may
be accounted for by the regular occurrence of reducing division in
sexual reproduction. But that the same or similar variations may
occur in several, it may be in many, ids at the same time must depend
upon similar general influences which affect the plant as a whole, as
happens through cultivation, manuring, and so on. I shall return to
this when discussing the influence of the environment.
In some quarters this whole conception of germinal selection has been
characterized as the merest figment of imagination, condemned on this
ground alone, that it is based on the differences in nutrition between
such extremely minute quantities of substance as the chromosomes of
nuclear substance within the germ-cell. The quantity of substance
is certainly minute, but it needs nutriment none the less, and can
we believe that the stream of nourishment for all the invisibly
minute vital elements is exactly alike? It may be admitted that the
nourishment outside the ids is usually abundant, although undoubtedly
fluctuations occur in it also, but it certainly does not follow from
this that every vital unit within the id is similarly disposed in
relation to the nutritive supply, or has food in equal quantities at
its command, or even that each has as much as it can ever need. To make
an assertion like this seems to me much the same as if an inhabitant
of the moon, looking at this earth through an excellent telescope and
clearly descrying the city of Berlin with its thronging crowds and its
railways bringing in the necessaries of life from every side, should
conclude from this abundant provision that the greatest superfluity
prevailed within the town, and that every one of its inhabitants had as
much to live upon as he could possibly require.
We certainly ought not to conclude from the fact that we cannot see
into the structure and requirements and methods of nutrition of a
very minute mass of substance that its nutrition cannot be unequal,
and that it cannot, by its inequalities, give rise to very material
differences, especially when we are dealing with a substance to which
we must attribute an extraordinarily complex organization built up of
enormous numbers of extremely minute particles. That this complexity
is undeniable is now admitted by many who formerly thought it possible
to believe in the simple structure of the germ-substance. How complex
not only the germ-substance but every cell of a higher organism is
in its structure, and how far below the limits of visibility its
differentiations and arrangements reach, is pressed upon our attention
by the most recent histological researches, such as those we owe to
Heidenhain, Boveri, and many others. The whole scientific world was
amazed when it came to know the mysterious nuclear spindle in the
seventies, and since then this has been quite thrown into the shade by
the discovery of the centrosphere, the centrosome, and more recently
even the centriole, and now we believe that these marvellous centres
of force may, or must, possess their own dividing apparatus! In the
face of discoveries like these no one is likely to be able to persist
in recognizing as existing only what is disclosed or even hinted at by
the most powerful lenses; no one can any longer doubt that far below
the limit of visibility organization is still at the basis of life,
and that it is dominated by orderly forces. To me, at least, it seems
more cogent to argue from the phenomena of heredity and variation to
an enormous mass of minute vital units crowded together in the narrow
space of the id, than to argue from the calculated size of atoms and
molecules to the number which we are justified in assuming to be
present in an id. In my book on the germ-plasm I made a calculation of
this kind, and I arrived at figures which seemed rather too small for
the requirements of the germ-plasm theory. This has been regarded as
a proof that I disregard the facts for the sake of my theory, but it
should rather be asked whether the size of the atoms and molecules is
a fact, and not rather the very questionable result of an uncertain
method of calculation. Undoubtedly modern chemistry has established
the _relative_ weight-proportions of the atoms and molecules with
admirable precision, but it can make only very uncertain statements
in regard to the _absolute_ size of the ultimate particles. It is
therefore admissible to assume that these have a still greater degree
of minuteness when the facts in another domain of science require this.
We _must_ assume determinants, and consequently the germ-plasm must
have room for these; the variations of species can only be explained
through variations of the germ-plasm, for these alone give rise to
hereditary variation. It is upon this foundation that my germinal
selection is built up; whether I have in the main reached the truth the
future will show: but that I have not exhausted this new domain, but
only opened it up, I am very well aware.
LECTURE XXVII
THE BIOGENETIC LAW
Fritz Müller's ideas--Development of the Crustaceans--Of
the Daphnidæ--Of Sacculina--Of parasitic Copepods--Larvæ
of the higher Crustaceans--Change of phyletic stages in
Ontogeny--Haeckel's _Fundamental Biogenetic Law_--Palingenesis
and Cœnogenesis--Variation of phyletic forms by interpolation in
a lengthened Ontogeny--Justification of deductions from Ontogeny
to Phylogeny--Würtemberger's series of Ammonites--Phylogeny of the
markings in the caterpillars of the Sphingidæ--Condensation of
Phylogeny in Ontogeny--Example from the Crustaceans--Disappearance
of useless parts--The variation of homologous parts, according
to Emery--Germ-plasmic correlations--Harmony with the theory of
determinants--Multiplication of the determinants in the course of the
phylogeny.
What I propose to discuss in this lecture should have been considered
at an earlier stage, if we had pledged ourselves to adhere strictly
to the historical sequence of scientific discovery, for the phenomena
which we are about to deal with attained recognition shortly after
the revival of the evolution idea, and indeed they formed the first
important discovery which was made on the basis of the Darwinian
Doctrine of Descent. I have introduced them at this stage because
they have to do with phenomena of inheritance and modifications of
these, the understanding of which--in as far as we can as yet speak
of understanding at all--is only possible on the basis of a theory
of inheritance. Therefore, in order to examine these phenomena and
their causes, it was necessary first to submit a theory of heredity,
as I have done in the germ-plasm theory. We have to treat of the
connexion between the _development_ of many-celled individuals and
the _evolution_ of the species, between germinal history and racial
history, or, as we say with Haeckel, between ontogeny and phylogeny.
Long before Darwin's day individual naturalists had observed that
certain stages in the development of the higher vertebrates, such as
birds and mammals, showed a likeness to fishes, and they had spoken
of a fish-like stage of the bird-embryo. The 'Natural Philosophers'
of the beginning of the nineteenth century, Oken, Treviranus, Meckel,
and others, had, on the basis of the transmutation theory of the time,
gone much further, and had professed to recognize in the embryonic
history of Man, for example, a repetition of the different animal
stages, from polyp and worm up to insect and mollusc. But von Baer
afterwards showed that such resemblances are never between different
types, but only between representatives of the same general type,
e.g. that of Vertebrata; and Johannes Müller maintained, from the
standpoint of the old Creation theory, that an 'expression of the
most general and simple plan of the Vertebrates' recurred in the
development of higher Vertebrates, giving as an instance that, at a
certain stage of embryogenesis, even in Man, gill-arches were laid down
and were subsequently absorbed. But why this 'plan' should have been
carried out where it was afterwards to be departed from remained quite
unintelligible.
An answer to this question only became possible with the revival of
the Theory of Descent, and the first to throw light in this direction
was Fritz Müller, who, in his work _Für Darwin_, published in 1864,
interpreted the developmental history of the individual, 'the
ontogeny,' as a shortened and simplified repetition, a recapitulation,
so to speak, of the racial history of the species, the 'phylogeny.' But
at the same time he recognized quite clearly--what indeed was plain to
all eyes--that the 'racial history' cannot be simply read out of the
'germinal history,' but that the phylogeny is often 'blurred,' on the
one hand by the fusing and shortening of its stages, since development
is always 'striking out' a more direct course from the egg to the
perfect animal, while, on the other hand, it is frequently 'falsified'
by the struggle for existence which the free-living larvæ have to
maintain.
For the establishment of these views Fritz Müller relied chiefly upon
larvæ, and in particular upon those of Crustaceans, and the facts,
which were in part new and in part interpreted in a new manner, were
so striking that it was impossible to deny their importance. In
particular, he drew attention to the fact that in several of the lower
orders of Crustaceans the most diverse species have a similar form when
they leave the egg, all of them being small, unsegmented larvæ, with
a frontal eye and a helmet-like upper lip, and with three pairs of
appendages, the two posterior pairs being two-branched swimming-legs
beset with bristles. In the size and form of the body, and especially
of the chitinous carapace, these larvæ differ in the various systematic
groups; thus, for instance, the larvæ of the Copepods are simply
oval, while those of the Cirrhipedes are produced anteriorly into two
horn-like processes, and so on, but in essentials they are all alike,
and for a long time these larval forms had been distinguished by the
special name of 'Nauplius' (Fig. 109).
The development of the perfect animal begins with the longitudinal
growth of the Nauplius; the posterior end lengthens and becomes
segmented, between the anterior portion and the tail more segments are
interpolated, and on these new pairs of limbs may grow. The number
of these segments and limbs varies according to the group to which
the animal belongs. Thus the body of the perfect animal in the little
Cyprids always consists of eight segments, seven of which bear a pair
of limbs apiece; in the Branchiopods, on the other hand, the number
of segments varies from twenty to sixty, with ten to over forty pairs
of legs; in the Daphnids or water-fleas there are about ten segments,
with seven to ten pairs of limbs, and in the Copepods about seventeen
segments with eleven pairs of limbs. The difference between the orders
depends not only upon the differences in the number of segments and
limbs, but quite as much upon the form and development of the segments,
and above all of the limbs, and in this connexion it is worthy of note
that the additional limbs which grow out usually appear at first as
biramose swimming-legs, and are subsequently modified in form. Thus
the pairs of jaws, three in number, which appear in the Copepods are
developed from such swimming-legs, and so also is the second pair of
antennæ in the Copepods and the jaws of the Branchiopods, Cirrhipedes,
&c.
[Illustration: FIG. 108. Nauplius larva of one of the lower
Crustaceans. After Fritz Müller. _Au_, the frontal eye; _I_, first pair
of limbs, corresponding to the future antennæ; _II_ and _III_, two
biramose swimming appendages.]
If then we have before us in the 'germinal history' (ontogeny) a fairly
precise repetition of the 'racial history' (phylogeny), we may deduce
from this that the primitive forms of the Crustacean race were animals
which consisted of few segments, and that from these, in the course
of the earth's history, the very diverse modern groups of Crustaceans
have arisen, by the addition of new segments, and the adaptation of
the limbs upon them, which were at first biramose swimming-legs, to
different kinds of functions, one becoming an antenna, another a jaw
or a swimming-arm, a third, fourth, fifth, and so on, a jumping-leg, a
copulatory organ, an egg-bearer, a gill-bearer, or a tail-fin.
[Illustration: FIG. 109. Metamorphosis of one of the higher Crustacea,
a Shrimp (_Peneus potimirim_), after Fritz Müller. _A_, the nauplius
larva with the three pairs of appendages: _I_, the antennæ; _II_ and
_III_, the biramose swimming-feet. _Au_, the single eye. _B_, first
Zoæa stage, with six pairs of appendages (_I-VI_). _Skn_, area where
new segments are being formed.]
That the development has in general followed those lines is made clear
chiefly by the fact that the members of all these different orders of
Crustaceans still arise from nauplius larvæ, even in those cases in
which the perfect animal possesses a structure differing widely from
the usual Crustacean form. _All_ Crustaceans arise from the _nauplius
form_, even those of the higher orders, though they may not arise from
a nauplius _larva_. But this very circumstance, that in most of the
higher and many of the lower Crustaceans, the young animal, when it
emerges from the egg, already possesses more numerous segments and
limbs than a nauplius larva, again points to the connexion between
phylogeny and ontogeny, for in these cases the nauplius stage _is gone
through within the ovum_. The whole difference between this and the
forms we considered first lies in the fact that, in the latter, the
development is greatly shortened, condensed, as we might say, so that
the nauplius stage forms a part of the _embryonic_ development, and
that new segments and limbs develop in the embryo nauplius within the
egg, so that the young animal leaves the egg in a more advanced state,
nearer to that of the perfect animal, to which it can, therefore,
attain in a shorter time.
[Illustration: FIG. 109. _C_, second Zoæa stage. The thorax is now
divided into cephalothorax (_Cph_) and abdomen (_Abd_); seven pairs of
appendages are developed, and five more (_VIII-XII_) are beginning to
appear. _Au_, paired eyes.]
We should expect that this shortening of the larval period would be
associated with a prolongation of embryogenesis, especially in those
Crustaceans which possess a large number of segments and limbs, that
is--in the higher forms--and in the main this is the case. But there
are exceptions in two directions; in the first place there are some,
even among the lower Crustaceans, which leave the egg not as a nauplius
but in the perfect form of the adult, and secondly, there are, among
the higher Crustaceans, certain species which emerge from the egg not
in the more mature form but still in the primitive nauplius form.
Fritz Müller was the first to furnish an example of this last case,
a Brazilian shrimp, _Peneus potimirim_. Like the lowest Copepods or
Branchiopods, this species, which belongs to the highest order of
Crustaceans, goes through the whole long development, from the nauplius
through a series of higher larval forms up to the perfect animal,
and all _outside of the egg_, as an independent free-swimming larva
(Fig. 109, _A-E_). This is in sharp contrast to its near relative, the
freshwater crayfish, which goes through this whole development within
the egg, and emerges perfectly formed.
[Illustration: FIG. 109. _D_, Mysis-stage. Thirteen pairs of appendages
are now formed: _I_ and _II_, antennæ; _III_, mandibles; _IV_ and
_V_, maxillæ; _VI-XIII_, swimming appendages with one branch or with
two. _Abd_, abdomen. _Sfl_, tail-fin. _E_, the fully-formed Shrimp,
with thirteen pairs of appendages on the cephalothorax (_Cph_); _I_
and _II_, the two pairs of antennæ; then follow the maxillæ and
maxillipedes (_III-VIII_), the last of which is visible in the figure,
and the five pairs of walking-legs (_IX-XIII_) of which the third bears
a long chela. On the abdomen there are now six pairs of appendages
(_XIV-XIX_).]
We see from this example that it is not some inward necessity which
thus, in the higher and more complicated organism, contracts the
ontogeny into the embryonic state, but that this depends upon external
adaptive factors. Here again we have adaptation, mainly to the
conditions of larval life. The elimination of the larvæ by enemies,
for instance, will, other things being equal, be so much the more
incisive the longer the larval development is protracted, but in that
case the general ratio of elimination of the species, and the degree
of fertility the species must possess in order to hold its own in the
struggle for existence, will also play a part in determining the mode
of development. For the higher the ratio of elimination the more eggs
the female must produce, and the more eggs that have to be produced the
smaller will be the quantity of nutritive material for the building
up of the young embryo which each egg can be furnished with. I know
of no records in regard to the eggs of that Brazilian shrimp in which
embryonic development ends with the nauplius stage, but we shall
certainly not be wrong in predicting that the eggs in this case will be
very small and very numerous, in contrast to those of the freshwater
crayfish, which are large and, as compared with others known to us, not
very numerous.
It is a point of undeniable theoretical significance which the
life-histories of these Crustaceans disclose, that embryogenesis is
not condensed according to hidden internal laws when the structure
increases in complexity, but that the condensation of the ontogenetic
stages depends upon adaptation, and may be quite different in nearly
related species. It shows us anew that all biological occurrences are
dominated by the process of selection.
I have already mentioned that exceptions to the usual mode of
development occur even among the lower Crustaceans, and I was thinking
at the time of the Daphnids, which leave the egg as fully formed little
animals, already equipped with all their segments and limbs. The
nauplius stage is passed through in the egg, and it is an interesting
indication that the ancestors of the modern species were in the way of
moulting, that this embryo nauplius moults within the egg by forming
a fine cuticle which is shed after a time. If it be asked why there
should be direct development in the case of these small and not very
complex water-fleas, while related species, the Branchiopods, which are
much richer in segments and in limbs, should emerge from the egg in
the form of a nauplius, and then pass through a longer larval period,
we may answer that the reason probably lies in the fact that, in the
former case, very few eggs are produced, sometimes only one, often two,
seldom more than a dozen, that these eggs can thus be relatively well
equipped with yolk, and that the formation of the little body which
bears only from seven to nine pairs of limbs can be easily completed
within this egg. Other things being equal, the direct development
would always be an advantage, because reproduction can begin sooner in
the young generation and the number of individuals will thus increase
more rapidly. And this is of particular importance in the case of the
water-fleas.
But if it be asked, further, why so few eggs are produced in this
case, and whether these animals have no enemies, we must answer that,
on the contrary, they are preyed upon and eaten in thousands by
fishes and other freshwater animals, but that the drawback of the
scanty production of eggs is counteracted on the one hand by their
habit of reproducing parthenogenetically for the greater part of the
year, and on the other hand by their habit of concealing the eggs in a
special brood-chamber. This is the case not only in the summer eggs,
to which nourishment is conveyed in the brood-chamber from the blood
of the mother (Fig. 70), but also in the winter or 'lasting' eggs,
which receive within the chamber a protecting covering (the shell or
ephippium).
[Illustration: FIG. 70 (repeated). Daphnella. _A_, summer ovum, with an
oil-globule (_Oe_). _B_, winter ovum.]
In almost all the Daphnids the winter egg develops into a perfect
animal just like that to which the summer egg gives rise, although
it no longer receives any nourishment after it passes into the
brood-chamber. But it receives a larger supply of yolk on this account,
so that the nutritive provision within the egg is sufficient to develop
the perfect animal. There is only one exception to this, and it is
of special theoretical interest, because it shows more plainly than
any other fact that the greater or less degree of condensation in the
ontogeny depends upon the combined effect of the external conditions of
life. The largest of the Daphnidæ, _Leptodora hyalina_, a beautifully
transparent inhabitant of our lakes, which measures about a centimetre
in length (Fig. 110), also emerges from the summer egg as a perfect
animal, but from the winter egg, which floats freely in the water and
has only a small provision of yolk, it emerges as a nauplius, which
then undergoes larval metamorphosis before it becomes a perfect animal
(Fig. 111).
[Illustration: FIG. 110. The largest of the Daphnids (_Leptodora
hyalina_), with summer ova (_Ei_) beneath the shell (_Sch_). _I-IX_,
the appendages. _II_, the oars (second antennæ) which always remain
biramose in Daphnids. _sb_, setæ. _ov_, ovaries. _Schl_, œsophagus.
_Ma_, stomach. _a_, anus. _H_, heart. _Au_, eye. _nG_, natural size.]
Fritz Müller concluded from the repetition of the nauplius form in all
orders of Crustaceans that the primitive form of the Crustacean must
have been a nauplius, and that from it all the modern Crustaceans must
have evolved phyletically by the addition of segments varying in number
and differentiation. Now, however, it is doubted whether there ever
were nauplioid types capable of reproduction. But even if the nauplii
only _represent what have been the larval_ forms from very early times,
they are equally important in illustrating the relations between
ontogeny and phylogeny; they at any rate represent the primitive
pre-cambrian larval form from which all modern Crustaceans are derived.
This is borne out not only by the facts to which we have already
referred, but also by those Crustacean-groups which have diverged far
from the usual Crustacean habit and type.
[Illustration: FIG. 111. Nauplius larva from the winter egg of
_Leptodora hyalina_; after Sars.]
Thus the sessile Cirrhipedes, with their mollusc-like shells, their
soft, unsegmented bodies, degenerate heads, and their twelve vibratile
food-wafting limbs, emerge from the egg as nauplius larvæ. But the
remarkable parasites on the shore-crabs and the hermit-crab deviate
much further from the type of the rest of the Crustaceans, for they
hang like a sac or formless sausage-like soft mass to the abdomen of
their host, growing into it by fine, pale, root-like threads, through
which they suck up the blood of their hosts (Fig. 112, _C. Sacc._).
They possess neither head, nor thorax, nor abdomen, not even an
indication of segmentation, no limbs of any kind, neither antennæ, nor
mouth parts, nor swimming-legs. Nevertheless they are Crustaceans;
indeed, we can say with certainty that they belong to the order of
Cirrhipedes, for they leave the egg in the form of a nauplius larva
(_A_), with 'horns' on their carapace which no other forms except
themselves and the Cirrhipedes possess. That they are of the same
stock as these is also proved by their further development, for the
nauplius grows first, just as in the case of the Cirrhipedes proper,
into a 'Cypris-like larva' (_B_), so called because it bears a certain
resemblance to the Ostracods of the genus Cypris, and only from this
point do their paths of development diverge. The Cypris-like larva of
the true Cirrhipedes settles down somewhere, attached by its antennæ;
it grows, and its body becomes that of the perfect Cirrhipede; but the
Cypris-like larva of the Sacculinæ bores its way into the inside of a
crab or hermit-crab, at the same time losing its limbs, segmentation,
and its chitinous covering; and within the body of its host it is
transformed into the sac-like organism we have already described. After
a time it emerges again on the surface, and remains attached to the
abdomen of its host (Fig. 112, _C. Sacc._), drawing its nourishment
from the blood which it sucks up by means of its numerous delicate
roots (_W_, _W_).
[Illustration: FIG. 112. Development of the parasitic Crustacean
_Sacculina carcini_, after Delage. _A_, Nauplius stage. _Au_, eye.
_I_, _II_, _III_, the three pairs of appendages. _B_, Cypris-stage.
_VI_-_XI_, the swimming appendages. _C_, mature animal (_Sacc_),
attached to its host, the shore-crab (_Carcinus mænus_), with a
feltwork of fine root-processes enveloping the crab's viscera.
_s_, stalk. _Sacc_, body of the parasite. _oe_, aperture of the
brood-cavity. _Abd_, abdomen of the crab with the anus (_a_).]
From all this we may conclude that certain Cirrhipedes in times long
past adopted a parasitic habit in the Cypris-larva stage, and that they
gradually underwent adaptations to this mode of life, and that these
went further and further, until the animal was transformed into the
singular creature which we now see in the sexually mature form.
The same is the case with the numerous fish-parasites of the order
Copepoda. They all leave the egg as nauplius larvæ, however greatly
they may be modified later on by adaptation to a parasitic habit,
and in them we can still observe, in the fully developed animals,
a whole series of grades of transformation. Thus many genera, like
_Ergasilus_, are distinguished from the free-swimming Copepods only
by the modification of their jaws into piercing and sucking organs,
and of a single pair of antennæ into hooks, by means of which they
attach themselves to the fish on which they feed. In other genera the
degeneration and modification go further; the antennæ, the eye, and
the appendages degenerate more or less, and very remarkable attaching
organs are sometimes developed, in the form of hooks or of knobbed
pincers, or of actual suckers. In several types the degeneration and
modification go so far that the segmentation of the body disappears,
and the animal looks more like an intestinal worm than like a
Crustacean (_Lernæocera_ and others). In all these forms adapted to a
parasitic mode of life it is always only the mature animal which has
been transformed in this manner, for previously it has gone through a
series of stages which are quite similar to those of the free-swimming
Copepods, beginning with the nauplius, and ending with the so-called
Cyclops stage, that is, a larval form which possesses antennæ, eyes,
and swimming-legs similar to our freshwater Copepods of the genus
_Cyclops_.
Here again we see in the ontogeny the repetition of a series of
phyletic stages before the mature form is assumed. Why these stages
should have persisted it is easy enough to understand, for how could an
animal which emerged from the egg as a worm-shaped _Lernæocera_ find a
fresh fish which would serve it as host? Yet these parasites could not
possibly go on preying upon the same fish generation after generation.
To secure the existence of the species it was therefore indispensable
that the faculty of swimming should be retained at least in the young
stages; in other words, that the free-swimming ancestral stages should
be preserved in the ontogeny. In all these cases it is therefore beyond
doubt that the germinal history recapitulates a series of stages
comparable to those of the racial history, although not quite unchanged
but adapted to the modern conditions of life, for instance in having
shorter antennæ, smaller eyes, and with four instead of the usual five
swimming-legs. The search for a host does not seem to last long, for
fishes are usually found in large numbers together, and thus the young
parasitic Crustacean does not require to make a long journey before it
finds a refuge.
It is noteworthy that the males of parasitic Crustaceans are not
only much smaller than the females (Fig. 113), but that they are
also much less modified, and resemble the ancestral free-swimming
Copepods to a much greater degree. They usually possess small but
well-developed swimming-legs, and by means of these they seek out the
female, dying after fertilization is accomplished. They are thus not
sessile parasites at all, and have therefore to go through the stages
of the free-swimming Copepods much more completely than the females,
whose task is to accumulate within themselves from the blood of the
fish as much material as possible for the forming of the eggs, and to
produce the largest possible number of these. These therefore greatly
surpass the free-swimming Copepods in fertility, as is evidenced by the
enormous egg-sacs they bear at the posterior end of the body (Fig. 113,
_ei_).
Even among the higher Crustaceans, the so-called Malacostraca, the
germinal history not infrequently exhibits more or less of the racial
history in distinct recapitulation.
[Illustration: FIG. 113. The two sexes of the parasitic Crustacean
_Chondracanthus gibbosus_, enlarged about six times; after Claus.
The main figure is that of the female, whose body bears quaint blunt
processes. At its genital aperture (♂) a dwarf male is situated.
_F_ and _F´_, the two pairs of appendages. _ei_, the long egg-sacs,
portions of which have been cut off in the figure.]
It is true however, as we have already shown, that there are only a
few of the higher Crustaceans which emerge from the egg in the form
of a nauplius; in most of them this stage has been shunted backwards
in the ontogeny, and most of the crabs and hermit-crabs leave the egg
in a higher larval form, that of the so-called Zoæa (Fig. 114). This
term is applied to a larva which already exhibits two main divisions
of the body, a head and thorax portion (cephalothorax, _Cph_) and
an abdomen (_abd_). The cephalothorax is frequently equipped with
remarkable long spines (_st_), and it always bears from five to eight
pairs of limbs, anteriorly the antennæ (_I_ and _II_), then the
mandibles (_III_), further back swimming-legs (_IV_, _V_), and behind
these can be recognized the primordia of the other legs (_VI_-_XIII_),
which will grow freely out later on. Large facetted and stalked eyes
(_Au_) are borne on the head. This Zoæa form is not now found as a
mature Crustacean form, so we cannot maintain with any confidence
that it lived as a mature animal at an earlier period of the earth's
history, but a second still more complex larval form of the higher
Crustaceans is preserved for us in a group of marine Crustaceans, the
Schizopods. These are Crustaceans which, though small, approach in
external appearance our freshwater crayfish, only they have, instead
of the ten walking-legs, biramose swimming-legs, by means of which
they move freely in the water. The number of these branched legs is
even greater than ten, there are sixteen of them (Fig. 109 _D_, p.
164, _VI_-_XIII_). In the aquaria of the Zoological Station at Naples
one may often see these dainty little creatures swimming about in
large companies. Here they are of interest to us chiefly because their
structure occurs in the ontogeny of the highest Crustaceans, the
Decapods; that is, the phyletic stage represented by the Schizopods
appears as an ontogenetic stage, just before the final metamorphosis of
the larva to the perfect animal. This is the case in most of the marine
Decapods, in those forms which do not go through the whole course of
their development within the egg, but emerge as Zoæa larvæ, or even,
as in _Peneus potimirim_, as nauplii. In the last-named species (Fig.
109) the ontogeny contains at least three stages which must have
lived, perhaps not as mature forms, but as primitive larval forms, for
unthinkable ages--the stage of the nauplus (Fig. 109 _A_), that of the
Zoæa (Fig. 109 _B_ and _C_), and that of the Schizopod (Fig. 109, _D_);
from this last the fully developed Decapod Crustacean arises (Fig. 109,
_E_).
We are, therefore, justified in saying that here the racial evolution
is recapitulated in the individual development, although condensed
and shortened in proportion as more numerous stages of the phyletic
development are gone through within the egg, for there the different
stages can succeed each other more rapidly and directly than in a
metamorphosis of the free-swimming larvæ, since these must procure
their own material for their further growth and their metamorphosis,
while the yolk of the egg supplies a store of material which is
sufficient for the production of a whole series of successive stages.
[Illustration: FIG. 114. Zoæa-larva of a Crab, after R. Hertwig.
_I_-_V_, the already functional anterior appendages--antennæ,
mandibles, and swimming-legs. _VI_-_XIII_, rudiments of the posterior
appendages of the cephalothorax (_Cph_). _Abd_, the abdomen. _st_,
spine of the carapace. _Au_, eye. _H_, heart.]
For this reason it inevitably resulted that the sharply defined
characters of the phyletic stages were more and more lost as soon as
they were transferred from larval stages to stages in embryogenesis.
For, in the first place, these sharply defined characters, such as
the spines of the Zoæa larva, or the swimming bristles of the 'oars,'
or the shape of thorax or abdomen characteristic of certain species,
are adapted to a free life, and would be valueless in an embryonic
stage; and secondly, in the transference of the free larval stages
to embryonic development the greatest possible condensation and
abbreviation of the stages must have been striven for, which could only
come about by a continual mutual adaptation of the embryonic parts to
one another, involving the suppression of everything superfluous.
Otherwise the transference of the free stages to the embryogenesis
would have brought no advantage, but rather a most prejudicial
protracting of the development.
We must not, therefore, expect to find the stages of the phylogeny
occurring unaltered in every ontogeny in the way we have found the
nauplius, Zoæa, or Mysis stages in the larval development of the
Decapods. I have noticed already that in the water-fleas (Daphnidæ) and
other Crustaceans without metamorphosis the nauplius stage is still
passed through, but within the egg, and as an embryonic stage, and
this is quite true, but nevertheless it would hardly do to liberate a
nauplius like this from its shell and place it in the water, for the
influence of the water upon the delicate embryonic cells of its body
would soon cause it to swell, and would destroy it utterly. And, even
apart from this, it has no hard and resistant chitinous covering, no
fully-developed appendages, but only the stump-like blunt beginnings
of these without swimming-bristles and without muscles capable of
function, so that it could not even move. Nevertheless it is a nauplius
with all its typical distinctive characters, only it is not a perfect
nauplius capable of life, but rather a 'schema' of one, which must be
retained in the embryogenesis that it may give rise to the later stages.
Shall we therefore say that the statement that phylogeny repeats itself
in ontogeny is false, that the nauplius stage within the embryo is
not a true nauplius at all? That would be pushing precision beyond
reasonable limits, and would obscure our insight into the causal
connexion between phylogeny and ontogeny, which, as we have seen,
undoubtedly exists.
A few years after the appearance of Fritz Müller's work _Für
Darwin_, Haeckel elaborated Müller's idea, and applied it in a much
more comprehensive manner. He formulated it under the name of 'the
fundamental biogenetic law,' and then he used this 'law' to deduce
from the ontogeny of animals, and more particularly of Man, the
paths of evolution along which our modern species have passed in the
course of the earth's history. In doing so the greatest caution was
necessary, since ontogeny is not an actual unaltered recapitulation
of the phylogeny, but an 'abridged' and in most cases--in my own
belief, in all cases--_a greatly modified recapitulation_. Therefore
we cannot simply accept each ontogenetic stage as an ancestral stage,
but must take into consideration all the facts supplied to us by other
departments of biological inquiry which afford help in the decision
of such questions, especially those brought to light by comparative
morphology and by the whole range of comparative embryology.
Haeckel was quite well aware of this difficulty, and repeatedly
emphasized it by laying stress on the fact that a 'blurring' of the
phyletic stages of development had arisen through the abridgement of
the phylogeny in the ontogeny, and a 'falsification' of it through
the secondary adaptation of individual ontogenetic stages to new
conditions of life. He therefore distinguished between 'Palingenesis,'
that is, simple though abridged repetition of the ancestral history,
and 'Cœnogenesis,' that is, modification of the racial history by
later adaptation of a few or many stages to new conditions of life.
As an example of cœnogenetic modification, I may cite the pupæ of
butterflies. Since these can neither feed nor move from one spot, they
can at no time have been mature forms, and cannot, therefore, represent
independent ancestors of our modern Lepidoptera; they have originated
through the constantly increasing difference between the structure of
the caterpillar and that of the moth or butterfly. Originally, that is,
among the oldest flying insects, the mature animal could be gradually
prepared within the larva as it grew, so that finally nothing was
necessary but a single moult to set free the wings, which had in the
meantime been growing underneath the skin, and to allow the perfect
insect to emerge, complete in all its parts. This is the case even now
with the grasshoppers and crickets. In these forms the larval mode of
life differs very little, if at all, from that of the perfect insect,
and the main difference between the two is the absence of wings in the
larva. But when the perfect insect adapted itself to conditions of life
quite different from the larval conditions, as was the case with the
nectar-sucking bees and butterflies adapted entirely for flight, while
the larvæ were still adapted exclusively to an abundant diet of leaves
and other parts of plants, and to a very inactive life upon plants, the
two stages of development ultimately diverged so widely in structure
that the transition from one to the other could no longer be made at
a single moulting, and a period of rest had to be interpolated, in
order that the transformation of the body could take place. In this
way arose the stage of the resting and fasting pupa, a 'cœnogenetic'
modification of the last larval stage, _not a recapitulation of an
ancestral form_, but a stage which has been interpolated, or better,
has 'interpolated itself' into the ontogeny on account of the widely
different adaptations of the early and the final stages.
This is a perfectly clear idea, and Haeckel's distinction between
palingenesis and cœnogenesis is undoubtedly justified.
But it is quite a different matter to be able to decide whether
a particular stage or organ has arisen palingenetically or
cœnogenetically with the same certainty as in the case of the
insect-pupa, or even with any degree of probability, and we must admit
that in very many cases, perhaps even in most cases, it is impossible.
This is so chiefly because pure palingenesis is hardly likely to occur
now; the ancestral stages were bound to be modified in any case if
they were to be compressed into the ever-shortening ontogeny of later
descendants, and particularly so if they were to be shunted back into
embryogenesis. In the latter case they would not only be materially
shortened, and, as I have already shown, modified by the mutual
adaptations of the different developing parts, but time-displacements
of embryonic parts and organs would be necessary, as has been very
clearly proved by the excellent recent investigations, which we owe
in particular to Oppel, Mehnert, and Keibel. A shunting forward or
backward of the individual organs takes place--conditioned apparently
by the decreasing or increasing importance of the organ in the finished
state; for in the course of the phylogeny everything may vary, and
not only may a new, somewhat modified, and often more complex stage
be added on at the end of the ontogeny, but each one of the preceding
stages may vary independently, whenever this is required by a change
in its relations to the other stages or organs. Adaptation is effected
at every stage and for every part by the process of selection, for all
parts of the same rank are ceaselessly struggling with one another,
from the lowest vital units, the biophors, up to the highest, the
persons. If we reflect that, in the course of the phylogeny of every
series of species, a number of organs always become superfluous and
begin to disappear in consequence, we can understand what great changes
must take place gradually as such a series of phyletic stages is
compressed into the ontogeny, for all organs which are no longer used
are gradually shifted further and further back in the ontogeny till
ultimately they disappear from it altogether. But, while the primary
constituents of these 'vestiges' play their part in ontogeny for a
shorter and shorter time, new acquisitions are being more and more
highly developed, and thus, in the course of the phylogeny, numerous
time-displacements of the parts and organs in ontogeny must result, so
that ultimately it is impossible to compare a particular stage in the
embryogenesis of a species with a particular ancestral form. _Only the
stages of individual organs can be thus compared and parallelized._
But we must not on that account 'empty out the child with the bath,'
and conclude that there is no such thing as a 'biogenetic law' or
recapitulation of the phylogeny in the ontogeny. Not only is there
such a recapitulation, but--as F. Müller and Haeckel have already
said--ontogeny is nothing but a recapitulation of the phylogeny,
only with innumerable subtractions and interpolations, additions
and displacements of the organ-stages both in time and place. It
would be a great mistake to conclude from the fact of these manifold
alterations that the whole proposition of the recapitulation of the
phylogeny in the ontogeny is erroneous, or at least valueless. If its
only use were to enable us to read the racial history of a species out
of its germinal history, it is intelligible enough that we might be
led to give it up in despair, but I think that the main thing is to
get some insight into the history of the ontogeny, and there can be
no doubt that this can have been built up on no other foundation than
upon the racial history. What is new could only have arisen from what
was already in existence, and everything in ontogeny, not only the
palingenetic stages which still represent in some measure the facies of
fully-formed ancestral stages, but also the cœnogenetic stages, like
the pupa-stage we have already discussed, have arisen historically,
nothing _de novo_, but all in connexion with what was already present.
But what was first present was in all cases the stages of the ancestral
forms.
It is undoubtedly of the greatest value to be able to penetrate more
and more deeply into embryonic development, and to discover more
precisely the changes that have taken place throughout its course in
the originally existing material of ancestral forms. But it must not
be forgotten that, all transformations notwithstanding, so much of
the racial history is still very plainly indicated in the germinal
history, that this must always remain for us a most important source
from which to draw conclusions in regard to the phyletic development of
any animal group. I admit that these conclusions have sometimes been
drawn with too great confidence, but even if we cannot regard as well
founded Haeckel's view that in the ontogeny of Man there are fourteen
different ancestral stages recognizable, a protist stage, a gastræa
stage, a prochordate, an acranial, a cyclostome, a fish-stage, and so
on, we must recognize that the unicellular stage of ontogeny, with
which even now the development of every human being begins, undoubtedly
repeats the facies of an ancestor, although greatly altered; for we
must be descended from unicellular organisms. The essential part of
this ancestral stage is thus preserved in the ontogeny, and only what
is special and in some measure due to chance, that is, to adaptation to
special conditions of existence, has been modified.
It has been supposed that the proposition that phylogeny is
recapitulated in the ontogeny is disproved, because the ontogenetic
stage must always contain within it the primordia of the later stages
which have been added since the corresponding phylogenetic stage. It
is certain that the egg-cell or the sperm-cell of Man contains, though
in a form not recognizable by us, all the determinants of the perfect
human body, but this neither affects its nature as a cell nor its
particular form as ovum or spermatozoon. It is essentials that are
important in this comparison, not accessories. Neither can I agree
with Hensen's argument when he says that the 'recapitulation-idea' is
erroneous, because the actual course of ontogeny is the 'best and only
possible one,' which, apart from previous history altogether, must of
necessity be followed. Certainly the actual course is the best, and
under the given circumstances the only possible one, but that does not
exclude recapitulation, on the contrary it implies it, for ontogeny
could at no time have arisen from a _tabula rasa_, but only from what
was historically existent.
I do not propose to examine each of Haeckel's ancestral stages in
Man's pedigree, or to estimate the degree of probability with which
they may be deduced from the ontogeny; but that Man's ancestry does,
in a general way, include such a series of phyletic stages may be
admitted, even if we grant that many of these stages are now no longer
represented in the ontogeny as stages of the developing organism as
a whole, but only by stages of individual organs or group of organs.
Thus it may be disputed whether there is still a fish-stage in
human development, but it cannot be disputed that the rudiments of
'gill-arches' and 'gill-clefts,' which are peculiar to one stage of
human ontogeny, give us every ground for concluding that we possessed
fish-like ancestors.
As we now know that the history of a given mode of embryogenesis
has involved numerous time-displacements of the organ-rudiments, we
must attach all the more weight to the developmental history of the
individual parts and characters, in which the phylogeny can often be
read more clearly than in the stages of the organism as a whole, and we
can probably find out important laws in this way.
As far back as 1873 Würtemberger investigated the fossil ammonites with
special reference to this point. He was concerned even more at that
time with finding proofs of the theory of descent in general, and this
was the first case in which any one succeeded in demonstrating phyletic
transformation-series of species, deposited one above the other in a
corresponding series of geological strata, and connected by transition
forms lying between these. In studying this interesting material, of
which many examples were at his disposal, Würtemberger proved that the
variations which had taken place in the spirally coiled shell in the
course of ages appeared first on the last whorl, and then subsequently
extended to the one before this, and thence to the still younger whorls
of the shell. Meanwhile the last whorl not infrequently exhibited
another new character. Thus, for instance, protuberances on the shell
were shifted in the course of the phylogeny from the last convolution
to the second last, and later to the third last, and so on, while at
the same time the last convolution showed the protuberance changed into
spines. In other words, the new phyletic acquirements first appeared in
the mature animal (in the last-formed whorl or chamber of the shell),
but were subsequently shifted back in the ontogeny to younger stages in
proportion as new transformations of the mature animal appeared. Thus
there was, so to speak, a retraction of the phyletic acquisitions of
the mature animal deeper and deeper into the germinal history of the
species.
About the same time--in the seventies--I obtained similar results
from living species when I was attempting to work out the ontogeny
of the markings on the external skin of the caterpillars of certain
butterflies, and I should like to submit a short account of these.
In one of the early lectures we discussed the protective and defensive
colours of caterpillars in general, and those of caterpillars of the
Sphingidæ in particular. I showed that those naked caterpillars which
live on plants among the grass, or on the grass itself, are often
not only green, like fresh grass-stalks, or yellowish-grey, like dry
ones, but all the larger forms also exhibit light, usually white,
longitudinal lines, which, by mimicking the sharp light reflections on
the grass-stems, heighten the protective resemblance.
We also spoke of the light transverse stripes, often marked with pink
or lilac-blue, of many of the large green caterpillars which live
on trees and bushes, and whose likeness to the leaves is heightened
by this imitation of the lateral veining of a leaf; and finally we
mentioned the warning coloration indicative of unpleasant or nauseous
taste, among which must be classed not only vivid contrasts of colour,
but also specially conspicuous elements of colour such as light
ring-spots upon a dark ground. These different colour schemes which
protect the caterpillars from their enemies are usually only to be
found in the adolescent caterpillar, not in the very small one which
has just emerged from the egg, and the development of the markings in
the individual life clearly shows that the phylogeny of the markings is
more or less obviously contained in the ontogeny.
There are three different schemes of marking which occur in the
caterpillars of hawk-moths or Sphingidæ--longitudinal striping,
obliquely transverse striping, and spots. Longitudinal striping pure
and unmixed is now found only in a few species, for instance in the
caterpillar of the _Macroglossa stellatarum_ (Fig. 115), in which a
white longitudinal line, beginning at the tip of the tail, runs up each
side of the body to the head as a 'sub-dorsal stripe' (_sbd_). These,
with other two similar stripes, effectively secure the fairly large
caterpillar from discovery when it is among grass and herbs.
[Illustration: FIG. 115. Caterpillar of the Humming bird Hawk-moth,
_Macroglossa stellatarum_. _sbd_, the sub-dorsal line.]
Transverse striping occurs as the sole mode of marking in species
which live on bushes and trees whose leaves have strong lateral veins,
such as willows, poplars, oaks, privet, syringa, and so on, and these
markings associated with the leaf-green of their colouring protect them
most effectively from discovery.
The third scheme of marking, namely by spots, occurs in various forms
in species of the genera _Deilephila_ and _Chærocampa_, and it varies
in its biological significance; in many species the spots serve as a
warning colour, by making the caterpillar conspicuous and easily seen
from a distance (_Deilephila galii_, Fig. 117); in others they imitate
the eyes of a larger animal, and have a 'terrifying' effect, as we have
already said (Fig. 4); in still other and rarer cases they heighten the
resemblance of the caterpillar to its food-plant by mimicking parts of
it, as, for instance, the red berries of the buckthorn (_Deilephila
hippophaës_, Fig. 8, _r_).
[Illustration: FIG. 3 (repeated). Full-grown caterpillar of the Eyed
Hawk-moth, _Smerinthus ocellatus_. _sb_, the sub-dorsal stripe.]
Thus all three modes of marking possess a biological value, and
protect the soft and easily wounded animal in some way, and, in the
case of at least two of them, it is clear that they must have arisen
at the very end of the caterpillar's development, since they can only
be effective as the animal is approaching full size, and would be
valueless in the very young caterpillar. The transverse striping only
makes the caterpillar like a leaf when the stripes bear about the same
relation to each other as those on the leaf, and eye-spots can only
scare away lizards and birds when they are of a certain size. Only
longitudinal striping is effective as a protection in the case of young
caterpillars, supposing, that is, that they live in or on the grass
(Fig. 116).
[Illustration: FIG. 4 (repeated). Full-grown caterpillar of the
Elephant Hawk-moth, _Chærocampa elpenor_, in its 'terrifying attitude.']
[Illustration: FIG. 8 (repeated). Caterpillars of the Buckthorn
Hawk-moth, _Deilephila hippophaës_. _A_, Stage III. _B_, Stage V. _r_,
annular spots.]
Let us consider the ontogeny of these different forms of markings,
beginning with the eye-spots. It appears that these develop from a
sub-dorsal stripe, which appears in the young caterpillar in the
second stage of its life, and from it, in the course of the further
development, two pairs of large eye-spots are formed. Even in the
young caterpillar, scarcely one centimetre in length (Fig. 116), it
can be observed that the fine, white sub-dorsal line takes a slight
curve upwards on the fourth and fifth segments (_C_), and on the
lower edge of these curves a black line is laid down (_D_). This is
then continued to the upper side (_E_), and encloses the piece of the
sub-dorsal stripe (_F_ and _G_), and thus there arises a white-centred,
black-framed spot which only requires to grow and to differentiate a
blackish shadow-centre, the pupil (_G_), to give the impression of a
large eye. This occurs as the caterpillar goes on growing, and after
the fourth moult or ecdysis the eyes have already some effect, as the
animal is six centimetres in length, but they become even more perfect
in the fifth and last stage. During this development of the eye-spot
the sub-dorsal stripe disappears completely from the greater part of
the caterpillar, persisting only on the first three segments (Fig. 116,
_B-F_).
[Illustration: FIG. 116. Development of the eye-spots in the
caterpillar of the Elephant Hawk-moth (_Chærocampa elpenor_). _A_,
Stage I, still without marking, simply green. _B_, Stage II, with
sub-dorsal stripe (_sbd_). _C_, sub-dorsal line somewhat later, with
the first hint of the eye-spot (_Au_) on segments 4 and 5. _D_,
eye-spots in Stage III of the caterpillar, somewhat further developed
than in _E_, the third stage. _F_, Stage IV. _G_, the anterior eye-spot
at the same stage.]
When we consider that this stripe in the little caterpillar a
centimetre long, which lives on the large leaves of the vine, or on
the obliquely ribbed willow-herb (_Epilobium hirsutum_), is quite
without protective value, its occurrence at that stage can only be
regarded as a phyletic reminiscence due to the fact that the ancestors
of these species of _Chærocampa_ possessed longitudinal stripes in the
adult state, probably because at that time they lived on plants among
the grass, and that, later, when the species changed their habitat to
plants with broad leaves which had arisen in the meantime, eye-spots
were developed in addition to the green or brown protective colouring
which they retained. Thus the modern development of these spots mirrors
their phyletic evolution very faithfully; on the two segments there
were formed, from pieces of the sub-dorsal line, first white spots
ringed round with black, then unmistakeable eyes with pupils (_C_, _D_,
_G_). This transformation can only have begun in the fairly well-grown
caterpillar, because it was only of any use to it; but later on it was
shunted further back in the ontogeny, from the sixth and fifth to the
fourth and third caterpillar stage, not in its complete development,
but in more and more incipient form; and nowadays the first traces of
eyes, as we have already seen, are visible in the course of the second
stage. The marking of the more remote ancestors, the longitudinal
striping, is now lost in proportion as the eye-spots develop, perhaps
because the former would take away from the full effect of the latter.
The longitudinal stripes are still quite plainly visible on the first
three segments, but these segments are drawn in and are scarcely
noticeable when the caterpillar assumes a defiant attitude (Fig. 4).
In the case of marking with ring-spots, which is found especially in
species of the genus _Deilephila_, the ontogeny discloses that it has
developed phyletically from the sub-dorsal stripe; in the young stage
of this caterpillar also, the sole marking is longitudinal striping;
in _Deilephila zygophylli_, from the steppes of Southern Russia,
this persists apparently through all the stages, but in the others
it disappears almost completely in the later stages, but only on the
segments on which the spot-marking has developed from it. This happens
in a manner similar to that in which the eye-spot in _Chærocampa_
arises, a piece of the white sub-dorsal stripe is enclosed above and
below by a semicircle of black, and later these semicircles unite, and
cut off the portion of the sub-dorsal line, and form a black spot with
a light centre within which a red spot frequently appears (Fig. 117,
_A_).
[Illustration: FIG. 117. Caterpillar of the Bed-straw Hawk-moth
(_Deilephila galii_). _A_, Stage IV, sub-dorsal stripe still distinct,
the annular spots are still incompletely enclosed in it. _B_,
fully-formed caterpillar without trace of a sub-dorsal stripe, but with
ten annular spots.]
In most species these ring-spots occur on many segments (10-12) (Fig.
117, _B_), and in cases where they are of importance in making the
caterpillar conspicuous and easily seen they sometimes form a double
row. But we know one species, _Deilephila hippophaës_, in which only
a single ring-spot exists, and it is a large brick-red spot on the
second last segment, mimicking the red berry of the buckthorn (Fig.
8, _A_ and _B_, _r_). But individuals also occur in which there are,
on the five or six segments in front, smaller ring-spots which become
less distinct the further forward they are, and in most caterpillars
it is possible, on careful examination, to recognize little red dots
on the faded sub-dorsal stripes of these segments (Fig. 8, _B_). We
might be disposed to think, on this account, that the ancestors of
_D. hippophaës_ bore rings on all the segments, and that these had
gradually become vestigial on the majority of them, because they had
lost their earlier biological importance, and now, by adaptation to the
buckthorn, could only be of use on the second last. But when we take
the ontogeny also into account we find in the young caterpillar only a
simple sub-dorsal line, upon which, in the third stage, the red spot of
the tail-horn segment appears (Fig. 8, _A_).
No spots ever occur on the other segments at this stage; they only
appear in the last stage, but as they may be entirely wanting, they
must have arisen as the result of internal laws of correlation, that
is, they must be recapitulations of the hindmost spots which arose
in the phylogeny through natural selection. We may conclude this, at
least, if we believe in the truth of the fundamental proposition of the
biogenetic law, and admit that there is in the ontogeny some more or
less distinct recapitulation of the phylogeny.
[Illustration: FIG. 118. Two stages in the life-history of the Spurge
Hawk-moth (_Deilephila euphorbiæ_). _A_, first stage, the caterpillar
dark blackish-green, without marking. _B_, second stage, the row of
spots is distinctly connected by a light streak, the vestige of the
sub-dorsal stripe.]
This proposition may be recognized as true in the case of _Deilephila_
also, if we compare the different species with one another as regards
their ontogeny. We find here too that not only the sub-dorsal, that is,
the phyletically oldest marking of the Sphingid caterpillars, occurs
everywhere in the young stages, but also that it is being shunted back
to younger and younger stages, in proportion to the degree of the
development of the spot-marking reached in the full-grown caterpillar.
Thus, for instance, in the caterpillar of _Deilephila euphorbiæ_ the
highest form of spot-marking is reached, and in this species the
sub-dorsal line is no longer the sole marking element at any stage.
Leaving out of the question the absolutely unmarked little caterpillar
which emerges from the egg (Fig. 118, _A_), there appears at once in
the second stage a series of ring-spots connected by a fine white
sub-dorsal line (Fig. 118, _B_). In the following stage, the third,
this sub-dorsal line disappears without leaving a trace, and there
remains only the spot-marking, which is subsequently duplicated.
Let us compare with this the ontogeny of the bed-straw hawk-moth,
_Deilephila galii_ (Fig. 117). The full-grown caterpillar possesses
only a single row of ring-spots (_B_), and accordingly the young stages
of the caterpillar up to the fourth show a distinct sub-dorsal line
(_A_), although spots are seen upon it. A still earlier phyletic stage
of development is illustrated by _Deilephila livornica_, in which the
ring-spots are all connected by the sub-dorsal line.
It can thus hardly be doubted that the biogenetic law is guiding us
aright when we conclude from a comparison of the ontogeny of the
different species of _Deilephila_, that the oldest ancestors of the
genus possessed only the longitudinal stripes, and that from these
small pieces were cut off as ring-spots, and that these were gradually
perfected and ultimately duplicated, while at the same time the
original marking, the longitudinal stripe, was shunted back further and
further in the young stages, until it finally disappeared altogether.
Let us now refer for a moment to the third form of marking in the
caterpillars of the Sphingidæ--transverse striping. This has not arisen
out of the sub-dorsal line, but quite independently and at a later
date. This is proved with great certainty by the ontogeny of species
of the genus _Smerinthus_. The full-grown, and usually also the young
caterpillars, of these species have quite regularly the seven broad
oblique stripes which run in the direction of the tail-horn at equal
intervals on the lateral surfaces of the body (Fig. 3). They are
absent only from the three anterior segments, and upon these a part
of the older marking, the sub-dorsal stripe, has persisted. But we
find this fully developed in the youngest stages of other species. In
_Smerinthus populi_, the little caterpillar, which has no markings at
all when it leaves the egg, very soon shows the white sub-dorsal line,
and simultaneously with it the seven transverse stripes, which cut
obliquely through it; in the older caterpillars the sub-dorsal then
disappears (Fig. 119).
When I was investigating these matters at the beginning of the
seventies I did not succeed in procuring eggs of the species of the
genus _Sphinx_, which likewise almost all exhibit the oblique striping
in their full-grown stages. But from what I knew of the ontogeny of
_Smerinthus_ species I was able to predict that, among the young
stages of _Sphinx_, there must be some with sub-dorsal lines. This
was confirmed later, for Poulton found in _Sphinx convolvuli_ that in
the first stage there are no oblique stripes, but only the sub-dorsal
stripe, while in _Sphinx ligustri_ both kinds of marking were present
at the same time.
[Illustration: FIG. 119. Caterpillar of _Smerinthus populi_, the Poplar
Hawk-moth, at the end of the first stage, showing both the complete
sub-dorsal stripe and the oblique stripes.]
From all these facts, which I have summarized as briefly as possible,
we see that the older phyletic characters are gradually crowded by the
newer into ever-younger stages in the ontogeny, until ultimately they
disappear altogether. We have now to ask to what this phenomenon is
due; is it a simple crowding out of the old and less advantageous by
the new and better characters as a result of natural selection, or is
there some other factor at work? It is clear in regard to these forms
of marking that they can have been developed at first only in the
almost full-grown larva by natural selection, because they are of use
only there, and that, at the same time, the old marking must have been
set aside through the influence of the same factor, in as far as it
prejudiced the effect of the new adaptation. This seems to be indicated
by the persistence of the sub-dorsal line on those segments which are
drawn in when _Chærocampa_ assumes a terrifying attitude, or which do
not bear oblique stripes in the leaf-like caterpillars, e.g. the three
anterior segments in the species of _Sphinx_ and _Smerinthus_. When
newly acquired schemes of marking like the eye-spots of _Chærocampa_
are transmitted from the last stage to the stage before, this can be
explained by following the same train of thought, for the caterpillar
is already of sufficient size to be able to inspire terror with its
eyes; but in still younger stages the spots would not be likely to
have that effect, and yet they occur in quite small animals (20 mm.).
More obvious still is the uselessness of the oblique striping in the
young stages of the _Sphinx_ and _Smerinthus_ caterpillars, for in the
earliest stages of life the caterpillars are much too small to look
like a leaf, and the oblique stripes stand much closer together than
the lateral ribs of any leaf. Moreover, the little green caterpillars
require no further protection when they sit on the under side of a
leaf; they might then very easily be mistaken _in toto_ for a leaf-rib.
_Thus it is certainly not natural selection which effects the shunting
back of the new characters._ Nor can this be caused by the fact that
the new character can only be developed gradually and in several
stages, for the oblique striping at any rate arises in the ontogeny all
at once. There must therefore be some mechanical factor in development
to which is due the fact that characters acquired in the later stages
are gradually transferred to the younger stages. But this shifting
backwards can be checked by the agency of natural selection as soon as
it becomes disadvantageous for the stage concerned.
It is in this way that I explain the fact that the majority of the
caterpillars of the Sphingidæ are absolutely without markings when
they emerge from the egg. Thus, for instance, the caterpillars of
_Chærocampa_ (Fig. 116, _A_), of _Macroglossa_ (Fig. 115), and of
_Deilephila_ (Fig. 118, _A_), as well as those of the _Smerinthus_
species, are at first without stripe or mark of any kind; they are
of a pale green colour, almost transparent, and very difficult to
recognize when they sit upon a leaf. How very greatly the different
stages _can be_ independently adapted to the different conditions of
their life, when that is necessary for the preservation of the species,
is shown in the most striking manner by many species. Thus the little
green caterpillar of _Aglia tau_, when it leaves the egg, bears five
remarkable reddish rod-like thorns, which in form and colour resemble
the bud-scales of the young beech-buds among which they live, and which
disappear later on; the full-grown caterpillar shows nothing of these,
but is leaf-green, marked with oblique stripes. Even if the use of
these reddish thorns be other than I have indicated, we have in any
case to deal with a special adaptation of _one_, and that the first
caterpillar-stage, and what can happen at this stage is possible also
at every other. Nor is it only animals which undergo metamorphosis that
can exhibit independent phyletic variation at every stage, but those
also with direct development, and indeed, in the case of these, we may
assume adaptation of this kind at almost every stage in the history
of the organs, as we have already seen, because the great abridgement
of the phylogeny into the ontogeny necessitates a very precise
mutual adaptation of the organ-rudiments and of the diverse rates of
development.
We have thus been led by the facts discussed--and numerous others
from other groups in the animal kingdom might be ranked along with
them--to two main propositions, which express the relation of phylogeny
to ontogeny. The first and fundamental proposition is the one already
formulated. The ontogeny arises from the phylogeny by a condensation of
its stages, which may be varied, shortened, thrown out, or compressed
by the interpolation of new stages. The second proposition refers to
individual parts, and may run as follows: As each stage can undergo
new adaptations by itself, so can every part, every organ; such new
adaptations very often show a tendency to be transferred to the
immediately antecedent stage in ontogeny.
It is not my intention to formulate the laws of ontogeny just now,
otherwise many others might be added to these, such as that of the
regular transference of characters acquired at one end of a segmented
animal to the other segments: I must confine myself here to bringing
the two main propositions into harmony with the principles of our
theory of heredity.
How phylogeny is condensed in ontogeny can be understood readily
enough in a general way, although we cannot profess to have any
insight into the detailed processes. The continuity of the germ-plasm
brings about inheritance, in that it is continually handing over to
the germ-plasm of the next generation the determinant-complex of the
preceding one. Every new adaptation at any stage whatever depends
on the variation of particular determinants within the germ-plasm,
and this in its turn depends on germinal selection, that is, on the
struggle of the different determinant-variants among themselves, and
on the variation in a definite direction which arises from this, as
we have already shown. A new kind of determinant can never arise of
itself, but always only from already existing determinants, and through
variation of these. But as spontaneous variation never causes all the
homologous determinants of a germ-plasm to vary in quite the same way,
but only a majority of them, there always remains a minority of the
old determinants, which may, under certain circumstances, predominate
again, as is proved by the aberrations in _Vanessa_ species due to
cold, and by many other kinds of reversion.
But it is not this variation which leads to the prolongation of
ontogeny, and the repetition of the phyletic stages within it. In this
case it is rather that a new character takes the place of an old one,
not that it is added to it. A black spot may arise instead of a red
one, but not first a black spot and then a red one. Of course we still
know far too little in regard to the intimate succession of events in
the stages of ontogeny to be able to say definitely that, in such
apparently simple transformations, the older stage does not, in every
ontogeny, precede the more recent one as a preparation for it, though
it may be only for a brief and transient period.
It is certain, however, that variations such as the addition of a new
stage in ontogeny are undergone, and that this implies the occurrence
of something really quite new. Therefore such a new stage can arise
only from the germ-plasm, by the duplication, and in part variation, of
the determinants of the preceding stage. If, for instance, the body of
a Crustacean be lengthened by a segment, this must be due to a process
of this kind, and in such a case it is intelligible enough that the new
segment can be formed in the ontogeny only after the development of the
older preceding one, for its determinants come from that, and are from
the beginning so arranged that they are only liberated to activity by
the formation of the preceding segment.
Now, if in the course of the phylogeny numerous new segments were
added to the body of the Crustacean, the ontogeny would be materially
prolonged, and condensation would become necessary in the interests of
species-preservation. To bring this condensation about, whole series of
segments which were added successively in the phylogeny succeeded each
other with gradually increasing rapidity in the ontogeny, until finally
they appeared _simultaneously_: the determinants of the segments _n_,
_n_ + 1, _n_ + 2, ... _n_ + _x_ varied in regard to their liberating
stimuli, and were roused to activity no longer successively, but
simultaneously, in the cell complexes controlled by them. We have thus
recapitulation, but with abridgement and compression, of the phyletic
stages in the ontogeny. Thus in the nauplius of _Leptodora_ we see the
rudiments of five of the pairs of legs of the subsequent thorax (Fig.
111, _IV-VIII_), and in the Zoæa larva the rudiments of six thoracic
legs may be seen behind the already developed swimming-leg (Fig. 114,
_VI-XIII_).
But in the course of the phylogeny a segment may also become
superfluous, and we know that it then degenerates and is ultimately
eliminated altogether. Thus in a parasitic Isopod, which lives
within other Crustaceans, a segment of the thorax is wanting in the
relatively well-developed larva, and in the Caprellidæ among the
Amphipod Crustaceans the whole abdomen of from six to seven segments
has degenerated to a narrow, rudimentary structure. In such cases
the gradual degeneration of the relative determinants has preceded
step for step the degeneration of the part itself, and when this is
complete the ontogeny shows nothing of what was previously present,
and so we may speak of a 'falsification' of the phylogeny. But that
the complete disappearance of the determinants only comes about with
extreme slowness, so that whole geological periods are sometimes not
enough for its accomplishment, we have already learnt from our study
of rudimentary organs, instances of which can be demonstrated in every
higher animal, bearing witness to the presence of the relevant organs
or structures in the ancestors of the species.
We can infer with certainty, from the observational data at our
disposal, that the disappearance of useless parts is regulated by
definite laws; but it is too soon to attempt to formulate these laws,
or even to trace them back to their mechanical causes. As we have
already said, a much more comprehensive collection of facts, and
above all one which has been made on a definite plan, is a necessary
preliminary condition to this. But so much at least we may gather
from the facts before us, that the degeneration of an organ begins
at the final stage, and is transferred gradually backwards into the
embryogenesis. Thus the two fingers of birds which have disappeared
since Cretaceous times are still indicated in every bird-embryo,
though they subsequently degenerate. In various mammals 'pre-lacteal
tooth-germs' have been demonstrated in the jaws of embryos, which show
us that not only did ancestors exist whose dentition was the modern
'milk-teeth,' but that still more remote ancestors possessed another
set of teeth, which was crowded out by the 'milk-teeth'; thus the teeth
of the ancestors of the modern right whale (_Balæna mysticetus_) are
only represented in the embryo of to-day in the form of dental pits.
And, as we saw already, the Os centrale so characteristic of the wrist
of lower vertebrates only appears in Man at a very early embryonic
stage, and disappears again as such in the further course of the
embryogenesis.
We may perhaps give a preliminary statement of this law as follows: It
is impossible that any part or organ should be removed suddenly from
the ontogeny without bringing the whole into disorder, and the least
serious disturbance of the course of development will undoubtedly be
caused if the final stage of the part in question become rudimentary
first. Only after this has happened, and the neighbouring parts have
adapted themselves to the disappearance, can this extend to the stages
immediately preceding it, so that these too degenerate, and allow
the surrounding parts to adapt themselves. The further back into the
ontogeny the disappearance extends the greater will be the number of
other structures affected in some way or other by the degeneration, and
these must not all be brought suddenly into new conditions, else the
whole course of development would suffer. Thus at first only those
determinants may disappear--and can disappear according to the laws
of germinal selection--which control the final form of the useless
organ, then those just preceding them, which controlled, let us say,
its size, and thus more and more of the previously active determinants
disappear, and hand in hand with this disappearance there is variation
of all the parts correlated with the dwindling condition of the organ,
so that their own development and that of the animal as a whole suffers
no injury. If it were otherwise, if when a part became useless its
collective determinants were all to disappear at the same time, the
whole ontogeny would totter, in fact it would be much as if a man who
wished to remove the breadth of a window from a house standing on
pillars were to begin by taking away the foundation pillar.
It is, of course, to be understood that these processes go on so
exceedingly slowly that personal selection takes a share in them, at
least at the beginning. Later on, the further degeneration of a useless
organ or rudiment has no effect on the individual's power of life, and
therefore depends solely upon the struggle of the parts within the
germ-plasm (germinal selection).
If we could see the determinants, and recognize directly their
arrangement in the germ-plasm and their importance in ontogeny, we
should doubtless understand many of the phenomena of ontogeny and their
relation to phylogeny which must otherwise remain a riddle, or demand
accessory hypotheses for their interpretation. Several years ago Emery
rightly pointed out that the phenomena of the variation of homologous
parts might be inferred by reasoning from the germ-plasm theory. If
one hand has six fingers instead of five, it not infrequently happens
that the other also exhibits a superfluity of fingers, and sometimes
the foot does so too. The phyletic modification of the limbs in the
Ungulates has taken place with striking uniformity in the fore and
hind extremities; no animal has ever been one-hoofed in front and
two-hoofed behind. Although I might suggest that this primarily depends
on adaptation to different conditions of the ground, and that the
Artiodactyls were evolved in relation to the soft marshy soil of the
forest, and the Perissodactyls for the steppes, it cannot be denied
that germinal conditions may have co-operated in bringing about this
uniformity of the direction of variation, especially as the whole
structure of the fore- and hind-limbs exhibits such marked similarity.
Emery is inclined to refer this to 'germ-plasmic correlations,' and
we have assumed from the very first that the different determinants
and groups of determinants do indeed stand in definite and close
relations to one another. But it seems to me premature to say anything
more precise and definite than that in the meantime. I should like,
however, to say that determinants or groups of determinants which
had in old ancestral germ-plasms to give rise to a series of quite
similar structures by multiplication during the ontogeny, and therefore
only needed to be present _singly_ in the germ-plasm, would, in
later descendants, have to shift their multiplication back into the
germ-plasm itself, if necessity required that the homologous parts
which they controlled should become _different_ from each other.
Then the previously single group of determinants in the germ-plasm
would have to become multiple. But as new determinants can only arise
from those which already exist, these new ones must have had their
place beside the old, and would therefore probably be exposed to
any intra-germinal causes of variation in common with them--that is
to say, they will tend to vary even later in a similar manner. For
instance, we might think of the segments of primitive Annelids, which
in form and contents are for the most part alike, as arising from one
germ-rudiment, from which, when, in the higher Annelids, the various
regions of the body had to take a different form, several primary
constituents of the germ-plasm separated themselves off; and in a
similar way the much higher and more complex differentiation of the
somatic segments in the Crustaceans must have been brought about. Thus
we understand how the determinant groups of the germ-plasm multiplied
according to the need for increasing differentiation, but remained in
intimate relation, which exposed them in some measure to a common fate,
that is, to common modifying influences, and in many cases determined
them to similar variation.
But we cannot see directly into the germ-plasm, and are therefore
thrown back on the inductions we can make from the facts presented
to us by the phenomena of visible living organisms. As yet the
material for such inductions is scanty, because it has been got
together haphazard, and not collected on a definite plan. I therefore
refrain for the present from attempting any further elaboration of
my germ-plasm theory. It is only when an abundance of observation
material, collected according to a definite plan, lies at our disposal
that anything more in regard to the intimate structure of the
germ-plasm, or the mutual influences and relations of its determinants
and its modification in the course of phylogeny can be deduced with
any certainty. Meanwhile, we must content ourselves with having,
through the hypothesis of determinants, made intelligible at least
the one fundamental fact, how it is possible that in the course of
the phylogeny single parts and single stages can be thrown out or
interpolated, or even only caused to vary, without giving rise to
variation in all the rest of the parts and stages of the animal.
A theory of epigenesis cannot do this, for, if no representative
particles were contained in the germ-plasm, then every variation of
it would affect the whole course of development and every part of the
organism, and variations of individual parts arising from the germ
would be impossible.
LECTURE XXVIII
THE GENERAL SIGNIFICANCE OF AMPHIMIXIS
Twofold import of amphimixis--It conditions the continual changing
of individuality--Analogy from game of cards--The germ-plasm is at
once variable and persistent--The two roots of individual variation:
germinal selection and new combinations of the ids--'Harmonious'
adaptation conditions amphimixis--Difference between adaptation
and mere variation--Is a 'direct' use of amphimixis to be insisted
upon?--Ceaseless intervention of personal selection in the lineage of
the germ-plasm--Far-reaching effects of personal selection--Fixing
of the arrangements for amphimixis in the course of generations
of species--Increase of the constancy of a character with its
duration--Characters in the same species variable in different
degrees--The upper and under surfaces of Kallima--Wild plants brought
under cultivation do not at first vary--Amphimixis very ancient,
therefore very firmly established--Does amphimixis bring about
equalization (Hatschek, Haycraft, Quetelet)?--Galton's frequency
curves--Ammon's free scope for variations--De Vries' asymetrical
curves of frequency.
We have already made ourselves familiar with the process which in
unicellular organisms is called conjugation and in multicellular
organisms fertilization, and we have seen that its most obvious
significance lay in the fact that through it the germ-plasms of two
individuals are united. Since, according to our view, this germ-plasm
or idioplasm is the bearer of the hereditary tendencies of the
organism concerned, the mingling or amphimixis of two germ-plasms
brings together the hereditary tendencies of two individuals, and the
organism whose development is derived from this mingled germ-plasm
must therefore exhibit traits of both parents, and must to a certain
extent be made up of the traits of both. This is one result attained by
amphimixis.
But we went further than this, and saw that there is a second result
implied in amphimixis, namely, that the individual character of
the germ-plasm is being continually altered by new combinations of
the ids contained in it. We inferred from what I believe to be the
demonstrated hypothesis that the germ-plasm is composed of ids, that
its reduction to half the original mass must mean a reduction of the
ids to half the number, and as the ids contain primary constituents
which are individually different, this must effect a new arrangement,
a new mingling of these individual peculiarities. The reduction of the
germ-plasm to half, that is, the diminution of the number of its ids
to half, is a phenomenon generally associated with amphimixis, and
has been established in the case of all animals which have hitherto
been investigated, and of all the most carefully studied plants,
and finally, it has been shown to be very probable in unicellular
organisms, for the processes of conjugation in Infusorians and many
other Protozoa include phenomena very similar to those of reducing
division in the higher animals. The prediction made on theoretical
grounds has here been verified by observation, and it is obvious that
the assumption of ids, that is, of units in the germ-plasm which
are handed on from one generation to the succeeding one, involves a
reduction of their number in each amphimixis. Without this the number
of ids would be doubled at each amphimixis, and would therefore
gradually amount to something enormous. We see therefore why this
normally recurrent reduction of ids before each amphimixis was
established in the course of evolution, and we see that it inevitably
involves that a new combination of ids should be associated with each
amphimixis.
If nothing persists unless it be purposeful, that is, necessary, what
is the meaning of the fact that arrangements for amphimixis occur over
almost the whole known domain of life, from the very simple organisms
up to the highest, in unicellular and multicellular organisms, in
plants and animals alike? Why is it that this arrangement has been
departed from only in a few small groups of forms, while it occurs
everywhere else, in almost every generation, so indissolubly associated
with reproduction that it has even been regarded--with a lack of
clearness--as itself a form of reproduction, and is even now generally
called 'sexual reproduction'? And why is it that in many organisms,
especially lowly ones, it is not associated with every reproduction,
though it recurs at regular or irregular intervals? Such a universal
arrangement must undoubtedly be of fundamental importance, and we
have to ask wherein this importance lies. That is the problem to the
solution of which we must now apply ourselves.
So much we may say at once: The significance of amphimixis cannot be
that of making multiplication possible, for multiplication may be
effected without amphimixis in the most diverse ways--by division of
the organism into two or more, by budding, and even by the production
of unicellular germs. Even though these last are usually in various
ways so organized that they must undergo amphimixis before they can
develop into new organisms, yet there are numerous germ-cells which
are not subject to this condition (e.g. spores), and there are--as we
have seen--many germ-cells, adapted for amphimixis, which always, or in
certain generations, or even only occasionally, emancipate themselves
from this condition under certain external influences: I refer to
egg-cells which develop parthenogenetically.
If amphimixis is not a universal preliminary condition of reproduction,
wherein lies the necessity for its general occurrence among living
organisms?
We have already learned that there are two results produced without
exception by amphimixis; one of these is the antecedent reduction
of the original number of ids by one half, and the consequent new
combination of ids which results from this; the other is the union
of two such halved germ-plasms from two different individuals. The
first we may, with Hartog, compare to the removal of half of a pack
of cards previously mixed, the second to the combination of two
such halves from different packs. The first process brings nothing
new into the complex of primary constituents, but rather removes a
part--larger or smaller--of its characters: not necessarily exactly
half of these, since each individual kind of id may be represented by
doubles or multiples. But the reduction simplifies the composition
of the germ-plasm, and might by itself, through the struggle of the
ids in ontogeny, lead to a resultant different from the parent, that
is, to a new individuality. Through the second process, however, new
individual traits are of necessity added, and make the resultants still
more markedly diverse, that is, if the ids of both parents attain to
expression in the struggle of ontogeny, and this, as we have already
seen, is usually the case, though not always and certainly not always
in all parts. Thus amphimixis, together with the preparatory reduction
of the ids, secures the constant recurrence of individual peculiarities
through the ceaseless new combinations of individual characters already
existing in the species.
When sixteen years ago I first inquired into the actual and ultimate
significance of sexual reproduction, I thought I had found it in
this ceaseless production of new individualities. This seemed to me
a sufficient reason for the introduction of amphimixis into nature,
since the difference between individuals is the basis of the process
of selection, and thus the basis of all the transformations of
organisms, which we may refer to natural or sexual selection. Now
these differences of selection-value are--as I believed then, and do
still--not only by far the most frequent organic changes, but also the
most important, since they not only initiate, but control new lines of
evolution. Therefore I still regard amphimixis as the means by which a
continual new combination of variations is effected a process without
which the evolution of this world of organisms so endlessly diverse in
form and so inconceivably complex, could not have taken place.
But I do not regard this amphimixis as the real root of variation
itself, for that must depend not on a mere exchange of ids, but rather
upon a variation of the ids. The ids of a worm of the primitive
world could not without variation now make up the germ-plasm of an
elephant, even if it be true that mammals are descended from worms.
The ids must have been meanwhile transformed times without number by
the modification, degeneration, and new formation of determinants.
Amphimixis, that is, the union of two germ-plasms, does not of itself
cause variation of the determinants, it only arranges the ids (the
ancestral plasms) in ever-new combinations. If the origin of variation
were limited to that alone, a transmutation of species and genera
would only be possible on a very limited scale; there could at most be
a narrow circle of variations, just as in the example already given
of the packs of cards; even if the taking away and mixing up of the
halves were repeated a thousand times, a definite though undoubtedly
large number of card-combinations would in the long run recur again and
again. But the case is different with the germ-plasm and amphimixis,
where there is an infinitely more varied series of results, because
the individual cards--the ids--are variable, even between one time of
sifting and shuffling and another, and therefore infinitely productive
of variety in the course of numerous repetitions of the shuffling.
I have been frequently and persistently credited with maintaining that
the germ-plasm is invariable--a misunderstanding of my position, due
perhaps to a somewhat too brief and terse statement which I made at an
earlier period (1886). I had described the germ-plasm as 'a substance
of great power of persistence,' and as varying with difficulty and
slowly, basing this statement upon the age-long persistence of many
species in which the specific constitution of the germ-plasm must have
remained unchanged. The idea of 'germinal selection,' of a ceaseless
struggle between the 'primary constituents' of the germ, and of
the resulting continual slight and invisible rising and falling of
individual characters, had not yet dawned upon me, nor had I at that
time formulated the conception of 'determinants.' I was even doubtful
at that time whether development, heredity, and variation were not
interpretable on the assumption of an undifferentiated substance
without primary constituents. But at no time was I unaware that the
whole phyletic evolution of the organic world is only conceivable
on the assumption of continual variation of the germ-plasm, that it
actually depends upon this, even if these variations come about with
exceeding slowness, and are thus in a certain sense 'difficult.'
Now that I understand these processes more clearly, my opinion is
that the roots of all heritable variation lie in the germ-plasm,
and furthermore, that the determinants are continually oscillating
hither and thither in response to very minute nutritive changes, and
are readily compelled to _variation in a definite direction_, which
may ultimately lead to considerable variations in the structure of
the species, if they are favoured by personal selection, or at least
if they are not suppressed by it as prejudicial. But selection is
continually keeping watch over both kinds of variation, and if the
conditions of life do not further the variation or do not even allow it
to persist, selection eliminates everything that lessens the purity of
the specific type, everything that transgresses the limits of utility,
or that might endanger the existence of the species. Thus we understand
how the germ-plasm may be variable, and yet at the same time remain
unvaried for thousands of years, how it is ready and able to furnish
any variation that is possible in a species if that is required by
external circumstances, and yet is able to preserve the characters of
the species in almost absolute constancy through whole geological ages;
in short, how it can be at once readily variable and yet slow to vary.
The importance which amphimixis thus has in connexion with the
adaptation of organisms lies, if I mistake not, in the necessity
for co-adaptation, that is, in the fact that in almost all
adaptations it is not merely a question of the variation of a single
determinant, but of the correlated variations of many--often very
numerous--determinants, of 'harmonious adaptation,' as we have already
said. Many-sided adaptation of this kind seems to me impossible without
a continually recurrent sifting and recombining of the germ-plasms, and
this can only be effected by amphimixis.
It may be objected that, apart from amphimixis, variation can be
brought about in many parts of an organism, as in purely asexual
reproduction. A plant, for instance, may vary when it is transferred
to a strange soil or climate; and even in that case the variations
seem to be harmonious, at least the harmony of the parts is so far
maintained that the plant continues to flourish, at any rate under
cultivation. A plant species may be incited by abundant nourishment
to gigantic growth, and caused to vary in many of its parts, and the
abundant food may even directly affect the germ-plasm so that all or
some of these variations may become hereditary; and yet this is far
from being a case of adaptation, it is merely a case of simultaneous
variations, and it is questionable whether they will make the continued
existence of the plant under the new conditions possible or not. It
might easily happen, for instance, that the plant, though it became
larger and bore more abundant blossoms, would be sterile, and therefore
unfitted for continued existence in a natural state. Variations are not
necessarily adaptations; the latter can never come about solely through
direct influence upon the germ-plasm. What direct influence upon the
germ-plasm could, for instance, make the hind-legs of a mammal long and
strong and the fore-legs short and weak? Obviously neither an increase
nor decrease in the food-supply, nor a higher or lower temperature--in
short, no direct influence, because all these affect the germ-plasm as
a whole, and therefore cannot possibly influence two homologous groups
of determinants in opposite directions.
This, it seems to me, is only possible when amphimixis brings about
in one individual a favourable coincidence of the chance germinal
variations of the determinants of the fore- and hind-limbs; and just
as it is with the two variations in this simple hypothetical case,
so it will be in the actual processes of adaptation where there are
involved numerous--we know not how numerous--variations essential to a
'harmonious adaptation.'
It need not be objected that the very number of variations necessary to
a 'harmonious adaptation' makes its occurrence impracticable; for it
is _the complete_ harmony of the parts that makes the adaptation, and
without this the individual was only imperfectly adapted, and therefore
incapable of survival. It is certainly not mathematically demonstrable
that this is the case, but as the whole process of transformation which
makes an old adaptation into a new one begins with minimal fluctuations
of the determinants, which must first be brought by germinal selection
to the level of selection-value, and must then be subject to personal
selection, so the whole process goes on so gradually and by such
small steps that the harmony of the parts is maintained by functional
adaptation during the individual life in a great number of individuals.
But these are just the individuals which survive in the struggle for
existence, and at the same time possess at every stage of the process
_the best combination of favourably varying determinants_. As these
favourable variations are, in consequence of germinal selection, not
mere isolated variations of fluctuating importance, but variations in
a _definite direction_, the whole process of variation must persist
in every single part in the direction imposed upon it by personal
selection. But since at every reducing division the ids of the
germ-cells are brought down to half their number, a possibility is
offered for gradually removing the unfavourable ids from the germ-plasm
of the species, since the descendants resulting from the most
unfavourable id-combinations always perish, and so from generation to
generation the germ-plasm gets rid of its unfavourably varying ids, and
the most propitious combinations afforded by amphimixis are preserved,
till ultimately there remain only those combinations which are varying
appropriately, or at least only those in which the appropriately
varying determinants are in the majority, and so have controlling
influence.
Logically this deduction is undoubtedly indisputable, from the
standpoint of the germ-plasm theory; but whether it may be regarded as
a sufficient reason for the introduction of amphimixis, and for its
extremely tenacious persistence throughout the course of the long and
intricate phylogeny, cannot be maintained without special investigation.
Against my position the objection has often been urged that an
arrangement cannot arise or be maintained through natural selection
unless it is _of direct use_ to the individual in which it occurs.
Sexual reproduction cannot therefore have been established simply
because it advances, or even because it makes possible the adaptations
of species, for these adaptations only came about occasionally, perhaps
once in a thousand generations or even less frequently; thus the
intervening generations could derive no advantage of any kind from the
arrangement in question, and therefore, according to the law of the
degeneration of unused characters, it must have long since been lost. I
mentioned this objection before, but was obliged to postpone a detailed
consideration of it until we had discussed germinal selection.
We admit, of course, that characters are only preserved intact as
long as they are of advantage sufficient to turn the scale in favour
of their possessors, and that they begin to fall from their height of
perfection when that is no longer the case; we admit also that new
adaptations are not continually necessary, but are so only at intervals
of long series of generations, and yet the objection cited seems to me
baseless.
Leaving out of account, for the moment, the first introduction of
amphimixis, let us deal with it as an existing occurrence, for the
tenacious persistence of which we wish to find reasons.
Is it really the case that amphimixis is only of importance in
connexion with the new adaptation of a species, and that it has nothing
to do with the persistence of the species in the state of adaptation
already attained? According to the conception of the processes within
the germ-plasm which we have already stated, it is impossible that this
should be the case, for continual slight fluctuations are occurring in
the determinants in consequence of the fluctuations of the nutritive
stream, and these slight variations, plus or minus, do not in many
cases equalize one another or counteract one another by turning again
in a contrary direction; they go on increasing in the direction in
which they have begun. It is only when personal selection opposes them
that they come to a standstill, and this can only happen when they
attain to selection-value, that is to say, when they reach a level at
which they become disadvantageous in the struggle of persons. But as
germinal variations of this kind are continually occurring, personal
selection must keep continual watch over them, and eradicate them as
soon as they have attained selection-value.
Therefore, when a species is most perfectly adapted to its conditions,
it would of necessity begin to degenerate if personal selection were
not continually guarding it, and setting aside everything that is in
excess or deficient as soon as it begins to be prejudicial. But the
adaptation of a species does not depend upon _one_ character persisting
at its normal level, but on the persistence of very many, and many of
these vary simultaneously upwards or downwards, and reach the limit of
selection-value at one time or another. If there were no amphimixis,
then either all individuals with any excessive variant would be at
once eliminated, or the species would go on deteriorating until this
excessive variant was so numerously and strongly represented in all
its individuals that it would perish through degeneration. But even
in the first of these cases the species would drift towards the fate
of extinction, because excessive variations do occur even in every
asexual generation, and would appear in an increasingly large number
of determinants if there were no possibility of rejecting them and
eliminating them from the lineage of the species.
This is made possible through the periodic intervention of amphimixis;
it is actually effected thereby; and in this way alone the species
is kept at its high-water mark of adaptation. It is not necessary
to assume that every single determinant which is varying in an
unfavourable direction is at once eliminated as soon as it becomes
prejudicial, that is, reaches negative selection-value, or--to make
use of an expression introduced by Ammon--as soon as it oversteps the
boundaries of the 'playground of variations,' the limits within which
variations are neither favourable nor unfavourable. But _in the course
of generations_ they are unfailingly eliminated, especially when a
large number of unfavourably varying determinants are coincident in
the germ-plasm. Then the individuals which arise from a germ-plasm
thus composed must perish in the struggle for existence, and thus the
id-combinations with excessive determinants are eliminated from the
germinal constitution of the species. As this is repeated as often as
excesses of the ids occur, the species is kept pure.
It might be objected that, through such a continual weeding-out of
rebellious determinants, the germ-plasm would become so constant in
its constitution that it would ultimately be secure from all such
aberrations of it on the part of its determinants, and therefore would
in time become quite incapable of diverging from its proper path at
all, and would thus no longer require this continual correction through
amphimixis.
I do not wish to contradict this conclusion; indeed, I believe that the
constitution of the species becomes more and more constant in the way
I have indicated, and that an ever more perfect and stable equilibrium
of the whole determinant system is thus brought about. It follows that
in the course of generations the diverse determinants of the germ-plasm
will vary within a progressively shortened radius, and will thus more
and more rarely overstep the limits of the 'variation-playground'--and
yet I still believe that this justifiable conclusion tells in favour of
my interpretation of the utility of the persistence of amphigony once
introduced.
Let it be remarked, in the first place, that it is by no means
essential to the preservation of a useful institution that it should
practically justify its utility in _every_ generation. Although, for
instance, the warm winter coat of a species of mammal may be necessary
to its survival, it does not disappear at once when a winter happens
to occur which is so warm that even individuals with poor pellage can
survive. Indeed, several such mild winters might occur in succession,
in which there was no weeding-out of the individuals with poor fur, and
yet the thickness of the winter fur of the species would not become
less fixed, just because this character no longer varies perceptibly
in an old-established species which has long been perfectly adapted,
and it could only be brought into a state of marked fluctuation very
slowly through direct influence on the germ-plasm, or through panmixia.
But exactly the same thing is true in regard to the determinants of the
reproductive cells, in respect of their adaptation to amphimixis, only
very much more emphatically.
Before going further, I should like to show that the conclusion
we have just deduced from the theory, namely, that the equilibrium
of the determinant system of a species increases in stability with
the duration of its persistence, holds good not only for the whole
system, but for its individual parts, that is, for the individual
characters and adaptations. Experience teaches that characters are the
more exactly and constantly transmitted the older they are; generic
characters are more constant than species-characters, order-characters
more persistent than family-characters--this is implied even in their
name. But we are able to show even in relation to the characters of
a species that those which have been fixed for a long time are most
precisely and purely transmitted; that is, that their determinants are
least inclined to overstep the limits of the 'variation-playground'
either in an upward or downward direction.
Two groups of facts prove this: first the observed fact that the very
different degree of variability which the different species exhibit is
by no means common to all the characters of the species in the same
measure; for individual characters may be variable or constant in very
different degrees.
Many years ago[21] I drew attention to the fact that the different
stages in the life-history of insects, especially of Lepidoptera,
might be variable in quite different degrees. Thus, for instance,
the caterpillar might be very variable, and yet the butterfly which
arises from it might be extremely constant. I concluded from this--what
probably no one now will dispute--that the various stages may vary
phyletically independently of one another, that, for instance, the
caterpillar may adapt itself to a new manner of life, a new food-plant,
a new means of defence, while the butterfly, unaffected by this, goes
on quietly as it was before. Every new adaptation necessarily implies
variability, and so the stage which is in process of transformation
must have its period of variability, which gradually returns again to
greater constancy, and this the more completely the longer the series
of generations through which the weeding out of the less well-adapted
has endured.
[21] _Studien zur Descendenztheorie_, Leipzig, 1876.
But it is not only the individual stages of development that may be
unequally variable; the same is true of the characters of a species
which occur simultaneously. The most striking example of this known to
me is the leaf-butterfly, which I have already mentioned many times
in the course of these lectures--the Indian _Kallima paralecta_.
In this species the brown and red upper surface is almost alike in
colour and marking in all individuals, but the under surface, the
colour and marking of which is so deceptively mimetic of a leaf, is
variable to such a degree that it is difficult, among a large number
of specimens, to find even a few which are as like one another as are
the members of species in which the under side is constant. It need
not be urged that this is due to the complexity of the marking on the
under side. In many of our indigenous butterflies the under side is
just as complex in coloration and marking, and nevertheless it is very
constant, being almost identical in all individuals, as for instance
in _Vanessa cardui_. In _Kallima_ the great variability of the under
surface certainly depends not merely on the fact that the mimetic
character has been only recently acquired (phyletically speaking), but
chiefly on the fact that the dead leaves to which they approximate are
themselves very diverse in appearance, for many are dry, others moist
and covered with mould, and that the adaptations have therefore gone in
different directions, and as yet, at least, have neither combined to
form a single constant type, nor diverged to form two or three distinct
types. The various 'leaf-pictures' seem equally effective in concealing
the insects from their enemies, and thus there is still a continual
crossing and mingling of the different essays at leaf-picturing.
A second group of facts, which indicates that old-established
characters have less tendency to overstep the limits of the neutral
'variation-playground,' is to be found in the experience of breeders,
and especially that of gardeners who have brought wild plants under
cultivation in order to procure varieties.
It has been proved that the wild plants often exhibit no hereditary
variations for a long series of generations, notwithstanding the
greatly altered conditions of life, but that then a moment comes in
which isolated variations crop up, which may then be intensified by
the manipulations of the breeder to form sport-species with large
conspicuously coloured flowers, or with some other distinctive
character. Darwin called this a shattering of the constitution of the
plant; but the stable and slowly varying 'constitution' simply means
that in old-established and well-adapted species the determinants
possess only a very restricted 'variation-playground,' and because of
their firmly based harmonious correlation are not easily and never very
quickly induced to overstep its limits in any marked degree.
Let us now apply all this to the institution of amphimixis and
amphigony, and it is immediately obvious that these determinants of the
germ-plasm which control the characters relating to sexual reproduction
must be _more stable and less variable than all others which a species
possesses, for they are infinitely older_. They are older than all
species-characters, older than the characters of the genus, of the
family, of the class, and indeed of the whole series or phylum to
which a higher animal, a vertebrate, for instance, belongs. We cannot
wonder, therefore, that amphigony has persisted through hundreds and
thousands of generations, even if it had not been reinforced in the
germ-plasm during this period by selection. We should rather wonder
that an institution so primaeval, and so firmly engrained in the
germ-plasm, can ever be departed from, even when its abandonment is to
the advantage of the species, as has happened in parthenogenesis.
I have entered upon this long discussion because I believe that we
require to appreciate this power of persistence on the part of the
sexual determinants before we can explain the general occurrence of
amphigony. The occurrence of pure parthenogenesis, unaccompanied by
any degeneration of the species, can hardly be understood except on
the assumption that the constancy of the species, when it has once
been attained, may be preserved without the continual intervention of
amphimixis. How long it can be preserved is another question, which it
is difficult or impossible to answer, since species exhibiting _pure_
parthenogenesis are rare, and since we cannot tell with certainty
how long it is since amphimixis ceased to occur in them. Generally
speaking, the answer in regard to the few species which have to be
taken into account in this connexion would be 'not long,' but whether
this 'not long' signifies hundreds of generations or thousands of
generations we must leave undecided. So much only we can say, that
in all species of animals in which the male sex has quite died out
or has dwindled to a minimal remnant, there are as yet no traces of
degeneration to be found, and that even organs which have fallen into
disuse and become functionless because amphigony has disappeared, are
nevertheless in several cases retained in perfect completeness. I shall
return to this subject later on, but in the meantime I wish to work
out our conception of the actual efficacy of amphigony or ordinary
sexual reproduction, and thereby increase our understanding of its
significance and power of persistence.
We have seen that amphigony not only renders possible the novel
'harmonious adaptations' which are continually required, but that it
also leads, by a continual crossing of individuals, simultaneously with
the elimination of the less fit, to a gradually increasing constancy of
the species. This has been regarded by some writers as its sole effect;
thus recently by Hatschek, whose view has already been refuted.
Haycraft also finds the significance of amphigony simply in the
equalizing or neutralizing of individual differences which it effects.
Quetelet and Galton have attempted to show that intercrossing leads to
a mean which then remains constant. Haycraft supposes that a species
can only remain constant if its individuals are being continually
intercrossed, and that otherwise they would diverge and take different
forms, because the 'protoplasm' has within itself the tendency to
continual variation. The transformation of species is effected by
means of this variation tendency, and the persistency and constancy
of species which are already adapted to the conditions of their life
are secured by the constant intercrossing of the individuals, and the
consequent neutralization of individual peculiarities.
Although the cases already mentioned in which great constancy of
species is associated with purely parthenogenetic reproduction do
not tell in favour of the accuracy of the view just stated, yet
the fundamental idea, that amphigony is an essential factor in the
maintenance and even in the evolution of species, is undoubtedly sound.
We should certainly find neither genera nor species in Nature if
amphigony did not exist; but we cannot simply suppose that amphigony
and variation are, so to speak, antipodal forces, the former of which
secures the constancy of the species, the latter its transformation.
In my opinion, at all events, there is no such thing as a 'tendency'
of the protoplasm to vary, although there is a constant fluctuation of
the characters--dependent on the imperfect equality of the external
influences, especially of nutrition. This certainly results, as far
as it takes place within the germ-plasm, in a continual upward and
downward variation of the hereditary tendencies, and it would lead to
increasing dissimilarity of the individuals were it not that amphigony
is continually equalizing the differences by a constantly repeated
mingling of individuals. Quetelet and Galton have shown that the
tendency of this mingling is towards the establishment of a mean; the
characters of Man, such as bodily size, fluctuate about a mean, which
at the same time shows the maximum of frequency; and the frequency
curve of the various bodily sizes assumes a perfectly symmetrical
form, so that the average size is the most frequent, and deviations
from it upwards or downwards occur more rarely in proportion to the
amount of deviation, the largest and the smallest sizes occurring least
frequently.
Thus an equalizing of variations by means of amphimixis really exists,
and the question we have to ask is, How does it come about? The case
is assuredly not the same as that in which equal quantities of red
and white wine are mixed to make a so-called 'Schiller.' This is
proved even by the fact that the mixture may turn out quite different
even when the wines--the two parents--are alike: for the children
of a pair are often dissimilar. And while the 'Schiller' cannot be
separated again into red wine and white, this happens often in sexual
reproduction, and sometimes to such an extent that the grandchild
exactly resembles one or other of the grand-parents, as is most clearly
proved in the case of plant-hybrids.
There is thus a deep-seated difference, depending on the fact that
what is mingled in amphigony is not simple but composite, not a
simple uniform developmental tendency associated with a simple and
definite substance, but a combination of several or many developmental
tendencies, associated with several equivalent but different material
units. These units are the ids or ancestral plasms, and we have seen
how they are not only halved by reducing division, but are also
arranged in new combinations in amphimixis.
These ids differ very little within the same germ-plasm; in species
which have long been established the majority probably only differ
in correspondence to the individual differences of the fully-formed
organisms, but they are only absolutely alike in the case of two
ids which have been formed by the division of a mother-id. Let us
disregard this for the moment, and assume that all the ids of a
germ-plasm are different: the germ-plasm of a father, _A_, will be
composed of ids _A_ 1-100, that of the mother, _B_, of the ids _B_
1-100. But in each mature germ-cell of these two parents only fifty
ids are contained, and if we assume that the mingling of the ids is
controlled solely by chance, then in the various germ-cells _A_ × _B_
the most diverse combinations of ids may be contained; for instance,
_A_ 1, 3, 5, 7, 9, 11, ...to 99, or _A_ 1-10 and 20-30, and 40-50,
and so on, and similarly in the germ-cells _B_. If all germ-cells
produced by _A_ and by _B_ attained to development, or even if all the
ova succeeded, the thousand or hundred thousand children of this pair
would necessarily exhibit every possible mingling of their characters,
and each in the same number according to the rules of probability
calculations. But it is well known _that this does not happen_; of
the thousands of human ova, for instance, which come to maturity in
the course of the life of a female individual more than ten rarely
develop, and more than thirty never, and these are determined solely
by chance and quite independently of the mixture of ids which they
contain. It is thus purely a matter of chance which of the complexes
of primary constituents contained in the germ-plasm of an individual
are transmitted to descendants, and it is also purely a matter of
chance which combination of ids comes to be developed. Therefore we may
say that no regular neutralizing of contrasts, either in the primary
constituents of the parents or as regards the differences in their
characters, can occur. In one case there is a blended inheritance;
in another the child takes after the father or after the mother; in a
third, and this probably occurs most frequently, the child resembles
the father in some characters and the mother in others.
But how then does Galton's curve of frequency of variations come about?
Why does the mean of any character occur by far the most frequently,
and why does the frequency of a variation diminish regularly in
proportion to its approximation to either extreme? To this it is
answered: Because the process of mingling through amphigony goes on
through numerous generations, and thus an elimination of chance, and
the establishment of an average, must be brought about.
But this does not quite suffice to explain matters, for experience
shows that asymmetrical frequency-curves of variations also occur, even
in species with sexual reproduction. As De Vries has recently shown,
there are also 'half-Galton curves,' that is, curves which suddenly
break off at their highest point. We must conclude from this that the
frequency of the different variations depends not only on their degree,
but also on the greater or less facility with which they arise from the
constitution of the species.
This consideration can be readily elucidated with the help of
Ammon's exposition, and especially of his graphic representation
of the 'playground of variations.' If we think of the indifferent
variations occurring in any character of a species as arranged in
a series ascending from the smallest to the largest, this line may
be regarded as the abscissal-axis, and from it ordinates may be
drawn which express the frequency of the variation in question by
the differences in their length. If the tips of these ordinates
be united, we have the curve of frequency (Fig. 120, _A_), which
according to Galton ought to be symmetrical, and in most cases really
is so. Ammon calls the space between the smallest and the largest
variations the 'variation-playground,' that is, the playground within
which all variations are equally advantageous to the species. This
is not co-extensive with the variation-area, for there may be more
marked deviations below the beginning or above the upper end of the
variation-playground, but these, being disadvantageous, fall under
the shears of personal selection. The variation-playground may also
be called the area of indulgence of variation, because the variations
falling within it are spared from the eliminating activity of
selection, or the variation-area of survivors, because on an average
only those survive whose variations do not overstep the limits of this
area.
[Illustration: FIG. 120. _A_, symmetrical, and _B_, asymmetrical
curve of frequency; after Ammon. _U_, minimal, _O_, maximal limit of
individual variation. _U-O_, the 'variation-playground.' _M_, the mean
of variation. _H_, the greatest frequency or mode of variation.]
This implies that variations below _U_ (the lower limit of the area
of exemption) and above _O_ (the upper limit) can occur, but do not
survive and leave descendants, and we can therefore easily understand
why characters, of which different degrees arise with equal ease from
the constitution of the species, must gradually develop a symmetrical
curve of frequency because of the constant crossing. Obviously
those individuals which stand just upon the borders of admissible
variation will, other conditions being equal, leave behind them
fewer descendants than those which approximate to the middle of the
area of exemption; for as the characters concerned can vary in the
offspring in both directions, there will always be at the lower end
some of the descendants of a pair which will fall below the limits of
exemption, and at the upper end some which will rise above it. This
will happen even when pairing takes place between parents at the middle
or at the other end of the abscissa, for there are always cases of
the preponderance of one parent in heredity. A higher percentage of
the descendants of individuals on the borderline will therefore be
eliminated, and their frequency _must therefore be less_. Even if at
the beginning of the series of observations a condition obtained in
which all the ordinates of the area of exemption were equally high,
those nearest the boundaries would of necessity very soon become lower,
and this in proportion to their distance from the boundary, and the
frequency-curve, which at first would be a straight line (according
to our assumption, which of course does not tally with natural
conditions), would become a symmetrical curve, highest in the middle
and falling equally at either side.
Ammon has worked out the hypotheses on which the curve of frequency
would become asymmetrical. Firstly, when the fertility is greater
towards the upper or lower limit of the area of exemption; secondly,
when germinal selection forces the variation in a particular direction,
upwards or downwards; and thirdly, 'when natural selection intervenes
diversely at the upper or lower limit.' Of these three possibilities
the first two must be acknowledged as quite probable, but the third,
it seems to me, could only cause a temporary asymmetry of the curve,
lasting, that is, only until a state of equilibrium has again been
reached; but that may in certain conditions take a long time.
Asymmetrical curves of frequency (Fig. 120, _B_) therefore arise, for
instance, when the intra-germinal conditions (the 'constitution of the
species') more easily and therefore more frequently produce extreme
variations. In this case the area of exemption can only extend on one
side, and must remain in this state. In _Caltha palustris_, the marsh
marigold, we may find, according to De Vries, among a hundred flowers,
those with five, six, seven, and eight petals, in the following
proportions:--
Petals 5 6 7 8
Number of flowers 72 21 6 1
and thus there is an asymmetrical curve of frequency. But if we take
the whole area of variation as the area of exemption, that is, if we
assume that it is indifferent for the species whether the flowers
have five, six, seven, or eight petals, the preponderance of the
five-petalled flowers may have its reason in the fact that it is much
easier for five than six or more petals to be produced because of the
internal structure of the whole plant.
In this case the maximum of frequency lies at the lower limit of
variation, but it may also lie at the upper. Thus, according to De
Vries, the blossoms of _Weigelia_ vary, in regard to the number of
their petal-tips, in the following manner. Six-tipped corollas were not
found, and among 1,145 flowers there were the following proportions:--
Tips of the corolla 3 4 5
Number of flowers 61 196 888
It is thus clear that amphimixis is an essential factor in the fixing
of forms, but that it certainly does not of itself determine these, and
that it is not always the average of the variations that is the most
frequent, but that the form of the curve of frequency is determined by
other factors also, namely, by germinal and personal selection and by
the directive control which these exert on variations.
The equalizing effect of amphigony may perhaps be expressed
thus: In the case of every new adaptation there is at first a
large area of variation, but this gradually decreases owing to
a continual restriction on the part of natural selection, until
ultimately--when the highest degree of constancy of the character or
species has been attained--it only extends very little beyond the
'adaptation-playground' or the 'area of exemption.'
One of the effects of amphimixis is thus to bring about an increasing
restriction of the area of variation, or, as we usually say, a
constancy of the facies of a given form, a condensation into a species.
How far this result is necessary or useful, and therefore how far
it may be regarded as accounting for the persistence of amphimixis,
we shall discuss in the chapter on the formation of species. My own
view is that even the fact that new adaptations are rendered possible
through amphimixis and amphigony, the mode of reproduction associated
with it, affords in itself a sufficient reason why amphimixis should
have been retained when once it had been introduced.
LECTURE XXIX
THE GENERAL SIGNIFICANCE OF AMPHIMIXIS (_continued_)
Association of amphimixis with reproduction--Origin of amphimixis--Its
lowest forms--Amphimixis in Coccidia--Chromosomes in unicellular
organisms--_Coccidium proprium_--'Amœba-nests' as a preliminary
stage to amphimixis--Plastogamy of the Myxomycetes--Result: a
strengthening of the power of adaptation--Strengthening of the power
of assimilation--Use of complete amphimixis--Proof of its constant
efficacy to be found in the rudimentary organs of Man--Allogamy--Means
taken to prevent the mingling of nearly related forms--Amphimixis is
not a 'formative' stimulus--Attraction of the germ-cells--Effects of
inbreeding compared with those of parthenogenesis--Nathusius's case of
injurious inbreeding--Hindrances to fertilization in the crossing of
species--Probable reason for the injurious effect of inbreeding.
We have endeavoured to understand why amphimixis should have been
established among the processes of life, and we have now to turn to the
question when and how, that is, in what form, it was first introduced.
But first I should like to refer for a little to the association of
amphimixis with reproduction, which we find in all multicellular
organisms, and among the higher types so unexceptionally that, until
not very long ago, amphimixis and reproduction were looked on as
one and the same thing, and all multiplication was believed to be
associated with 'fertilization.' We have seen that this is not the
case, that on the other hand the two processes are quite distinct,
and may be called contrasts rather than equivalents, for reproduction
always means an increase in the number of individuals, while amphimixis
implies--originally at least--their diminution by a half.
Accordingly we found that, in unicellular organisms, amphimixis
is not associated with reproduction, but interpolated between the
divisions, and not even in such a manner that amphimixis precedes every
multiplication by division, but so that the conjugation of two animals
occurs only from time to time, after numerous divisions, sometimes
hundreds, have occurred. It is obvious that this must be so, since, if
amphimixis occurred regularly between every two divisions, no increase
in the number of individuals would be brought about, at least if the
fusion of the two conjugating individuals were complete.
Why, then, is there such an intimate, and in the case of the higher
types, such an indissoluble, association between reproduction and
amphimixis that 'fertilization' appears to be a _sine qua non_ of
reproduction, and not very long ago seemed to us to be the 'quickening
of the ovum,' the 'burning spark' which causes the powder-barrel to
explode?
The reason of this is not difficult to discover; it lies in the
structure of multicellular animals, and in their differentiation
according to the principle of division of labour, for since only
particular cells are capable of reproduction, that is, of giving
rise to the whole, it is in these necessarily that the process of
amphimixis has to occur if its significance lies in its effects on the
succeeding generations. It is true that in the lowest multicellular
organisms, such as the species of _Volvox_, there are, in addition
to the sex-cells, other reproductive cells quite similar to the ova,
whose development into a new colony takes place without amphimixis,
but the higher we ascend in the animal and plant series the rarer are
these 'asexual' germ-cells or 'spores,' and in the highest animal types
they are entirely absent and reproduction occurs only by means of the
'sex-cells.'
I am inclined to look for the cause of this striking phenomenon mainly
in the fact that, if amphimixis had to be retained, this was effected
with increasingly great difficulty the more highly and complexly
differentiated the organisms became, and that more complicated
adaptations were therefore necessary in order that the union of the
two germ-cells might be rendered possible at all. There is first of
all the separation into two kinds of sex-cells, whose far-reaching
differentiations and precise adaptations to the most minute conditions
we have already discussed; then follow the innumerable adaptations
to bring about the meeting of the sex-cells, the arrangements for
copulation, and, finally, the instincts which draw the two sexes
together, the means of attraction which are employed, whether
decorative colours or attractive shapes, stimulating odours or musical
notes, in short, all the diverse and intricate arrangements, which
seem to be more subtly elaborated the higher the organism stands upon
the ladder of life. When we call to mind that sexual differentiations
finally go so far that they dominate the whole organism, alike in
its external appearance and in its internal nature, its feelings,
inclinations, instincts, its will and ability, as well as its structure
down to the finest nerve-elements, we can understand that a mode of
reproduction which demands such a composite disposition of details,
involving a moulding of the whole organism, so to speak, from birth
till death, must of necessity remain the only one, and that there
was no room for the persistence of any essentially different mode
of reproduction with quite different adaptations. Or, to speak
metaphorically, the power of adaptation which is innate in the organism
so exhausted itself in the establishment of this marvellous amphimixis
adjustment that the possibility of any other was totally excluded.
It is true that it is only among the Vertebrates that we find
'the reproductive apparatus' so highly developed, but even among
Molluscs and Arthropods 'sexual' reproduction, that is, reproduction
associated with amphimixis, is the prevailing mode. In these, indeed,
parthenogenesis does occasionally occur, that is to say, sexually
differentiated female germ-cells are, by means of some slight
variations in the maturation of the egg, rendered capable of developing
without previous amphimixis, but this happens only in quite special
cases as an adaptation to quite special circumstances, and can only
be regarded as a temporary cessation of the association between
reproduction and amphimixis. In some cases it is a moiety of the ova
adapted for amphimixis which develop parthenogenetically, as it is the
same sexually differentiated animals, true females, which produce both
sorts, and this is often true to some extent when the differentiation
in the direction of parthenogenesis has advanced further, and the
ova have been separated into those requiring fertilization and those
which are parthenogenetic (e.g. the winter and the summer eggs of the
Daphnidæ). Parthenogenesis is not asexual but unisexual reproduction,
a mode of multiplication which shows us that even in highly
differentiated animals the apparently indissoluble association between
reproduction and amphimixis can be dissolved if circumstances require
it.
But if amphimixis had to be retained in the higher animal forms--and we
have seen reasons why this must be--it could only be effected by means
of unicellular germs, for amphimixis is in essence a fusion of nuclei,
and this is the reason why 'vegetative' reproduction, so-called,
becomes less and less prominent in animals at least, and above the
level of the Arthropods disappears almost entirely.
Let us now return to the question we asked at the beginning--When
and in what form was amphimixis first introduced into the world of
organisms? The best way to answer this is by observation. We must turn
to the lowest forms which now exhibit it, and see whether it occurs
in them in a simpler form, so that we may draw conclusions as to its
origin and its primitive significance, for it would be possible, _a
priori_, that this was something different from what it is now in
the relatively higher organisms, and that a change of function has
gradually come about.
Assuredly the whole intricate complex of adaptations which is now
exhibited on the conjugation of the two sex-cells in animals and
plants, the differentiation of two kinds of 'sexually' antagonistic
cells, with all their special adaptations, the reduction of the
chromosomes, the institution of the karyo-kinetic apparatus, together
with the centrospheres and so on, cannot possibly have arisen all at
once by fortuitous variation, but can only have arisen gradually,
step by step, and as the result of 'innumerable external and internal
influences.' But why should not these arrangements, nowadays so
complex, have had a simple beginning? Why might not this beginning have
been the simple union of the protoplasmic bodies of two non-nucleated
Monera; followed, after the origin of nuclear substances, by the union
of these, and, finally, after the differentiation of a nucleus with
a definite number of chromosomes, with a dividing apparatus, with a
membrane, and so on, by complete amphimixis as we now know it? And how
many transition stages may not be added to fill up the gaps between
these three main stages?
But how much we can actually prove in regard to these conceivable
preliminary stages of amphimixis is another matter. If we take a survey
of the observations that have been made up till now, we are confronted
at first by the undoubtedly striking fact that very little is known
about it as yet, for in fact the whole process is gone through even
in quite lowly forms of life in a manner very similar to that in the
higher forms. Amphimixis has been shown to be widespread even among
unicellular organisms, yet not in an _essentially_ simpler mode than
among multicellulars. We have seen that even in ciliated Infusorians
reducing division obtains, and that of the four nuclei which arise
from twofold division of the original nucleus three break up again,
and only the fourth, by a further division, separates into a male and
a female pronucleus, 'which then complete the amphimixis with the
corresponding pro-nuclei of another animal' (compare Fig. 85, 4-7, vol.
i. p. 321). This, and the existence of a dividing apparatus and of
chromosomes, make the process appear very little less complicated than
the fertilization of higher animals. The case is similar even in much
lower unicellular organisms, such as _Noctiluca_ (Fig. 83, vol. i. p.
317). In this form and in Rhizopods it is true that reducing divisions
have not yet been made out, but their occurrence in the lower Algæ
(_Basidiobolus_), and above all in those simple unicellular organisms
which give rise to malaria, and their allies, which live as 'Coccidia'
in the blood-cells and intestinal cells of animals, leads us to expect
that they may prove to be of general occurrence among unicellulars.
In the Coccidia, which are extremely simple unicellular organisms,
equipped, however, with a nucleus, the adaptations relating to
amphimixis are more extensive and more complex than in the Rhizopods.
For while in the latter the two conjugating cells are absolutely alike
in external appearance, in the former the male cell is distinct from
the female, and indeed the differences are as marked as those that
usually occur in multicellular animals.
[Illustration: FIG. 121. Life-cycle of _Coccidium lithobii_, a
cell-parasite of the centipede _Lithobius_; after Schaudinn. 1, a
'sporozoite'; 2, the same penetrating into an intestinal epithelial
cell; 3, the same growing into a 'schizont' capable of division; 4, the
same dividing, and 5, breaking up into numerous pieces which separate
from the 'residual body' in the centre, and either, as in 1, migrate
into epithelial cells and repeat the history, or pass on to the phase
of sexual reproduction. In the latter case, after eliminating a portion
of the nucleus (reduction) in 6 and 6 _a_, they form the 'macrogamete'
(the ovum); or within the mother-cell they produce microgametes (or
sperm-cells), 7 and 7 _a_. The penetration of a sperm-cell into an
egg-cell (amphimixis) is shown in 8, the fertilized egg-cell (9)
becomes the so-called oocyst or permanent spore, from which by repeated
division (10 and 11), new sporozoites, as in 1, arise, and begin the
cycle afresh.]
We owe our present knowledge of these processes especially to Schuberg,
Schaudinn, and Siedlecki, and, because of their theoretical importance,
I should like to summarize the essential points.
One of these Coccidia lives in the intestinal cells of a small
centipede, _Lithobius_; in Fig. 121 the parasite is shown as a
so-called 'Sporozoite,' that is, as a minute sickle-shaped cell,
which at first moves freely about the intestine of the host (1), but
then soon penetrates into an epithelial cell (2). There it grows to
a spherical shape (3), and then, after having devoured the cell,
it gives rise, by a peculiar process of division (Schizogony), to
a number of very minute nucleated pieces, again sickle-shaped, the
Schizonts, each of which bores its way into an epithelial cell as in
2, and follows the same path of development, so that a large number of
cells in the intestine of the same host are attacked in this manner.
But there is still another mode of reproduction, with which amphimixis
is associated, which leads directly to the formation of 'lasting'
germs which are enclosed in a capsule or cyst, reach the exterior with
the excrement of the host, and thus spread the infection to other
centipedes. The Schizonts which take this course develop into so-called
macro-gametes and microgametes, the former being the female, the latter
the male germ-cells. Then follows the penetration of a male gamete,
actively motile because of its two flagella, into the female gamete
(8). Amphimixis is accomplished, and the product of the fusion of the
two sex-cells (9) surrounds itself with a thinner cyst, within which
it multiplies by twofold division into four cells (10). These are the
'lasting' spores, which may dry up within the voided excrement of the
centipede (11), and if they be eaten by another animal of the species,
they infect it, for the sporozoites which have been formed by the
previous divisions creep out, and in form 1 begin the life-history anew.
We have thus an alternation of four generations which are all
unicellular, and of which one series (1-5) shows multiplication by
fission, while the other (6-11) includes, besides multiplication
by fission and as a condition of this, the process of amphimixis.
Amphimixis _must_ occur in order that the formation of 'lasting' spores
and new sporozoites may result. We have thus a regular alternation
of 'asexual' and 'sexual' reproduction, and the latter shows great
resemblance to that of multicellular organisms. The macrogamete
corresponds to the ovum, the microgametes to the spermatozoa, and they
resemble these also in their greater numbers and in their structure.
But the resemblance goes even further. The ovum is much larger than the
sperm-cell, and undergoes a kind of reduction of its nuclear substance;
shortly before fertilization the ovum-nucleus ('the germinal vesicle')
comes to the surface--just as in the case of animal ova--bursts, and
extrudes a part of its substance in the form of a sphere (Fig. 121, 6
and 7). A reduction of the nuclear substance in the male cell has not
been demonstrated in all cases, but in one of the _Lithobius_-Coccidia,
_Adelea ovata_, the relatively large microgamete (the sperm-cell, Fig.
122, _Mi_) places itself close to one pole of the female macrogamete
(the egg-cell) and then divides twice in succession, so that four small
cells arise (Fig. 122, _A-C_); of these only one penetrates into the
egg-cell (_D_, ♂_K_) and unites with it, the other three come to nought
(_D_, _Mi_). What a surprising resemblance this bears to the twofold
division of the mother sperm-cell in multicellular animals, through
which the number of chromosomes is reduced to half! In the conjugation
itself the thread-like chromosomes of the female nucleus are plainly
recognizable, while those of the male remain coiled up (Fig. 122, _D_).
That the nuclear substance can be separated into chromosomes (ids) even
in lowly unicellular organisms was probably first demonstrated by R.
Hertwig for _Actinosphærium_, a Heliozoon or freshwater sun-animalcule,
then by Lauterborn in regard to Diatoms, by Blochmann for an indigenous
Rhizopod, _Euglypha_, and by Ishikawa for the marine _Noctiluca_. Fresh
cases have been added in the last decade, so that we can now say that
a considerable number of unicellulars, from the ciliated Infusorians
and lower Algæ down to the Coccidia and Diatoms, exhibit a germ-plasm
composed of ids. These structures behave in the same way as those in
higher organisms, and Berger was able to demonstrate, in 1900, in the
case of a Radiolarian, their multiplication by spontaneous splitting.
[Illustration: FIG. 122. Conjugation of a Coccidium (_Adelea ovata_),
after Schaudinn and Siedlecki. _A_, the microgamete (sperm-cell)
(_Mi_) has become closely apposed to the macrogamete (_Ma_). _B_, the
reduction division of the nucleus of the macrogamete has been effected;
_Rk_, directive corpuscles. In the microgamete the first division of
the nucleus has begun. _C_, four nuclei in the microgamete, of which
three come to nought. _D_, the fourth microgamete-nucleus (♂_K_)
has become apposed to the nucleus of the ovum, in which distinct
chromosomes are seen.]
From our point of view all this cannot surprise us, since all these
organisms, though only single cells, possess great complexity of
structure; we need only call to mind the extremely fine differentiation
of structure in numerous ciliated Infusorians, such as _Stentor_, which
has already been mentioned, or the bell-animalcule (_Vorticella_) with
its long and peculiarly ciliated gullet, its retractile ciliated disk,
its muscular or myophane layer, its spirally retractile stalk with the
ribbon-like, rapidly acting muscular axis; or the regular geometrically
constructed flinty skeleton of the Radiolarians, with their radially
disposed sword-like or rod-like needles and their complex interlacing
lattice-work shells. In the latter case the complexity of the living
substance becomes visible only through its product, the shell, for the
protoplasm itself does not show any visible intricacy, and the same is
true of the Coccidium whose life-history we have just been tracing,
for in each of its stages it seems to be of very simple organization,
though the succession of numerous different forms shows that its
germ-substance must be composed of numerous determinants.
We cannot doubt, however, that, in all unicellular organisms, the
protoplasm can be hardly less complicated as regards its minute
invisible structure, since otherwise it would be impossible that the
delicate vital processes which we observe in them should run their
course. In this I agree, at least in principle, with the beautiful
picture drawn by Ludwig Zehnder in his recent book[22] already
mentioned, though he reached it in quite a different way, namely,
by a purely synthetic method. He made the daring attempt to build
up the organic world from below, starting from atoms and molecules,
and ascending from these to the lowest vital units, our biophors, to
which he attributes a tubular shape and therefore calls fistellæ. He
imagines the cell to be made up of a large number, perhaps millions,
of different kinds of fistellæ, of which one presides over the power
of turgidity, another over endosmosis, a third over contraction, a
fourth over the conduction of stimuli, &c., so that there results a
high degree of cellular complexity, a composition out of numerous
kinds of biophors arranged on a definite architectural plan. All this
corresponds perfectly with the views I have so long championed, and
which alone make the existence of a nucleus intelligible, if it is
composed--as I assume--essentially of an accumulation of determinants,
that is, of hereditary substances. And that such a high degree
of complexity of structure is not a mere fanciful picture we see
occasionally even in the case of unicellular organisms. Thus, for
instance, in _Coccidium proprium_, parasitic in the newt (_Triton_),
the macrogamete or egg-cell (Fig. 123, _Ma_) before fertilization by
the sperm-cell or microgamete (Fig. 123, _Mi_) surrounds itself with a
capsule, at one pole of which a minute opening, the micropyle, remains
for the entrance of the male cell. This proves, it seems to me, that
this particular spot of the capsule is hereditarily determined, just
as much and just as definitely as the ray of the flint-skeleton of a
Radiolarian. But if any spot of the capsule can vary by itself alone,
may not numerous other points in the animal also be hereditarily
determinable? With such complexity of the invisible structure it
would not greatly surprise us if we should find amphimixis occurring
in all unicellular organisms, and in many of them at a high level of
elaboration. These apparently lowly and simple organisms are obviously
very far from being the lowliest and simplest, as we shall discover
later in a different connexion. But that amphimixis is found as a
periodically recurring process even among these, must depend upon the
fact that here too the preservation of the best-adapted structure, as
well as adaptability to new conditions, requires that the best variants
of many different parts of the cell should be brought together,
and since the hereditary substance lies in the ids of the nucleus,
the union of the ids of two unicellulars will make harmonious and
many-sided adaptation materially easier. It will thus give an advantage
in the struggle for existence, and we may therefore expect to find that
the nuclear substance in all unicellular organism is made up of ids.
[22] Zehnder, _Die Entstehung des Lebens_, Freiburg-i.-Br., 1899.
[Illustration: FIG. 123. Conjugation of _Coccidium proprium_, a
cellular parasite of the newt (_Triton_), after Siedlecki. _A_, a
microgamete (_Mi_) in the act of penetrating the shell of a macrogamete
(_Ma_) through the micropyle. _B_, the male and the female nuclear
constituents are uniting (♂ _chr_ and ♀ _chr_).]
The observations hitherto made do not, however, appear to bear this
out, for in the lower Flagellata and Algæ the nuclear substance does
indeed consist of chromatin, but--as far as it can be made out--of a
compact unarranged mass of it. But even though deeper investigations
should succeed in demonstrating chromosomes in many of these, the
nucleus _must have arisen at some time_, and we must assume that it
did so through a more intimate union of previously loose aggregates of
determinants, which were gradually arranged and bound together by the
combining forces (affinities) we have assumed to obtain among them,
thus giving rise to the first chromosomes or ids which were complete in
themselves. Then came the multiplication of these ids by the process of
division, and only then was the state arrived at from which amphimixis,
as we now know it, could have arisen, namely, the existence of a
considerable number of identical ids, half of which could be exchanged
for the identical ids of another individual in conjugation.
But as to our question, In what organisms did amphimixis first arise,
and how? there seems, from what we have already learned with regard
to the Coccidia, little prospect of our being able to give a definite
answer, for if amphimixis occurs even in these lowly organisms, and
occurs, too, in the same manner as in the higher unicellular organisms,
and not very much more simply than among the highest multicellular
organisms, we may conclude that the preliminary stages will now be very
difficult or impossible to detect, either because they are extinct, or
because they occur only in ultra-microscopic organisms.
Nevertheless there do appear to be preliminary stages, and they are
exactly those which we should have assumed if we had been obliged to
construct them theoretically.
The first phenomenon of this kind is the mere juxtaposition of two or
more unicellular organisms, without the occurrence of fusion. This was
probably first observed by Gruber in Amœbæ, and it was theoretically
interpreted at a later date by Rhumbler. As many as fifty Amœbæ gather
together to form a 'nest,' and remain closely apposed to each other for
a fortnight. Although no fusion took place, and there were no visible
results of this juxtaposition, it may be concluded that the animals had
some sort of attractive effect upon each other, and it may be supposed
that some sort of advantage must have been associated with this state
of quiet, close apposition against one another. Cytotropism, the
mutual attraction of similar cells, which Wilhelm Roux first observed
in the segmentation-cells of the frog's egg, seems to occur also in
unicellular organisms, and this may help us to understand how a fusion
of cell-bodies may have come about.
Fusion of this kind was demonstrated in the Myxomycetes almost forty
years ago by De Bary, and it has been observed more recently in various
unicellular organisms, especially in Rhizopods and in Heliozoa.
These last often place themselves close together in pairs, threes,
or even more at a time, and then the delicate cell-bodies coalesce,
though no fusion of the nuclei takes place. With Hartog, we call this
process 'Plastogamy,' but we cannot agree with that observer when he
regards the importance of the process as consisting in the fact that
the nuclei thus come into contact with fresh cell-substance, after
having been surrounded for a very long time with the same cytoplasm.
If this were the import of amphimixis, then an _exchange of nuclei_
would take place, and this we find nowhere even among the lowest forms
of life, for everywhere there is a union of the nuclear substance of
two individuals. But this is by the way! Further cases of plastogamy
have been observed in many of the limy-shelled Rhizopods. A union of
this kind does not usually lead to any visible consequences, but in
some Foraminifera a group of young animals is developed within the
cell-bodies by the division of the nuclei and the cell-body; thus
multiplication follows the fusion just as in perfect amphimixis, and
we may therefore assume that there is a causal connexion between the
two. In the slime-fungi, too, the union of several amœba-like cells
into a multi-nucleated plasmodium is followed later by the development
of numerous encapsuled spores, but only after the plasmodium, which to
begin with is microscopically small, has grown to a macroscopically
visible, reticulated mass (_Æthalium_) sometimes a foot in extent. In
this case the fungus, creeping slowly over its foundation of decaying
substance, takes up nourishment from it, and it is not possible to
tell whether the union of the amœbæ yields any further advantage than
that of facilitating the spreading over large uneven surfaces, and
through this, later, the development of large fruit-bodies. But in the
case of the Foraminifera the plastogamy has obviously another effect,
unknown and mysterious, which as yet no one has ever been able to
define precisely. Words like 'stimulus to growth,' 'stimulating of
the metabolism,' and even 'rejuvenation,' give no insight into what
happens, but that something happens, that through the fusion of two or
more unicellulars a stimulus is exerted, which reveals itself later in
increased rapidity of growth, we may, and indeed must assume, because
this process has become a permanent arrangement in so many unicellular
organisms. Only what is useful survives, and the uniting individuals
must derive some advantage from the process of fusion, and it remains
to be seen whether we can find out with any clearness what this
advantage may be.
Till within a few decades ago it was believed that in this process
one individual devoured the other, but this can now no longer be
maintained. If any one still seriously considers this possible,
Schaudinn's observations would convince him of his error, for in
_Trichosphærium_, a marine, many-nucleated Rhizopod, he observed, on
the one hand, the union of two or more animals, i. e. plastogamy, and
on the other hand, the swallowing and digesting of a smaller member
of the species by a larger one--two processes which are absolutely
different, for in the first case the cell-bodies of the two animals
remain intact, while an animal that is eaten becomes surrounded by a
food-vacuole, and is dissolved and digested within it. In the former
case the vital units (biophors) of each animal obviously remain intact
and capable of function; in the second, those of the over-mastered
animal are at once dissolved and chemically broken up; as biophors,
therefore, they cease to exist. Whether one or the other process takes
place may perhaps depend on whether the two animals differ greatly in
size, so that the smaller can be quite surrounded by the larger.
In a former lecture I have emphatically expressed my dissent from the
view which interprets amphimixis as a process of rejuvenation, meaning
thereby a necessary renewal of life, and I need not go into this again
in detail: for that the metabolism can continue through uncounted
generations without being artificially stimulated--that is, in any
other way than by nutrition--is proved by all those lowly organisms
which exhibit neither plastogamy nor complete amphimixis, and also by
the occurrence of purely parthenogenetic reproduction, &c. In what,
then, can the advantage lie which the conjugating unicellular organisms
derive from conjugation? Obviously not in that they impart to each
other what each already possessed, but only in the communication of
something special and individual, something that was peculiar to each,
and becomes common to both.
Haberlandt believed that the development of auxo-spores in Diatoms
pointed towards the processes which form the deepest roots of
amphimixis. As is well known, the hard and unyielding flinty shell
of these lowly Algæ involves a diminution of the organism at every
division, so that the Diatoms become smaller and smaller as they go
on multiplying, and if that went on without limit they would come
rapidly to extinction. But a corrective is supplied in the periodical
occurrence of conjugation of two organisms which have already
materially diminished in size, and this is followed by the growth of
the two fused individuals to the original normal size of the species.
It is, of course, obvious that in this case the union of two organisms
which have become too small may be of advantage in bringing them back
to the requisite normal size; but this is an isolated special case,
which certainly does not justify our regarding conjugation as a means
whereby diminished bodily size may be brought back to its normal
proportions. By far the greater number of unicellular organisms are not
permanently diminished in size by division, and even in the Diatoms the
mass of the two fused individuals does not amount to the normal size of
the species, so that even in this case there must be growth subsequent
to the conjugation before the normal is re-attained. It may be doubted,
therefore, whether the increase in mass is, even in the case cited, the
essential event in conjugation, and whether there are not other effects
which we cannot clearly recognize. Here, too, there must be differences
between the two conjugating individuals, as we have just seen, for if
they only communicated something similar to each other, the result
would be an increase only in their mass, not in their qualities.
Although we cannot demonstrate differences of this kind in the case
of the lowly organisms with which we are now dealing, we may assume
their existence from analogy with the higher organisms. We know,
especially through G. Jäger, that in Man every individual has a
specific exhalation, his particular odour, and that in the secretions
of his glands there are incalculably minute differences in chemical
composition, which justify the conclusion that the living substance
of the secreting cells themselves exhibits such differences, and that
all the various kinds of cells in an individual are not absolutely
identical with the corresponding cells of another individual, but
that they are distinguished from them by minute yet constant chemical
differences. The assumption that differences of this kind exist even
in unicellulars, and in all lowly organisms generally, is not a merely
fanciful one, but has much probability.
How far the combination of these individual differences of chemical,
and at the same time vital, organization is able to quicken, to
strengthen the metabolism, to bring about 'physiological regeneration,'
or whatever we may choose to call it, we do not yet understand. It has
been said that in plastogamy an exchange of 'substances' takes place;
that each gives to the other the substances which it possesses and
the other lacks, and that this causes an increase of vital energy.
But it is unlikely that we have here to do merely with chemical
substances, although these, of course, as the material basis of all
vital processes, are indispensable; it seems to me more probable that
the vital units (biophors) themselves in their specific individuality
must play the chief part. But even this is saying very little, for we
have not yet reached an understanding of these processes, and if we
were not forced by the fact of plastogamy to the conclusion that this
union must have some use, no one would have been likely to postulate it
as useful, still less as necessary. It has, of course, been frequently
suggested that multiplication by fission, if long-continued, results
in 'exhaustion,' and that this is corrected by amphimixis, but who
can tell why this 'exhaustion' might not be remedied, and even more
effectually remedied, by a fresh supply of fuel, that is, of food?
One might have thought that the vital processes would be thus more
readily recuperated than by the co-operative combination of two already
'exhausted' cells. Two exhausted horses may perhaps be able to pull the
load that one of them was no longer equal to, but in the case we are
considering it is the combined burdens of two units that have to be
borne, although each was no longer equal to its own share! That is more
than we can understand.
Zehnder has recently defined the effect of amphimixis as a
'strengthening of the power of adaptation,' and he infers that the
'digestive fistellæ' (Biophors) of two individuals, which have
somewhat different powers of digestion, are, when they combine, able
to assimilate more kinds of food than either was able to assimilate
by itself. But I confess that I do not see how an advantage for the
whole would be gained through this alone, since half of the digestive
biophors would have to work for the nutrition of the mass of the
individual _A_, the other half of the differently constituted biophors
for that of the individual _B_, and the nutritive capacity would thus
remain exactly what it was before conjugation. Nevertheless I believe
that Zehnder was right in his supposition that conjugation is concerned
with strengthening the power of adaptation, and I have long maintained
and defended this interpretation with regard to true amphimixis in
nucleated organisms. In these cases it is quite obvious that the
communication of fresh ids to the germ-plasm implies an augmentation
of the variational tendencies, and thus an increase of the power
of adaptation. Under certain circumstances this may be of _direct_
advantage to the individual which results from the amphimixis, but
in most cases the advantage will be only an indirect one, which may
not necessarily be apparent in the lifetime of this one individual,
but may become so only in the course of generations and with the aid
of selection. For amphimixis must bring together favourable as well
as unfavourable variations, and the advantage it has for the species
lies simply in the fact that the latter are weeded out in the struggle
for existence, and that by repetition of the process the unfavourable
variational tendencies are gradually eliminated more and more
completely from the germ-plasm of the species.
But this cannot have been the efficient cause in the introduction of
amphimixis into the series of vital phenomena; the reason for this
must be found in some _direct_ advantage, such as that it improved and
increased the assimilating power, the growth, and the multiplication
of the particular individual, so that it gained an advantage over
individuals which had not entered into conjugation. This advantage must
exist, at least in the lower forms of conjugation, in pure plastogamy,
i. e. in the mere coalescence of the protoplasmic bodies. But, as it
seems to me, we have not yet clearly recognized what the advantage
precisely is; we do not yet see how such a mingling or combination
of two plasms should every time be of advantage for the combined
conjugate. If we assume with Zehnder that two kinds of 'nutritive'
biophors are brought together which differ slightly from each other
in digestive capacity, three cases may occur. Either the food _a_,
adequate for the animal _A_, is just as abundant as the food _b_,
suitable for the animal _B_, and then half the conjugated animal will
be nourished by means of the biophors _a_, the other half by means
of the biophors _b_, and the state of matters is the same as it was
before conjugation; or the food _b_ is more abundant than the food
_a_, or conversely, and then the biophors _b_ will have to take the
larger share in the nourishment of the conjugate _A_ + _B_, and they
will therefore multiply more rapidly and the biophors _a_ will decrease
relatively in number. Nutrition and growth will then go on more slowly
for a time, but will soon attain to their former intensity. The
combined individual _A_ + _B_ has then certainly gained an advantage
over the isolated animal _A_, and the living substance of _A_ which,
if left to itself would probably have perished, can continue to live
in combination with _B_. But in that case it is not obvious where
the advantage in the union can lie, as far as _B_ is concerned. An
advantage to _B_ only results if there be a combination not of _one_
kind of biophor only, but of several or many kinds of biophors. If for
instance _A_, whose digestive biophors were weak, brought with it into
the partnership 'secretory' or nervous biophors stronger than those of
_B_, then there would be an advantage for both in the combination, and
it is thus that, in the meantime, I interpret the direct benefit which
results from pure plastogamy. This benefit must be the more important
and far-reaching the longer multiplication by fission continues without
the occurrence of conjugation.
We thus reach what is perhaps a not wholly unsatisfactory conception of
amphimixis, in so far at least that we do not require to assume that
there has been a fundamental change in its significance between its
expression in the lowest organisms and in the higher and even highest
forms. Everywhere it is the same advantage: an increase in the power of
adaptation; but it sometimes finds expression directly in the product
of conjugation, sometimes only indirectly, sooner or later, among the
descendants of the product.
How far below the Myxomycetes pure plastogamy reaches we do not know;
whether it also occurs among non-nucleated organisms (Haeckel's
Monera) we cannot tell from experience, since these assumed organisms
have not yet been observed with certainty. Perhaps they all lie
below the limits of visibility, and then we could never do more than
_suppose_ that plastogamic processes occur among them. Logically and
purely theoretically we may _suppose_ that amphimixis occurred first
between the plasmic bodies of non-nucleated Monera, then between the
cell-bodies of true cells, and finally between the nuclei of cells.
Let us hold fast to what we have found to be probable, namely, that
the fusion of individually different simple organisms must or may
bring about a direct advantage--a stimulation of the metabolism, and
at the same time an improvement of the constitution in different
directions, and let us go on to the consideration of cell-fusion
combined with nuclear fusion, or complete amphimixis. In this something
is added which we can recognize as an important advantage, namely,
the combination of two hereditary substances, and thus the union of
two variation-complexes which, according to our view, is necessary if
transformation of species is to take place. In mere plastogamy such a
union of two hereditary masses could only take place in Monera, not in
nucleated organisms. If then there are really unicellular organisms
which exhibit plastogamy without karyogamy (certain Foraminifera),
we have a further proof that these processes of plasmic fusion imply
direct advantage, which is distinct from the indirect advantage lying
in the mingling of two different hereditary contributions, since
in these cases of plastogamy there is no demonstrable mingling of
hereditary bodies, no karyogamy.
But as soon as karyogamy or nuclear fusion was associated with mere
plastogamy, complete amphimixis could never be lost again, because it
alone made it possible that there should be harmonious transformation
and adaptation in organisms which were becoming ever more complex; the
primary effect of the mingling would be more and more transcended,
since, without amphimixis, transmutation with harmonious adaptation
in all directions would be less and less possible as organisms became
more complex in structure. I have already referred to the manifold
details in the structure and development of the lowest organisms which
make this conclusion appear luminous to us, but we can also infer the
necessity for an unceasingly active selection, from a quite different
set of facts, namely, from what we know of rudimentary organs in Man.
We may regard Mankind as a species which has its local races and
sub-races, but which is fixed in its essential characters, and only
fluctuates hither and thither in individual variation in each sub-race,
just like any other modern mammal, such as the marmot or the hare.
Nevertheless we know that Man, as regards certain fairly numerous
parts, is continually and persistently varying in a definite direction.
Wiedersheim, in his book _On the Structure of Man_[23], enumerates a
long series of parts and organs of the human body, which are in process
of gradual degeneration, and of which it may be predicted that they
will disappear from the human structure since they have lost functional
significance. Among these dwindling structures are the two last ribs,
the eleventh and twelfth, while the thirteenth has already disappeared,
and only occurs exceptionally as a small vestige in the adult human
being of to-day. The series includes also the seventh cervical rib,
the _os centrale_ of the wrist, the wisdom teeth, and the vermiform
appendix of the intestine. The last is much larger in many mammals,
and represents an important part of the digestive apparatus, but in
Man it has dwindled to an unimportant appendage, which is a source of
danger when foreign bodies (cherry stones and such like) lodge in it
and set up inflammation. The variations in its length warrant us in
concluding that it is still in process of degeneration; its average
length is about 8½ cm., but it varies from 2 cm. to 23 cm. in length,
and in about 25 per cent. of cases a partial or entire closing up of
its opening into the intestine may be observed.
[23] _Ueber den Bau des Menschen_, 2nd ed., Freiburg-i.-Br., 1893.
Trans. London, 1896.
Wiedersheim enumerates nearly a hundred parts thus in process of
degeneration: this means that nearly a hundred structures in Man
are at the present time in process of variation, and this could not
be so unless amphimixis were continually mingling the hereditary
contributions anew from generation to generation, so that the
minus-variations of the parts in question, starting from the germ-plasm
in which they arose at one time as chance variations, and confirmed in
their direction by means of germinal selection, are gradually being
transmitted to all the germ-plasms of the species. We thus see that
even in a period of species-life, which we may fairly call a period of
constancy, variations of a phyletic kind are continually in process,
which could not become general without the co-operation of amphimixis.
Now, we have already seen that personal selection plays no part, or, at
least, no important part in such degenerations, because the variations
which are here concerned do not usually attain to selection value,
but it is just such variations proceeding with infinite slowness that
occur in functionally important organs likewise, and in the progressive
advance of which personal selection and mutual adaptation probably play
a part, so that in this way we can understand why the preservation of
amphigony by natural selection must be effected. It is impossible--for
obvious reasons--to name particular instances with certainty, as we
can do in the case of the rudimentary organs, but even on general
considerations we might expect that among the incipient variations
of the determinants of the germ-plasm there would be some which were
in an ascending direction, and that among these there would be some
which, advanced by germinal selection, would go on ascending until
they attained selection value. Wiedersheim reckons, for instance,
the gradually increasing differentiation of the cortical zone of the
human brain among the parts which are still in process of ascending
variation, and he is probably right in doing so.
But if variations, so slow as to be unnoticeable, are still of abundant
occurrence in Man, we have no reason to doubt that similar processes
are going on in other animals; among the higher Vertebrates at least
there is hardly a species which does not exhibit regressive variations
even now, and in many cases progressive variations also are occurring,
although we cannot give definite proofs of this.
The appearance of fixity which most species have is, therefore,
illusory; in reality they exhibit a slow flux, gradually setting aside
the superfluities they received from their ancestors, perfecting the
important parts to more precise adaptation and greater functional
capacity, and at the same time endeavouring to maintain all the parts
in constant harmony. We can understand that as long as this process of
gradual perfecting goes on, amphimixis will not readily be given up.
Those that retain it must always, in the long run, have the preference.
Moreover, as we have seen, _it cannot be given up_, when it has existed
through æons, because of the power of persistence which the germ-plasm
has gradually acquired in the course of such a long hereditary
succession. It could only be given up if an advantage decisive as to
survival were associated with its abandonment, such as can be actually
recognized in most cases of parthenogenesis, among animals at least.
In my opinion this indirect effect of amphimixis, that is, the
increasing of the possibilities of adaptation by new combinations of
individual variational tendencies, is the main one, while the direct
nutritive effect of the two germ-cells upon one another is quite
subsidiary. In this opinion I find myself in opposition to the views of
many if not most naturalists, who assume that amphimixis has a direct,
sometimes, indeed, only a direct effect, and believe that they can
prove it by facts.
In support of this position it has been pointed out that allogamy, that
is, the mingling of individuals of different ancestry, occurs even
among lowly unicellulars, and then higher up among most organisms;
but the question has not been asked whether this mutual attraction of
the unlike really expresses a primary characteristic of organisms,
and may not possibly be a secondary acquisition adapted to ensure the
occurrence of amphimixis. If we examine the facts we find that even in
the lowly Algæ, such as _Pandorina_ and _Ulothrix_, only the migratory
cells or swarm-spores of different cell-colonies conjugate with one
another, but not those of the same lineage, and this phenomenon may be
observed in many unicellular plants and animals. We are justified in
concluding from this that a fairly large degree of difference between
the conjugating gametes secures the best results, whether this result
is to be looked for in a 'rejuvenation' or in an increased adaptive
capacity; but it is erroneous to regard the stronger attraction
between individuals of different descent as a direct outcome of this.
To me, at least, it seems to be an adaptive arrangement. The whole
of the long and complex phylogenetic history of the sex-cells, the
gametes, shows clearly that we have here to deal with a succession of
adaptations, and that the degree of attraction which obtains between
gametes has gradually been increased and specialized in the course of
the phylogeny. I need only briefly recall what we have discussed in a
former lecture, that at first the copulating cells were exactly alike
in appearance and size, that then one kind of cell became rather larger
than the other, and that only gametes which were thus different in size
were mutually attractive--the micro-gametes and the macro-gametes,
or male and female germ-cells; we have seen that these differences
between the two became more and more accentuated, that the female
cell continued to grow larger than the male, and to accumulate more
and more nutritive material for the building up of the young organism
which arises from its union with the male cell, and that the male cells
became smaller, but more numerous, as was essential if their chances of
finding the often remote female cell were not to disappear altogether.
And besides, there are all the innumerable adaptations of the egg-cell
to the countless special circumstances which obtain in the different
groups, and the innumerable varieties in the form of the sperm-cell,
with all its delicate and complicated adaptations to the special
conditions under which the egg-cell can be reached and fertilized in
this or that group of organisms. Of a truth, he is past helping who
does not regard with wonder and admiration the adaptations which have
been worked out in this connexion in the course of evolution! But if
all these details are adaptations, so is the beginning of the whole
process of differentiation; allogamy, the attraction of conjugating
cells of different lineage, is not a primary outcome of individual
diversity; gametes of different descent did not strongly attract each
other of themselves, but they were equipped with a strong power of
mutual attraction, because the union of very different individualities
was the more advantageous.
This is an important distinction, for the adaptation to allogamy is
widely distributed, and its latest manifestations have frequently
been misunderstood in the same way as its beginnings. The widespread
occurrence of allogamy has been interpreted as evidence in favour of
the rejuvenation theory, and the endeavour on Nature's part to secure
the union of the unlike has been associated with the hypothetical
'rejuvenating' power of amphimixis, and regarded as a direct and
inevitable outcome of this. That this view is erroneous we shall see
even more clearly from what follows.
As among unicellular Algæ it is frequently only gametes of different
lineage which conjugate, so among animals and plants there are numerous
cases in which the union of nearly related gametes is more or less
strictly excluded, both by the prevention of self-fertilization
(autogamy) in hermaphrodites, or by the prevention of inbreeding, that
is, the continued pairing of near relatives. Now all the preventive
measures which effect this are of a secondary nature; they are
adaptations which result from the advantage involved in the mingling of
unrelated germ-plasms, even though it sometimes seems as if they were
an outcome of the primary nature of the germ-cells.
The primary result of the mutual chemical influence of the two
germ-cells upon one another is--apart from the impulse to development
which the centrosphere of the sperm-cell supplies--as far as I see,
only the more favourable or the more unfavourable mingling of the
biophor- or determinant-variants, and the resulting increase or
decrease in adaptive capacity, which leads to the better thriving of
the offspring, or conversely to its degeneration. Everything else
is secondary and depends upon adaptation, effected in very diverse
ways, to secure the most favourable mingling of the germ-plasms for
the particular species concerned. Undoubtedly the parental ids united
through amphimixis have an effect upon each other, since throughout the
building up of the organism of the child the homologous determinants
struggle with one another for food, but they do not affect each other
in the way that many prominent physiological and medical writers
suppose, namely, that the union of the parental germ-plasms sets up a
'formative stimulus' which 'advances' or even 'greatly advances' the
process of development in the egg.
Parthenogenetic development goes on just as rapidly, sometimes even
more rapidly than that of the fertilized ova of the same species! How
can the supposed 'formative stimulus' be so entirely dispensed with in
this case?
Of course I am well aware that the two kinds of germ-cells have a
strong attraction for each other, and that the protoplasm of the ovum
actually exhibits tremulous movement when the spermatozoon penetrates
through the micropyle. I myself observed this in the case of the
lamprey (_Petromyzon_) when Calberla instituted his investigations on
the fertilization of that animal, but has that anything to do with
a formative stimulus? Is it anything more than the result of the
chemotactic stimulus exerted by the substance of the ovum upon that
of the spermatozoon and conversely? And have we any ground for seeing
anything more in this than an adaptation of the sex-cells to the
necessity of mutually finding each other out and thereafter combining?
Two quite different things are often confused with one another in this
connexion: the mutual attraction of the two kinds of sex-cells which
tends to secure their union, and the results of this union. A more
exact distinction is necessary between the effects and the advantages
which allogamy brings in its train and the means by which it is secured
in the different species.
If amphimixis really set up a 'formative' stimulus, and if the amount
of this was regulated by the differences between the two parental
germ-plasms, then parthenogenesis, which implies the entire absence
of the mingling of two parental cells, would necessarily be even less
advantageous than amphimixis between near relatives; but this is not
the case. Continued inbreeding leads in many cases to the degeneration
of the descendants, and particularly to lessened fertility and even
to complete sterility. Thus in my prolonged breeding experiments with
white mice, which were later carried on by G. von Guaita, strict
inbreeding, effected throughout twenty-nine generations, resulted in
a gradually diminishing fertility, and similar observations have been
made by Ritzema Bos and others. But why does not the same thing happen
in pure parthenogenesis? My experiments in breeding parthenogenetic
Ostracods (_Cypris reptans_) shows that these crustaceans, in the
course of the eighty generations which I have observed till now[24],
have lost nothing of their prolific fertility and vital power; and
the same is true in free nature of the rose-gall wasp (_Rhodites
rosæ_), which enjoys the greatest fertility notwithstanding its purely
parthenogenetic reproduction, the females not infrequently laying
a hundred eggs in a single bud. How does it happen that 'the mutual
influence of two different hereditary substances which so powerfully
promotes individual development' can be here altogether dispensed with?
Only because it does not really exist, except in the imagination of my
opponents, still influenced by the old dynamic theory of fertilization.
[24] The cultures were begun in 1884 and are still continued (March 6,
1902), still multiplying as abundantly as at the outset. I reckon that
there are on an average five generations in a year, which means about
eighty generations in sixteen years.
But it may be asked, whence come the injurious results of inbreeding,
if not from the union of two nearly related germ-plasms? They certainly
do arise from that cause, but it is not through a 'formative stimulus,'
_too slight_ in this case, exercising a direct formative chemical
effect upon the two hereditary substances, but through the indirect
influences exerted by these _too similar_ hereditary contributions
during the development of the new individual. Lest it be imagined that
I am tilting against windmills, I will refer to one of the numerous
examples of the evil effects of inbreeding which have been submitted
to me as specially corroborative of the conception of amphimixis as a
'formative stimulus' whose strength depends upon the difference between
the germ-substances. The renowned breeder, Nathusius, allowed the
progeny of a sow of the large Yorkshire breed, imported from England
when with young, to reproduce by inbreeding for three generations. The
result was unfavourable, for the young were weakly in constitution
and were not prolific. One of the last female animals, for instance,
when paired with its own uncle--known to be fertile with sows of a
different breed--produced a litter of six, and a second litter of five
weakly piglings. But when Nathusius paired the same sow with a boar of
a small black breed, which boar had begotten seven to nine young when
paired with sows of his own breed, the sow of the large Yorkshire breed
produced in the first litter twenty-one and in the second eighteen
piglings.
How could this really remarkable difference in the fertility of the
sow in question be the result of a formative stimulus, exercised by
the sperm-cells of the unrelated boar upon the ova of the female
animal? If the progeny of the sow had been more fertile than herself,
then we should have been at least logically justified in concluding
that this was the case, but it is not intelligible that the egg-cells
of this mother sow should be increased twice or three times because
they were fertilized by a new kind of sperm as they glided from the
ovary. The number of ova which are liberated from the ovary depends
in the first instance upon the number of mature ova contained in
it; and unless we are to make the highly improbable assumption that
the crossing with the strange boar had as an immediate result the
maturing of a large number of ova, we must look elsewhere than in the
ovary of the animal for the cause of this sudden fertility, possibly
in chance circumstances which we are unaware of and which make the
ovary occasionally more productive, possibly however in the fact that
inbreeding may have brought about various slight structural variations
in the animal, and among these some which made the fertilization of
the abundantly produced ova by the sperm of the related boar less
easy, and caused it to fail more frequently. As will be readily
understood, I cannot say anything definite on this point, but we know
that very slight variations in the sperm-cell or the ovum may make
fertilization difficult, or may even prevent it. I need only remind
you of the interesting experiments in hybridization which Pflüger
and Born made with Batrachians nearly thirty years ago, which showed
that in two nearly related species of frog the ova of the species A
were frequently fertilized by the sperms of the species B, but not
conversely, the ova of the species B by the sperms of A. This is the
case, for instance, with the green edible frog (_Rana esculenta_),
and the brown grass-frog (_Rana fusca_), and the reason of this
dissimilarity in the effectiveness of the sperm lies simply in 'rough
mechanical conditions,' in the width of the micropyle of the ovum,
and the thickness of the head of the spermatozoon. If each species
possesses a micropyle which is exactly wide enough to admit of the
passage of the spermatozoon of its own species, another species will
only be able to fertilize these eggs if the head of its spermatozoon
be not larger than that of the first species. Thus, as experiment has
proved, the spermatozoa of _Rana fusca_ fertilize the ova of almost
all other related species, for they have the thinnest head and it is
at the same time very pointed. In this case, therefore, it depends
upon the microscopic structure of the ovum whether fertilization can
take place or not, and we can imagine that similar or perhaps other
minute variations had taken place in the ova in the case of Nathusius's
sow, and that these made it difficult for the sperms of boars of the
same family to effect fertilization. These variations may have arisen
as a result of the continued inbreeding, because the same ids were
constantly being brought together in the fertilized ova, and thus any
unfavourable directions of variations which existed were strengthened.
It seems to me that in this way alone can the injurious effect of
inbreeding be made intelligible. From both parents identical ids meet
in the fertilized ovum, in greater numbers the longer inbreeding
continues, for at the maturation of every germ-cell the number of
different ids is diminished by a few, and their number must therefore
gradually decrease, and it is conceivable that ultimately it may
sink to one kind of id, that is, that the germ-plasm may then consist
entirely of identical ids. If chance variations of certain determinants
in unfavourable directions occur in some of the ids composing the
germ-plasm, these are brought together in the offspring from both the
maternal and the paternal side, and will occur in an increasing number
of ids the longer the inbreeding has gone on, that is, the smaller the
number of different ids has become. The unfavourable variation-tendency
is therefore persisted in, and its influence upon the development of a
new descendant will be the greater the larger the number of identical
ids with these unfavourable variations. It is obvious that the crossing
of an animal, which is thus, so to speak, degenerating slightly, with
a member of an unrelated family must immediately have a good effect
upon the descendants, for in this way quite different ids with other
variations of their determinants are introduced into the inbred
germ-plasm which had become too monotonous.
From this theoretical interpretation of the injurious consequences of
inbreeding we may at once infer that not every inbreeding necessarily
implies degeneration, for the occurrence of unfavourable variational
tendencies in the germ-plasm is presupposed as the starting-point of
degeneration, and if these do not exist there can be no degeneration.
This harmonizes with the fact that the evil effects of inbreeding are
observed _to vary greatly in amount, and may not occur at all_. But
they are greatest in breeds artificially selected by man, which have
long been under unnatural, directly influential conditions, and are
also removed from the purifying influence of natural selection. In such
cases, therefore, there is every probability that diverse unfavourable
variational tendencies in the determinants will occur.
But how are we to understand the fact that pure parthenogenesis
may last through innumerable generations, and yet no degeneration
sets in? I believe very simply. In this case too, the same ids
which were peculiar to the mother of the race are contained in the
descendants, but they do not diminish in number, for in pure and
normal parthenogenesis, such as that of _Cypris reptans_, the second
maturation-division of the ovum does not take place, and this is
precisely the nuclear division which effects reduction. In addition,
the introduction of identical ids, which must take place in the case of
inbreeding at every amphimixis, does not occur, and, what is certainly
of great importance, all these cases are old species, living under
natural conditions--the same conditions under which they lived as
amphigonous species, and not newly formed breeds under artificial
conditions, as has probably always been the case in the experiments in
inbreeding.
It is true that even in old species, living in a state of nature,
unfavourable variations may arise in the germ-plasm, and may go on
increasing during purely parthenogenetic multiplication, for the ids
with unfavourably varying determinants will no longer be set aside
by means of reducing division. But those individuals in which the
unfavourable variational tendency increases until it has attained
selection-value will be subject to selection and will be gradually
eliminated; indeed, the weeding out of the inferior individuals will be
more drastic here than where amphigony obtains, because in this case
all the offspring of one mother are nearly alike, so that the whole
progeny is exterminated if the mother varies unfavourably.
On the other hand, a transformation in a favourable direction, an
adaptation to new conditions of life, as far at least as that implies
the simultaneous variation and harmonious co-adaptation of many parts,
cannot, as far as I can see, be effected in the course of purely
parthenogenetic reproduction, nor can a degeneration of complicated
parts which have become superfluous. For both these changes, in my
opinion, require that the ids of the germ-plasm should be frequently
mingled afresh, since apart from this there cannot be a harmonious
readjustment of complicated structures, nor can a uniform degeneration
affecting all parts set in. As an example of this last case we may take
that organ which became functionless in the purely parthenogenetic
species of Ostracods when amphigonous reproduction was given up--the
sperm-pocket or receptaculum of the female. All these species still
possess an unaltered receptaculum seminis, a large pear-shaped bladder
with a long, narrow, spirally coiled entrance-duct, very well adapted
for allowing the enormous spermatozoa of the males to make their way
in singly, and to arrange themselves within the receptacle side by
side in the most beautiful order, like a long ribbon, and finally to
migrate out again singly to fertilize the liberated ova. In _Cypris
reptans_ and several other species, however, no males have been found
in any of the places which have been carefully searched, and the
receptaculum of the female is always found to be empty. Nevertheless
it shows no hint of degeneration. It is possible enough that, as in
_Apus cancriformis_, which is of similar habit, the males have become
extinct in most colonies of these species, but that nevertheless they
do occur here and there from time to time in the area inhabited by the
species, and if this should prove to be the case, it would confirm the
conclusion, which is very probable on other grounds, that the pure
parthenogenesis of these species has not existed in most of their
habitats for a long time, speaking phylogenetically. For this reason we
must not over-estimate the significance of the complete persistence of
the receptaculum even with exclusively parthenogenetic reproduction.
It proves, however, that degeneration of a superfluous organ does
not necessarily set in even after hundreds of generations, and in
this fact there is certainly a corroboration of the view that it is
'chance' germinal variations which give the impulse to degeneration.
These first induce a downgrade variation through germinal selection,
and this, if it concerns an organ of no importance to the survival of
the species, is not hindered in its progress by personal selection.
Whether degeneration of the receptaculum would have occurred in
these parthenogenetic species if they had retained even a periodic
sexual reproduction, as is actually the case in the generations of
the alternately parthenogenetic and sexually reproducing Aphides,
we cannot decide, since we know nothing in either case as to the
length of time that parthenogenesis has prevailed among them, nor
have we any method of computing the number of generations that must
elapse before a superfluous organ begins to vacillate. We only know
that the parthenogenetic generations of Aphides no longer possess
a receptaculum, while other forms with alternating bi-sexual and
parthenogenetic modes of reproduction, which are in this respect
possibly more modern, e. g. some of the gall-wasps, possess one similar
to that of the Ostracods.
[Illustration: FIG. 79 (repeated). The two maturation divisions of the
'drone eggs' (unfertilized eggs) of the bee, after Petrunkewitsch.
_Rsp_ 1, the first directive spindle. _K_ 1 and _K_ 2, the two
daughter-nuclei of the same. _Rsp_ 2, the second directive spindle. _K_
3 and _K_ 4, the two daughter-nuclei. In the next stage _K_ 2 and _K_ 3
unite to form the primitive sex-nucleus. Highly magnified.]
I must refer to one other case of parthenogenesis, since it has been
hitherto regarded as a formidable puzzle for the germ-plasm theory,
and has only recently found its solution, I mean the facultative
parthenogenesis of the queen-bee. As the 'male' eggs of the bee remain
unfertilized, and yet undergo two reducing divisions, which must
diminish the number of ids in the ovum-nucleus by a half, the number of
ids in the germ-plasm of the bee must be steadily decreasing, and this
state of things has therefore been regarded by some English biologists
as convincing evidence of the untenability of the conception of ids and
of the whole germ-plasm theory. Apparently, indeed, it is contradictory
to the theory, and we must inquire whether the contradiction is merely
an _apparent_ one, disappearing when the facts are more precisely
known. It was mainly on this ground that I instituted the researches
carried out by Dr. Petrunkewitsch, the results of which I have already
in part communicated in a former lecture. These results confirmed
the previous conclusions that the 'male' eggs of the queen-bee
remain unfertilized, that two reducing divisions occur, and that in
consequence the ovum-nucleus only contains half the normal number of
chromosomes. That these increase again by division to the normal
number does not save the theory, for only _identical_ ids can arise in
this way, while the significance of the multiplicity of the ids lies
mainly in their difference. The halving of the number of ids in each
'male' ovum would necessarily lead, if not to a permanent diminution
in the number of ids, at least to a monotony of the germ-plasm, since
the number of _different_ ids would be steadily decreasing and the
number of _identical_ ids as steadily increasing. This too would be a
contradiction of the theory. But Dr. Petrunkewitsch's investigations
have shown that, of the four nuclei which are formed by the two
reducing divisions, the two middle ones (Fig. 79, _K_ 2 and _K_ 3)
recombine with one another, and fuse into a single nucleus, and that
from this copulation-nucleus in the course of development the primitive
germ-cells of the embryo arise. Now all the ids which were originally
present in the nucleus of the immature ovum may be reunited in this
'polar copulation-nucleus' if the two nuclei _K_ 2 and _K_ 3 turned
towards each other in Fig. 79 contain different ids. That this is the
case cannot of course be seen from the ids themselves, but it seems to
me extremely probable, since it is dissimilar poles of the two nuclear
spindles which here unite, namely, the lower pole (daughter-nucleus) of
the upper spindle and the upper pole of the lower spindle. In the first
directive or polar spindle there lay thirty-two chromosomes, which had
increased by duplication from sixteen, and of these sixteen passed over
into the first polar nucleus, while sixteen formed the basis of the
second directive spindle. These two sets of sixteen chromosomes must
have been quite similar, since the two sets arose by division of the
sixteen mother-chromosomes. Let us call the chromosomes _a_, _b_, _c_,
_d-q_, then similar sets of chromosomes must have been contained in the
two nuclear spindle figures depicted in Fig. 79 at the beginning of the
division, and eight of these went to each daughter-nucleus. Now, if
_a-k_ migrated to the upper pole of the spindle and _l-q_ to the lower
pole, then the union of _K_ 2 with _K_ 3 would bring together again
all the ids that had before been present. In consideration of this I
predicted to Dr. Petrunkewitsch that this copulation-product might be
the basis of the formation of the germ-cells in the drone-bee, and his
painstaking and difficult researches have confirmed this prediction,
strange though it may seem, that the male germ-cells have a different
origin from the female germ-cells. But this discovery gives a strong
support to the germ-plasm theory. It may, of course, be objected that
the assumed regular distribution of the ids in the two daughter-nuclei
cannot be proved, but we know already that this dividing apparatus
does _very exact_ work, and we are at liberty to assume it in an even
higher degree. Moreover, what other interpretation of the unexpected
development of the germ-cells discovered by Petrunkewitsch could be
given if this had to be rejected? A clearer proof of the individual
differences of ids and of their essential importance could not be
desired, than lies in the fact that in the 'male' eggs of the queen-bee
a different and novel mode of germ-cell formation is instituted, after
half the ids have been irrecoverably withdrawn from the ovum-nucleus.
We see from this that for individual development a duplication of
individual ids may suffice, but that for the further development of the
species a retention of the diversity of the ids is important.
LECTURE XXX
INBREEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION, AND THEIR
CONSEQUENCES
The separation of the sexes exists even among the
Protozoa--Conditions determining the occurrence of
Hermaphroditism--Tape-worms, Cirrhipeds--Primordial males--Advantages
of parthenogenesis--Alternation with bi-sexual generations--In
Gall-wasps--In Aphides--Cross-fertilization secured in
plants--Self-fertilization is avoided whenever possible--The
mechanism of fertilization and the mingling of germ-plasms must
be clearly distinguished from one another--Cases of persistent
self-fertilization--The effects of inbreeding compared with those
of parthenogenesis--The effect of purely asexual reproduction--In
sea-wracks--In lichens and fungi--In cultivated plants--Degeneration
of the sex-organs--Summary.
We have seen that continued inbreeding must make the germ-plasm
monotonous, and therefore unplastic as regards the requirements of
adaptation. Accordingly, we found that the gametes of many unicellulars
are so constituted that they only possess a power of attraction for
gametes of a different lineage, not for those of their own stock. Among
multicellular organisms the most intense mode of inbreeding is to be
found in the uninterrupted self-fertilization of hermaphrodites: in
such cases the monotony of the germ-plasm must reach extreme expression
more readily than in the case of ordinary inbreeding. We can thus
understand why, in the scale of organisms, there is such an early
occurrence of gonochorism, the separation of the species into male and
female individuals. Even among unicellular plants or Protophytes this
occurs occasionally, as it does in the Vorticellids among Infusorians.
In the Metazoa and Metaphyta the separation of the sexes finds emphatic
expression; it is absent from no important group, and in many, such
as, for instance, among the Vertebrates, it has become the absolutely
normal condition, with hardly any exception. But in many divisions of
the animal and plant kingdoms hermaphroditism also plays an important
part, as, for instance, in terrestrial snails and in flowering plants.
Obviously the sexual adaptations of a species are definitely related
to the conditions of its life, and, though Nature's endeavour to
prevent inbreeding and to secure cross-fertilization is evidenced by
the occurrence of separate sexes in such a multitude of forms yet
in many cases gonochorism has been relinquished, and always where
this was necessitated by the conditions of life to which the group
concerned was subject. In such a case inbreeding is regulated as
far as possible, for instance, by an arrangement which ensures that
individuals shall be crossed at least from time to time. But cases of
exclusive and constant self-fertilization do also seem to occur, and
even these may be brought into harmony with our conception, according
to which cross-fertilization is an advantage, but only an advantage
which must be weighed against others, and which may eventually be given
up in favour of greater advantages. This occurrence of persistent
autogamy can no more be reconciled with the rejuvenation theory than
can continuous parthenogenesis, because, according to this theory,
the mingling of different individuals is a _sine qua non_, for the
continued life of the species.
It is impossible for me here to discuss in detail all the deviations
from pure gonochorism or bi-sexuality which occur in nature, but I must
at least attempt to take a general survey, and to arrange the chief
phenomena of these various modes of 'sexual reproduction' in an orderly
scheme. I must take a survey of both plants and animals, but I shall
give the precedence to animals, as being to me more familiar ground.
Where do we find, in the animal kingdom, that Nature has departed from
gonochorism, from the separation of the sexes, and for what reasons was
this departure necessary? And further, what means does Nature take to
compensate for this renunciation of the simplest method of securing the
continual cross-fertilization of individuals?
Let us glance over the animal kingdom with special reference to these
questions: we find that hermaphroditism prevails chiefly among species
which at maturity have lost their power of free locomotion, and have
become sedentary, such as oysters, barnacles among Crustaceans, the
Bryozoa, and the sea-squirts (Ascidians) which are fixed to the rocks
at the bottom of the sea. For forms such as these it must often have
been advantageous that each individual could function both as male
and as female, especially when it was capable of self-fertilization,
since individuals which settled down singly, or in very small numbers
together, would not be lost as regards the persistence of the species.
The continuance of the species is thus better secured than it would
be by separation of the sexes, because in the latter case it might
frequently have happened that the animals which had settled beside
each other by chance were of the same sex, and would therefore remain
unfertile. But many of these species do not fertilize themselves, but
fertilize each other mutually; and this, too, carries a great advantage
with it, because in sedentary animals the sperms will fertilize
twice as many individuals, if each contains eggs, than if half were
exclusively male. It is thus to some extent an economy of sperms, but
at the same time also of ova, which is effected by hermaphroditism:
the result is that these valuable products are wasted as little as
possible. On this account we find that not only sedentary, but also
sluggish, slow-moving animals are equipped with male and female organs
of reproduction, as, for instance, all our terrestrial snails. They
fertilize each other mutually: when two meet it is always as males and
females, and notwithstanding the sluggishness of movement, it is not
likely to happen that a snail does not attain to reproduction because
it has not found a mate. The same is true of the earthworms, which
are likewise not adapted for making long journeys in search of the
opposite sex; they, and the leeches also, function as male and female
simultaneously, while their nearest relatives, the marine Chætopods,
are of separate sexes, which may be associated with their much greater
power of free movement in the water.
In these cases self-fertilization is often absolutely excluded; it
may be physically impossible, and hermaphroditism therefore secures
cross-fertilization in such cases just as effectively as if the sexes
were separate. Similarly, in many hermaphrodite flowers, as we have
already seen, the pollen is so constituted and so placed within the
flower that it cannot of itself make its way to the stigma. In oysters,
for instance, the young animal is male, and liberates into the water an
enormous quantity of minute spermatozoa, and therewith fertilizes the
older individuals, functioning only as females, which have grown upon
the same bank. At a later stage of its development the oyster which was
male becomes female, and produces only ova. This state of affairs, of
which I shall shortly mention another case, has been called _temporary_
hermaphroditism. In this case not only is self-fertilization excluded,
but close inbreeding also, since it is always a young generation
functioning simultaneously as males that mingles with an older
generation which has become female.
It is quite otherwise with parasites which live singly within the body
of a host: for these it was indispensably necessary that they should
not only produce both kinds of germ-cells, but that they should unite
the two kinds in fertilization, and they therefore possess the power
of self-fertilization. Thus, in the urinary bladder of the frog, there
occurs a flat-worm (_Polystomum integerrimum_) which possesses special
organs for pairing with another individual, but which is also capable
of self-fertilization when, as frequently occurs, it has no companion
in its place of abode. But this self-fertilization is always liable to
be interrupted by cross-fertilization, for not infrequently there are
two, three, or even four such parasites within the bladder of a single
frog.
In the tape-worms, too, cross-fertilization is not excluded, for there
are often two or more of these animals together in the intestine
of a host at the same time. But even where there is only one,
self-fertilization on the part of the joints, that is, the sexual
individuals, is prevented, and by the same device, metaphorically
speaking, as in the case of the oyster, for in each joint the male
elements mature first and the female elements afterwards. In certain
parasitic Isopods of the genus _Anilocra_ and related forms close
inbreeding is prevented in the same way--by a difference in the period
at which the two sets of gonads in the hermaphrodite individual become
mature (dichogamy).
This is secured in a different way in Crustaceans which have grown to
maturity in a sedentary state, like the Cirrhipeds. These animals,
known as 'acorn-shells' and 'barnacles,' are sedentary, sometimes
on rocks and stones, sometimes on a movable object, the keel of a
ship, floating pieces of wood, cork, or cane, or sometimes attached
to turtles or whales, and although they generally occur in great
numbers together, they are probably only able to fertilize each
other occasionally, and are therefore essentially dependent upon
self-fertilization. But Charles Darwin discovered long ago that many
of them, notwithstanding their hermaphroditism, have males which are
small, dwarf-like, and very mobile organisms, destined only for a
very brief life. These seemed quite superfluous in association with
hermaphrodite animals, and they have therefore long been regarded as
vestigial males, as the last remnant, so to speak, of a past stage of
the modern Cirrhipeds, in which the sexes were separate. It is obvious,
however, that we must now attribute to them a deeper significance,
for these so-called 'primordial males,' although extremely transitory
creatures without mouth or intestine, represent a means of securing
the cross-fertilization of the species. What importance nature
attaches to their preservation is shown especially by the parasitic
Cirrhipeds which have been so carefully studied by Fritz Müller
and Yves Delage--those sac-like Rhizocephalidæ or root-crustaceans
which are altogether disfigured by parasitism. The fully developed
animals are hermaphrodite and live partly in, partly upon crabs and
hermit-crabs (Fig. 112, _C_, _Sacc_). These hermaphrodites indeed
fertilize themselves, but in their youth they are of distinct sexes,
and the females are so constituted that they lay eggs for the first
time just when the males of the current year are appearing. Thus the
first batch of eggs liberated by the females are fertilized by the
minute free-swimming 'primordial males,' but after that the females
themselves develop testes, and then fertilize themselves; the males
die very soon after copulation, and only appear the following year
in a new generation. They are therefore far from being mere historic
reminiscences, vestiges of the early history of the modern species,
for they are the instruments of a regular cross-fertilization of
the species, and therefore of a constant mingling of new ids in
the germ-plasm. This is not the place to discuss the marvellous
life-history of these parasites in detail; I can only say that when
we inquire into the whole story, and appreciate the difficulties
associated with the persistence of these 'primordial males,' we can no
longer doubt that crossing is an indispensable feature of amphimixis--a
feature which must at least occasionally occur if amphimixis is to
retain its significance. This is shown, it seems to me, especially by
these numerous instances of what we may call compulsory retention of
ephemeral males in hermaphrodite, self-fertilizing animals; it follows
also from the theory, for with continued self-fertilization all the ids
in the germ-plasm of an individual would tend to become identical, and
the mingling of two germ-plasms which contained _identical_ ids would,
at least according to the germ-plasm theory, have no meaning at all.
[Illustration: FIG. 112 (repeated). Development of the parasitic
Crustacean _Sacculina carcini_, after Delage. _A_, Nauplius stage.
_Au_, eye. _I_, _II_, _III_, the three pairs of appendages. _B_,
Cypris stage. _VI-XI_, the swimming appendages. _C_, mature animal
(_Sacc_), attached to its host, the shore-crab (_Carcinus mænas_),
with a feltwork of fine root-processes enveloping the crab's viscera.
_S_, stalk. _Sacc_, body of the parasite. _oe_, aperture of the
brood-cavity. _Abd_, abdomen of the crab with the anus (_a_).]
Thus we see that in the animal kingdom hermaphroditism is always
associated with cross-fertilization in some way or other, even though
the latter may occur rarely, being usually periodically interpolated,
and thus bringing new ids into the germ-plasm which is rapidly becoming
monotonous or uniform. Adaptations quite analogous to these are found
in relation to parthenogenesis, and it will repay us to give a brief
summary of these.
Parthenogenesis effects a very considerable increase in the fertility
of a species, and in this increase the reason for its introduction
among natural phenomena obviously lies. By the occurrence of
parthenogenesis, the number of ova produced by a particular colony of
animals may be doubled, because each individual is a female, and as the
multiplication increases in geometrical ratio a few parthenogenetic
generations result in a number of descendants enormously in excess of
those produced by bi-sexual reproduction. We can therefore understand
why parthenogenesis should obtain among animals whose conditions of
life are favourable only for a short time, and are then uncertain and
dangerous for a long period. This is the case with the water-fleas,
the Daphnids (see Figs. 57 and 58), whose habitats--pools, ponds, and
marshes--often dry up altogether in summer, or freeze in winter, so
that it becomes almost if not quite impossible for the colonies to go
on living, and the preservation of the species can only be secured
by the production of hard-shelled 'lasting' eggs, which sink to the
bottom, dry up in the mud, or become frozen, or at least remain latent
in a sort of slumber. As soon as the favourable conditions reappear,
young animals which emerge from the eggs are all females and reproduce
parthenogenetically, so that after a few days there is a numerous
progeny swimming freely about, which in their turn are all females,
and reproduce after the same manner. In many Daphnids this goes on
for a series of generations, and there thus arises an enormous number
of animals, which may fill a marsh so densely that, by drawing a
fine net a few times through the water, one can draw out a veritable
animal soup. In our ponds and lakes these little Crustaceans form the
fundamental food of numerous fishes. But notwithstanding the enormous
havoc wrought among them by enemies, large numbers remain at the end
of a favourable season, and these produce the lasting eggs, _after
fertilization_. For shortly before the end of the season males appear
among the progeny of the hitherto purely parthenogenetic females.
Although each female will only produce a few of these 'lasting' eggs,
which require fertilization and are richly supplied with yolk, the
whole number in each colony is a very large one, because the number of
individuals is very large; and it must be so, since the eggs, though
secure against cold and desiccation, are very imperfectly protected
against the numerous enemies which may do them injury.
Of course the number of individuals which form a colony may vary
greatly in the different species, and the same is true of the number
of parthenogenetic generations which precede the bi-sexual generation.
I have already shown in detail that this depends precisely on the
average duration of the favourable conditions, so that, for instance,
a species which lives in large lake-basins will produce many purely
parthenogenetic generations before the bi-sexual one, which only
appears towards autumn, while species which live in quickly-drying
pools have only a few parthenogenetic generations, and the true
puddle-dwellers give rise to males and sexual females along with the
parthenogenetic females as early as the second generation.
We thus find in the Daphnids an alternation, regulated and made
normal by natural selection, of purely parthenogenetic with bi-sexual
generations, and the result is that the uniformity of the germ-plasm,
which is the necessary consequence of pure parthenogenesis, is
interrupted after a longer or shorter series of generations by
the occurrence of amphimixis. That the number of parthenogenetic
generations may be so varied, though with a definite norm for each
species, indicates again that amphimixis is not an absolute condition
of the maintenance of life, not an indispensable rejuvenation, designed
to counteract the exhaustion of vital force--whether this be meant
in a transcendental sense or otherwise--but that it is an important
advantage calculated to keep the species at its highest level, and that
its influence appears whether it occurs in the species regularly, or
frequently, or only rarely.
This kind of alternation of generations, that is, the alternation
between unisexual (female) and bi-sexual generations, has been called
heterogony. In the Daphnids, certainly, a difference in form between
the parthenogenetic and the bi-sexual generation does not exist,
for the same females which produce eggs requiring fertilization can
also produce parthenogenetic ova, although these are very different
from each other, as we have already seen. The difference between
generations, therefore, does not lie in their structure, but in their
tendency to parthenogenetic or to amphigonous reproduction, and in the
absence or presence of male individuals. There are, however, other
cases of alternation of generations in which the different generations
diverge from each other in structure. One of the most remarkable of
these is that of the gall-wasps (Cynipidæ). In many of these little
Hymenoptera, which form galls on leaves, blossoms, buds, and roots,
especially of the oak, two generations occur annually, one in summer,
the other in early spring, or even in the middle of winter. The latter
consists of females only and reproduces parthenogenetically. We can
readily understand this from the point of view of adaptation to
particular conditions, since the young wasps which emerge from their
galls in winter, or in the middle of a raw spring, are exposed to many
dangers and are terribly decimated before they can succeed in laying
their eggs in the proper place on the plant. Moreover, much precious
time would be lost by the mutual search of the sexes for each other,--a
search which would often be entirely without result. Thus the wingless
female of _Biorhiza renum_ (Fig. 124, _A_), which is not unlike a plump
ant, attempts, without taking food, and often interrupted by a spell
of cold or a snowstorm, to reach a neighbouring oak-shrub, creeps up
on it, and lays its eggs in the heart of a winter bud, whose hard
protecting scales it laboriously perforates by means of its short,
thick, sharp ovipositor.
[Illustration: FIG. 124. Alternation of generations in a Gall-wasp.
_A_, winter generation (_Biorhiza renum_). _B_ and _C_, summer
generations (_Trigonaspis crustalis_). _B_, male. _C_, female. After
Adler.]
After it has succeeded in sinking its ovipositor into the heart of the
bud, it goes on working for hours, piercing the delicate tissue with a
multitude of fine canals, one close beside the other, and then deposits
an egg in each of these. The whole detailed piece of work requires,
according to Adler, uninterrupted active exertion for about three days,
even though in the end only two buds may be filled with eggs. If at
every egg-laying the arrival of a male had to be waited for, an even
larger number of females would fall victims to the unfavourable weather
and other dangers, while at the same time the number of emerging
females could be only half as large as it is. It is obvious that in
this case parthenogenesis is of very great advantage.
In summer the climatic conditions are incomparably more favourable for
the gall-wasps, and accordingly we find that the summer generation
is bi-sexual, but, strangely enough, is so different from the winter
generation that the relationship of the two forms was for a long time
overlooked. The antennæ, the legs, and particularly the ovipositor,
the whole shape of the animal, its size, the length of the abdomen,
the structure of the thorax, and many other points are so different
that as long as the structural features afforded the only criterion
of relationship, the systematists quite naturally placed the winter
and summer forms in different genera. It was only when Dr. H. Adler
succeeded in breeding the one form from the other that people were
convinced that such marked differences in structure could be found
within the same life-cycle.
[Illustration: FIG. 125. The two kinds of Galls formed by the species.
_A_, the many-chambered galls produced by the parthenogenetic
winter form, _Biorhiza renum_. _B_, those produced on oak-leaves by
_Trigonaspis crustalis_, the bi-sexual form. After Adler.]
But we see here quite clearly why the two generations had to become so
different; simply because the winter generation had to adapt itself
to different conditions from the summer generation, above all as to
the laying of its eggs within the tissues of a plant of a different
constitution. In our example, the winter form _Biorhiza renum_ pierces
the terminal buds of the oak, and lays in each of them a large number
of eggs, sometimes as many as 300, so that a very large gall is
formed, in which a great many larvæ can find food, and grow on to the
pupa-stage. From this spongy gall, something like an inverted onion in
shape, and about the size of a walnut (Fig. 125, _A_), there emerge in
July the slender, delicately formed male and female gall-wasps which
were long known as _Trigonaspis crustalis_. Both males and females
are winged, and fly rapidly about in the air (Fig. 124, _B_ and _C_).
The sexes pair, and the females lay their eggs in the cell-layers
on the under side of an oak-leaf, on which arise small, wart-like,
kidney-shaped galls (Fig. 125, _B_) which fall to the ground in autumn,
and from which there emerge, in the middle of winter, the plump,
wingless females, to which, as we have already seen, the name _Biorhiza
renum_ was given.
One generation, therefore, lays its eggs in the parenchyma of tender
leaves, and has only to pierce through a thin layer of plant-tissue,
while the other must penetrate deep down into the hard winter bud,
to be able to deposit its eggs in the proper place, and we therefore
find that in the two kinds of female the ovipositor differs in length,
thickness, and general structure, and so also does the whole complex
apparatus by which the ovipositor is moved. But these differences are
associated with the form of the abdomen, in which the ovipositor lies,
and with the strength and shape of the legs, which must be shorter
and stronger when the boring has to be performed through a hard
plant-tissue or to a considerable depth. We can readily understand how
numerous must be the secondary variations which a transformation of the
ovipositor brings in its train when we compare the ovipositor apparatus
in the two generations of one of these species (Fig. 126).
[Illustration: FIG. 126. Ovipositor and ovum of the two generations
of the same species of Gall-wasp. _A_, those of the winter form,
_Neuroterus læviusculus_. _B_, those of the summer-form, _Spathegaster
albipes_. _st_, ovipositor. _ei_, ovum. Similarly magnified. After
Adler.]
Figure 126 shows the ovipositor of another gall-wasp, of which the
winter form, _Neuroterus læviusculus_, also perforates the hard winter
buds of the oak, while the summer form, _Spathegaster albipes_, lays
its eggs in the tender young leaves of the same tree. The ovipositor of
the former is thin and long, that of the latter short and strong (Fig.
126, _A_ and _B_), and corresponding also to the depth at which the egg
must be sunk, or, so to speak, sown in the tissue of the plant, the egg
of the summer generation differs from that of the winter generation
by having a much shorter stalk (Fig. 126, _ei_). These little wasps
thus afford a beautiful example of the way in which even marked
changes in the conditions of life of a generation may be associated
with transformations in bodily structure, and we understand how it
was possible that by means of processes of selection the generations
which alternate periodically in the year should come to diverge very
considerably in structure. The example may also serve to illustrate how
diverse are the harmonious co-adaptations which such transformations
require, and how necessary, therefore, the continual re-combination of
the ids of the germ-plasm by means of amphimixis must be. We understand
why bi-sexual reproduction was only abandoned in one generation, and
that the one in which parthenogenesis was of considerable advantage.
But such transformations must have come about with extreme slowness,
since they were the result of climatic changes which only come about
very gradually. We thus come again to the same conclusion to which
we were led by our study of vestigial organs in Man, that numerous
species which appear to be at a standstill are continually working
towards their own improvement. But for this amphimixis is essential;
consequently the descendants which have arisen through amphimixis, and
whose ancestors have arisen in the same way, have an advantage over
those of parthenogenetic origin. On the whole, at least, this must be
so; in special cases it may be otherwise, namely, when the advantage
offered by parthenogenesis in respect to the maintenance of the species
preponderates over the advantage which amphimixis implies as regards
possibilities of transformation.
As far as we have seen from the case of the gall-wasps, the absence of
amphimixis in every second generation implies no disadvantage in regard
to the capability for transformation which the species exhibits. As to
whether any disadvantage would ensue if the number of parthenogenetic
generations in the life-cycle were greater we can only guess, since no
case is known which enables us to decide this point, _pro_ or _con_,
with any certainty. The heterogony of the plant-lice, the Aphides, and
their relatives might be cited as against the probability, for in this
case a long series of parthenogenetic generations often alternates
with a single bi-sexual one, but the difference in structure is not so
great in this case, although it does exist, and moreover we can quite
well assume that the adaptation to parthenogenesis was effected at the
beginning of heterogony, when it still consisted of a cycle of only
two generations, and that further virgin generations were interpolated
subsequently.
This assumption is supported by the fact that in some species of our
indigenous Ostracods, in _Cypris vidua_ and _Candona candens_, in
contrast to the Daphnids, several bi-sexual generations alternate
with one parthenogenetic generation. But in this case again there is
no difference whatever in the structure of the two generations, the
parthenogenetic generation being distinguished from the bi-sexual
generation simply by the absence of males.
The alternation of generations in the plant-lice is particularly
instructive, because it emphatically indicates how much Nature is
concerned with the retention of amphimixis, and how little mere
multiplication has to do with this. This is especially striking in the
case of the bark-lice; for instance, in their notorious representative,
the vine-pest, _Phylloxera vastatrix_.
[Illustration: FIG. 127. Life-cycle of the Vine-pest (_Phylloxera
vastatrix_), after Leuckart and Nitsche, and Ritter and Rübsamen. _A_,
the fertilized ovum. _B_, the resulting apterous and parthenogenetic
Phylloxera. _C_, its eggs, from which, as the uppermost arrow
indicates, there may arise similar apterous, parthenogenetic forms, or,
as the horizontal arrow indicates, winged forms (_D_), which produce
'female' and 'male' ova (_E^1_ and _E^2_); from these the sexual
generation arises, the female (_F^1_) and the male (_F^2_); the former
lays the fertilized ovum (_A_).]
As in all plant-lice, the advantage for the sake of which sexual
reproduction was given up depends upon the fact that a practically
unlimited food supply is at the disposal of these parasites of the
vine, which can be made full use of during the proper season, and
which, since every animal is female and produces eggs, results in an
enormous increase in the number of individuals, and thus secures the
continuance of the species. These insects emerge in spring from small
fertilized eggs, which have lain dormant throughout the winter (Fig.
127, _A_), and they develop rapidly into wingless females (_B_), which,
sucking the juice of the vine, multiply by producing large numbers
of little white eggs (_C_). These develop without fertilization into
similar wingless females. Several generations of females succeed each
other, but then, usually from August onwards, differently formed winged
females (_D_) make their appearance, and these, flying from plant to
plant, effect the distribution of the species. But these, too, lay
parthenogenetic eggs (_E^1_ and _E^2_), and from these there emerge,
late in autumn, the members of the single bi-sexual generation, males
and females (_F^1_ and _F^2_), both very minute and wingless, without
a piercing proboscis, and thus incapable of taking food. These pair,
and the female lays a single egg (_A_) under the bark of the vine,
from which the leaves are now falling; this egg survives the winter,
and from it in the following April or May there emerges once more a
parthenogenetic female.
It could hardly be more plainly shown than it is by this case that the
importance of amphimixis is something quite apart from reproduction
and multiplication, for here the number of individuals is not only not
increased by amphimixis, but is materially diminished, being indeed
lessened by a half. By the retention of amphimixis, the species gains
in this case no advantage _except_ the mingling of two germ-plasms.
Something similar occurs in plants which exhibit alternation of
generations, for instance the ferns, in which the sexual generation,
the so-called prothallium or prothallus, contributes nothing to
the _multiplication_ of the plant, since only a single egg-cell
is developed; and the same is true of the mosses. In both cases
_multiplication_ depends solely on the asexual generation, which, as
the so-called 'moss-fruit' or 'fern-plant proper,' produces an enormous
number of spores, in addition to multiplying by runners.
To sum up: we have seen that self-fertilization does occur in
hermaphrodite animals, where otherwise the species would be in
danger of extinction, but this is never the _sole_ and exclusive
mode of fertilization[25], for hermaphrodite species have always the
possibility of securing inter-crossing of individuals, and that in
various ways, whether by the intervention of 'primordial males' or
by an occasional or a periodic alternation of self-fertilization and
mutual fertilization. Pure parthenogenesis enduring through innumerable
generations does appear to occur, but in most cases unisexual
generations alternate with bi-sexual, so that a stereotyping of the
germ-plasm with complete uniformity of ids is obviated.
[25] As to the cases Maupas has brought into notice, of permanent and
apparently exclusive self-fertilization in Rhabditidæ (round worms),
it seems fair to say that they have not been as yet sufficiently
investigated to admit of a secure appreciation of their value in their
theoretical bearings. Cf. _Arch. Zool. Exper._, 3rd ser., vol. viii,
1900.
We must now briefly consider the higher plants with reference to the
maintenance of diversity in the germ-plasm through crossing.
We saw in an earlier lecture that most flowers are hermaphrodite,
but that they do not fertilize themselves, and are adapted for
crossing, since the pollen of one flower is carried by insects to
the pistil of another, which cannot be reached by its own pollen,
either because it ripens too early or too late, or because the
stigma, notwithstanding its proximity, is so placed as to be out of
reach of the pollen from the adjacent stamens. I showed, following
the fundamental investigations of Sprengel, Charles Darwin, Hermann
Müller, and other successors of Darwin, that the flowers may in a sense
be regarded as the resultants of the insect-visits, since all their
accessory adaptations--large coloured petals, fragrance, nectar, and
even little minutiæ of colour and markings (honey-guides)--as well
as their detailed shape, as seen in 'landing stages,' corolla tubes,
and so on, are only intelligible when we refer their existence to
natural selection. We assume that each of these adaptations secured
some advantage for the species concerned, and that therefore their
first beginnings as slight germinal varieties were accepted, and were
brought gradually to their full expression by the united operation
of germinal and personal selection. This at least is how we should
express ourselves now that we have become acquainted with the factor
of germinal selection. The advantage secured by every such improvement
in a flower's means of attracting insects is obvious, as soon as it
is established that cross-fertilization is more advantageous for the
species than self-fertilization.
We have discussed this already; we saw that experiments instituted
by Darwin proved that seedlings which had arisen through
cross-fertilization were superior to those arising through
self-fertilization, and that in many cases the mother-plant itself
produced fewer seeds when self-fertilized than when cross-fertilized.
This discovery afforded an explanation of the cross-fertilization
of flowers by insects which Sprengel had previously observed. We
understand how the flowers must have become so adapted through
processes of selection that they were unable to fertilize themselves,
but attracted insects, and, so to speak, compelled these to dust them
with pollen from another plant of the same species. We also understand
how self-fertilization remained possible for many flowers in the
event of cross-fertilization through insects not being effected,
since after a certain period of waiting, a curvature of the stamens
or the pistil may take place and lead to the stigma being dusted with
the pollen of the same flower. Obviously the development of _fewer_
seeds is preferable to complete sterility. It is a well-known fact
that peculiar inconspicuous and closed flowers, designed solely for
self-fertilization, may occur along with the open flowers, as in the
case of the so-called cleistogamous flowers of the violet (_Viola_)
and the little dead-nettle (_Lamium amplexicaule_), and the phyletic
origin of these becomes intelligible as soon as it is established that
cross-fertilization is more advantageous than self-fertilization.
Now, however, it seems as if the fundamental proposition of this theory
of flowers will have to be rejected. Not only do the cleistogamous
flowers just mentioned exhibit a great fertility, not at all less
than that of the open flowers of the same species which are adapted
for cross-fertilization, but there is a small number of plants which
produce seeds by _self-fertilization alone_. Thus in _Myrmecodia_
cross-fertilization is absolutely prevented by the fact that the
flowers never open, and according to Charles Darwin _Ophrys apifera_
also reproduces by self-fertilization alone, and is nevertheless a
thoroughly vigorous plant. There are several other cases of this sort,
and particularly among the orchids, though the whole of the structure
of their flowers is specially adapted for pollination by insects.
Many of them are only rarely visited by insects, some not at all, we
know not why, but it is readily intelligible that in such cases they
should have adapted themselves to self-fertilization wherever that was
possible. For this _no great_ variation was necessary; it was enough
that the pollinia, which formerly only became detached from their
attachment at a touch or a push from an insect, should free themselves
spontaneously. And this, according to Darwin, is what happens,
for instance, in _Ophrys scolopax_, which at Cannes is frequently
self-fertilizing. For the development of seed, however, it is not
enough that the pollen should reach the stigma; the pollen-grain has
to send out its tube and penetrate into the ovary, and in many orchids
this does not happen; they are infertile with their own pollen. Various
other plants are also non-fertile with their own pollen, for instance
the common corydalis, _Corydalis cava_, or the meadow cuckoo-flower,
_Cardamine pratensis_ (Hildebrand).
How are we to reconcile these apparently absolutely contradictory
facts? On the one hand, the innumerable devices for securing crossing
lead us to conclude that it is necessary, or at least advantageous,
and on the other we find a small number of plants which reproduce
continually by self-fertilization and yet remain strong and vigorous.
And again there are many plants which yield seed when fertilized with
their own pollen, and others which remain absolutely sterile in the
same circumstances, yielding no seed or very little, and there is
indeed one on which its own pollen has the effect of a poison, for if
it reaches the stigma the flower dies. If there is anything injurious
in self-fertilization (Darwin), we can understand that it will be
avoided, but how can it be continued so long in many cases, and even
become in others the exclusive method of fertilization without visible
evil results?
It seems to me that in these facts, established by observation, the
results of two quite different processes have been confused, and that
we can only gain clearness by studying them apart from one another; I
mean the processes involved in the mechanism of fertilization and those
involved in the mingling of the germ-plasms.
In many cases self-fertilization is said to yield less seed and weaker
seedlings. Let us for the present take this statement as the basis of
our consideration; it does not seem to me conceivable, though here I
am not in agreement with views that have been expressed by others,
that both effects should depend upon the same causes, for the smaller
number of seeds cannot possibly depend upon the mingling of the two
parental germ-plasms, and thus not upon the process of amphimixis
itself, since the effect of the mingling does not make itself felt
until the organism of the offspring is being built up. Of course the
plant seed is the embryo of the young plant, but it will hardly be
thought probable that its development could be absolutely prevented
by the too close relationship of the two germ-cells, and thus the
number of the developing seeds cannot depend on the quality of the ids
co-operating in the segmentation-nucleus, but presumably on the number
of ova awaiting fertilization in the ovary, which are reached by a
pollen-tube and then by a paternal sex-nucleus. This again will depend
upon the impelling and attracting forces of the pollen-grain on the one
hand, and of the stigma and 'embryo-sac' of the flower on the other. In
other words, the fertility of a flower with its own pollen will depend
upon whether the two products of the flower are adapted for mutual
co-operation, and in what degree they are so. We are here dealing not
with the _primary_ reactions of the germ-plasms, which are as they are
and cannot be varied, but with secondary relations, which may be thus
or thus--in short, with _adaptations_.
By what adaptations the pollen of a flower can be made ineffective
for that flower is a question which we must leave the botanists to
answer; in any case it must have been possible, and we see clearly
that it depends upon adaptation when we consider the numerous stages
which occur--from the rare case of the actually poisonous influence
of self-pollination already noticed, to complete sterility, and from
lessened fertility to greater or even perfect fertility. It is possible
that chemical products, secretions of the stigma or the pollen-grain,
or the so-called synergid-cells, have to do with this, or that the
size and therewith the penetrating power of the pollen-cell in
self-fertilization stand in inverse ratio to the length of the pistil,
as has been proved in regard to heterostylism by Strasburger; but in
any case it was possible for Nature, by means of slight variations in
the characters of the male and female parts of the flower, to diminish
the certainty of the meeting of the two germ-cells, even to the total
exclusion of the possibility of any union of these.
If, then, self-fertilization had to be guarded against or at least
rendered difficult because its consequences were injurious, all
variations pointing in the direction of safeguarding would necessarily
be preserved and increased. In many cases variations in the structure
of the flower were sufficient; but when, as in _Corydalis cava_, the
pollen could not readily be prevented from falling upon the stigma, the
pollen might be made sterile as far as its own flower was concerned
by a process of selection, in which on an average those plants would
remain successful which produced the largest number of cross-fertilized
seeds, and in this case those which did so were those whose pollen
reacted most feebly to the stimulus of their own stigma.
[Illustration: FIG. 128. Heterostylism (_Primula sinensis_), after
Noll. Two heterostylic flowers from different plants. _L_, the
long-styled form. _K_, the short-styled form. _G_, style. _S_, anthers.
_P_, _p_, pollen-grains. _N_, _n_, stigmatic papillæ of the long-styled
and short-styled forms respectively. _P_, _p_, _N_, _n_, magnified 110
times.]
That self-sterility in all these different degrees is not a primary
character of the species, but an adaptation to the advantages of
cross-fertilization, is apparent--if indeed it seems doubtful to any
one--especially from cases of heterostylism. I refer to the dimorphism
and trimorphism which Darwin discovered in many flowers, and which
shows itself in the fact that flowers otherwise almost exactly
alike, as, for instance, primroses, may exhibit a long style in some
individuals, and in others a short one (Fig. 128). At the same time,
there is a difference in the position of the stamens, which are placed
higher up in flowers with short styles, and much lower down in those
with long styles. Experiments have proved that the dusting of the
stigma has the best results if pollen from the short-styled reaches
the stigma of the long-styled form, or if pollen from the long-styled
form reaches the stigma of the short-styled. Thus we have again to deal
with an arrangement for crossing, an adaptation to the advantages of
cross-fertilization, and we can in this case see the reason why the
pollen has a different effect upon the two stigmas; the pollen-grains
of the flowers with short style are larger than those of the flowers
with long style, and as the length of the pollen-tube that can be sent
out must depend upon the mass of protoplasm within the pollen-grain,
it follows that the smaller pollen-grains will send out too short a
tube to reach through the long style to the embryo-sac. In addition to
this there is a difference in the papillæ of the stigmas, and it is
possible that these may form an obstacle to the penetrating of pollen
from a similar type. The process of selection which gives rise to such
arrangements as we find in Primulas may easily be imagined, as soon as
we are able to assume that cross-fertilization is more advantageous
than self-fertilization as regards progeny, that is, as regards the
continuance of the species.
We have already seen that uninterrupted self-fertilization is unknown
among animals, but that it is not even very rare among plants, and
this emphatically corroborates our previous conclusion, that the
reason for which amphimixis was introduced as a normal event in nature
is not to be sought for in the necessity for a renewing of life, or
'rejuvenation.' It cannot be a necessity, but only an advantage, which
can in certain circumstances be dispensed with.
Although it is obvious enough that continued inbreeding in its
most extreme form, self-fertilization, does not imply an absolute
abandonment of amphimixis, the adherents of the rejuvenescence theory
have regarded the unfavourable consequences of pure inbreeding as a
confirmation of their assumption, according to which amphimixis is
indispensable to the continuance of the life of the species, and it
is therefore an important fact, if it can be proved, that continued
self-fertilization can occur persistently, among plants at least, and
yet not cause any injurious results to the species.
But how can this fact be understood from our point of view? How does it
happen that crossing is striven after in so many different ways and yet
so often given up again, and continued self-fertilization resorted to?
To this it may be answered, in the first place, that it is not,
as far as we can see, for _internal_ reasons that persistent
self-fertilization becomes the rule; there is no peculiar condition
of the germ-plasm which makes it disadvantageous or superfluous
that the diversity of the id-combinations should be maintained;
self-fertilization is due to _external_ influences which bring it about
that the plant has only the alternative of producing no seeds at all
or of producing them by self-fertilization. In this connexion Darwin's
experiments with orchids are particularly noteworthy.
In this very diversified order of plants there are numerous species
whose flowers are infertile with their own pollen, although it
does not reach the stigma in natural conditions, and therefore
there was no necessity--as far as we can see--for guarding against
self-fertilization by 'self-sterility.' These flowers are thus doubly
adapted, so to speak, for crossing by means of insects. But as regards
many of these, as well as many other modern orchids, insect-visits are
very rare, and in some cases do not occur at all, and therefore these
species cannot produce seed or can do so only exceptionally.
This is true of most of the Epidendra of South America, and of
_Coryanthus triloba_ of New Zealand, two hundred blossoms of which only
yielded five seed-capsules, and also of our _Ophrys muscifera_ and _O.
aranifera_, the latter of which yielded only a single seed-capsule
from 3,000 flowers gathered in Liguria. We might expect that the
species in question must have become very rare, but this is not always
the case, since each of these capsules contains an enormous number
of seeds, sometimes many thousands. As soon as the visits of insects
cease altogether, the species must die out in the particular locality
concerned, unless it can revert to self-pollination and self-fertility.
There is a whole series of species in which the stigma of the flower
is sensitive to its own pollen, and in many of these an adaptation
to self-fertilization has actually been effected, for the pollinia
detach themselves from their anthers at maturity and fall upon the
stigma. I have already mentioned _Ophrys apifera_, which, according to
Charles Darwin, is no longer visited by insects, although its flowers
still possess the structure required for insect-fertilization. This
species has saved itself from extinction by the normal occurrence of
self-fertilization.
This seems to me noteworthy in two respects. In the first place, it
shows that pure self-fertilization need not necessarily result in a
weakening of the species, and secondly, it affords a clear instance
of a species being transformed in one minute character only, all the
other characters remaining unaltered. In this case it was only the
pollinia that required to vary a little in their mode of attachment and
maturation, in order to effect the transformation of the flower for
self-fertilization, and in point of fact that is all that has varied.
The case is not relevant to our investigation at this moment, but
cases of the kind can so rarely be clearly demonstrated that I cannot
lose the opportunity of calling attention to it. The germ-plasm of
this _Ophrys_ must have varied at an earlier stage, for otherwise the
detachment of the pollinia would not have become normal and hereditary,
but it can only have varied to the extent that the structure of this
one small part of the flower was affected by the variation; something
must have varied in the germ-plasm that had no influence upon the other
parts of the flower, that is, solely the _determinants_ of the pollinia.
Let us return after this digression to our previous train of thought;
we have to inquire how we can interpret the fact of continued
self-fertilization without any visible injurious results to the
species. If cross-fertilization be a material advantage as regards the
continuance of the species, how can it be transformed into its opposite
without evil effects? And there are no visible evil effects in _Ophrys
apifera_. It is indeed not so abundant as _Ophrys muscifera_, or other
allied species, but it certainly does not follow from that that it is
on the way to extinction; certainly no decrease either of vigour of
growth or of fertility can be observed.
If we inquire from the standpoint of our theory, how the composition
of the germ-plasm must have altered through continual inbreeding,
we have already found the answer--that through the reduction of the
number of ids at the maturation of every germ-cell the diversity of the
germ-plasm would gradually be lessened, that the number of different
ids would thereby be lessened possibly even to the identity of the
whole of the ids.
The consequences of such extreme uniformity of the germ-plasm would
not, according to our theory, necessarily be that the species would
be incapable of continued existence, but it would be that the species
would become incapable of adaptations in many directions. Adaptations
in one direction, such, for instance, as the variation in the mode of
attachment and detachment of the pollinia of an Orchid, would still be
possible. Thus a species which has long been perfectly adapted will be
able to make the transition to inbreeding without injury to its chances
of continued existence, if it be compelled by circumstances to do so.
Species, on the other hand, which are still undergoing considerable
transformations in many directions must be exposed by these to the
danger of degeneration, just as happens in the artificial experiments
with domesticated animals, whose secret weaknesses are greatly
exaggerated by inbreeding.
We might be inclined to regard the effects of inbreeding as similar to
those of parthenogenesis; they are certainly analogous, for both modes
of reproduction must lead to a certain degree of uniformity in the
germ-plasm. But there seems to me to be a difference and one which is
not without importance.
In parthenogenesis no amphimixis occurs, but neither does any reduction
of the number of the ids to one-half; all the ids present at the
beginning of parthenogenesis are retained; they are only no longer
mingled with strange ids. In inbreeding both amphimixis and reduction
take place, but the former soon ceases to convey any really strange
ids to the germ-plasm, but only the same as those which it already
contains, so that a rapidly increasing monotony of the germ-plasm
must result. To this must be added the possibility that among the
few ids which now--many times repeated--form the germ-plasm, some
must occur which exhibit unfavourable variational tendencies in
one or many determinants, and then the same thing will occur which
usually occurs in experimental inbreeding of domesticated animals,
namely, _degeneration of the progeny_. In parthenogenesis the case is
otherwise; unfavourable variational tendencies, as soon as they attain
selection-value, are, so to speak, eliminated root and branch, because
the individuals which exhibit them, and their whole lineage, are
exterminated, without their having any effect upon the other collateral
lines of descent. A purely parthenogenetic species will, therefore, not
degenerate as long as individuals of normal constitution are present,
for these reproduce with perfect purity. But if in later generations
unfavourable variational tendencies crop up in the germ-plasm through
germinal selection, the process of personal selection will be
reinforced on these or on their descendants, and it is conceivable, and
even probable, that in perfectly adapted species parthenogenesis may
last for a very long time without doing any injury to the constitution
of the species.
The same is true of purely asexual reproduction, to the investigation
of which we shall now turn.
Let us leave out of account the simplest animals (Monera) without
amphimixis, which we have already discussed. In simple animals
reproduction by budding or by fission is frequent, or it occurs in
alternation with sexual reproduction; in higher animals, Arthropods,
Mollusca, and Vertebrates, asexual reproduction is wholly absent.
In plants it plays an enormously greater part, and what is called
'vegetative reproduction,' which is purely asexual without any
amphimixis, is to be found in all groups of plants, especially in
the form of budding and spore-formation, besides which there is
multiplication by runners, rhizomes, tubers, bulbs, and bulbils. In
most cases there is, in addition to the purely asexual reproduction,
so-called sexual reproduction associated with amphimixis, and often
the sexual and asexual generations alternate with each other, so that
'alternation of generations' occurs, as is common in lower animals,
especially polyps, medusæ, and worms.
But it sometimes happens among plants that the sexual reproduction is
absent, and that a species reproduces by the asexual mode only, and
this is the case which we must now consider more closely.
Let us first of all seek to gain clearness as to the composition of
the germ-plasm in the case of purely asexual multiplication, and what
conclusions may be drawn from this, and then let us compare these
with the known observational data, and it will be apparent that in
individuals which have arisen by budding the complete germ-plasm
of the species must be contained; the number of ids will not only
remain the same in the bud as it was in the mother plant, but the
number of _different ids_ will not be diminished. The case is
analogous to that of pure parthenogenesis, in which the absence of
the second maturation-division of the ovum allows the germ-plasm to
retain the full complement of ids. Charles Darwin held that purely
asexual multiplication was 'closely analogous to long-continued
self-fertilization,' yet, as we have seen, according to our theory
there must be a not inconsiderable difference between the two
processes, depending on the fact that in exclusive self-fertilization
the number of different ids is continually decreasing, while in purely
asexual reproduction the germ-plasm loses nothing of the diversity of
its ids. If, therefore, the germ-plasm in purely asexual reproduction
no longer receives fresh ids through amphimixis, it at least loses none
of those it formerly possessed. Although we cannot consider it adapted
for entering upon new adaptations in many directions, yet we may expect
that the species will continue to reproduce unchanged for longer
than in the case of exclusive self-fertilization, the more so since
all unfavourable variational tendencies which crop up are eliminated
as soon as they attain to selection-value, and, as in the case of
parthenogenesis, they are eliminated without being mingled with other
lines of descent.
Let us take, for instance, the purely asexual reproduction which
obtains in Algæ of the genus _Laminaria_, in regard to which it is
stated that it multiplies only through asexual swarm-spores. There
are quite a number of species of this large tangle, and if it should
be established that in all these the spore-cells really do not
conjugate, then the case would prove that the species of a genus can
maintain a well-defined existence for a long time after amphimixis has
been given up. But this would not be a proof of the possibility of
_species-formation_, for that the ancestral forms of the Laminarians
must have possessed amphigony may be assumed, since their nearest
relatives exhibit it still. It cannot be proved, but there seems
nothing against the assumption that these tangles have existed for a
long time under uniform conditions, and have become adapted to these
with a high degree of constancy.
The conditions are similar in the marine Algæ of the genus _Caulerpa_,
the nearest relatives of which reproduce sexually, though they
themselves, as far as is known, reproduce only by spores.
In the Lichens, which represent, as we have already seen, a
life-partnership between Fungi and Algæ, amphimixis appears not to
occur at all; the unicellular Alga reproduces by cell-division, the
Fungus by producing a great number of swarm-spores, which do not
conjugate with one another. As far as the Alga is concerned we might
perhaps suppose that the simplicity of its structure makes it possible
for it to dispense with a constant recombination of its few characters
to bring about the most favourable composition in its idioplasm; in
support of this we may note that even the life-long combination with
the Fungus has caused no visible variation in the Alga, as we must
conclude from the fact that these Algæ can also live independently, and
that the same species of Alga may combine with several different Fungi
to form different species of lichen, just as the same Fungus may also
form part of several species of lichen. We might also imagine that we
have here no more than a direct influence of the Alga and Fungus upon
one another, and that there is no adaptation to the new conditions of
life at all, yet that can hardly be seriously maintained in regard to
species which live under such definite and diverse conditions. It now
seems to be established--contrary to the older statements--that the
lichen-fungus only reproduces asexually, and in face of this it seems
to me that nothing remains except to make the assumption that lichens
formerly possessed sexual reproduction, but that they have lost it,
though whether all have done so is, perhaps, not yet quite certain.
The same assumption must be made in regard to the Basidiomycetes among
the Fungi, and for most of the Ascomycetes, for in these groups of
Fungi sexual reproduction has only been demonstrated 'with certainty in
a few genera.' That in these cases also there has been a degeneration
of amphigony, until it has completely disappeared, seems probable from
the two other groups of Fungi, the Zygomycetes and Oomycetes, since in
these 'a reduction of sexuality amounting in some cases to complete
disappearance' can be demonstrated even in existing forms. But whether
it may be assumed that the Fungi which are now asexual are no longer
capable of new adaptations, and whether their parasitic habit may be
regarded as making up in some way for the lack of the remingling of
the germ-plasm, as the botanist Möbius supposes, I am not able to
decide. It is obvious that data in regard to amphimixis among the Fungi
are still incomplete, and recent investigations lead us to suspect
that sexual mingling may not be absent, but only disguised. Dangeard,
Harold Wager, and others have observed that a fusion of nuclei precedes
the formation of spores, and this may be regarded as amphimixis,
although the conjugating nuclei belong to cells of the same plant and
sometimes even to the same cell. But although we are here dealing
with a set of facts which cannot yet be satisfactorily formulated in
terms of our theory, it is nevertheless not contradictory to it that
amphimixis should be wholly absent in the higher Fungi. But the fact
would be contradictory to the unadulterated rejuvenescence-theory, for
if amphimixis were really a condition of the continuance of life, no
species--as we have already said--could continue to exist without it
for countless generations.
[Illustration: FIG. 38 (repeated). A fragment of a Lichen (_Ephebe
kerneri_), magnified 450 times. _a_, the green alga-cells. _P_, the
fungoid filaments. After Kerner.]
The same argument holds true for the higher plants, which have become
purely asexual under the influence of cultivation. I refer to many
of the well-marked varieties of our cultivated plants which multiply
exclusively, or almost exclusively, by means of tubers and slips,
as is the case with the potato, the manioc, the sugar-cane, the
arrowroot-plant (_Maranta arundinacea_), and others. All these facts
can easily be reconciled with our interpretation of the meaning of
amphimixis, although the attempt to range them as evidence against
our theory has more than once been made. We have thus arrived at the
conclusion that while many-sided adaptations, that is, variations
which transform the plant in accordance with the indirect influences
of new conditions of life, cannot be brought about without a
persistent mingling of germ-plasms, simple modifications may readily
appear although amphimixis is altogether absent. If a wild plant be
permanently transferred to a well-manured culture-bed, it is probable
that certain changes will occur in it, either gradually or at once.
But these are not adaptations; they are, so to speak, direct reactions
of the organism which do not even require selection to make them
increase, but depend upon the influencing of certain determinants of
the germ-plasm, and which, like all germinal variations, will follow
their course steadily until a halt is called either by germinal or by
personal selection. When a given plant is exposed to these new and
artificial conditions, the changes in question make their appearance
sooner or later, and follow their course, and go on increasing as long
as that is compatible with the harmony of the structure and functioning
of the plant, this depending, as in all individual development, on the
struggle between the parts, that is to say, on histonal selection.
Only in this respect is the utility or injuriousness of the change of
importance, for personal selection, the struggle between individuals,
does not affect plants which are under cultivation.
That such modifications may increase and may persist through many
generations, even with asexual multiplication, depends upon the fact
that the budding cells contain germ-plasm, as well as the germ-cells,
and if particular determinants of the germ-plasm in general are caused
to vary by these new influences, the variation may be transmitted
from bud to bud, from shoot to shoot, and so go on increasing as long
as the new conditions persist, as well as in amphigonic (bisexual)
reproduction, where they are transmitted from germ-cell to germ-cell.
It is not inconceivable that an individual adaptation, that is to
say a useful adjustment, might be effected in the course of asexual
reproduction, although it is improbable that direct influences
would give rise to just those changes which would be useful under
the new conditions. But there are a number of cases which have been
interpreted in this way. In several of the cultivated plants named,
the reproductive organs have themselves degenerated, either only the
male, or only the female, or both at the same time; and some observers,
accepting the hypothesis of an inheritance of functional modifications,
have regarded this as the direct result of disuse during the long
period of asexual reproduction.
Leaving out of account this erroneous presupposition, we may ask how
asexual reproduction, such as that of the potato by tubers instead of
by seed, which has gone on exclusively for several centuries, could
exercise any influence upon the flowers and seed-forming of this
species? In point of fact it has exercised none in most potatoes, for
the flowers and seeds are just as fertile now as they were when the
potato was first discovered.
Whether the pollen of a flower is utilized in one or other of its
thousands of pollen-grains by reaching the stigma of another plant
of the same species, or whether all the pollen-grains are uselessly
scattered abroad, cannot possibly affect the flower so as to cause
degeneration; the theory of disuse cannot be applied in this case.
What is true of the potato holds good also of the manioc (_Manihot
utilissima_), but, on the other hand, many of the best varieties of
common fruits--pears, figs, grapes, pine-apples, and bananas--are
seedless. In _Maranta arundinacea_ 'the whole wonderful structure
of the flower has persisted, but the pollen-grains, that is the
germ-cells, are wanting.' Whether this implies a permanent degeneration
of the sexual organs, that is to say, one that is embodied in the
primary constituents of the species, or whether it is only the result
of over-abundant nourishment, or of other causes in the circumstances
affecting the particular plant, can only be decided by experiment.
Probably both occur. The common ivy, for instance, does not now blossom
in the northern parts of Sweden and Russia, but it does so still in
the southern provinces. If plants were brought to us from the most
northerly zone of distribution, they would in all probability flower
and bear fruit with us, and in that case the absence of bloom in these
plants must have been a direct effect of the cold climate. But it is
quite conceivable that cultivated plants have in many cases become
hereditarily infertile, when they are constantly propagated only by
means of buds, layering, and so on, not however because of any direct
effect of this mode of propagation, but through chance germinal
variations. For in regard to many of them man has lost all interest in
the flowers and fruit, as, for instance, in the case of the potato; in
other cases he is even interested in procuring seedless fruits.
In the first case he will quite readily make use of plants with
imperfect flowers for propagating, if they are otherwise fit and
exhibit what he wants in other respects; in the second case, he will
give a preference to individuals with seedless fruits, and thus
increase and strengthen the tendency to degeneration of the seeds in
the race concerned.
All these cases are quite in harmony with our conception of amphimixis,
which, now that we have investigated the facts throughout the animate
kingdom, we may sum up in the following propositions. In the whole
organic world, from unicellular organisms up to the highest plants
and animals, amphimixis now means an augmentation of the organism's
power of adaptation to the conditions of its life, since it is only
through amphimixis that simultaneous harmonious adaptation of many
parts becomes possible. It effects this by the mingling and constant
recombination of the germ-plasm ids of different individuals, and thus
gives the selection-processes the chance of favouring advantageous
variational tendencies and eliminating those which are unfavourable,
as well as of collecting and combining all the variations which are
necessary for the further evolution of the species. This indirect
influence of amphimixis on the capacity of organisms for surviving
and being transformed is the fundamental reason for its general
introduction and for its persistence through the whole known realm of
organisms from unicellulars upwards.
The reason for its _first_ introduction among the lower forms of life
must have been a direct effect which had a favourable influence on the
metabolism, and this is so far coincident with the subsequent import of
amphimixis, inasmuch as it may be regarded not only as a heightening of
the power of adaptation, but as an immediate and direct increase and
extension of the power of assimilation. In any case, amphimixis is not
necessary to the actual preservation of life itself, but it does bring
about a wealth and diversity of organic architecture which without it
would have been unattainable.
If amphimixis has been abandoned in the course of phylogeny by isolated
groups of organisms, this has happened because other advantages accrued
to them in consequence, which gave them greater security in the
struggle for existence; but it must be admitted that they thereby lost
their perfect power of adaptation, and that they have thus bartered
their future for the temporary securing of their existence.
In addition to this variational influence, amphimixis has also played a
part in the evolution of sharply defined organic types, especially of
specific types; but of this we shall have more to say later on.
LECTURE XXXI
THE INFLUENCES OF ENVIRONMENT
Different modes and grades of selection--Changes due to the influences
of environment--Superfluity and lack of food--The horses and cattle
of the Falkland Islands--Angora animals--Protection against cold
in Arctic and marine mammals--Plant-galls--Nägeli's _Hieracium_
experiments--Experiments with _Polyommatus phlæas_--Artificially
produced _Vanessa_-aberrations--Vöchting's experiments on the
influence of light in the production of flower-forms--Heliotropism
and other tropisms--Primary and secondary reactions of
organisms--Herbst's 'lithium larvæ'--Schmankewitsch's experiments
with _Artemia_--Poulton's caterpillars with facultative colour
adaptation--Colour-change in fishes, chamæleon, &c.--Actual scope of
those influences which directly produce organic changes.
Through a long series of lectures we have devoted our attention to
those phenomena which bear some relation to the processes of selection;
we have attempted to gain clearness in regard to the modes and stages
of these, and we reached the result that all variations which have
taken place in organisms since the first appearance of living matter
are directed by processes of selection, that is, their direction and
duration are determined by these processes, although they may have
their roots in external influences. But it is not to be supposed that
this guidance is due solely to that one kind of selection which,
with Darwin and Wallace, we designate 'natural selection'; on the
contrary, we must regard this as only one of the different modes of
the processes of selection, necessarily occurring between all living
units which are equivalent to one another, and which, therefore, must
maintain a continual struggle with one another for space and food. If
the expression 'natural selection' were not already so firmly fixed
in its meaning, I should propose that it should be employed in the
most general sense for all the processes of selection collectively,
but we must keep to its original meaning and use it only for personal
selection.
We have seen that processes of selection take place even between the
elements of the germ-plasm in all organisms which possess a germ-plasm
as distinguished from the mass of the body, and that through these
processes there arise those hereditary individual variations which,
under some circumstances, form the basis of transformations in the
species.
Obviously this may come about in a twofold manner: firstly, a
variation movement originating in the germ-plasm may go on increasing
till it attains to selection-value, and then 'personal selection' steps
in, and seeks to make it the common property of the species. But it
is obviously also conceivable that variational tendencies arising in
the germ-plasm may never attain to selection-value at all, and then
in most cases they will only continue to exist through a longer or
shorter series of generations as individual distinguishing characters,
without being transmitted to a larger number of individuals or becoming
a constant character of the species. Their persistence will depend
essentially on the chance of mingling with other individuals, and on
the halving of the germ-plasm which precedes sexual reproduction.
Sooner or later these individual peculiarities disappear again, as may
often be observed in the case of abnormalities or morbid tendencies in
man, in as far as these do not weaken vitality. In the latter case they
attain selection-value, though only negatively.
But even quite indifferent germinal variations, which neither raise
nor lower the individual's power of survival, may, under some
circumstances, increase and lead to permanent variations of all the
individuals of a species, and this happens when they are conditioned by
external influences which affect all the individuals of a species, or
of the particular colony concerned, and it is this kind of organismal
change which we shall now study for a little in detail.
The ordinary never-ceasing, always active germinal selection depends,
we must assume, upon intra-germinal fluctuations of nutrition, or
inequalities in the nutritive stream which circulates within the
germ-plasm. The variations which it produces may, therefore, be
different in each individual, since these fluctuations are a matter
of chance and may affect the determinants _A_ in one individual and
the determinants _B_, _C_, or _X_ in another, or alternating groups
of these. Or it may be that the homologous determinants _A_ may vary
in a plus direction in one individual, and in a minus direction in
another, while in a third they may remain unchanged, and although the
same direction of variation of a determinant _N_ may occur in many
individuals, it will certainly not do so in all, and still less will
it occur in all along with the same combination of fluctuations in the
rest of the determinants. It is only if this occurs that the variation
can become a specific character.
We might expect on _a priori_ grounds that not only the chance
fluctuations of nutrition within the germ-plasm would cause its
elements to vary in this or that direction, but that there would also
be influences of a more general kind, especially those of nutrition and
climate, which would in the first place affect the body as a whole, but
with it also the germ-plasm, and which would therefore bring about
variations, either in all or only in certain determinants. In this case
all the individuals would vary in the same way, because all would be
similarly affected by the same causes of change.
This is actually the case; it is indubitable that external influences,
such as those emanating from the environment or media in which species
live, are able to cause direct variation of the germ-plasm, that is,
permanent, because hereditary variations. We have already referred to
this process and called it 'induced germinal selection.'
That such influences of environment may bring about changes in
_individual_ organisms is obvious enough; that, for instance, good
nutrition makes the body strong and vigorous, that too abundant food
makes it fat and causes degeneration, that insufficient food lessens
its stamina and vigour, are well-known facts. We have to inquire, on
the one hand, to what extent such influences are able to cause changes
in the individual body in the course of a lifetime, and, on the other
hand, more particularly, how far such changes or modifications of the
soma can call forth corresponding variations in the determinant system
of the germ-cells, and whether and under what circumstances they may be
transmitted; for where this is not the case there can be no permanent
hereditary variation of the whole species, and the variation will only
persist as long as the conditions which gave rise to it endure, and
will disappear again with these.
The influence of nutrition as a cause of variation has often been
over-estimated. The old statement which has gone the round of the
textbooks since the time of John Hunter, that the stomach of carnivores
may be transformed by vegetable diet into a herbivore stomach, is
absolutely unproved. Brandes at least, who not only subjected all the
statements in the literature on this point to a critical investigation,
but also instituted experiments of his own, regards the statement as
altogether unfounded. All the 'cases' cited, in which the stomach of a
gull or of an owl fed on grain became transformed into an organ with
stronger muscles and covered with horny plates, depend, according to
Brandes, upon inexact observation. There can therefore be no question
of any inheritance of this fictitious stomach-transformation, and the
idea that such a fundamental histological adaptation as the alleged
transformation of the stomach of the grain-eating bird should arise as
a direct effect of the food is wholly without foundation.
But it is quite otherwise with purely quantitative differences in
nutrition. That meagre diet influences individuals unfavourably is
indubitable, and we are certainly justified in considering whether
this may not have an effect on the germ-cells, and one which will
correspond to the changes induced on the body, so that if the
poor nutrition should last through many generations an hereditary
degeneration of the species would occur, which would not at once
disappear though the animals were transferred to more favourable
conditions.
We certainly know nothing of how far the minuteness of the determinants
of the germ-plasm, the whole quantity of the germ-plasm, or the reduced
size of the germ-cell, may bear an internal relation to the smallness
of the animal which develops therefrom, but it surely cannot be
regarded as absurd to suppose that there is some such relation. There
are no experiments known to me which prove that meagre diet brings
about a progressive decrease in the size of the body. Carl von Voit
has observed that dogs of the same litter grew to very different sizes
of body according as they received abundant or scanty food, but it
would be difficult to make animals small through scantiness of food
and at the same time to keep them capable of reproduction, and thus
proofs of the inheritance of the dwarfing are lacking. Moreover, the
experiments which Nature herself has made are never quite convincing,
because we never can definitely exclude the indirect effect of altered
circumstances. The case of the feral horses of the Falkland Islands, so
often cited since the time of Darwin, which have become small 'through
the damp climate and scanty food,' seems to me, of all known cases of
the kind, the one we should most readily attribute to the direct effect
of continued scanty diet; but even here we cannot altogether exclude
the possibility of the co-operation of adaptations of some kind to the
very peculiar conditions of life in these islands, as far as the feral
horses are concerned. I have not been able to find any record of more
modern exact investigations either regarding these feral horses, or in
regard to the others which are reared in the Falklands under conditions
of domestication. Darwin himself, however, in the Journal of his famous
voyage tells us much that is interesting in regard to the mammals of
the Falkland Islands. Cattle and horses were brought there in 1764 by
the French, and have increased greatly in numbers since that time; they
roam about wild in large herds, and the cattle are strikingly large
and strong, while the horses both wild and tame are rather small, and
have lost so much of their original strength that they cannot be used
for catching wild cattle with the lasso, and horses have to be imported
from La Plata for this purpose. From this contrast between the horses
and the cattle we may at least conclude that it cannot be 'scanty food'
alone which causes the horses to become smaller, but that the climatic
conditions as a whole are concerned in the matter. Whether the total
amount of variation which has taken place in the horses which have
lived wild there for a hundred years would take place in the course of
a single life, or whether it is a cumulative phenomenon, has still to
be decided.
Similar statements, for the most part still more uncertain, are made
in regard to changes in the hair of goats, sheep, cattle, cats, and
sheep-dogs, which are referred to climatic influence. The raw climate
of many highlands, like Tibet and Angora, is said to have directly
produced the long and fine-haired breeds. But there is a lack of proof
that adaptation or artificial selection did not also play a part, and
the fact that similar long-haired breeds have arisen among rabbits
and guinea-pigs in quite different places and under quite different
climatic conditions, but under the directing care of man, speaks in
favour of our supposition. But, on the other hand, it does not seem
impossible that the climate may have a variational influence upon
certain determinants of the germ-plasm, for we have already seen
that the influence of cultivation may incite plants and animals to
hereditary variations, and that slowly increasing disturbances in
the equilibrium of the determinant system may thereby be produced,
which may suddenly find marked expression as 'mutations.' But there
is little probability that _adaptations_, that is, transformations
corresponding to the altered climate, can arise in this way. The thick
fur of the Arctic mammals is assuredly not a direct effect of the cold,
although it has developed in all Arctic animals, not only in the modern
polar bears, foxes, and hares of the polar regions, but also in the
shaggy-haired mammoth of diluvial Siberia, whose tropical relatives of
to-day, the elephants, have an almost naked skin. Another interesting
case, recently brought to light, shows that a group of animals
which, in correspondence with their otherwise exclusively tropical
distribution, have only a moderately developed coat of hair, may, on
migrating to a cold country, grow as good a fur as the members of other
families. I refer to one of the higher apes, _Rhinopithecus roxellanæ_,
which live in companies in the forest on the high mountains of Tibet,
notwithstanding that the snow lies there for six months[26].
[26] See Milne-Edwards, _Recherches pour servir à l'histoire nat. d.
mammifères_, Paris, 1868-74.
But we should assuredly make a mistake if we were to regard the thick
fur of these apes as a direct reaction of the organism to the cold. We
see at once that this cannot be the case if we compare them with marine
mammals, which differ just as much from one another in this respect
and yet are exposed to the same low temperature. The whale and the
dolphin are quite naked, absolutely hairless, but the seals possess a
thick hairy coat. This striking difference is obviously connected with
the mode of life; the whales remain always in the water, the seals
leave it often and therefore require the hairy coat, especially in
colder climates, since otherwise they would be too rapidly cooled by
the evaporation of the water from their bodies. For the whales, on the
other hand, even a very thick hairy coat would not have sufficed as a
protection against cold, since water is a much better conductor of heat
than air, and so it was necessary for them to become enveloped with
the well-known thick layer of blubber, a deposit of fat lying under
the skin, and this--after it was once developed--made the hairy coat
superfluous, so that it disappeared. The seals certainly also possess
a layer of fat under the skin, but it is only in the largest of them
that it affords sufficient protection against the cooling effect of
evaporation when they go upon land or on the ice, and it is therefore
only in these larger ones that the hairy coat has markedly degenerated,
as, for instance, in the walrus and the sea-lion; in all the smaller
seals, in which the mass of the body is much less, the hairy coat is
necessarily very thick and protected from soaking by being very oily,
because the layer of fat under the skin would not be sufficient to
prevent excessive cooling when on land. But the thick coat of hair is
no more _produced_ by the cold than is the layer of fat. As Kükenthal
has shown, all these characters are adaptations, and may depend here as
elsewhere upon natural selection and upon the 'fluctuating' variations
of the germ-plasm upon which that process is based. They are directed
by personal selection because there is the need for them, and they are
produced and augmented by germinal selection.
In all these cases the _direct_ effect of external influences has
nothing to do with the matter, but in other cases that alone brings
about the whole change, which is then limited to the individual and
does not affect the species as a whole at all.
Plant-galls afford striking illustration of the extraordinary changes
that may be brought about in an organism or in its parts by external
influence in the course of the individual life. All possibility of
adaptation on the part of the plant is excluded in this case. The
gall can only depend upon the direct influence of a stimulus, which
is exercised by the young animal, the larva, upon the cells which
surround it; and yet these cells vary to a considerable extent, become
filled with starch or form a woody layer, secrete special substances,
such as tannic acid, in large quantities, or develop hairs, moss-like
growths, pigments, and so on, which do not otherwise occur in that
particular part of the plant. Since Adler and Beyerinck have proved
that it is not a poison conveyed by the mother animal into the leaf or
bud when laying the eggs, which gives rise to the gall-formation, the
matter has become rather clearer. We can now understand that different
stimuli in succession affect the cells which enclose the larva, and
that the ordered succession of these and the exactly graded stimulation
incite the cells to activity in various ways, whether to mere growth
and multiplication in a given direction, or to the secretion of tannic
acid, or to the formation of wood, or to the deposition of reserve
material, and so on. Even the feeble movements of the young larva may
form a stimulus that increases with its growth; then the movements
made by the larva in feeding, and not least the different secretions
emanating from the salivary glands of the animal, which must contain
some substances capable of acting as stimuli and probably changing
in character as time goes on. All these factors must act as specific
stimuli to the plant-cells, influencing and modifying their processes
of growth and metabolism in one direction or another. In principle
at least, if not in detail, we understand the possibility that
through the ordered succession and exact balancing of these different
cell-stimuli the really marvellous structure of the gall may be brought
about as the product of the direct influence, exercised only once,
of the gall-insect upon the plant's parts. But the animal's power of
exercising such a succession of finely graded stimuli upon the plant
must be referred to long-continued processes of selection, and the
structure of the gall, which is adapted to its purpose down to the
minutest details, can thus be understood. The assumption of substances
which can act even in minute quantities as specific cell-stimuli, which
we require to make in this attempt to explain galls, is no longer
without corroboration since we find analogies in the Iodothyrin of
Baumann, the specific secretions of the thymus and the supra-renal
bodies in the higher animals, not to speak of the 'anti-toxins' of the
pathogenic bacteria, which are only known by their effects.
The case of plant-galls is thus of great theoretical interest because
we can exclude all preparation of the plant-cells for the stimuli
exercised by the animal, since the gall is quite useless for the
plant, though many have endeavoured to discover some utility. We
have therefore here a clear case of modification due to the effect,
exercised once only, of external influences, an adaptation of the
animal to the mode of reaction of particular plant-tissues.
It might be supposed that if any inheritance of somatogenic
modifications, any transmission of the acquirements of the personal
part to the germinal part, were possible at all, it would occur in this
case, for many species of gall-insects attack plants, particularly
oaks, in great numbers every year. It has actually been maintained
that galls may arise spontaneously, that is without the presence of a
gall-insect. But no proof of this has ever been found, and the fact
that no one has paid any attention to the assertion probably implies an
unconscious condemnation of the hypothesis of the transmissibility of
acquired characters.
It has been proved by Nägeli's often discussed experiments on hawkweeds
(_Hieracium_) that much less specialized external influences can
give rise to changes which are not hereditary. The Alpine species of
hawkweed varied considerably in their whole habit in the rich soil of
the Botanic Gardens at Munich, but their descendants, when transferred
to a poor flinty soil, returned to the habit of the Alpine species.
The changes which occurred in garden soil were therefore somatic and,
as I have called them, 'transient,' and they did not depend upon
variations of the germ-plasm. It may be objected in regard to these
experiments that they were not continued long enough to prove that
hereditary variations would not also have cropped up in consequence of
the altered conditions. But in any case they prove that marked changes
in the whole body of the plant may occur without any obvious variation
of the germ-plasm. This does not mean, however, that the possibility of
variations of the germ-plasm through such direct external influences
is disputed. We must assume the occurrence of these on _a priori_
grounds, if we refer--as we have done--individual hereditary variation
to fluctuations in the nutrition of the individual determinants of the
germ-plasm. It is probable that many general nutritive variations or
climatic factors affect the germ-plasm as well as the soma, and it is
by no means inconceivable that it is not all, but only certain definite
determinants that are caused to vary.
A proof of this may be found in the results of experiments made upon
the little red-gold fire-butterfly (_Polyommatus phlæas_), to which
I have briefly referred in a former lecture. This little diurnal
butterfly of the family Lycænidæ has a wide distribution and occurs
in two climatic varieties. In the far north and also in the whole of
Germany the upper surface is red-gold with a narrow black outer margin,
but in the south of Europe the red-gold has been almost crowded out by
the black. I reared caterpillars in Germany from eggs of _P. phlæas_
found at Naples and exposed them directly after they had entered on
the pupa-stage to a relatively low temperature (10° C.). Butterflies
emerged which were not quite so black as those of Naples, but
considerably darker than the German form. Conversely, German pupæ were
exposed to greater warmth (38° C.), and these gave rise to butterflies
which were rather less fiery gold and considerably blacker than the
ordinary German form. If I had to repeat these experiments I should
use a much lower temperature in the case of the cold experiments,
because we now know from the experiments of Standfuss, E. Fischer, and
Bachmetjeff, that most of the pupæ of diurnal butterflies can stand a
temperature below zero for a considerable time; probably the results
would be even more marked then.
But even from the results of my former experiments we are justified
in concluding that the blackening of the upper surface of the wing is
really the direct result of the increased temperature during pupahood,
and that the pure red-gold results from the lowered temperature.
Similar experiments made by Merrifield with English _Phlæas_ pupæ agree
exactly with mine. But we may conclude further from these experiments
that both warmth and cold only give rise to slight variations in the
individual pupæ, and that the pure red-gold of the northern form
and the black of the southern are the result of a long process of
inheritance and accumulation, in which the germ-plasm has been caused
to vary in as far as the relevant determinants are concerned, so that
these yield the respective northern and southern forms even in less
extreme temperatures.
As it is to be assumed that these determinants are present not
only in the primordium of the wing in the pupa, but also in the
germ-cells, both must be affected by the varying temperature, and, in
accordance with the continuity of the germ-plasm, each variation of
these determinants, however slight, would be continued in the next
generation. It is thus intelligible that somatic variations like the
blackening of the wings through warmth appear to be directly inherited
and accumulate in the course of generations; in reality, however,
it is not the somatic change itself which is transmitted, but the
corresponding variation evoked by the same external influence in the
relevant determinants of the germ-plasm within the germ-cells, in other
words, in the determinants of the following generation.
This interpretation of these experiments, which I offered some years
ago, has been confirmed in several ways in regard to various other
diurnal Lepidoptera. By employing a temperature as low as 8° C. in
the case of fresh pupæ of various species of _Vanessa_ Standfuss
and Merrifield, and especially E. Fischer, succeeded in getting
great deviations in the marking and colour of the full-grown
insects,--so-called aberrations, such as had previously been found
only very rarely and singly under natural conditions. The deviations
from the normal must undoubtedly be ascribed to the effect of cold,
but it does not follow that they are new forms which have suddenly
sprung into existence, as many have assumed without further experiment.
Dixey, on the other hand, has attempted to establish, by a comparison
of the different species of _Vanessa_, the phyletic development of
their markings, and has found that these aberrations due to cold
are more or less complete reversions to earlier phyletic stages. As
regards the common small painted lady (_Vanessa cardui_), the small
tortoise-shell butterfly (_Vanessa urticæ_), the 'Admiral' (_Vanessa
atalanta_), the peacock (_Vanessa io_), and the large tortoise-shell
(_Vanessa polychioros_), I can agree with this interpretation, and I
do so the more readily because some years ago I suggested that the
alternation of differently coloured generations of seasonally dimorphic
Lepidoptera might be considered as a reversion. But this by no means
excludes the possibility that other than atavistic aberrations may be
produced by cold or heat. There is nothing against this theoretically.
Yet we must not, without due consideration, compare these abruptly
occurring variations to the sport-varieties of plants which we have
already discussed; there is an important difference between the two
sets of cases. In the Lepidoptera a single interference, lasting only
for a short time, modifies the wing-marking, but in the plant varieties
the visible appearance of the variation is preceded by a long period of
preparatory change within the germ-plasm. This period required for the
external influences to take effect was already recognized by Darwin,
and it has recently been named by De Vries the 'premutation period.'
We may explain these remarkable aberrations theoretically in
the following way: The determinants of the wing-scales in the
wing-primordium of the young pupa are influenced by the cold in
different ways, some kinds of determinants being strengthened by it,
others markedly weakened, even crippled so to speak, and in this way
one colour-area spreads itself out more than is normal on the surface
of the wing, and another less, while a third is suppressed altogether.
That this disturbance of the equilibrium between the determinants
leads usually to the development of a phyletically older marking
pattern leads us to the conclusion that in the germ-plasm of the
modern species of _Vanessa_ a certain number of determinants of the
ancestors must be contained in addition to the modern ones. We might
even inquire whether these were not better able to endure cold than
their modern descendants, since their original possessors, the old
species of the Ice age, were accustomed to greater cold, but this idea
is contradicted by the experiments of E. Fischer, which go to show that
the same aberrations are evoked by abnormally high temperature. That
the old ancestral determinants are present in _different_ numbers in
the germ-plasm of the modern species, I am inclined to infer from the
fact that among a large number of experiments made by me in the course
of several years the aberrations have always occurred in very different
numbers in the different broods, although the greatest care was taken
to have the conditions as nearly alike as possible; absolutely alike,
of course, they never can be.
But it would lead me too far if I were to enter on a detailed
discussion of these cases, which have not yet been fully worked up;
only one thing more need be mentioned, that is, that the aberrations
induced by cold are to a certain extent transmissible. Standfuss
first succeeded in making some aberrant specimens of _Vanessa urticæ_
reproduce, and from their eggs he procured butterflies which showed
a much slighter deviation from the normal, which however was still
so decided that it could not be regarded as due to chance. I myself
succeeded in doing the same, but the deviation in this case was much
slighter. But that these observed cases are rightly referred to the
cold to which their parents had been subjected is proved by other
observations recently published by E. Fischer. These refer to one of
the Bombycidæ (_Arctia caja_), which flies by day, and accordingly
has a gay and very definite marking and coloration. A large number
of pupæ were exposed to cold at 8° C., and some of these resulted in
striking and very dark aberrant forms (Fig. 129, _A_). A pair of these
yielded fertilized eggs; in the progeny, which were reared at a normal
temperature, there were among the much more numerous normal forms a
few (17) which exhibited the aberration of the parents, though to a
considerably less degree (Fig. 129, _B_).
This shows that the cold had affected not only the wing-primordia
of the parental pupæ, but the germ-plasm as well, and at the same
time that this latter variation was less marked than that of the
determinants of the wing-rudiments. This gives rise _to an appearance_
of the transmission of acquired characters.
In the case of many of these cold-aberrations in Lepidoptera the cold
gives rise to variations, but does so not by creating anything new,
but by giving the predominance to primary constituents which have long
been present, but are usually suppressed, and so it is also among the
plants. I have in mind, for instance, the interesting experiments of
Vöchting on the influence of light in the production of flowers in
phanerogams. These showed that the common balsam (_Impatiens noli me
tangere_) produces its familiar open flowers in a strong light, but
in weak light only bears small, closed, so-called 'cleistogamous'
flowers. But it would be utterly erroneous to suppose that the strong
or weak light is the real cause, the _causa materialis_, of these two
forms of flowers: the degree of illumination is merely the stimulus
which provokes one or other of the primary constituents to development,
both kinds being present in the constitution of the plant. As has long
been known, the balsam normally possesses two kinds of flowers, and
the slumbering primary constituents of these are so arranged that the
open flowers develop where there is a prospect of insect visits and
cross-fertilization, that is, in sunny weather or in a strong light,
while closed and inconspicuous flowers adapted for self-fertilization
develop in weak light, that is, in shady places and in concealed parts
of the plant, where insect visits are not to be expected.
[Illustration: FIG. 129. _A_, an aberration of _Arctia caja_, produced
by low temperature. _B_, the most divergent member of its progeny.
After E. Fischer.]
Among plants we find thousands of instances of such reactions of
the organism to external stimulus--reactions which are not of a
primary nature, that is, are not the inevitable consequences of
the plant's constitution, but which depend upon adaptations of the
special constitution of a species or group of species to the specific
conditions of its life. To this category belong all the phenomena of
heliotropism, geotropism, and chemotropism, which have been discovered
by the numerous and excellent observations of the plant physiologists.
That all these are adaptations and secondary reactions to stimuli is
proved by the fact that the same stimuli affect the homologous parts of
different species in very different, and often in opposite ways. For
instance, while the green shoots of most plants turn towards the light,
being positively heliotropic, the climbing shoots of the ivy and the
gourd are negatively heliotropic, which is an adaptation to climbing.
In this case the reason of the difference in the mode of reaction
must lie in the difference of constitution of the cellular substance
of the shoot, and since this may differentiate so very diversely in
its relation to light, the power of reaction which plant substance in
general has to light must not be regarded as a primary character, like
the specific gravity of a metal or the chemical affinities of oxygen
and hydrogen, but as adaptations of the living and varying substance
to the special conditions of life. And the origin of these adaptations
must depend upon processes of selection, and on these alone. This is
just the difference between living and non-living matter,--that the
former is variable to a high degree, the latter is not; it is the
fundamental difference upon which the whole possibility of the origin
of an animate world depends.
Among animals also we must distinguish between the direct effects of
external influence to which the organism is not already adapted, and
those reactions which imply a previously established adjustment to the
stimulus. That is, we must distinguish between primary and secondary
reactions.
For instance, Herbst made artificial sea-water in which the sodium was
partially replaced by lithium, and the eggs of sea-urchins developed
in this artificial sea-water into very divergent larvæ of peculiar
structure. We have here a primary reaction of the organism to changed
conditions of life--not an adaptation, not a prepared reaction.
Accordingly these 'lithium larvæ' eventually perished.
The increasing blackness of _Polyommatus phlæas_, which we have already
discussed, must also be regarded as a primary reaction, but not so
the variations--often misinterpreted--of those species of _Artemia_
which live in the brine-pools of the Crimea, in regard to which
Schmankewitsch showed that, when the amount of salt in the water is
diminished, they undergo certain changes which bring them nearer to
the fresh-water form _Branchipus_, while when the salt is increased
in amount they vary in the contrary direction. Probably these are
adaptations to the periodically changing salinity of their habitat.
There can be no doubt of this in the case of the caterpillars of
different families, in regard to which Poulton showed that in their
early youth they possess the power of adapting themselves exactly
to the colour of their chance surroundings. It is obvious that the
protection which the caterpillar would gain from being coloured
_approximately_ like its surroundings would be insufficient, for
instance because the surroundings may be very diverse, since
the species lives upon different, variously coloured plants and
plant-parts. Thus a facultative adaptation arose. Selection gave rise
to an extraordinarily specialized susceptibility on the part of the
different cell elements of the skin to differences of light, and the
result of this is that the skin of the caterpillar invariably takes on
the colouring which is reflected upon it in the first few days of its
life from the plants and plant-parts by which it is surrounded. Thus
the caterpillars of one of the Geometridæ, _Amphidasis betularia_, take
on the colours of the twig between and upon which they sit, and they
can be made black, brown, white, or light green quite independently of
their food, according to the colour of the twigs (or paper) among which
they are reared.
Colour-change in fishes, Amphibians, Reptiles, and Cephalopods, depends
upon much more complex adaptations. In their case a reflex-mechanism
is present which conducts the light-stimulus affecting the eye to the
brain, and there excites certain nerves of the skin; these in their
turn cause the movable cells of the skin which condition the colouring
to change and rearrange themselves in the manner necessary to bring
about the harmonization of colour. On this depends the colour-change of
the famous chamæleon, and also the scarcely less striking case of the
tree-frog, which is light green when it sits on trees, but dark brown
when it is kept in the dark. All these are secondary reactions of the
organism in which the external stimulus is, so to speak, made use of
to liberate adaptive variations, either permanently or transitorily.
In the caterpillars colour-changes are permanent, that is, it is only
the young caterpillar which takes on the colour of its surroundings;
later it does not change, even when it is exposed to different light,
or intentionally placed upon a food-plant of a different colour.
In fishes, frogs, and cuttlefishes, on the contrary, the reaction
of the colour-cells to light only lasts a little longer than the
light-stimulus, and it changes with it. The purposiveness of this
difference of reaction is obvious.
We cannot say to what degree the direct influence of external
conditions is effectively operative on the germ-plasm, or how far, by
persistently repeated slight changes, the determinants and the parts
of the body determined by them may be made to vary in the course of
generations; that is to say, how large a part this direct influence
of climate and food may play in the transmutation of species. We can
give no answer from experience, because there is an entire lack of
perfectly satisfactory and clear experiments; we only know in a few
cases how great the variations are which can be brought about in the
body during the individual life by means of any of these factors. In
most cases it is uncertain whether actually hereditary effects play any
part, that is, whether the germ-plasm itself is affected. But if we
wish to be theoretically clear as to how far direct climatic effects
may go, we may say this, that they may operate as long as they cause
no disturbance in the life of the species concerned, for at the moment
that such a direct effect begins to be prejudicial to the species
personal selection will step in, and, by preferring the individuals
which react least strongly to the climatic stimulus, will inhibit the
variation. If in any case this should be physically impossible, the
species would die out in the climate in question. That a species of
plant or animal has climatic limits indicates that individuals which go
beyond these are exposed to influences which make life impossible and
which natural selection is unable to neutralize. We are here brought
face to face with one of the limits to the scope of natural selection.
There is no doubt that the influences of the environment must always
have a powerful effect upon the soma of the individual, but we have
seen, in the case of Alpine plants and of galls, how very far this
effect may go without leaving any trace in the germ-plasm.
LECTURE XXXII
INFLUENCE OF ISOLATION ON THE FORMATION OF SPECIES
Introduction--Isolated regions are rich in endemic species--Is
isolation a condition in the origin of species?--Moriz Wagner,
Romanes--'Amiktic' local forms, the butterflies of Sardinia, of the
Alps, and of the Arctic zone--Periods of constancy and periods of
variation in species--Amixia furthered by germinal selection--The
thrushes of the Galapagos Islands--The intervention of sexual
selection--Humming-birds--Central American thrushes--Weaver-birds
of South Africa--Papilionidæ of the Malay Archipelago--Natural
selection and isolation--Snails of the Sandwich Islands--Influences
of variational periods--Comparison with the edible snail and with the
snail fauna of Ireland and England--Changed conditions do not always
give rise to variation--Summary.
In an earlier lecture I endeavoured to show, by means of Darwinian
arguments and examples, how important for every species, in relation to
its transmutation, is the companionship of the other species which live
with it in the same area. We saw that the 'conditions of life' operated
as a determining factor in the composition of an animal and plant
association quite as momentously as any climatic conditions whatsoever,
and, indeed, Darwin rated the influences of vital association even
more highly, and attributed to them an even greater power of evoking
adaptation than he granted to the physical conditions of life.
We are, therefore, prepared to recognize that even the transference of
a species to a different fauna or flora may cause it to vary, and this
occurs when the species gradually extends the area of its distribution,
so that it penetrates into regions which contain a materially different
association of forms of life. But these migrations are not necessarily
only gradual, that is, due to the slow extension of the original area
of distribution in the course of generations as the species increases
in numbers; they may also occur suddenly, when isolated individuals
or small companies of a species transcend in some unusual manner the
natural boundaries of the old area, and reach some distant new region
in which they are able to thrive.
Species-colonies of this kind may be due to the agency of man, who has
spread many of his domesticated animals and plants widely over the
earth, but who has also intentionally or unintentionally forced many
wild animals and plants from their original area to distant parts
of the earth, as, for instance, when the English humble-bees were
imported into New Zealand with a view to securing the fertilization
of the clover; but such colonies also occur in thousands of cases
independently of man's agency, and the means by which they are brought
about are very diverse. Little singing-birds are sometimes driven
astray by storms, and carried far away across the sea, to find, if
fortune favours them, a new home on some remote oceanic island;
fresh-water snails, which have just emerged from the egg, creep on to
the broad, webbed feet or among the plumage of a wild duck or some
other migratory bird, and are carried by it far over land and sea,
and finally deposited in a distant marsh or lake. This must happen
not infrequently, as is evidenced by the wide distribution of our
Central European fresh-water snails towards the north and south. But
terrestrial snails can also, though more rarely, be borne in passive
migration far beyond limits which are apparently impassable, as is
evidenced by the presence of land-snails on remote oceanic islands.
The Sandwich Islands are more than 4,000 kilometres from the continent
of America; they originated as volcanoes in the midst of the Pacific
Ocean; and yet they possess a rich fauna of terrestrial snails,
the beginnings of which can only have reached them by the chance
importation of individual snails carried by strayed land-birds.
Charles Darwin was the first who attempted to investigate the problem
of the colonization of oceanic islands by animal inhabitants, and the
chapter in _The Origin of Species_ which deals with the geographical
distribution of animals and plants still forms the basis of all the
investigations directed towards this point. We learn from these that
many land-animals, of which one would not expect it on _a priori_
grounds, may be carried away by chance over the ocean, either, as in
the case of butterflies and other flying insects, and of birds and
bats, by being driven out of their course by the wind, or by being
concealed--either as eggs or as fully-formed animals--in the clefts of
driftwood, where they can resist for a considerable time the usually
destructive influence of salt water. Thus eggs of some of the lowest
Crustacea (Daphnidæ), which are contained in large numbers in the mud
of fresh water, may be transported with some of the mud on the feet
of birds, and this may happen also to encysted infusorians and other
unicellulars, and to the much more highly organized Rotifers as well.
In all these cases, and in many others, it may happen occasionally that
single individuals, or a few at a time, may be carried far afield, and
may reach regions from which their fellows of the same species are
entirely excluded. If they thrive there they may establish colonies
which will gradually spread all over the isolated area as far as it
affords favourable conditions of life.
But oceanic islands are not the only cases of isolated regions;
mountains and mountain-ranges which rise in the midst of a plain also
form isolation-areas for mountain-dwelling plants or animals which
have not much power of migrating. In the same way marine animals
may be completely isolated from each other by land-barriers, as the
inhabitants of the Red Sea, for instance, are from those of the
Mediterranean, as has been clearly expounded by Darwin. The idea of an
isolated region is always a relative one, and the region which seems
absolutely insular for a terrestrial snail is not so at all for a
strong-flying sea-bird. There is no such thing as _absolute_ isolation
of any existing colony, for otherwise the colony could never have
reached the region; but the degree of isolation may be _absolute_ as
far as the time of our observation is concerned, if the transportation
of the species concerned occurs so rarely that we cannot observe it in
centuries, or perhaps in thousands of years, or if the extension of its
range could only take place through climatic or geological changes,
such as a subsidence of land-barriers between previously separated
portions of the sea, or, in the case of land animals such as snails,
the elevation of the sea-floor and the filling up of arms of the sea
which had separated two land-areas. But even the transportation of a
species by the accidental means already indicated will occur so rarely,
if the isolated insular region is very distant, that the isolation of
a colony by such a chance may be regarded as almost absolute as far as
the members of the same species in the original habitat are concerned.
If we examine one of these insular regions with reference to the
animal inhabitants which live upon it in isolation, we are confronted
with the surprising fact that it harbours numerous so-called endemic
species, that is to say, species which occur nowhere else upon the
earth, and that these species are the more numerous the further the
island is removed from the nearest area of related species. It looks
at first sight quite as if isolation alone were a direct cause of the
transformation of species.
The facts which seem to point in this direction are so numerous that I
can only select a few of them. The Sandwich Islands, to which we have
already referred, possess eighteen endemic land-birds, and no fewer
than 400 endemic terrestrial snails, all belonging to the family group
of Achatinellinæ, which occurs there alone.
The Galapagos Islands lie 1,000 kilometres distant from the coast
of South America, and they too harbour twenty-one endemic species
of land-birds, among them a duck, a buzzard, and about a dozen
different but nearly related mocking-birds, each of which is found
in only one or two of the fifteen islands. The group of islands also
possesses peculiar reptiles, and they take their name from the gigantic
land-tortoises, sometimes 400 kilogrammes in weight, which in Tertiary
times inhabited also the continent of South America, but are now found
in the Galapagos Islands only. The islands also possess endemic lizards
of the genus _Tropidurus_, and although the lizards can no more have
been transported across the ocean than the tortoises, but corroborate
the conclusion drawn from geological data, that the islands were still
connected with the mainland in Tertiary times, the occurrence of a
particular species of _Tropidurus_ upon almost every one of the fifteen
islands testifies anew to the mysterious influence of isolation, for
most of these islands are quite isolated regions for the different
species of lizard, even more than for the mocking-birds, which have
also split up into a series of species.
We are thus led to the hypothesis, which was first introduced into the
Evolution Theory by Darwin, that the prevention of constant crossing
of an isolated colony with the others of the same species from the
original habitat favours the origin of new endemic species, and his
conclusion is confirmed when we learn that islands like the Galapagos
group possess twenty-one endemic land-birds, but only two endemic
sea-birds out of eleven, for the latter traverse great stretches of
sea, and crossings with others of the same species on the neighbouring
continental coasts will often take place. The Bermuda Islands also
afford a proof that the development of endemic species is prevented by
regular crossing with other members of the species from the original
habitat, for although they are 1,200 kilometres distant from the
continent of North America--that is, further than the Galapagos Islands
from South America--they possess no endemic species of bird, and we may
undoubtedly associate this with the fact that the migratory birds from
the continent visit the Bermudas every year.
Madeira also confirms our conclusion, for only one of the ninety-nine
species of bird occurring there can be regarded as endemic, and it has
often been observed that birds from the neighbouring African mainland
(only 240 kilometres distant) are driven across to Madeira. Terrestrial
snails, on the other hand, will seldom be carried to Madeira by birds,
and accordingly we find there an extraordinary number of endemic
terrestrial snails, namely, 109 species.
Although these and similar facts indicate strongly that isolation
favours the evolution of new species, it would be erroneous to imagine
that every isolation of a species-colony conditions its transmutation
to a new species, or, as has been maintained, first by Moritz Wagner,
and later by Gulick and by Dixon, that isolation is a necessary
preliminary to the variation of species--that not selection but
isolation alone renders the transmutation of a species possible, and
thus admits of its segregation into several different groups of forms.
Romanes went so far as to regard the natural selection of Darwin and
Wallace as a sub-species of isolation, and isolation in its diverse
forms he regarded as the sole factor in the formation of species. He
assumed that it was only by the segregation of individuals which did
not vary that the constant reversion to the ancestral species could
be prevented, and he regarded the process of selection as essentially
resulting in the 'isolation' of the fittest through the elimination of
the less-fit. The idea is correct in so far, that selection undoubtedly
aids the favourable variation to conquest over the old forms, precisely
because the latter, being less favourably placed in the struggle for
existence, are gradually more completely overcome and weeded out, so
that a constant mingling of the new forms with the old is prevented,
just as it is by isolation of locality. Obviously the new and fitter
forms could not become dominant, could not even become permanent,
if they were always being mingled again with the old. But whether
it serves any useful purpose to bring this under the category of
'isolation,' and to say that mingling with the ancestral form during
transmutation is prevented by natural selection, in that favourably
varying individuals _are isolated_ by their superiority from the
inferior ones, that is, the non-varying individuals which are doomed
to elimination, is somewhat doubtful. For my part, I should prefer to
retain the original meaning of the word, and to call 'isolation' the
separation of a species-colony by spatial barriers.
Whether this factor by itself prevents the mingling with the ancestral
form as effectually as selection does, and whether isolation alone and
by itself can lead to the evolution of new forms, or perhaps must lead
to them, must now be investigated.
I look at this question from exactly the same point of view as I did
nearly thirty years ago, when in a short paper[27] I endeavoured to
show that, under favourable circumstances, an individual variation of a
species may become the origin of a local variety if it finds itself in
an isolated region. Suppose an island had no diurnal butterflies, until
one day a fertilized female of a species from the continent was driven
thither, found suitable conditions of life, laid its eggs, and became
the founder of a colony; the prevention of constant crossing between
this colony and the ancestral continental species would not in itself
be any reason why the colony should develop into a variety. But suppose
that the foundress of the colony diverged in some unimportant detail of
colouring, such as may at any time arise through germinal selection,
from the ancestral species; then this variation would be transmitted to
a portion of her progeny, and there would thus be a possibility that a
variety should establish itself upon the island which would be the mean
of the characters of the surviving progeny. The greater the divergence
was in the first progeny of the mother-colonist, and the stronger this
variational tendency was, the greater also would be the chance that it
would be transmitted further and become a characteristic aberration
from the marking of the original species. I then designated this effect
of isolation as due to _amixia_, that is, to the mere prevention of
crossing with the members of the same species in the original habitat.
[27] _Ueber den Einfluss der Isolirung auf die Artbildung_, Leipzig,
1872.
We have examples of this from the Mediterranean islands, Sardinia and
Corsica, which possess in common nine endemic varieties of butterflies,
most of which diverge from the species of the continent in a quite
inconsiderable degree, though quite definitely and constantly. Thus
there flies in these islands a variety (_Vanessa ichnusa_) of our
common little _Vanessa urticæ_ in which the two black spots on the
anterior wing exhibited by the original species are wanting. The large
tortoise-shell (_Vanessa polychloros_) also occurs there, but it has
not varied and still exhibits the black spots. Our little indigenous
butterfly (_Pararga megæra_), which is abundant on warm, stony slopes,
quarries, and roads, flies about in Sardinia, but as a variety
(_tigelius_), which is distinguished from the original species by the
absence of a black curved line on the posterior wings.
That of two nearly related and similarly marked species, like the large
and small tortoise-shell, one should remain unvaried, while the other
has become a variety, shows us that amixia alone does not necessarily
lead to the evolution of varieties in every case. It might of course be
objected that one species may have migrated to the islands at a much
earlier period than the other, and that it might be a direct effect
of the climate which found expression in this way. But we have other
similar cases in which one of two species has varied in an isolated
region, while the other has not, and in regard to which we can prove
definitely that both were isolated at the same time.
An instance of this kind is to be found in Arctic and Alpine
Lepidoptera, which inhabited the plains of Europe during the Glacial
period, and subsequently, when the climate became milder again,
migrated some to the north into countries within the Arctic zone, and
some to the south to the Alps to escape in their heights from the
increasing warmth. There are many diurnal Lepidoptera which now belong
to both regions, and of these some have remained exactly alike, so that
the Arctic form cannot be distinguished from the Alpine form; others
show slight differences, so that we can distinguish an Arctic and an
Alpine variety. To the former category belong, for instance, _Lycæna
donzelii_ and _Lycæna pheretes_, _Argynnis pales_, _Erebia manto_,
and others; to the second category belong, for instance, _Lycæna
orbitulus_, Prun., _Lycæna optilete_, _Argynnis thore_, and some
species of the genus _Erebia_.
This cannot be an instance of the direct effect of general climatic
influences, for in that case all the nearly related species of a genus
would have varied or not varied; nor have we to do with adaptations,
for the differences in marking are seen on the upper surfaces of
the wing, which do not exhibit protective colouring, at least in
these Lepidoptera. It can only have been the prevention of crossing
that has fixed the existing variational tendencies in the isolated
colonies--variations which would have been swamped and obliterated if
there had been constant crossing with all the rest of the members of
the species.
But there is another factor to be considered. Those Alpine Lepidoptera,
for instance, which have not remained exactly the same in the far
north, have formed local varieties in the rest of the area of their
distribution also, while species which have remained quite alike
in isolated regions, such as the Alps and the north, exhibit no
aberrations in other isolated regions, such as the Pyrenees, in
Labrador, or in the Altai. Thus one species must have had a tendency
in the Glacial period to form local varieties, and the other had not;
and I have already attempted to explain this on the hypothesis that the
former at the time of their migration and segregation into different
colonies were at a period of dominant variability, the latter at a
period of relatively great constancy. Leaving aside the question of
the causes of this phenomenon, we may take it as certain that there
are very variable and very constant species, and it is obvious that
colonies which are founded by a very variable species can hardly ever
remain exactly identical with the ancestral species; and that several
of them will turn out differently, even granting that the conditions of
life be exactly the same, for no colony will contain all the variants
of the species in the same proportion, but at most only a few of
them, and the result of mingling these must ultimately result in the
development of a somewhat different constant form in each colonial area.
If we were to try to imitate this 'amixia' artificially we should
only require to take at random from the streets of a large town a
number of pregnant bitches, and place each of them upon an island
not previously inhabited by dogs, and then a different breed of dog
would arise upon each of these islands, even if the conditions of life
were exactly similar. But if, instead of these variable bitches, the
females of a Russian wolf were placed on the islands, the developing
wolf-colonies would differ as little from the ancestral species as the
various Russian wolves do from one another--similar climate and similar
conditions of life being presupposed.
There is thus an evolution of varieties due to amixia alone, and we
shall not depreciate the significance of this if we consider that
individual variations are the outcome of the fluctuations in the
equilibrium of the determinant system of the germ-plasm, to which it
is always more or less subject, and that variations of the germ-plasm,
whether towards plus or minus, bear within themselves the tendency
to go on increasing in the direction in which they have begun, and
to become definite variational tendencies. In isolated regions such
variational tendencies must continue undisturbed for a long period,
because they run less risk of being suppressed by mingling with
markedly divergent germ-plasms.
The probability that variational tendencies set up in some ids of the
germ-plasm by germinal selection will persist and increase is obviously
greater the more the germ-plasms combining in amphimixis resemble each
other. For instance, let us call the varying determinants _Dv_, and
assume as a favourable case that these are represented in three-fourths
of all the ids in the fertilized eggs of a butterfly-female which has
been driven astray on to an island, that is, that they are present
in twelve out of sixteen ids; then of 100 offspring of the first
generation it is possible that seventy-five or more will contain the
determinants _Dv_, some of them in a smaller number of ids, some in a
great number than the mother, according as the reducing division has
turned out. If the pairing of the second generation be favourable--and
this again is purely a matter of chance--a third generation must arise
which would contain the variants _Dv_ throughout, and thus the fixation
of this particular variation on this particular island would be begun.
In other words, the possibility would arise, that, if individuals with
a majority of _Dv_ ids predominated, they would gradually come to be
the only ones, since by continual crossing with the minority which
possessed only the determinants _D_, they would mingle the varied ids
with those of the descendants of these last, till ultimately germ-plasm
with only the old ids would no longer occur.
In following out this process it is not necessary to assume that the
first immigrant possessed the variation _visibly_; if determinants
varying in a particular direction occurred in the majority of its ids,
these would, as a consequence of persistent germinal selection, go on
varying gradually until the externally visible variation appeared. This
would not have appeared at all if the animal concerned had remained
in the original habitat of its species, for there it would have been
surrounded by normal germ-plasms, and its direct descendants, even if
they had been as favourably situated for the origin of variations as
we have assumed, would not have reproduced only among themselves, and
therefore even in the next generation the number of _Dv_ ids would have
diminished.
Obviously it is to a certain extent a matter of chance whether in the
isolated descendants the variation or the normal form remains the
victor, for it depends on the number of _Dv_ ids originally present in
the fertilized eggs, then on the chances of reducing divisions, and
finally on the chance which brings together for pairing individuals in
which the similarly varied ids preponderate. The probability of the
conquest of the variation will depend in the main on the strength of
the majority of the varied ids in the fertilized eggs of the parents;
if this be an overwhelming majority, then the chances of favourable
reducing divisions and pairings will also be great. The origin of a
pure amixia variety will thus depend upon the fact that _the same_
variational tendency _Dv_ was present in a large number of the ids of
the ancestral germ-plasm. We need not wonder therefore that of the
numerous diurnal butterflies of Corsica and Sardinia only eight have
developed into endemic, probably 'amiktic,' varieties.
But since we know that so many species in oceanic islands and other
isolated regions are endemic or autochthonous, i.e. of local origin,
there must obviously be some other factor in their evolution in
addition to the mere prevention of crossing with unvaried individuals
of the same species. The variational tendencies which have arisen in
the germ-plasm through germinal selection may--as we have already
seen--gain the ascendancy in various ways; first, by being favoured
by the climatic influences, then by being taken under the protection
of personal selection, whether in the form of natural or of sexual
selection.
As the inhabitants of insular areas are not infrequently subject to
special climatic conditions, we may assume at the outset that many of
the 'endemic' species are climatic varieties, but in many cases this
explanation is insufficient. For instance, special local forms of
mocking-bird live on several of the Galapagos Islands, but this cannot
depend upon differences of climate, for the islands are only a few
kilometres apart, and resemble one another as regards the conditions
of life which they present. But as the differences between these
local forms show themselves especially in the male sex, as colour
variations of certain parts of the plumage, we must take account of
sexual selection, which, though with its basis in germinal selection,
has in many islands followed a path of its own. Sexual selection
operates especially in the case of sporadically occurring characters
which are in any way conspicuous. But it is just such variations as
these that are called into existence by germinal selection, whenever it
is allowed to continue its course undisturbed through a long series of
generations. Characters of this kind, such, for instance, as feathers
of abnormal structure or colour in a bird, or new colour spots in a
butterfly, make their appearance when a group of determinants has been
able to go on varying in the same direction for a long time unimpeded,
that is, without being eliminated as injurious by natural selection or
obliterated by crossing. This is very likely to happen in the case of
an isolated area, and as soon as the conspicuous character thus brought
about makes its appearance, sexual selection takes control of it, and
ensures that all the individuals, that is, all the germ-plasms which
possess it, have the preference in reproduction.
I believe, therefore, that a large number of the endemic species of
birds and butterflies in isolated regions result from amixia based
upon germinal selection, whose results have been emphasized by sexual
selection. Experience corroborates this, as far as I can see, for many
of the endemic species of birds in the Galapagos and other islands
differ from one another solely or mainly in their colouring, and in
many it is especially the males which differ greatly.
As to the humming-birds we may say, without going into details
regarding their sexual characters and their distribution, that the many
endemic species which inhabit the Alpine regions of isolated South
American volcanic mountains differ from one another chiefly in the
males and in the secondary sexual characters of these. The family of
humming-birds is characteristically Neotropical, that is, it has its
centre in the Tropics of the New World, and by far the greater number
of humming-bird species--there are about a hundred and fifty--occur
there only, while a few occur as migrants north of the Tropical zone,
and visit the United States as far north as Washington and New York.
We know that many of the most beautiful species have quite a small
area of distribution, that many are restricted to a single volcanic
mountain, living in the forests which clothe its sides. These species
are isolated there, for they do not migrate; apparently they cannot
endure the climate of the plains, but remain always in their mountain
forests. Without doubt they originated there, chiefly, I am inclined
to think, through the variation of the males due to sexual selection.
Any one who has seen Gould's magnificent collection of humming-birds
in the British Museum in London knows what a surprising diversity of
red, green, and blue metallic brilliance these birds display, what
contrasts are to be found in the diverse colour-schemes, and what
differences they exhibit in the length and form of the feathers of the
head, of the neck, of the breast, and especially of the tail. There
are wedge-shaped, evenly truncate, and deeply forked tails, some with
single long, barbless feathers, and so on. All these characters are
confined to the males, and are at most only hinted at in the female;
in no species does the female even remotely approach the male in
brilliance or decorativeness of plumage.
I do not believe that so many species with very divergent plumage in
the males could have developed if they had all lived together on a
large connected area. But here, distributed over a large number of
isolated mountain forests, the decorative colouring or the distinctive
shape which chances to arise through germinal selection on any of these
terrestrial islands can go on increasing, undisturbed by crossing with
individuals of the ancestral species, and furthered, moreover, by
sexual selection.
In this way, if I mistake not, numerous new species have arisen as a
result of isolation, and it is quite intelligible that several new
species may have arisen from one and the same ancestral species, as we
may see from the nearly related yet constantly different species of
mocking-bird on the different islands of the Galapagos group.
A number of similar examples might be given from among birds. Thus
Dixon calls attention to the species of the thrush genus _Catharus_,
twelve of which live in the mountain forests of Mexico and of South
America as far as Bolivia, all differing only slightly from one another
and all locally separated. They came from the plains, migrated to the
highlands, were isolated there, and then no longer varied together all
in the same direction, but each isolated group evolved in a different
direction according to the occurrence of chance germinal variations:
one developed a chestnut-brown head, another a slate-grey mantle, a
third a brown-red mantle, and so on. From what we have already seen in
regard to the importance of sexual selection in evolving the plumage of
birds, it is probable that this factor has been operative in this case
also.
Another example is afforded by the weaver-birds (_Ploceus_) of South
Africa, those ingenious singing-birds resembling blackbirds in size and
form, whose pouch-shaped nests, hanging freely from a branch, usually
over the water, and with their little openings on the under side, are
excellently protected from almost every form of persecution. These
birds have in South Africa split up into twenty or more species, but
the areas of each are not sharply isolated, and the division into
species cannot, therefore, be due to isolation. But it is not difficult
to guess upon what it depends, when we know that the males alone are
of a beautiful yellow and black colour, while the females are of a
greenish protective colouring all over.
Thus, in my opinion, sexual selection plays a part more or less
important in the origin of the numerous endemic species of diurnal
Lepidoptera which are characteristic especially of the islands of the
Malay Archipelago, and which make the Lepidopteran fauna there so rich
in individuality. A large number, indeed the majority of the types of
Papilionidæ, have a peculiar species, a local form, on most of the
larger islands, which is sharply and definitely distinguished from
those of the other islands, usually in both sexes, but most markedly in
the much more brilliantly coloured males.
Thus each of these types forms a group of species, each of which is
restricted to a particular locality, and has usually originated where
we now find it, although of course the diffusion of one of these
large strong-flying insects from one island to the other is in no way
excluded. As an example we may take the _Priamus_ group, the blackish
yellow _Helena_ group, the blue _Ulyssus_ group, and the predominantly
green _Peranthus_ group.
If we inquire into the causes of this divergence of forms and their
condensation into numerous species, we shall find that their roots lie
in this case, as in that of all transformations, in germinal selection
and the variational tendencies resulting therefrom, but we must
_regard their fixation us the result of isolation_, which prevented
the variational tendencies which happened to develop on any one island
from being neutralized and swamped by mingling with the variations of
other islands. But that sexual selection took control of these striking
colour-variations and increased them still further is obvious from
the rarely absent dimorphism of the sexes. Even if the females do not
consciously select mates from among the males, they will more readily
accept as a mate the one among several suitors which excites them most
strongly. And that will be the one which exhibits the most brilliant
colours or exhales the most agreeable perfume, for we know from their
behaviour in regard to flowers how sensitive butterflies are to both
these influences.
Although isolation has an important rôle in the formation of all
these species, it seems to me an exaggeration to maintain, as many
naturalists do, that the splitting up of a species is impossible
without isolation. Certainly the splitting up of species is, in
numerous cases, facilitated by isolation, and indeed could only have
been brought about in its present precision by that means, but it is
underestimating the power of natural selection not to credit it with
being able to adapt a species on one and the same area to _different_
conditions of life, and we shall return to this point later on in a
different connexion. But in the meantime it must suffice to point out
that the polymorphism of the social insects affords a proof that a
species may break up into several forms in the same area through the
operation of natural selection alone.
I am therefore of opinion, with Darwin and Wallace, that adaptation to
new conditions of life has, along with isolation, had a material share
in the evolution of the large number of endemic species of snail on the
oceanic islands. This brings us to the co-operation of natural selection
and isolation. If, thousands of years ago, by one of the rarest
chances, an _Achatina_-like snail was carried by birds to the Sandwich
Islands, it would spread slowly, at first unvaried, from the spot
where it arrived over the whole of the snailless island. But during
this process of diffusion it would frequently come in contact with
conditions of life which would not prevent it from penetrating further,
but to which it was imperfectly adapted, and in such places a process
of transformation would begin, which would consist in the fostering of
favourably varying individuals, and which would run its course quietly
by means of personal selection, based upon the never-ceasing germinal
selection, and unhindered by any occasional intrusion of still unvaried
members of the species from the original settlement on the island.
But these new conditions were not merely different from those of the
ancestral country; the island region itself presented very diverse
conditions, to which the snail immigrant had to adapt itself in the
course of time, as far as its constitution allowed. Terrestrial snails
are almost all limited to quite definite localities with quite definite
combinations of conditions; none of our indigenous species occurs
everywhere, but one species frequents the woods, another the fields;
one lives on the mountains, another in the valleys; one on gneiss
soil, another on limy soil, a third on rich humus, a fourth on poor
river-sand; one in clefts and hollows among damp moss, and another in
hot, dry banks of loess, and so on. Although we cannot see in the least
from the structure of the animal why this or that spot should be the
only suitable one for this or that species, we may say with certainty
that each species remains permanently in a particular place because
its body is most exactly adapted to the conditions of life there, and
therefore it remains victorious in the competition with other species
in that particular spot.
In this way the immigrants to the Sandwich Islands must have adapted
themselves in the course of time to their increasingly specialized
habitats, and in doing so have divided up into increasingly numerous
forms, varieties, and species, and indeed into several genera.
But this alone is not sufficient to explain the facts. According to
Gulick's valuable researches there live on one little island of the
Sandwich group no fewer than 200 species of Achatinellidæ, with 600-700
varieties! This remarkable splitting up of an immigrant species is
regarded by him as a result of the isolation of each individual species
and variety, and I do not doubt that this is correct as far as a
portion of these forms is concerned, and that isolation plays a certain
part in regard to them all. Gulick, who lived a long time upon the
island, attempts to prove that the habitats of all these nearly related
varieties and species are really isolated as far as terrestrial snails
are concerned; that intermingling of the snails of one valley with
those of a neighbouring one is excluded, and that the varieties of the
species diverge more markedly from one another in proportion as their
habitats are distant. On the other hand, species of different genera
of Achatinellidæ often live together on the same area; but they do not
intermingle.
Although Gulick's statements are worthy of all confidence, and though
his conclusions have great value as contributions to the theory of
evolution, I do not think that he has exhausted the problem of the
causes of this remarkable wealth of forms among the terrestrial snails
of oceanic islands. It is not that I doubt the relative and temporary
isolation of the snail-colonies at numerous localities in the island of
Oahu. But why have we not the same phenomenon in Germany, in England
or Ireland? Gulick anticipates this objection by pointing out the
peculiar habits of the Oahu snails. Many of the species there are
purely arboreal animals, living upon trees and never leaving them, even
during the breeding season, or in order to deposit eggs, for they bring
forth their young alive. Active migration from forest to forest seems
excluded by the fact that on the crests of the mountains there is a
less dense forest of different kinds of trees, and dry sunny air, which
could not be endured by the species of _Achatinella_ and _Bulimella_,
which love the moist shades of the tropical forests. Active migration
over the open grass-land at the mouths of the valleys is also excluded.
It must be admitted that the isolation of these forest snails in their
valleys is for the time being very complete, and that intermingling
of two colonies which live in neighbouring valleys _does not occur
by active migration, within the span of one or several human
generations_. It will also be admitted that our terrestrial snails in
Central Europe are much less isolated in their different areas, that,
for instance, they could get from one side of a mountain to the other
by active migration; but we must nevertheless repeat the question: how
does it happen that in Oahu every forest, every mountain-crest, and so
on, has its own variety or species, while our snails are distributed
over wide stretches of country, frequently without even developing
sharply defined local varieties? The large vineyard or edible snail
(_Helix pomatia_) occurs from England to Turkey, that is, over a
distance of about 3,000 kilometres, and within this region it is found
in many places which might quite as well be considered isolated as
adjacent forest valleys in Oahu. It occurs also on the islands of the
Channel and of the Irish Sea, and lives there without intermingling
with the members of the species on the mainland. But even on the
Continent itself it would be possible to name hundreds of places in
which they are just as well protected from intermingling with those of
other areas as they are in Oahu. There too the snails must _somehow_
have reached their present habitat some time or other, perhaps rather
in an indirect way, by means of other animals; but this is true also of
the snails of a continent, as we shall show more precisely later on.
In the meantime let us assume that this is so, and that the vineyard
snail (_Helix pomatia_), or some other widely distributed snail,
is relatively isolated. _Why then have not hundreds of well-marked
varieties evolved--a special one for each of the isolated areas?_
Obviously there must have been something in operation in the Sandwich
Islands which is absent from the continental habitats of _Helix
pomatia_, for this species shows fluctuations only in size, but is
otherwise the same everywhere, and the few local varieties of it which
occur are unimportant. I am inclined to believe that this 'something'
depends on two factors, and especially on the fact that the immigrant
snail enters upon a period of variability. This will be brought about
in the first place by the fact that the climate and other changes
in the conditions of life will call forth a gradually cumulative
disturbance in the equilibrium of the determinant system, and thus
a variability in various directions and in various combinations of
characters. To this must be added the operation of natural selection,
which attempts to adapt the immigrant to many new spheres of life, and
thus increases in diverse ways the variational tendencies afforded by
germinal selection. These two co-operating factors bring the species
into a state of flux or lability, just as a species becomes more
variable under domestication, likewise as a direct effect of change
of food and other conditions, such as the consciously or unconsciously
exercised processes of selection. It follows from this that, in the
gradual diffusion of snails all over the island, similar localities
would almost never be colonized by exactly similar immigrants, but
by individuals containing a different combination of the existing
variations, so that in the course of time different _constant forms_
would be evolved through amixia in relatively isolated localities.
But everything would be different in the diffusion of a new species
of snail in a region which was already fully or at least abundantly
occupied by snail-species. Let us leave out of account altogether
the first factor in variation, the changed climate, and we see that
a species in such circumstances would have no cause for variation,
because it would find no area unoccupied outside of the sphere to
which it was best adapted; it would therefore not be impelled to adapt
itself to any other, and in most cases could not do so, because in each
it would have to compete with another species superior to it because
already adapted.
The case would be the same if an island were suddenly peopled with
the whole snail-fauna of a neighbouring continent, with which a land
connexion had arisen. If the island had previously been free from
snails, all the species of the mainland would be able to exist there in
so far as they were able to find suitable conditions of life, but each
species would speedily take complete possession of the area peculiarly
suited to it, so that none of their fellow migrants would be impelled,
or would even find it possible, to adapt themselves to new conditions
and thus to become variable and split up into varieties. If Ireland
were at present free from snails, and if a land connexion between it
and England came about, then the snail-fauna of England would probably
migrate quite unvaried to Ireland, and in point of fact the snail-fauna
of the two islands, which were formerly connected, is almost the same.
For the same reason the fauna of England, as far as terrestrial snails
are concerned, is almost the same as that of Germany.
On the other hand, it may be almost regarded as a law that an
individual migrant to virgin territory must become variable. This could
not be better illustrated than by the geographical distribution of
terrestrial snails, which emphasizes the fact that a striking wealth of
endemic species is to be found on all oceanic islands. Moreover, the
fact that the number of these endemic species is greater in proportion
to the distance of the island from the continent, indicates that the
variability sets in more intensively and lasts longer in proportion to
the small number of species which become immigrants in the island, and
in proportion to the number of unoccupied areas which are open to the
descendants of the immigrant species. This is undoubtedly the reason
why the Sandwich Islands do not possess _a single species_ which occurs
elsewhere, and the segregation of the unknown ancestral form into many
species and several (four) sub-genera is also to be interpreted in the
same way. There was probably in this case only one immigrant species,
which found a free field, and adapted itself in its descendants to
all the conditions of snail-life which obtained there, and in doing
so split up into numerous and somewhat markedly divergent forms. But
the number of different forms is much greater than the number of
distinctive habitats, as Gulick indicates and substantiates in detail,
for similar areas, if they are relatively isolated from one another,
are inhabited not by the same forms, but by different though nearly
related varieties, and this depends on the fact that from the species
which was in process of varying a different combination of variations
would be sent out at different periods, and the temporary isolation
would result in the evolution of special local varieties.
But I do not believe that this would continue for all time. I rather
think that these--let us say--representative varieties would diminish
in numbers in the course of a long period. For the isolation of single
valley-slopes or of particular woods is not permanent, individuals are
liable to be carried from one to another in the course of centuries as
they were at the beginning of the colonization of the isolated woods;
forests are cleared or displaced by geological changes, connexions are
formed between places, which were formerly separated, and in the course
of another geological period the number of representative varieties,
and probably even of species, will have diminished considerably,--the
former will have been fused together, the latter in part eliminated.
Even now Gulick speaks regretfully of the decimation of rare local
forms by their chief enemies, the mice.
But even if the number of endemic forms in insular regions diminishes
from the time when they were first fully taken possession of, it
nevertheless remains a very high one, for even now Madeira possesses
104 endemic terrestrial snails, the Philippines have more than the
whole of India, and the Antilles as many as the whole American
continent.
Many naturalists believe that each isolated variety must diverge
further and further from its nearest relatives as time goes on.
Although I entirely admit that this is possible, for I have endeavoured
to show that variational tendencies which have once arisen in the
germ-plasm go on in the same direction until they are brought to a full
stop in some way or other, yet I cannot admit that this must always
be so. The species which has been carried to a strange area need not
always contain particular variational tendencies in its germ-plasm, and
need not in every case be impelled to such variations by the influence
of new conditions. We know species which have made their way into new
regions, and, without varying at all, have held their own with, or
even proved superior to, the species which were already settled there.
Many cases of this kind are known, both among plants and animals;
these have been brought by man, intentionally or by chance, from one
continent to another, and have established themselves and spread over
the new area. I need only recall the evening primrose (_Œnothera
biennis_[28]), whose fatherland is Virginia, but whose beautiful big
yellow blossoms now display themselves beside nearly every river in
Germany, having migrated stream-upwards along the gravelly soil; or
the troublesome weed (_Erigeron canadense_), which is now scarcely
less common in our gardens than in those of Canada; or the sparrow
(_Passer domesticus_), which was introduced into the United States to
destroy the caterpillars, but which preferred instead to plunder the
rich stores of corn, and in consequence of these favourable conditions
increased to such an extent that it has now become a veritable pest,
all imaginable means for its extirpation having been tried--as yet,
however, with no great results.
[28] This was written before the appearance of the researches which
De Vries has made on the variations of _Œnothera_ in Europe. Thus
the illustration may not be quite apposite, for it seems to remain
undetermined whether the 'mutations' which occur in Holland do not also
occasionally appear in America. See end of lecture xxxiii.
In all these cases the migration is certainly of recent date, and it is
quite possible that, when a longer time has elapsed, some variations
will take place in the new home, but in any case these instances
prove that an immigrant species can spread over its new area without
immediately varying.
Similarly, it must be admitted that species which have belonged to
two continents ever since Tertiary times need not have diverged since
that time, and we know, for instance, thirty-two species of nocturnal
Lepidoptera which are common to North America and to Europe and yet
exhibit no differences, while twenty-seven other nocturnal Lepidoptera
are, according to Grote, represented in America by 'vicarious' species,
that is, by species which have varied slightly in one or other of the
two areas, perhaps in both.
To sum up: we must undoubtedly admit that isolation has a considerable
influence in the evolution of species, though only in association
with selection in its various grades and modes, especially germinal
selection, natural selection, and sexual selection. We can say
generally that each grade and mode of selection will more readily lead
to the transformation if it be combined with isolation. Thus germinal
selection may call forth slight divergences in colour and marking,
which will be permanent if the individuals concerned are in an isolated
region. In isolation these variations will increase undisturbed, and
in some circumstances will be intensified by sexual selection, so that
the male sex will vary alone in the first place, though the female
may follow, so that ultimately the whole species will be transformed.
Finally, the most marked effect of isolation is seen when individual
members of a species are transferred to virgin territory which offers
unoccupied areas, suitable not to one particular species alone, but to
many nearly related species, so that the immigrant colony can adapt
itself to all the different possibilities of life, and develop into a
whole circle of species. But we saw that such an aftergrowth of new
forms, whether varieties, species, or even genera, may far exceed the
number of different kinds of localities, if there be relative isolation
between the different groups of immigrants within the insular region,
as happens in the case of slow-moving animals like the terrestrial
snails, or of small singing-birds, to which each island of a little
archipelago is a relatively isolated region (Galapagos).
We may thus fully recognize the importance of local isolation without
regarding the absence of crossing with the members of the species in
the original habitat as the sole cause of species-formation, without
setting 'isolation' in the place of the processes of selection.
These last, taken in the wide sense, always remain the indispensable
basis of all transformations, but they certainly do not operate
only in the form of personal selection, but, wherever indifferent
characters are concerned, in that of germinal selection. Here, too,
we see the possibility of reconciliation with those naturalists who
regard transformations as primarily dependent upon internal forces of
development. The fact is that _all variations depend upon internal
causes_, and their course must be guided by forces which work in an
orderly way. But the actual co-operation of all these forces and
variations is not predetermined, but depends to a certain extent upon
chance, for of the possible modes of evolution the one which gains
the upper hand in the play of forces at the moment is alone followed,
the better are everywhere preferred, from the most minute vital units
of the germ-plasm, up to the struggle between individuals and between
species.
LECTURE XXXIII
ORIGIN OF THE SPECIFIC TYPE
Transition species of Celebes snails, according to Sarasin--Possible
variations in the shell due to nutrition--Natural selection plays a
part--Germinal selection--Temporary transitions between species--The
fresh-water snails of Steinheim--How do sharply-defined species
arise?--Nägeli's Developmental Force--The species a complex of
adaptations--Adaptive differences between species--Adaptive nature of
specific characters--The case of Cetaceans--Of birds--Additional note:
the observations and theories of De Vries.
Our study of the influence which geographical isolation may have in
transforming old and giving rise to new forms of life has led us
naturally to a much more important problem, that of the origin of
species as more or less sharply defined groups of forms, and I wish
to make the transition to this problem by discussing another case of
species-splitting effected in association with, or, as is usually
said, _through_ isolation. The naturalists Paul and Fritz Sarasin,
well known through their excellent studies on many components of
the tropical fauna, have published in their latest work interesting
discoveries in regard to the terrestrial snails of Celebes. These
observations show that on this island a great transformation of snails
has taken place, even since the later Tertiary period. A large number
of new species of snail have arisen on this island since that time,
and this, as the authors show to be probable, in association with
the receding of the sea, that is, with the elevation of the island
further out of the water, and thus with the increase of its surface.
The modern terrestrial snails show chains of forms connected in many
ways so that a series of species is connected by transition forms, and
therefore does not really consist of separate species at all, although
the extremes would seem to be separate species if they were studied
by themselves without taking the transition forms into account. The
state of things is exactly as if a Tertiary snail had spread from any
small area over the whole island, and had been transformed slowly
and in a definite direction in accordance with its distance from its
starting-point. It is thus that we must interpret this discovery;
we have here, beside each other in space, and indeed often disposed
along geographical lines, the individual stages of a phyletic process
of transformation, which has reached different levels at different
places. One of the longest of these chains of forms is that of _Nanina
cincta_, which runs across the island from east to west, and, beginning
with the smallest and most delicate forms, ascends through many
intermediate stages to the giant form _N. limbifera_. Such chains of
forms have been previously recognized; thus Kobelt described one in the
case of the Sicilian land-snails of the genus _Iberus_, and other cases
are recorded in literature, but in all instances they refer to areas
which must be regarded as isolated for the snails, and which have been
colonized from a single starting-point.
We have now to inquire whether and how we can explain the origin of
these chains of forms. The cousins Sarasin tell us how they at first
attempted to refer the differences between the individual links of such
a chain to the diverse influence of the external conditions of life,
but in vain; neither the height above sea-level nor the character of
the soil was sufficient, and natural selection was no more so; 'for
why should a high _Obba_-form twisted like a beehive be either better
or worse equipped for the struggle for existence than a smaller and
flatter one?' It is true that we do not understand why, but this does
not seem to me any reason to doubt that natural selection should be
regarded as one of the causes of the divergence of these species,
for we could not answer the same question in regard to any of the
other structural differences between two species of snail, for the
simple reason that we have far too little knowledge of the biological
value of the parts of a snail. Or could any one tell of what use it
would be to a snail-species to have the horns slightly longer, the
foot somewhat narrower, the radula beset with rather larger or more
numerous teeth? We might indeed imagine many ways in which it might
be of advantage, but we are not in a position to say definitely why,
for instance, longer horns should be better for one species than for
another, and yet we do not believe that the structure of snails is
less well adapted to the life of each species than that of any other
animals. The snail's structure is certainly built up of hundreds and
thousands of adaptations, like that of every other animal species,
but while in many others we can, at least in part, recognize the
adaptations as such, we cannot do so at all in regard to the snail.
Simroth has pointed out that the spiral asymmetrical shell bears a
relation to the one-sided opening of the genital organs, but that only
states the general reason for the coiling of the shell. In studying the
differences in the shell one is apt to think of its external appearance
alone, of the protection which it affords to the soft internal organs
of the easily wounded animal; perhaps also of the distribution of
weight, which must be different in a high tower-like structure and a
low flat spiral; possibly, too, of the varied obstacles and resistance
the snail has to encounter in creeping into clefts and holes, or among
a tangle of plants, according to the form of its shell; but is it not
also conceivable that the form of the shell has been determined by its
contents? As Rudolph Leuckart taught, the snail may be regarded as
composed of two parts, one of which is formed by the head and foot,
the other by the so-called 'visceral sac': the former may be called
the animal half, because it chiefly contains the dominant organs--the
nerve-centres, almost the whole mass of muscle, and the sense-organs;
the latter the vegetative half, since it contains the main mass of the
nutritive and reproductive systems--the stomach and intestine, the
large liver, the heart, the kidneys, the reproductive organs, and so
on. The vegetative half of the animal is always concealed within the
shell; would not therefore any great variation in the size of liver,
stomach, intestine, and so on, bring with it a variation in the size
and form of the shell, as well as in the expansion or contraction of
its coils? And might not such variations become necessary because of
some change in the food-supply? It is only a supposition, but it seems
to me very probable that becoming accustomed to a new diet, less easily
broken up and dissolved and of diminished nutritive value, would cause
modification not only of the radula and jaw-plate, but also of the
stomach and the liver, the intestine and the kidneys, whose activity
is closely associated. The stomach must become more voluminous, the
liver which yields the digestive fluid must become more massive, and
so forth. I will not follow this hypothetical example further, for I
merely wished to recall the fact that the snail shell, to the form
of which no biological significance can be commonly attributed, is
actually a sort of external cast of the visceral sac, and consequently
dependent on the variations to which that is liable in accordance
with the conditions of its life. To give precise proofs for such
processes is certainly not yet possible, for we do not even know with
certainty what the diet of the various species of snail is, much less
the difference between the modes of nutrition in two varieties, or the
nutritive value of the materials used, or the changes in secretion,
absorption, assimilation, and excretion which must be brought about by
these differences. But we can at least see that variations in nutrition
must be enough in themselves to give rise to new adaptations in the
size, constitution, and mutual adaptation of the internal vegetative
organs, and we cannot overlook the possibility that the form and
size of the vegetative half, and therefore the form and size of its
secretion, the shell, may also be caused to vary[29]. The fact that we
cannot recognize, for instance, the beehive shape of an _Obba_ as an
adaptation, is thus no proof that it is not one. But let us assume for
the moment that it is not, and that it cannot be referred to natural
selection any more than the other variations in the Celebes chain of
forms, and we may further admit that they cannot be referred to sexual
selection, still less to some 'inherent principle of perfecting,' not
only because there is no question of perfecting in the matter, but
because such a mystical principle is outside of the scope of natural
history and its principles of interpretation.
[29] That this suggestion was not unjustified is evident from a
recent contribution by Simroth ('Ueber die Raublungenschnecken,'
_Naturwissenschaftliche Wochenschrift_, December 8 and 15, 1901). In
this paper the author, who is an expert as regards the biology of
Gastropods, shows that a change of diet may evoke many kinds of changes
in the structure of the food-canal, which may indirectly compel changes
in the shell. Thus in a small indigenous snail, _Daudebardia_, the
pharynx has grown so enormously in thickness and length in adaptation
to the predatory mode of life, that the head and the anterior part of
the body can no longer be retracted within the shelter of the shell.
For this reason, and also because of the snail's habit of following
earthworms into their burrows, the shell has been shunted far back and
obliquely downwards. It has at the same time markedly changed in its
shape, as may still be verified by comparing the form of the shell in
the young stages with that of the adult.
But that transformations in a definite direction can and must arise
from fresh disturbances in the equilibrium of the determinant system,
that is from germinal selection, we have already shown.
Even if the changes of form with which we are here dealing had really
no biological importance, they might quite well have been brought about
by germinal selection, and only one thing remains obscure, that is,
why the different stages on the path of distribution of a species are
at a different level of evolution and not all at the same level. Why
have not all been transformed? Why have some colonies remained near
the ancestral form, while others have varied only a little, and others
again a great deal? This cannot be explained by any assumption of an
internal power of development, and the explanation can only be found
in germinal selection associated with isolation, since the internal
processes in the germ-plasm can quite well run a different course in
different colonies. Nevertheless I am inclined to infer from these
differences in the individual colonies of these chains of forms, that
natural selection in the accepted sense has also played a part in the
evolution of these snail varieties.
Such series of forms are especially interesting, because they show us
the process of species-formation in its different stages beside each
other in space, and thus simultaneously. They represent, so to speak,
a horizontal branch of the genealogical tree of the species, as the
Sarasins well express it, that is, a series of species arising from
each other, which do not break off, but are all capable of life at
the same time, and so exist simultaneously on different areas; they
are species adapted to different localities, not to different times.
The same is true of the snails of other isolated regions, except that
the chains of forms are usually not simple, but split up into several
chains of forms arising from one ancestral form, and under certain
circumstances each of these may break up into two or more diverging
series. The great number of related species in Madeira, or in the
Sandwich Islands, compels us to this assumption, although the branching
of the genealogical trees can no longer be demonstrated with certainty.
This splitting up of forms into several series on a varied insular
region shows us once more that it is germinal selection alone which
forms the basis of all transformations, and that there is not, as
earlier naturalists, especially the botanists Nägeli and Askenasy,
maintained, any peculiar impelling Force of Development innate in
organisms. If there were such a force, a species would be obliged to
go on continuously in the same direction, exactly like the Sarasins'
chains of forms, but no breaking of the species into one or many forms
could occur. But this breaking up into series is easy to understand
when we take germinal selection into consideration, for the germ-plasm
contains many ids and determinants, and each of these can enter upon
new variations, so that one colony can vary in this direction, another
in that, and a great diversity of forms living in isolation must, or at
least may be the result, as we see in the case of the Sandwich Islands.
Let us delay a moment over the Sarasins' case of the Celebes snails. We
are dealing here with series of forms in regard to which the ordinary
conception of species fails us, for they contain varieties whose
extremes are as far apart as distinct species usually are, which are
not, however, distinct, since they are connected with one another by
one and often by several transition forms. Thus we can only break them
up into two or more 'species' by an arbitrary division at one place or
other. The phenomenon itself is not new to us; we have seen that even
Lamarck and Treviranus made use of similar series of forms, connected
by transition stages, in their attack upon the old theory of creation,
and sought to prove by means of these that the idea of species is an
artificial one, read into nature by man, and not innate in nature, and
that the forms of life were only apparently fixed and sharply defined,
being in reality in process of slow transformation. Such beautiful
and convincing examples as we now possess were not available at that
time, but it might even then be said that it was the easier to make
a new species the fewer examples one had to deal with, and the more
difficult the more numerous these became, because with the number of
individuals, especially if they come from a wide area, the number and
diversity of the divergences increases also, so that in many cases, as
in that of the Celebes snails, it becomes impossible to draw a line
between the different species.
There are, however, many animal and plant forms which do not show such
marked divergences, but rather exhibit a great harmony of individuals
even in detail, and the conception of species is more readily applied
to these. It would certainly be foolish to give it up, since we should
then lose all possibility of arriving at any sort of orientation
among the enormous wealth of forms in nature. But at the same time we
must not forget that these 'typical' species only appear so to our
short-sighted vision--short-sighted as far as time is concerned--and
that they are connected from long-past times with 'species' which
lived at an earlier date, by just such transition stages as connect
the Celebes species of to-day, which are all living at the same
time. The world of life on the earth only presents at any given time
a 'cross-section of its genealogical tree,' and according as its
branches grow out vertically or horizontally we receive an impression
of typical, sharply defined species or of circles or chains of forms.
In the first case the evolution of new species was associated with the
dying out of the horizontal branches, and the end-twigs of the branch
stand beside each other now apparently isolated and sharply defined;
in the other case only a portion of the ancestral species has been
transmuted, and the other part continues to live alongside of the
species derived from it, and perhaps repeats the process of giving off
a varied race of descendants.
The last thirty years have yielded much palæontological evidence of
the successive stages of species-transformation. In quietly deposited
horizontal strata of the earth's crust, lying one above another, the
whole phyletic history of a group of snail-species has repeatedly
been found in historic order, the oldest in the deepest layer, the
youngest in the uppermost, and the numerous and often very divergent
'species' of a particular deposit are connected by transition forms in
the intermediate strata. From the point of view of time, therefore,
these are not 'typical' species, but circles of forms in a state of
variability.
The most beautiful of such cases are the _Planorbis_ species from the
small lacustrine deposits of Steinheim in Swabia, the _Paludina_ strata
of Slavonia, and various groups of Ammonites.
These cases have been described and discussed so often that I need only
refer to their most essential features.
The _Planorbis_ strata of Steinheim were first investigated, from
the point of view of the theory of descent, by Hilgendorf (1866).
He described nineteen different varieties, which, as they are all
connected in chronological succession with each other, he grouped
together under the name of _Planorbis multiformis_. These little
freshwater snails are found in millions in the strata of the former
lake-basin of Steinheim, and they are arranged in so orderly and
regular a manner that two observers, working independently and at
different times, succeeded in building up the genealogical tree
in almost the same way. According to Alpheus Hyatt, the later
investigator, all the forms are derived from one ancestral form,
_Planorbis lævis_, from which four different series have descended, one
of them splitting up again into three subordinate series.
All the individual members of these series are connected by
intermediate forms in such a manner that a long period of constancy of
forms seems to be succeeded by a shorter period of transformation, from
which again a relatively constant form arises.
We see, therefore, that the idea of species is fully justified in
a certain sense; we find indeed at certain times a breaking up of
the fixed specific type, the species becomes variable, but soon the
medley of forms clears up again and a new constant form arises--_a new
species_, which remains the same for a long series of generations,
until ultimately it too begins to waver, and is transformed once
more. But if we were to place side by side the cross-sections of this
genealogical tree at different levels, we should only see several
well-defined species between which no intermediate forms could be
recognized; these would only be found in the intermediate strata.
The problem we have now to discuss is, how it comes about that
relatively sharply defined species exist which are connected with
ancestral forms further back, but which form among themselves an
exclusive, more or less homogeneous, host of individuals. How does it
happen that we everywhere find a specific type, and not an endless
number of individual forms connected with one another in all directions?
This would require no further explanation if a phyletic evolutionary
force impelled the forms of life to vary in a definite manner, and thus
to become transmuted into new forms in the course of generations. In
that case the whole genealogical tree of the organisms on the earth
must have been potentially contained in the lowest moneron, so that,
given time and the most indispensable general conditions of existence,
the living world just as we know it must have resulted. Nägeli was the
first to express this view, and he followed it out consistently, not
even hesitating to deny the existence of all processes of selection,
and to represent the whole of evolution as a process conditioned by
this phyletic force, which would have given rise to the world of
organisms which has actually arisen, even if the conditions of life at
the different periods of the earth's history had been other than they
were. I have always combated this idea, without however overlooking
that it is based upon facts which--at that time at any rate--gave it a
certain justification. We cannot pass it by without giving some other
interpretation of the facts. Following Nägeli, the botanist Askenasy
championed this view of 'variation in a different direction,' which
gives rise to new forms; and in more recent times Romanes, Henslow,
and Eimer expressed similar views, and--although they did not actually
dispute the existence of processes of selection--they attributed a much
less important rôle to them, and referred the phyletic genealogical
tree of organisms in the main to other and internal causes.
Like Nägeli himself, his followers have laid stress upon the fact that
natural selection cannot be the cause of the evolution and succession
of particular species, because the differences which separate species
from species are not of an adaptive nature, and therefore cannot
depend upon selection; but if the step from one species to the next
succeeding one does not depend upon adaptation, then the greater steps
to genera, families, and orders cannot be referred to it either,
since these can only be thought of as depending upon a long-continued
splitting up of species. Genera, families, and all higher groups
must be recognized as conventional categories, not as real divisions
existing in nature itself. Even Treviranus and Lamarck maintained that
the differences between genera depended just as much upon our estimate,
our intellectual convenience, as do the differences between species.
All forms were originally connected, though they may not be so now, and
if the species are really not distinguished by adaptive characters,
then neither are any other grades of our classificatory system,
neither order nor classes, since they all depend originally on the
transmutation of species. It was therefore quite consistent of Nägeli
to seek the mainspring of organic evolution, not in adaptation, but in
an unknown evolutionary force. Thus he refused to recognize adaptation
as a consequence of selection, but regarded it, as Lamarck had done,
as the direct effect of external conditions, and as an entirely
subordinate factor in the transmutation of forms.
Nägeli and his modern successors conceive of phyletic evolution as
depending upon definitely directed variation, resulting from internal
causes and occurring at definite times, which of necessity causes the
existing form to be transformed into a new one. To them the species
appears, so to speak, as a vital crystallization, or to use Herbert
Spencer's phraseology, as an equilibrium of living matter, which
becomes displaced from time to time, and passes over into a new state
of equilibrium, being transmuted into a new species, something like the
pictures in a kaleidoscope. The species is thus something conditioned
from within, which must be as it is and could not be otherwise, just
like a crystal which crystallizes in one particular system and not
in another; it must be just thus or it could not be at all. From the
point of view of this theory it would be easy to understand that the
thousands or millions of individuals composing a species all agree in
essentials--that a specific type exists.
But this conception can hardly be entirely correct, although there is
some truth at its foundation, namely, that germinal variations which
arise independently are the basal roots of all transmutation. But the
species is not simply the result of these internal processes, it is
not even mainly so; it is not the result of an internal, definitely
directed developmental force, even if we attempt to think out such a
force in a purely scientific or mechanical, instead of a mystical,
sense. It seems clear to me that the species is not a life-crystal in
the sense that it must, like a rock-crystal, take form in a particular
way and in no other for purely internal reasons and by virtue of
its physical constitution; the species is essentially a complex of
adaptations, of modern adaptations which have been recently acquired,
and of inherited adaptations handed down from long ago--a complex which
might quite well have been other than it is, and indeed must have been
different if it had originated under the influence of other conditions
of life.
But of course species are not exclusively complicated systems of
adaptations, for they are at the same time 'variation-complexes,'
the individual components of which are not all adaptive, since they
do not all reach the limits of the useful or the injurious. All
transformations arise from a basis of spontaneous chance variations,
just as all forest plants grow from the soil of the forest, but do not
all grow into trees, the adaptive forms which determine the essential
character of the forest; for many species remain small and low, like
the mosses, grasses, and herbs; and these too have a share, though a
subordinate one, in determining the character of the forest, which
depends definitely, though only partially, on the loftier growths.
According to my view all adaptation depends on an alteration in
the equilibrium of the determinant system, such as must arise from
intra-germinal or even general fluctuations in the nutritive supply,
affecting larger or smaller groups of determinants and causing
variation in them to a greater or less degree. And these variations
may be in a quite definite direction, persisted in for internal
reasons, as we have already seen in the section dealing with germinal
selection. These variations are the building-stones out of which,
under the guidance of personal selection, a new specific type, that
is, a new complex of adaptations, can be established. In this type
many indifferent characters are involved, which are just as constant
characters of the species as the adaptations.
The opponents of the selection theory have often urged against it
this constancy of indifferent characters, but as soon as we cease to
restrict the principle of selection to 'persons,' and extend it also
to the lower categories of vital units, the occurrence of indifferent
characters is easily understood. To illustrate characters of this kind,
Henslow has recently called attention to the species of gentian, whose
flowers have a corona split into five tips in some species and into
six or seven in others, and we cannot possibly ascribe any biological
significance to these specific characters. It is quite possible that
they possess none; but did not even Darwin express his belief that many
peculiarities of form 'are to be attributed to the laws of growth, and
to the mutual influence of parts,' forces which he rightly refrained
from including under 'natural selection' in his sense of the word,
but which we now regard as an expression of intra-selection or of
histonal selection? It is this, in our opinion, which brings about the
co-adaptation of the parts to form a harmonious whole, which admits of
the primary adaptations to the conditions of life being followed or
accompanied by correlative secondary variations, and which plays an
important part in directing the course of every individual development,
and is therefore uninterruptedly active within the organism. We cannot
analyse the factors precisely enough to be able to demonstrate in an
individual case why the corona should be divided into four in one
species of gentian and into five in another, but we can understand
in principle that all adaptations of a species which are not primary
are determined by the compelling influence of intra-selection. And
we need not now rest content even with that, for we know that this
intra-selection--as we have already seen--is active within the
germ-plasm, and it is only a logical consequence of the principle of
germinal selection to suppose that variations of definite determinants
due to personal selection may in the germ-plasm itself give rise
to correlative variations in determinants next to them or related
to them in any way, and that these may possess the same stability
as the primary variation. This seems to me a sufficient reason why
biologically unimportant characters may become constant characters of
the species. Correlation is not effected only in the perfect organism;
it exists at every period of its life, from the germ till death, and
what it brings about is quite as inevitable as what is evoked through
adaptation by means of personal selection.
We can thus also understand that indifferent characters may be
contained not only in individual ids of the germ-plasm, but also
coincidentally in a great majority of them, as soon as we think of
them as dependent upon the characters established through personal
selection, for these must be contained in a majority of the ids.
But there is still another reason why indifferent characters should
become stable, and that is the effect of general variational influences
on all the individuals of a species, as, for instance, in many climatic
varieties, and probably also in many cultivated varieties.
But even when we have fully recognized that, from the arcana of the
germ-plasm, new minimal variations are continually cropping up, which
are biologically indifferent, and nevertheless become variational
tendencies, and may increase even to the extent of causing _visible_
differences, and that therefore varieties of snails or of butterflies,
or of any animal or plant whatever, may originate through germinal
selection alone, it cannot for a moment be supposed that the
transmutation of species depends upon this process exclusively or even
preponderantly. This was Nägeli's mistake, and that of his followers as
well, that he ascribed to his 'principle of perfecting' the essential
rôle in directing the whole movement of evolution, while the general
structure of all species shows us that they are, so to speak, built
up of adaptations. But adaptations could not be--or could only be
fortuitously and exceptionally--the _direct_ result of an internal
power of development, since the very essence of adaptational changes is
that they are variations which bring the organism into harmony with the
conditions of its life. We are therefore forced either to underestimate
greatly the part played by adaptation in every organism--and that
is what Nägeli did--or to leave the standpoint of natural science
altogether, and assume a transcendental force which varied and
adapted the species of organisms _pari passu_ with the changes in the
conditions of life during the geological evolution of our earth. This
would be a sort of pre-established harmony, through which the two
clocks of evolution--that of the earth and that of organisms--kept
exact time, although they had quite different and independent works!
But that the determining significance of adaptations in organic forms
is underestimated even now is evidenced by the continually repeated
statement that species differ, not in their adaptive characters, but in
purely morphological characters, whereas it is obvious that we are far
from being able to estimate the functions of a part with sufficient
precision to be able to say definitely whether the differences between
two nearly allied species are or are not adaptations to different
conditions. The same is true with regard to the other side of the
problem--the conditions of life. These are often to all appearances
identical in two allied species, but even where they are visibly
different it is often difficult to assert that the differences between
the two species can be interpreted with certainty as adaptations to
the specific conditions of life. At an earlier stage we discussed
the protective coloration of butterflies, and we saw that the forest
butterflies of the Tropics frequently mimicked a dry leaf on their
under surfaces. In the various regions of the extensive forest
districts of the Orinoco and the Amazon in South America there are
fifty species of the genus _Anæa_ alone, and in the resting pose all
these bear a most deceptive resemblance to a leaf, yet each of them
differs from the rest in the mingling of its colours, its brilliance,
and usually in markings when these are present. If we wished to be able
to decide whether these specific differences were of an adaptive nature
or not, we should first of all require to know in what kind of forest
two neighbouring species lived, and in what places, among what sort
of leaves, they were in the habit of settling. Even then we should at
best only know whether the species _A_ was better protected, as far as
our own eyes were concerned, among the leaves of the forest _A´_ than
the species _B_, and conversely; but we could not tell whether they
_required_ this protection, or whether the species _A_, if transferred
to the forest _B´_, would be more frequently discovered and destroyed
by its enemies than in its own forest-home, and that alone could
prove the difference to be biologically important, that is, to have
selection-value. The difficulty, indeed the impossibility, of arriving
at such decisions can perhaps be better illustrated by an example from
our indigenous fauna. No one doubts that the upper surface of the
anterior wing in the so-called banner-moth (_Catocala_) possesses a
very effective protective colouring; by day the moths rest with wings
spread out flat upon tree trunks, wooden fences, walls, &c., and they
are so excellently suited to their environment that they are usually
overlooked both by man and animals. But each of the twelve German
species of _Catocala_ has a special protective colouring; in _Catocala
fraxini_ it is a light grey, in _Catocala nupta_, a dark ash-grey, in
_Catocala elocata_ rather a yellowish-brown grey, in _Catocala sponsa_
an olive brown, in _Catocala promissa_ a mingling of whitish-grey
and olive brown, and so on. All these colourings are protective; but
could any even of our most experienced and sharp-sighted entomologists
prove that each of these different shades of colour depends upon
adaptation to the usual resting-place of the particular species to
which it belongs? And yet it is on _a priori_ grounds highly probable
that this is the case. But even this would by no means dispose of the
whole problem, for each of these protective colour schemes is composed
of several, often many, tints; it must be so if they are to fulfil
their end at all, for a uniformly coloured wing would contrast with
the bark of every tree and with every wooden fence. The wing-surface
must therefore bear on a lighter background a number of lines and
streaks varying from brown to black, and usually running zigzag across
the wing; beside these are spots of lighter colour, which complete
the deceptive picture. This 'marking' of the wing is similar in all
twelve species, and yet in each it is different in detail. It is
constant in each, and thus is a specific character. But who would
venture to undertake the task of proving that each of these streaks,
spots, zigzag lines, &c., is or is not adaptive--that the details are
necessary adaptations to the resting-place which had become habitual
to the species, or, on the other hand, simply expressions of the
variational tendencies of the elements of marking, depending upon
germinal selection? This would be an impossible task, and yet we are
here dealing with a character which, _as a whole_, is undoubtedly
adaptive; in many of the differences between other species even that is
not certain.
It seems to me, therefore, hardly reasonable to talk of the
'insufficiency of natural selection' because we are not able to
demonstrate that the minutiæ of specific characters are adaptational
results. Personal selection intervenes whenever the variations produced
by germinal selection attain to selection-value; and whether we can
determine the exact point at which this takes place in individual cases
is, as I have said before, theoretically quite indifferent.
Moreover, there are cases in which we _can_ prove that specific
differences are of an adaptive nature. When, of two nearly related
species of frog, the spermatozoon of one possesses a thick head and
that of the other a thin head, and when at the same time the micropyle
through which alone the spermatozoon can make its way into the ovum is
wide in the first species and narrow in the second, we have before us a
specific character which is obviously adaptive.
In order to gain clearness as to the significance of natural selection
in the restricted sense, that is of personal selection, it seems to
me much more important to study the different groups of animals and
plants with special reference to what they undoubtedly exhibit in the
way of adaptation. For that reason I discussed different groups of
adaptations in detail in some of the preceding lectures, although, or
rather because, they all teach us that every part of every species,
whether animal or plant, even every secretion, and indeed every habit,
every inherited instinct, is subject to adaptation to the conditions of
life. It seems difficult to refuse to admit that this is the natural
impression which this study conveys, and it is strengthened as our
knowledge increases; _that every essential part of a species is not
merely regulated by natural selection, but is originally produced
by it_, if not in the species under consideration at the time, then
in some ancestral species; and, further, that every part can adjust
itself in a high degree to the need for adaptation. It was not without
a purpose that I discussed the phenomenon of mimicry so fully, for
it, above all others, teaches us how great a power of adaptation the
organism possesses, and what insignificant and small parts may be
transformed, in a remarkable degree, in accordance with some actual
need. We saw that a butterfly might assume a colouring which diverged
entirely from that of its nearest relatives, but which caused it to
resemble an immune species of a different family, and thereby protected
it more effectively from persecution. Such a case can no more be due to
a dominating phyletic force than to a chance and sudden displacement
of the state of equilibrium of the determinant system; it can depend
only on natural selection, that is, on a sifting out of the diverse
variations offered by germinal selection, and the unhampered expression
and augmentation of those favoured.
But it is not only these minute variations, insignificant in relation
to the whole structure of the animal, which can be determined by
natural selection. The same applies to the phyletic evolution as a
whole; even that is not directed by the assumed internal principle of
development.
Adaptations, from their very nature, can only depend upon selection,
and not upon an internal principle of evolution, since that could
take no account whatever of external circumstances, but would cause
variations in the organism altogether independently of these. Thus, in
considering the origin of any of the larger groups of animals, we may
exclude a phyletic power as the guide of its evolution as soon as we
can prove that all its essential structural relations, as far as they
diverge from those of nearly related groups, are adaptations. We may
not be able to do this for nearly all of the animal groups, and it will
hardly be possible in regard to a single group of plants, because our
insight into the _biological_ significance of characters, which means
more than the functional significance of the individual parts, and
their correlation as parts of a whole, is seldom sufficiently intimate
or thorough. But among animals we can do this in regard to some groups;
one of these is the order of whales or Cetaceans.
[Illustration: FIG. 130. Skeleton of a Greenland Whale with the contour
of the body. _Ok_, upper jaw. _Uk_, lower jaw. _Sch_, shoulder-blade.
_OA_, upper arm. _UA_, bones of fore-arm. _H_, hand. _Br_, vestige of
the pelvis. _Fr_, vestige of the femur. _Tr_, vestige of the lower part
of the leg. After Claus.]
Cetaceans, as is well known, belong to the Mammalia, that is to
say, to a class whose structure was built for life on the land. The
ancestors of Cetaceans were similar to the other mammals, and possessed
a coat of hair and four legs, and a body the mass of which was so
distributed that it could be borne by those four legs. But all the
modern Cetaceans live in the sea, and they have therefore entirely
changed their bodily form; they have become spindle-shaped like fishes,
well adapted for cleaving the water, but incapable of moving upon land.
At the same time, their hind-legs have completely disappeared, and can
now be demonstrated only as rudiments within the mass of muscle (Fig.
130, _Br_, _Tr_, _Fr_), while the fore-legs have been transformed
into flippers, in which, however, the whole inherited, but greatly
shortened, skeleton of the mammalian arm is concealed (_OA_, _UA_,
_H_). The skin has lost its covering of hair so completely that in
some cases no traces of it are demonstrable except in the embryo. All
these changes are adaptations to an aquatic life, and could not have
been produced independently of the influence of external conditions.
But there is much more than this. A thick layer of blubber under the
skin gives this warm-blooded animal an effective protection against
being cooled down by the surrounding water, and at the same time gives
it the appropriate specific gravity for life in the sea; an enormous
tail-fin similar to that of fishes, but placed horizontally, forms the
chief organ of locomotion, and for this reason the hind-legs became
superfluous and degenerated. Similarly, the muscles of the ear have
also disappeared, for the hearing organ of this aquatic type is no
longer suited for receiving the sound-waves through an air-containing
trumpet, but receives them by a shorter route from the surrounding
water, directly through the bones of the skull. Remarkable changes
in the respiratory and circulatory organs make prolonged submersion
possible, and the displacement of the external nares from the snout to
the forehead enables the animal to draw breath when it comes up from
the depths to a frequently stormy surface. It would take a long time to
enumerate all that can be recognized as adaptive in these remarkable
aquatic mammals to a life in what to their ancestors would have been a
strange and hostile element. Let us study particularly the case of the
whalebone whales, for instance the Greenland whale, and we are at once
struck by the enormous size of the head, which makes up about a third
of the whole body (Fig. 130). Can this, which has such an important
effect in determining the whole type of animal, be an outcome of some
internal power of development? By no means! It is rather an adaptation
to the mode of nutrition peculiar to this swimming mammal, for it
does not, like dolphins and toothed whales, feed on large fishes and
Cephalopods, but on minute delicate molluscs--Pteropods and pelagic
Gastropods, on Salpæ, and the like, which often cover the surface
of the Arctic Ocean in endless shoals, sometimes extending for many
miles. To enable the whale to sustain life on such minute morsels it
was necessary that it should be able to swallow enormous quantities;
teeth were therefore useless, and they have become rudimentary, and can
only be demonstrated in the embryo as rudiments (dental germs) in the
jaw; but in place of these there hang from the roof of the mouth-cavity
great plates of 'whalebone,' a quite peculiar product of the mucous
membrane of the mouth, the ends of which are frayed into fibres, and
form a sieve-net for catching the little animals which are engulfed
with the sea-water. The mouth-cavity itself has become enormous, so
that great quantities of water at a time can be strained through the
net of whalebone-plates.
When I mention that peculiar changes have occurred also in the internal
organs, that the lungs have elongated longitudinally and thus enable
the animal more readily to lie in the water in a horizontal position,
that peculiar arrangements exist within the nostrils and the larynx
which enable the animal to breathe and swallow simultaneously, and that
the diaphragm lies almost horizontally because of the length of the
lung, I think I have said enough to indicate that not only does almost
everything about the whale diverge from the usual mammalian type, but
that all these deviations are adaptations to an aquatic life. If
everything that is characteristic, that is, typical either of the order
or of the family to which animals belong, depends upon adaptation,
what room is left for the activity of an internal power of evolution?
How much is left of the whale when the adaptations are subtracted?
Nothing more than the general scheme of a mammal; but this was implicit
in their ancestors before the whales originated at all. But if what
makes whales what they are, that is, the whole 'scheme' of a whale,
has originated through adaptation, then the hypothetical evolutionary
power--wherever its seat may be--has had no share in the origin of this
group of animals.
I said all this more than ten years ago, but the idea of an internal
directive evolutionary force is firmly rooted in many minds, and
new modifications of the idea are always cropping up, and of these
the most dangerous seem to me to be those which are not clear in
themselves, but suppose that the use of a shibboleth like 'organic
growth' means anything. That organic growth is at the base of the
phyletic evolution of organisms may be maintained from any scientific
standpoint whatsoever, from ours as well as from Nägeli's, for no one
is so extreme and one-sided as to regard the process of evolution as
due _solely_ to internal or _solely_ to external factors. The process
may thus always be compared to the growth of a plant, which likewise
depends on both internal and external influences. But that is saying
very little; we have still to show how much and how little is effected
by these internal and external factors, what their nature precisely
is, and what relation they bear to one another. There is thus a great
difference between believing, with Nägeli, that 'the animal and plant
kingdoms must have become very much what they actually are, even
had there been on the earth no adaptation to new conditions and no
competition in the struggle for existence,' and sharply emphasizing, in
accordance with the facts just discussed, that, in any case, a whole
order of mammals--the Cetaceans--could never have arisen at all if
there had been no adaptation.
The same thing could be proved in regard to the class of Birds, for
in them too we are able to recognize so many adaptive features, that
we may say everything about them that makes them birds depends upon
adaptation to aerial life, from the articulations of the backbone
to the structure of the skull and the existence of a bill; from the
transformation of the fore-limbs into wings, and of the hind-limbs
to very original organs of locomotion on land or swimming organs in
water, to the structure of the bones, the position, size, and number
of the internal organs, down even to the microscopic structure of
numerous tissues and parts. What could be more characteristic of a
class of animals than feathers are of birds? They alone are enough
to distinguish the class from all other living classes; an animal
with feathers can now be nothing but a bird, and yet the feather is
a skin-structure which has arisen through adaptation, a reptilian
scale which has been so transformed that an organ of flight could
develop from its anterior extremity. We find it thus even in the two
impressions of the primitive bird _Archcæopteryx_, which have been
preserved for us in the Solenhofen slate since the Jurassic period in
the history of the earth. And into what detail does adaptation go in
the case of the feathers! Is not the whole structure, with its quill,
shaft, and vane, precisely adapted to its function, although that is
purely passive? What I have just said of the whole class of Birds holds
true for this individual structure, the feather; everything about it
is adaptation, and indeed illustrates adaptation in two directions,
for in the first place the feathers, by spreading a broad, light,
and yet resistant surface with which to beat the air, act as organs
of flight, while they are also the most effective warmth-retaining
covering conceivable. In both these directions their achievements
border on the marvellous. I need only recall the most recent discovery
in this domain, the proof recently given by the Viennese physiologist,
Sigmund Exner, that the feathers become positively electric in their
superficial layer, and negatively electric in their deeper layer,
whenever they rub against one another and strike the air. But they
are rubbed whenever the bird flies or moves, and the consequence
of the contrast in the electric charging of the two layers is that
the covering feathers are closely apposed over the down-feathers,
while, on the other hand, the similar charging of the down-feathers
makes them mutually repel each other, with the result that a layer
of air is retained between them, and thus there is between the skin
and the covering feathers a loose thicket of feathers uniformly
penetrated by air--the most effective warmth-preserver imaginable.
The electric characters of the feathers--and the same is true of the
hairs of animals--are thus not indifferent characters, but with an
appreciable biological importance, and the same is true of the almost
microscopical series of little hooklets which attach the barbs of the
covering feathers to one another, and thus form a relatively firm but
exceedingly light wing-surface which offers a strong resistance to
the air. But as we must regard these hooklets as adaptations, so must
we also regard the electrical characters of the feathers, and we must
think of them as having arisen through natural selection, as Exner
himself has insisted.
If we are able to recognize all the more prominent features of
the organization in Cetaceans and Birds as due to adaptation, we
must conclude that, in the rest of the great groups of the animal
kingdom, the main and essential parts of the structure are adaptations
to the conditions of life, even although the relations between
external circumstances and internal organization are not so readily
recognizable. For if there were an internal evolutionary force at all,
we should be able to recognize its operation in the origin of the races
of Cetaceans and Birds; but if there be no such power, then even in
cases where the conditions of life are not so conspicuously divergent
as in Cetaceans and Birds, we must refer the typical structure of the
group to adaptation. Thus everything about organisms depends upon
adaptation, not only the main features of the organization, but the
little details in as far as they possess selection-value; it is only
what lies below this level that is determined by internal factors
alone, by germinal selection; but this is not an imperative force in
the sense in which the term is used by Nägeli and his successors, for
it is capable of being guided; it does not necessarily lead to an
invariable and predetermined goal, but it can be directed according to
circumstances into many different paths. But it is precisely this that
constitutes the main problem of the evolution theory--how development
due to internal causes can, at the same time, bring about adaptation to
external circumstances.
This lecture had been transcribed so far, and was ready for the
press, when I received the first volume of a new work by De Vries, in
which that distinguished botanist develops new views in regard to the
transformation of species, based upon numerous experiments, carried on
for many years on the variation of plants. As not only his views, but
the interesting facts he sets forth, seem to contradict the conclusions
as to the transmutation of organisms which I have been endeavouring to
establish, I cannot refrain from saying something on the subject.
De Vries does not believe that the transformation of species can
depend on the cumulative summation of minute 'individual' variations;
he distinguishes between 'variations' and 'mutations,' and attributes
only to the latter the power of changing the character of a species.
He regards the former as mere fluctuating deviations which may be
increased by artificial selection, and may even, with difficulty,
if carefully and purely bred for a long period, be made use of to
give character to a new breed, but which play no part at all in the
natural course of phylogeny. As regards phylogeny, he maintains that
only 'mutations' have any influence, that is, the larger or smaller
saltatory variations which crop up suddenly and which have from the
very first a tendency to be purely transmitted, that is, to breed true.
The facts upon which these views are mainly based are observations
on and breeding experiments with a species of evening primrose
(_Œnothera_) which was found in quantities on a fallow potato-field
at Hilversum in Holland. It had been cultivated previously in a
neighbouring garden, and had sown itself thence in the field. The
numerous specimens of this _Œnothera lamarckiana_ growing there were
in a state of marked 'fluctuating' variability, but in addition there
grew among them two strongly divergent forms which must have arisen
from the others, and which led De Vries to bring the parent stock
under cultivation, in the hope that it would yield new forms, in the
Botanical Gardens at Amsterdam. This hope was fulfilled; in the second
cultivated generation there were, among the 15,000 plants, ten which
represented two divergent forms, and in the succeeding generations
these forms were repeated several times and in many cases, and five
other new forms cropped up, most of them in several specimens and in
different generations of the original stock. All these new forms, which
De Vries calls 'elementary species,' breed true, that is to say, when
they are fertilized with their own pollen they yield seed which gives
rise to the same 'elementary species.' The differences between the
new forms are usually manifold, and of the same kind as those between
the 'elementary' species of the wild Linnæan species. But, according
to De Vries, what we have been accustomed since the time of Linné to
call a 'species' is a collective category, whose components are these
'elementary' species which De Vries has observed in his experiments
with _Œnothera_. In other species, such as _Viola tricolor_ and _Draba
verna_, true-breeding varieties have long been known to botanists, and
these have been studied carefully and tested experimentally, especially
by A. Jordan, and more recently by De Bary.
All 'species,' according to the Linnæan conception, consist, De Vries
maintains, of a larger or smaller number (in _Draba_ there are two
hundred) of these 'elementary' species, and these arise, as is proved
by the case of _Œnothera_, by saltatory or discontinuous 'variations'
which occur periodically and suddenly break up a species into many
new species, because the variations of the germ-plasm, which are for
a time merely latent, suddenly find expression in the descendants of
one individual or another. According to this view, species must be
the outcome of purely internal causes of development, which reveal
themselves as 'mutations,' that is as saltatory variations, which are
stable and transmissible from the very first, and among which the
struggle for existence decides which shall survive and which shall
be eliminated. For the mutations themselves occur in no particular
direction; they are sometimes advantageous, sometimes indifferent,
sometimes even injurious (for instance, when one sex is left out), and
so it is always only a fraction of the mutations, often only a few,
which prove themselves capable of permanent existence. Thus 'species do
not arise through the struggle for existence, but they are eliminated
by it' (p. 150); natural selection does nothing more than weed out
what is unfit for existence, it does not exercise any selective, in
the sense of directive, influence on the survivors. A difference in
the nature of variations was previously maintained by the American
palæontologist Scott, though for different reasons and also with a
different meaning. He believed that variations in a definite direction
were necessary to explain the direct course of development which many
animal groups, such as the horses and the ruminants, have actually
followed, and which he thought could not be ascribed to cumulative
adaptation to the conditions of life. The 'mutations' of De Vries are
not distinguished from the 'fluctuating' variations by following a
definite direction, but in that they are strictly heritable, that they
'breed true.' It is true that 'fluctuating' individual differences are
also transmissible, and can be increased by artificial selection, but
they lack one thing that would make them component parts of a natural
species, namely, constancy; they do not breed true, and are therefore
never independent of selection, but require to be continually selected
out afresh in order that they may be kept pure. They form 'breeds,' not
species, and if left to themselves they soon revert to the characters
of the parent species, as is well known of the numerous 'ennobled
races' among our cereals. De Vries therefore denies absolutely that a
new species could be developed by natural selection from 'fluctuating'
variations, and not alone because there is no constancy of character,
but also because the capacity of the character for being increased is
very limited. Usually nothing more can be achieved than doubling of
the original character, and then progress becomes more difficult and
finally ceases altogether.
These are incisive conclusions, based upon an imposing array of weighty
facts. I readily admit that I have rarely read a scientific book
with as much interest as De Vries's _Mutationstheorie_. Nevertheless
I believe that one might be carried away too far by De Vries, for
he obviously overestimates the value of his facts, interesting and
important as these undoubtedly are, and under the influence of what
is new he overlooks what lies before him--the other aspect of the
transmutation of species, to which the attention of most observers
since Darwin and Wallace has been almost exclusively devoted--I mean
the origin of adaptations. Not that he does not mention these, he
assumes in regard to his mutations 'a selection working in a constant
direction,' and seeks to interpret them in terms of it, but as the
mutations occur from purely internal reasons--I mean without any
connexion with the necessity for a new adaptation--and occur only
in a small percentage of individuals, and in no definite direction,
they cannot possibly suffice to explain _adaptation_, which seems to
dominate the whole organic world. But this is precisely the point at
which many botanists cease to understand the zoologists, because among
plants there are fewer adaptations than among animals; or, in any
case, adaptations in plants are not so readily demonstrated as among
animals, which not infrequently seem to us to be entirely built up of
adaptations.
In this book, and in this chapter itself, I have discussed adaptations
and their origin so much already that I need only refer to these pages
for convincing evidence that we cannot think of them as being brought
about by the accumulation and augmentation of individually occurring
saltatory 'mutations.' Not even if we assume that the leaps of mutation
can be increased in the course of generations; in short, even if we
say that mutations are all those variations which breed true and lead
to the development of species, while variations are those which do
not. This would only be playing with words, so let us say that the
fluctuating variations are really different in their nature, that is,
in their causes, from mutations. De Vries lays great stress on the
fact that these two kinds of variations must be sharply distinguished
from one another, and this may have been useful or necessary for the
first investigation of the facts before him, for we must first analyse
and then recombine, but that variations and mutations are in reality
different in nature can assuredly not be assumed, since innumerable
adaptations can only have arisen through the augmentation of individual
variations. These must therefore be able to become 'pure breeding,'
even although they may not have done so in the cases of artificial
selection which have hitherto been observed. How is it possible that
chance mutations, in no particular direction, occurring only rarely and
in a small percentage of individuals, can explain the origin of the
leaf-marking of a _Kallima_ or an _Anæa_--the shifting of the original
wing-nervures to form leaf-veins, and the exact correlation of these
veins across the surfaces of both pairs of wings? And even if we were
to admit that a mutation might have occurred which caused the veins of
the anterior and posterior wings to meet exactly by chance, that would
still not be a leaf-adaptation, for there would still be wanting the
instinct which compels the butterfly, when it settles down, to hold
the wings in such a position that the two pictures on the anterior
and posterior wings fit into each other. Correlated mutations of the
nervous system suited to this end are required, but that is too much to
attribute to happy chance! The same holds true in regard to the whole
leaf-picture on the two wings, for it could not possibly have arisen as
a whole by a sudden mutation. The whole litany of objections which have
been urged throughout several decades against the Darwin-Wallace theory
of natural selection, which were based on the improbability that chance
variations not in a definite direction should yield suitable material
for the necessary adaptations, may be urged much more strongly against
mutations, which make their appearance in much smaller numbers and with
less diversity.
But it is--as we have already seen--in regard to the necessity which
exists almost everywhere for the co-adaptation of numerous variations
of the most different parts, that the 'mutation theory' breaks down
utterly. The kaleidoscopic picture, the mutation, is implicit from
the first, and must be accepted or rejected just as it is in the
struggle for existence; but harmonious adaptation requires a gradual,
simultaneous, or successive purposive variation of all the parts
concerned, and this can be secured only through the fluctuating
variations which are always occurring, and are increased by germinal
selection and guided by personal selection.
Many naturalists, and especially many botanists, regard adaptation
as something secondary, something given to species by the way, to
improve the conditions of their existence, but not affecting their
nature--comparable perhaps to the clothing worn by man to protect
himself from cold; but that is hardly the real state of the matter.
The deep-sea expedition conducted by Chun in 1898 and 1899 made many
interesting discoveries in regard to animals living in the depths of
the ocean, all of which exhibit peculiar adaptations to the special
conditions of their life, and especially to the darkness of the great
depths. One of the most striking of these discoveries was that of
the luminous organs which are found not in all but in a great many
animals living on the bottom of the abyssal area, and also among the
animals occurring at various levels above the floor of the abyss.
These are sometimes glands which secrete a luminous substance, but
sometimes complex organs, 'lanterns' which are controlled by the will
of the animal, and suddenly evolve a beam of light and project it in
a particular direction, like an electric searchlight. These organs
have a most complex structure, composed of nerves and lenses, which
focus the light, and on the whole are not unlike eyes. That this sort
of structure should have arisen all at once through a 'mutation' is
inconceivable; it can have originated only from simple beginnings by a
gradual increase of its structure along with continual strict selection
among the variations which cropped up. They all depend upon complicated
'harmonious' adaptation, and cannot possibly have been derived from
mutations, that is, from ready-made structural 'constellations,' unless
we are to call in the aid of the miraculous. But lanterns of this kind
are found in many different kinds of animals--in Schizopod Crustaceans,
in shrimps, in fishes of different genera and families. Many fishes
have long rows of luminous organs on the sides and on the belly, and
these probably serve to light up the sea-floor and facilitate the
finding of food; in others the luminous organs are placed upon the
snout just above the wide voracious mouth, and in that position they
have undoubtedly the significance attributed to them by Chun, namely,
that they attract small animals, just as the electric lamps allure all
sorts of nocturnal animals, and especially insects, in large numbers
to their destruction. But not fishes only, but molluscs, e.g. the
Cephalopods of the great depths, have developed luminous organs, and
one species of Cephalopod has about twenty large luminous organs, like
gleaming jewels, ultramarine, ruby-red, sky-blue and silvery, while
in another the whole surface of the belly is dotted over with little
pearl-like luminous organs. Even if we cannot be quite clear as to the
special use of these lanterns of deep-sea animals, there can be no
doubt that they are adaptations to the darkness of the great depths,
and when we find the _same_ adaptations (in a physiological sense)
in many animals belonging to the most diverse groups, there is no
possibility of referring them to sudden mutations which have arisen
all at once in these groups with no relation to utility, and yet have
not occurred in any animals living in the light. Only 'variations'
progressing and combining in the direction of utility can give us the
key to an explanation of the origin of such structures.
The same is true of the eyes of deep-sea animals. It was believed at
one time that all the inhabitants of dark regions had lost their eyes.
This is the case with many cave animals and the inhabitants of the
lightless depths of our lakes, but in the abyssal zone of the sea it
is only some fishes and Crustaceans whose eyes have degenerated to
the vanishing point. Moreover, the disappearance apparently occurs in
species which are restricted to the ocean-floor in their search for
food, which therefore can make more use of their tactile organs than of
their eyes, for while the ocean-floor undoubtedly contains over wide
areas an abundance of food for these mud-eaters, it is only partly
illuminated, that is, only in places where there are luminous animals
such as polyp-colonies, &c. The fact that so many of the animals of the
great depths are luminous obviously conditions, not that most of the
immigrants into the abyssal zone should lose their eyes as useless,
but that they should adapt them to the light which is very weak in
comparison with that of the superficial layers. The eyes of deep-sea
fishes, for instance, are either enormously large, and therefore suited
for perceiving the faint light of the depths, or they have varied in
another and very characteristic manner: they have become elongated
into a cylinder, which projects far beyond the level of the head. It
looks almost as if the animals were looking through an opera-glass, and
Chun has called these eyes 'telescope-eyes.' A. Brauer has recently
shown what far-reaching variations of the original eye of fishes were
necessary in order to transform it into an organ for seeing in the
dark. These variations, however, have occurred in the eyes of the most
diverse animals in the deep sea, and not only do different families
of deep-sea fishes possess 'telescope-eyes,' but Crustaceans and
Cephalopods as well. Even our owls possess quite a similar structure,
although it does not project beyond the head in the same way. Here
again we have to deal with the phenomenon which Oscar Schmidt in
his time called _convergence_, that is, corresponding adaptations
to similar conditions in animal forms not genealogically connected
with one another. These telescope-eyes are not all descended from
one species which chanced in one of the 'mutation-periods' suddenly
to produce this combination of harmonious adaptations, but they have
risen independently through variation progressing step by step in
the direction of the required end, that is to say, through natural
selection based upon germinal selection. Only thus can their origin be
understood.
But what is time of eyes adapted to darkness is true in some measure
of all eyes, for the eyes of animals are not mere decorative points
which might be present or absent; they cannot have arisen in any
animal whatever through sudden mutation--they have been laboriously
acquired with difficulty, by the slow increase of gradually perfecting
adaptations; they are parts which bear the most precise internal
correlation with the whole organization of the animal, and which can
only cease to exist when they become superfluous. Thus the origin of
eyes seems to me only conceivable on the basis of germinal selection
controlled towards what is purposeful by natural selection, that is to
say, on a basis of fluctuating variation, and not through chance.
This is the case with all adaptations. Just as the eyes of animals
are adaptations which utilize the light-waves in the interest of the
organism and its survival, the same is true of all the sense-organs,
tactile organs, smelling and tracking organs, organs of hearing, and so
on. The animal cannot do without these; first the lower sense-organs
arose and then the higher; the increasingly high organization of the
animal conditioned this, and a multicellular animal without sensory
structures is inconceivable. The same may be said of the nervous system
as a whole, whose function it is to translate into action the stimuli
received through the sense-organs, whether directly or by means of
intervening nerve-cells, which form central organs of ever-increasing
complexity of composition. As telescope-eyes have evolved in some
groups of deep-sea animals, independently of one another, and certainly
not through the fortuitous occurrence of a mutation, but under the
compulsion of necessity in competition, so all the organs we have just
named, the whole nervous system with all its sense-organs, must have
arisen through the same factors of evolution in numerous independent
genealogical lines. And it must not be supposed that this is all; what
is true of the sense-organs--that they are necessities--is undoubtedly
true also of all parts and organs of the animal body, both as a whole
and in every detail. It cannot be demonstrated in all cases, but it
is nevertheless certain that this applies also to all the organs of
movement, digestion, and reproduction, to all animal groups and also
to the differences between them, even although these may not always be
obvious adaptations to the conditions of life. What part is left for
mutation to play if almost everything is an adaptation? Possibly the
specific differences; and these in point of fact cannot in many cases
be interpreted with certainty as adaptations, though this can hardly
be taken as a proof that they are not. Possibly also the geometrical
skeletons of many unicellulars, in which again we cannot recognize any
definite relation to the mode of life. It is easy enough to conceive
of the wondrously regular and often very complex siliceous skeleton
of the Radiolarians or Diatoms as due to saltatory mutations, and
'leaps' of considerable magnitude must certainly have been necessary
to produce some of the manifold transformations here as everywhere
else. But whether these are or are not without importance for the
life of the organisms, we are in the meantime quite unable to decide.
Here too it is well to be cautious in concluding that these organic
'crystallizations' are without importance, and therefore to infer that
they have arisen suddenly from purely internal causes. One of the
experts on Diatoms, F. Schütt, has shown us that differences in length
in the skeletal process of the Peridineæ have a definite relation to
their power of floating in the sea-water, that the long skeletal arms
or horns which these microscopic vegetable organisms extend into the
surrounding water form a float-apparatus, for their friction against
the particles of the water prevents sinking and enables them to
float for a considerable time at approximately the same level. These
skeletal forms are thus adaptations, and Chun has recently been able to
corroborate the conclusion that this adaptation is exactly regulated,
for the length of these horns varies with the specific gravity of
the different ocean-currents, species with 'monstrously long' horns
occurring, for instance, in the Gulf of Guinea, which is distinguished
by its low salinity and high temperature (Fig. 131, _A_), while in
the equatorial currents with higher salinity and cooler water, and
thus a higher specific gravity, there is a predominance of species
of Peridineæ with 'very short' processes and relatively undeveloped
float-apparatus (Fig. 131, _B_). It could be seen clearly in the course
of the voyage that the long-armed Peridineæ became more abundant as the
ship passed from the North Equatorial current into the Gulf of Guinea,
and that by and by they held the field altogether, but later, when the
'Valdivia' entered into the South Equatorial current, they disappeared
'all at once.' Thus in this case, in which the veil over the relations
between form and function in unicellular organisms has been lifted a
little, we recognize that the smallest parts of the cell-body obey
the laws of adaptation, and consistent thinking must lead us to the
conviction that even in the most lowly organisms the whole structure in
all its essential features depends upon adaptation.
[Illustration: FIG. 131. Peridineæ: species of _Ceratium_. _A_, from
the Gulf of Guinea. _B_, from the South Equatorial currents. After
Chun.]
If the horns of the Peridineæ grow to twelve times the usual length
in adaptation to life in sea-water with a salinity increased to the
extent of .002 per cent., then undoubtedly not only the protoplasmic
particles of the body which form the horns, but all the rest as well,
may be capable of adaptation; and if the _Peridinium_ protoplasm has
this power of adapting itself to the external conditions, then the
capacity for adaptation must be a general character of all unicellular
organisms, or rather of all living substance. As will be seen later
on, we shall be brought to the same conclusion by different lines of
evidence. But a recognition of this must greatly restrict the sphere of
operation which we can attribute to saltatory mutations in the sense
in which the term is used by De Vries, for adaptations from their very
nature cannot arise suddenly, but must originate gradually and step by
step, from 'variations' which combine with one another in a definite
direction under the influence of the indirect, that is, selective
influence of the conditions.
According to the theory of De Vries it seems as if 'variations,'
augmented by selection, could never become constant, and that even the
degree to which they can be augmented is very limited. As far as this
last point is concerned, De Vries seems to me to overlook the fact
that every increase in a character must have limits set by the harmony
of the parts, which cannot be exceeded unless other parts are being
varied at the same time. Artificial selection, in fact, in many cases
reaches a limit which it cannot pass, because it has no control over
the unknown other parts which ought to be varied, in order that the
character desired may be increased still further. Natural selection
would in many cases be able to accomplish this, provided that the
variation is useful. But of what use is it to the beetroot when its
sugar-content is doubled, or to the Anderbeck oats to be highly prized
by man? And yet many individual characters have been very considerably
increased in domesticated animals by selection: of these we need only
call to mind the Japanese cock with tail-feathers twelve feet long.
But undoubtedly these artificial variations do not usually 'breed true'
in the sense that De Vries's mutations of _Œnothera lamarckiana_ did,
that is to say, they only transmit their characters in purity with the
continual co-operation of artificial selection. This at least appears
to be the case, according to De Vries, in the ennobled cereal races,
which, if cultivated in quantities, rapidly degenerate. In many animal
breeds, however, this is not the case to the same degree; many, indeed
the majority, of the most distinct races of pigeon breed true, and only
degenerate when they are crossed with others.
De Vries regards it as a mistake to believe that artificial selection,
persevered in for a long time, will succeed in producing a breed
which will--as he expresses it--be independent of further selection
and will maintain itself in purity. Experience cannot decide this, as
we have not command over the unlimited time necessary for selection,
but theoretically it is quite intelligible that a variation which had
arisen through selection would be more apt to breed true the longer
selection was practised, and there is nothing to prevent it becoming
ultimately quite as constant as a natural species. For, at the
beginning of breeding, we must assume that the variation is contained
in only a small number of ids; as the number of generations mounts up,
more and more numerous ids with this variation will go to make up the
germ-plasm, and the more the breed-ids preponderate the less likelihood
will there be that a reversion to the parent-form will be brought about
by the chances of reducing-division and amphimixis. That most if not
all breeds of pigeon still contain ids of the ancestral form in the
germ-plasm, although probably only a small number of them, we see from
the occasional reversion to the rock-dove which occurs when species
are repeatedly crossed, but that ancestral ids may also be contained
in the germ-plasm of long-established natural species is shown by the
occurrence of zebra-striping in horse-hybrids. We can understand why
these ancestral ids should not have been removed long ago from the
germ-plasm by natural selection, since they are not injurious and may
remain, so to speak, undetected. It is only when they have an injurious
effect by endangering the purity of the new species-type that they can
and must be eliminated by natural selection, and this does not cease
to operate, as the human breeder does, but continues without pause or
break.
I therefore regard it as a mistake on the part of De Vries to exclude
fluctuating variation from a share in the transformation of organisms.
Indeed, I believe that it plays the largest part, because adaptations
cannot arise from mutations, or can only do so exceptionally, and
because whole families, orders, and even classes are based on
adaptations, especially as regards their chief characters. I need only
recall the various families of parasitic Crustaceans, the Cetaceans,
the birds, and the bats. None of these groups can have arisen through
saltatory, perhaps even retrogressive, 'mutation': they can only have
arisen through variation in a definite direction, which we can think of
only as due to the selection of the fluctuations of the determinants of
the germ-plasm which are continually presenting themselves.
The difference between 'fluctuating' variability and 'mutation' seems
to me to lie in this: that the former has always its basis only in
a small majority of the ids of the germ-plasm, while the mutation
must be present in most of the ids if it is to be stably transmitted
from the very first. How that comes about we cannot tell, but we
may suppose that similar influences causing variation within the
germ-plasm may bring about variation of many ids in the same direction.
I need only recall what I have already said as to the origin of
saltatory variations, such as the copper-beech and similar cases. The
experiments made by De Vries seem to me to give a weighty support to
my interpretation of these phenomena. De Vries himself distinguishes a
'pre-mutation period,' just as I have assumed that the variations which
spring suddenly into expression have been in course of preparation
within the germ-plasm by means of germinal selection for a long time
beforehand. At first perhaps only in a few ids, but afterwards in
many, a new state of equilibrium of the determinant-system would be
established, which would remain invisible until the chance of reducing
division and amphimixis gave predominance to a decided majority of the
'mutation-ids.' In the experiments made by De Vries the same seven new
'species' were produced repeatedly and independently of one another in
different generations of _Œnothera lamarckiana_, and we thus see that
the same constellations (states of equilibrium) had developed in many
specimens of the parent plant, and that it depended on the proportion
in which the ids containing these were represented in the seed whether
one or another of the new 'species' was produced.
My interpretation, according to which a larger or smaller number of
ids were the bearers of the new forms, receives further support from
the experiments, for the new species did not always breed true. Thus
De Vries found one species, _Œnothera scintillans_, which only yielded
35-40 per cent. of heirs, or in another group about 70 per cent.; the
other descendants belonged to the forms _lamarckiana_ or _oblonga_, but
the number of pure heirs could be increased by selection!
I cannot devote sufficient space to go fully into these very
interesting experiments; but one point must still be referred to:
the parent form, _Œnothera lamarckiana_, was very variable from
the beginning, that is, it exhibited a high degree of fluctuating
variability. This tells in favour, on the one hand, of a deep-rooted
connexion between 'variation' and 'mutation,' and, on the other hand,
it indicates that saltatory variation may be excited by transference
to changed conditions of life--as Darwin in his day supposed, and as
I have endeavoured to show in the foregoing discussion. De Vries
assumes mutation-periods, I believe rightly; but they are not periods
prescribed, so to speak, from within, as those who believe in a
'phyletic force' must suppose; they are caused by the influences of the
environment which affect the nutritive stream within the germ-plasm,
and which, increasing latently, bring about in part mere variability,
in part mutations, just as I have indicated in the section on Isolation
(vol. ii. p. 280), and indeed, in one of my earliest contributions to
the theory of descent[30]. In that essay I suggested the conclusion
that periods of constancy alternate with periods of variability,
basing my opinion on general considerations, and on Hilgendorf's study
of the Steinheim snail-shells. According to De Vries's _Œnothera_
experiments we may assume that periods of increased variability may
lead to the marked variations sometimes affecting several characters
simultaneously, and occurring in many ids, which have hitherto been
called 'saltatory' variations, and which we should perhaps do well to
call in future, with De Vries, _mutations_.
[30] _Ueber den Einfluss der Isolirung auf die Artbildung_, Leipzig,
1872, p. 51.
We cannot yet determine how far the influence of such mutations
reaches. I think it is plain that De Vries himself overestimated it,
but how many of the species-types which we find to-day depend upon
mere mutation can only be decided with any certainty after further
investigations. For the present it is well to be clear as to the
validity of the general conclusion, that all 'complex,' and especially
all 'harmonious,' adaptation, must depend, not upon 'mutation,' but
only upon 'variation' guided by selection. As species are essentially
complexes of adaptation, originating from a basis of previous complexes
of adaptation, there remains, as far as I can see, only the small field
of indifferent characters to be determined by mutations, unless indeed
we are to include under the term 'mutation' all variations which gain
stability, but this would be merely a play upon words. In my opinion
_there is no definable boundary line between variation and mutation,
and the difference between these two phenomena depends solely on
the number--larger or smaller--of ids which have varied in the same
direction_.
LECTURE XXXIV
ORIGIN OF THE SPECIFIC TYPE (_continued_)
Illustration of phyletic evolution by an analogy--Reconciliation
of Nägeli and Darwin--Unity of the specific type furthered by
climatic variation--By natural selection: illustration from aquatic
animals--Direct path of evolution--Natural selection works in
association with amphigony--Influence of isolation in defining the
specific type--Duration of the periods of constancy--The Siberian
pine-jays--Species are, so to speak, 'variable crystals'--Gradual
increase of constancy and decrease of reversions--Physiological
segregation of species through mutual sterility--Romanes's
physiological selection--Breeds of domestic animals mutually fertile,
presumably therefore 'amiktic' species also--Mutual fertility
in plant species--Mutual sterility certainly not a condition of
the splitting up of species--Splitting up of species without
amphigony--Lichens--Splitting up of species apart from isolation and
mutual sterility, _Lepus variabilis_.
Our train of thought in the last lecture brought us back again to the
so-called 'indifferent' characters, whose occurrence is so often used
as evidence against natural selection, as a proof that evolution is
guided essentially by internal forces alone. But this objection was
based on a fallacy, for the fact that the first small variations are
due to internal processes in the germ-plasm does not imply that the
whole further course of evolution is determined by these alone, any
more than the fact that a sleigh requires a push to start it on its
descent down an inclined plane would imply that its rapid descent is
due only to the force of the push, and not at the same time to the
attraction of gravity. But the analogy is not quite sound, for the
processes within the germ-plasm which condition and direct variation
do not merely give them the first shove off; they are associated with
every onward step in the evolutionary path of the species, they impel
it further and further, and without these continual impulses the
progressive movement would cease altogether. We have seen that, for
internal reasons, germinal selection continues to impel the varying
determinants further along the path on which they have started, and
thus gives them cumulative strength, and that it is in this way that
adaptations to the conditions of life are brought about. The evolution
of the character of a species may thus be compared to the course of
a sleigh upon a level snow-surface which it could traverse in any
direction, but it is moved only by the impulses received through
germinal selection. The conditions of life to which the varying parts
have to adapt themselves may be thought of as the distant goal, and
the processes in the germ-plasm which give rise to variation in a
definite direction may be compared to numerous human beings scattered
irregularly over the surface of the snow. If the sleigh receives from
one of these a push which chances to be in the direction of the goal,
it rushes on towards this and ultimately reaches it if the person
pushing continues to push in the same direction. So far, then, it
seems as if the transformation of the part concerned depended upon
germinal selection alone, but we must not forget that the germ-plasm
does not contain only one determinant for every part of the body, but
as many determinants as there are ids. We must therefore increase the
number of our sleighs, and now it is obvious that the pushers of the
sleighs, that is, germinal selection, may push one sleigh on toward
the goal, but others in the opposite or in any other direction. If we
assume that all the sleighs which have taken a wrong direction must
reach dangerous ground, and ultimately plunge into an abyss, but that
from a neighbouring point sleighs were being dispatched to replace all
that came to grief, that these in their turn might attempt to reach
the goal, it would ultimately come about that the requisite number
of sleighs would arrive at the goal--that is to say, that the new
adaptation would be attained.
The abysses represent the elimination of the less favourable
variational tendencies, and the constant replacing of sleighs
represents the intermingling of fresh ids through amphimixis. If all
the sleighs run in the wrong direction they all come to grief, that
is, the individual concerned is eliminated with all the ids of its
germ-plasm--it disappears altogether from the ranks of the species. But
if only a portion of them run in the right direction, care is taken
that in following generations, that is, in the continuation of the
sleigh-race, this portion combines with those of another group which
are also running in the right direction--that is, with the half number
of ids from another germ-plasm in amphimixis.
It is not possible to follow the analogy further, but perhaps it may
serve to illustrate how germinal selection may be the only impelling
force in the organisms, and yet only a small part of its results
are determined by itself, and by far the larger part by external
conditions. We understand how a variation in a definite direction can
exist, and yet it is not that which creates species, genera, orders,
and classes; it is the selection and combination of the variational
tendencies by the conditions of life, which occurs at every step.
There was no variational tendency leading from terrestrial mammals to
Cetaceans, but there was a variational tendency moving the nostrils
upwards towards the forehead, the hind-limbs towards diminution,
the lungs towards lengthening, the tail towards broadening out. But
each of these variational tendencies was always only one of several
possibilities, and that the particular path which led towards the
'goal' was followed was due to the fact that the others plunged into
the abyss to which the wrong paths led, that is to say, they were
weeded out by selection. Thus germinal selection offers a possibility
of reconciliation between Nägeli's and Darwin's interpretations, which
seem so directly contradictory; for the former referred everything to
the hypothesis of an internal evolutionary force, the latter rejected
this, and regarded natural selection as the main, if not the exclusive
factor in evolution. The internal struggles for food, which we have
assumed as occurring in the germ-plasm, represent an internal force,
though not in the sense of Nägeli, who thought of determining influence
operative from first to last, but still an impelling force, which
determines the direction of variation for the individual determinants,
and must therefore do the same for the whole evolution up to a certain
point; for it is only the _possible_ variations of the determinants in
a germ-plasm which can be chosen, selected, combined, and increased by
natural selection, and every germ-plasm cannot give rise to all sorts
of variations; the determinants contained in it condition what is
possible and what is not, and this is an important limitation to the
efficacy of natural selection, and to a certain extent also implies a
guiding and determining power on the part of the internal mainspring,
to wit, _germinal selection_.
The essential difference between Darwin's view of the transformation
of forms and my own lies in the fact that Darwin conceived of natural
selection as working only with variations which are not only due to
chance themselves, but the intensification of which also depends in
its turn solely upon natural selection, while, according to my view,
natural selection works with variational tendencies which become
intensified through internal causes, and are simply accumulated by
natural selection in an ever-growing majority of ids in a germ-plasm
through the selection of individuals.
This affects our view of the establishment of a specific type in so
far that my intra-germinal variational tendencies are not necessarily,
and not always due to chance, though they are so in most cases. If
certain determinants are impelled to vary in a particular direction
through climatic or any other influences, as we have seen to be the
case, for instance, with the climatic varieties of many Lepidoptera,
then the corresponding determinants in all the individuals must vary in
the same direction, and thus all the individuals of the species which
are subject to the same influence must undergo the same variation.
Transformations of this kind have exactly the appearance of resulting
from 'an internal evolutionary force,' such as Nägeli assumed, and the
unity of the specific type will not be disturbed by them.
Nor will this occur, as far as I can see, if the transformation of
a species depends solely upon new adaptations and their internal
consequences, for if a particular organism has to adapt itself to
special new conditions, it will usually be able to do so only in _one_
way, and thus natural selection will always allow _the same_ suitable
variational tendencies to survive and reproduce, so that the unity
of the specific type will not be permanently disturbed in this way
either. The more advantageous the new conditions of life prove, and the
more diverse the ways in which they can be utilized, the more rapidly
will the species first adapted to them multiply, and the more will
their descendants be impelled to adapt themselves specially to the
_different_ possibilities of utilizing the new situation, and thus,
from a parent species adapted in general to the new conditions, there
arise forms adapted to its more detailed possibilities. I must refer
again to the previous instance of the Cetaceans which originated from
vegetarian littoral, or fluviatile mammals, and have evolved since the
Triassic period into a very considerable number of species-groups.
All are alike in their _general_ adaptation, and these adaptations to
the conditions of life of aquatic animals--the fish-like form, the
flippers, the peculiarities of the respiratory organs and the organs of
hearing--when once acquired would not and could not be lost again; but
each of the modern groups of whales has its particular sphere of life,
which it effectively exploits by means of subordinate adaptations.
Thus there are the dolphins with their bill-like jaws and the two
rows of conical teeth, their active temperament, rapid movements, and
diet of fish; and the whalebone whales with their enormous gape, the
sieve-apparatus of whalebone-plates, and a diet of small molluscs and
the like. But each of these groups has split up into species, and if we
again regard the principle of adaptation as determining and directing
evolution, we are no nearer being able to prove the assumption in
regard to individual cases, for we know too little about the conditions
of life to be able to demonstrate that the peculiarities of structure
are actually adaptations to these. Theoretically, however, it is quite
easy to suppose that adaptation to a particular sphere of life was
the guiding factor in their evolution, and if this be so--as we have
already proved that it is in regard to the two chief groups and the
whole class--then the harmony of the structure must be due solely to
the continued selection of the fittest. We require no further principle
of explanation for the establishment of a specific type.
This 'type' is thus not reached by any indefinite varying of the parent
form in all directions, but in general it is reached by the most direct
and shortest way. The parent form must indeed have become to some
extent fluctuating, since not only the variational tendencies 'aiming
at the goal,' but others as well, must have emerged in the germ-plasm;
but gradually these others would occur less and less frequently, being
always weeded out afresh by selection, until the great majority of the
individuals would follow the same path of evolution, under the guidance
of germinal selection, which continues to work in the direction that
has once been taken. After a short period of variation, which need not,
of course, involve the whole organism, but may refer only to certain
parts of it, a steady direct progress in the direction of the 'goal,'
that is, of perfect adaptation, will set in, as we have seen in the
case of the _Planorbis_ snails of Steinheim.
We must not forget, however, that natural selection works essentially
upon a basis of sexual reproduction, which with its reduction of the
ids and its continually repeated mingling of germ-plasms, combines the
existing variational tendencies, and thus diffuses them more and more
uniformly among the individuals of a whole area of occupation. Sexual
reproduction, continual intermingling of the individuals selected
for breeding, is thus a very effective and important, if not an
indispensable, factor in the evolution of the specific type.
But it is not only in the case of species transformations due to
new adaptations that sexual mingling operates; it does so also in
the case of variations due to purely intra-germinal causes. We have
already seen in discussing Isolation that isolated colonies may come
to have a peculiar character somewhat different from that of the
parent form, because they were dominated by some germinal variational
tendency which occurred only rarely in the home of the parent form, and
therefore never found expression there. On the isolated area this would
indeed be mingled with the rest of the existing germinal variational
tendencies, but the result of this mingling would be different, and the
further development of the tendency in question would probably not be
suppressed.
We need not wonder, therefore, that specific types occur in such
varying degrees of definiteness. If a species is distributed over
a wide connected territory, sporadically, not uniformly, it will
depend partly upon the mutual degree of isolation of the sporadic
areas whether the individual colonies will exhibit the same specific
type or will diverge from one another. If the animal in question is
a slow-moving one like a snail, the intermingling from neighbouring
sporadic colonies will be much slower than in the case of a resident
bird such as a woodpecker. Many interesting results would undoubtedly
be gained if the numerous careful investigations into the geographical
distributions of species and their local races were studied with
special reference to this point, and much light would undoubtedly
be thrown upon the evolution of the specific type. But it would be
absolutely necessary to study carefully all the biological relations
of the animals concerned, to trace back the history of the species as
far as possible, and to decide the period of immigration, the mode and
direction of distribution, and so on.
Nothing shows more plainly the enormous duration of the period of
constancy in species than the wide distribution of the same specific
type on scattered areas or even over different areas absolutely
isolated from one another. If, as we saw, the same diurnal butterflies
live in the Alps and the far North, they must have remained unvaried
since the Glacial period, for it was the close of that period that
brought them to their present habitats, and while other diurnal
butterflies now living on the Alps differ from their relatives in
the Arctic zone (Lapland, Siberia, and Labrador) in some unimportant
spot or line, and must therefore have diverged from one another in
the course of the long period since the Glacial epoch, they have
done so only to a minimal degree, and in characters which possibly
depend solely upon germinal selection and can hardly be regarded as
adaptations.
I should like, however, to cite one of the few cases known to me in
which a slight deviation from the specific type undoubtedly depending
upon adaptation has occurred on an isolated region. The nut-jay
(_Caryocatactes nucifraga_) lives not only upon our Alps and in the
Black Forest, but also in the forests of Siberia, and the birds there
differ from those with us in small peculiarities of the bill, which
is longer and thinner in them, shorter and more powerful in ours.
Ornithologists associate this difference with the fact that in this
country the birds feed chiefly on hard hazel nuts, which they break
open with their bill, and on acorns, beechmast, and, in the Alps,
on cembra-cones, while in Siberia, where there are no hazel nuts,
they feed chiefly upon the seeds of the Siberian cedar, which are
concealed deep down in the cones. Thus we find that in Siberia the
bill is slender, and that the upper jaw protrudes awl-like beyond the
lower, for about 2.5 mm., and probably serves chiefly to pick out the
cedar nuts from behind the cone-scales. In the Alps the birds (var.
_pachyrhynchus_) break up the whole cone of the cembra-pine with their
thick, hard bill, and in the Upper Engadine, where the nut-jay is
abundant, I have often seen the ground underneath the cembra-pines
covered with the débris of its meal. In addition to these differences
between the two races, the Alpine form is stronger in build, the
Siberian form is daintier; in the former the white terminal band on
the tail is narrow (about 18 mm.), in the Siberian form it is broader
(about 27 mm.).
Such cases of variation of individual parts in different areas seem to
me very important theoretically, because they furnish us with an answer
to the view which represents the species as a 'life-crystal,' which
must be as it is or not at all, and which therefore cannot vary as
regards its individual parts. The case of the nut-jay has the further
interest that it is one of the few in which we find the new adaptation
of a single character without variation of most of the other characters.
It is only in an essentially different sense that we can compare the
species, like any other vital unit, to a crystal, in so far as its
parts are harmoniously related one to another, or, as I expressed
myself years ago, are in a state of equilibrium, which must be brought
about by means of intra-selection. This analogy, however, only applies
to the actual adjustment of the parts to a whole, and not to their
casual adjustment. Species are variable crystals; the constancy of a
species in all its parts must be regarded as something quite relative,
which may vary at any time, and which is sure to vary at some time
in the course of a long period. But the longer the adaptation of
a species to new conditions persists the more constant, _ceteris
paribus_, and the more slowly variable will it become, and this for
two reasons: first, because the determinants which are varying in a
suitable direction are being more and more strictly selected, more
and more precisely adapted, and are thus becoming more like each
other; and secondly, because, according to our theory, the homologous
determinants of all the ids do not vary in the required direction, and
a portion of the unvaried ancestral ids is always carried on through
the course of the phylogeny, and only gradually set aside by the
chances of reducing division. But the more completely these unvaried
ids are eliminated from the germ-plasm the less likely will they be
to find expression in reversions or in impurities of the new specific
characters. I may recall the reversions of the various breeds of
pigeons to the rock-dove, those of the white species of _Datura_ to the
blue form, and the _Hipparion_-like three-toed horse of Julius Cæsar,
and so on. The unvaried ancestral ids, which in these cases find only
quite exceptional expression, will make the new 'specific character'
fluctuating, as long as they are contained in the germ-plasm in
considerable numbers, but they must become more and more infrequent in
the germ-plasm as successive generations are passed through the sieve
of natural selection, and the oftener these germ-plasms, to which the
chances of reducing division and amphimixis have assigned a majority
of the old determinants, are expelled from the ranks of the species by
personal selection. The oftener this has occurred in a species the less
frequently will it recur, and the more constant, _ceteris paribus_,
will the 'type' of the species become.
If we add to this idea the fact that adaptations take place very
slowly, and that every variation of the germ-plasm in an appropriate
direction has time to spread over countless hosts of individuals, we
gain some idea of the way in which new adaptations gradually bring
about the evolution of a more and more sharply-defined specific type.
So far, however, we have only explained the morphological aspect of the
problem of the nature of species, but there is also a physiological
side, and for a long time this played an important part in the
definition of the conception of species. Until the time of Darwin it
was regarded as certain that species do not intermingle in the natural
state, and that, though they could be crossed in rare cases, the
progeny would be infertile.
Although we now know that these statements are only relatively correct,
and that in particular there are many higher plants which yield
perfectly fertile hybrids, it is nevertheless a striking phenomenon
that among the higher animals, mammals, and birds the old law holds
good, and hybrids between two species are very rarely fertile. The two
products of crossing between the horse and the ass, the mule and the
hinny, are never fertile _inter se_, and very rarely with a member of
the parent stock.
We have to ask, therefore, what is the reason of this mutual sterility
of species; whether it is a necessary outcome of the morphological
differences between the species, or only a chance accessory phenomenon,
or perhaps an absolutely necessary preliminary condition to the
establishment of species.
The last was the view held by Romanes. He believed that a species could
only divide into two when it was separated into isolated groups either
geographically or physiologically, that is, when sexual segregation
in some form is established within the species, so that all the
individuals can no longer pair with one another, but groups arise which
are mutually sterile. It is only subsequently, he maintained, that
these groups come to differ from one another in structure. To this
hypothetical process he gave the name of 'physiological selection.'
This view depends--it seems to me--upon an underestimate of the power
of natural selection. Romanes believed that when a species began to
split up, even the adaptive variations would always disappear again
because of the continual crossing, and that only geographical isolation
or sexual alienation, that is, physiological selection, would be
able to prevent this. But even the fact that there are dimorphic and
polymorphic species proves sufficiently that adaptation to two or even
several sets of conditions can go on on the same area. In many ants we
find many kinds of individuals--the two sexual forms, the workers, and
the soldiers, and these last are undoubtedly distinguished by adaptive
characters which must be referred to selection. The same is true of
the caterpillars, whose coloration is adapted to their surroundings
in two different ways. If the individuals of one and the same species
can be broken up into two or more different forms and combinations
of adaptations, while they are mingling uninterruptedly with one
another, natural selection must undoubtedly be able, notwithstanding
the continual intermingling of divergent types, to discriminate between
them and to separate them sharply from one another. Assuredly then a
species can not only exhibit uniform variation on a single area, but
may also split up into two without the aid of physiological selection.
Theoretically it is indisputable that of two varieties which are
both equally well suited to the struggle for existence, a mixed form
arising through crossing may not be able to survive. Let us recall, for
instance, the caterpillars, of which some individuals are green and
some brown, and let us assume that the brown colour is as effective
a protection as the green, then the two forms would occur with equal
frequency; but though a mixed hybrid form which was adapted neither to
the green leaves nor to the brown might occasionally crop up, it would
always be eliminated. It would occur because the butterflies themselves
are alike, whether they owe their origin to green caterpillars or to
brown, and thus at first, at least, all sexual combinations would be
equally probable.
I do not believe therefore in a 'physiological selection,' in Romanes's
sense, as an indispensable preliminary condition to the splitting of
species, but it is a different question whether the mutual sterility so
frequently observable between species has not conversely been produced
by natural selection in order to facilitate the separation of incipient
species. For there can be no doubt that the process of separating two
new forms, or even of separating one new form from an old one, would
be rendered materially easier if sexual antipathy or diminished
fertility of the crossings could be established simultaneously with the
other variations. This would be useful, since pure and well-defined
variations would be better adapted to their life-conditions than
hybrids, and would become increasingly so in the course of generations.
But as soon as it is useful it must actually come about, if that is
possible at all. It may be, however, as we have said before, that the
two divergent forms depend merely upon quantitative variations of the
already existing characters; sexual attraction, whether it depends
upon very delicate chemical substances, or on odours, or on mutual
complementary tensions unknown to us, will always fluctuate upwards and
downwards, and plus or minus determinants, which lie at the root of
these unknown characters in the germ-plasm, must continually present
themselves and form the starting-point for selection-processes of a
germinal and personal kind, which may bring about sexual antipathy and
mutual sterility between the varieties. I therefore consider Romanes's
idea correct in so far that separation between species is in many cases
accompanied by increasing sexual antipathy and mutual sterility. While
Romanes supposed that 'natural selection could in no case have been
the cause' of the sterility, I believe, on the contrary, that it could
only have been produced by natural selection; it arises simply, as
all adaptations do, through personal selection on a basis of germinal
selection, and it is not a preliminary condition of the separation
of species, but an adaptation for the purpose of making as pure and
clean a separation as possible. It is obviously an advantage for both
the divergent tendencies of variation that they should intermingle
as little as possible. This is corroborated by the fact that by no
means all the marked divergences of species are accompanied by sexual
alienation, and that the mutual sterility so frequently seen is not an
inevitable accompaniment of differences in the rest of the organism.
That this is not the case is very clearly proved by our domesticated
animals. The differences in structure between the various breeds of
pigeon and poultry are very great, and breeds of dog also diverge
from one another very markedly, especially in shape and size of body.
Yet all these are fertile with one another, and they yield fertile
offspring. But they are products of artificial selection by man, and
he has no interest in making them mutually sterile, so that they have
not been selected with a view to sexual alienation, but in reference
to the other characters. The segregation of animal species into
several sub-species on the same area is probably usually accompanied
by sexual antipathy, since in this case it would be useful although
not indispensable. But the matter is different in the case of the
transformation of a colony upon a geographically isolated region.
'Amiktic' forms, such as _Vanessa ichnusa_ of Corsica, are hardly
likely to be sexually alienated from the parent form; we have here
to do only with the preponderance of a fortuitous and biologically
valueless variation and its consequent elevation to the rank of a
variety. The new form was not an adaptation, but only a variation, and
as it was of no use, it was not in a position to incite any process of
selection favouring its advancement.
But even adaptive transformations on isolated regions from which the
parent species is excluded are not likely to develop rapidly any sexual
antipathy as regards the parent stock, and I should not be surprised if
experiments showed that there is perfect mutual fertility between, for
instance, many of the species of _Achatinella_ on the Sandwich Islands
or of _Nanina_ on Celebes, or between the species of thrushes on the
different islands of the Galapagos Archipelago, or between these and
the ancestral species on the adjacent continent, if that species is
still in existence. For there was no reason why sexual antipathy to
the parent form should have developed in any of these adaptation forms
which have arisen in isolation, and therefore it has probably not been
evolved.
That our view of the mutual sterility between species, as an adaptation
to the utility of precise species-limitation, is the correct one is
evidenced not only by our domesticated races, but even more clearly by
plants, in regard to which it is particularly plain that the sexual
relations between two species are adaptational. We have already seen
in what a striking way the sensitiveness of the stigma of a flower is
regulated in reference to pollen from the same plant, that some species
are not fertilizable by their own pollen at all, that others yield very
little seed when self-fertilization is effected, and that others again
are quite fertile--as much so as with the pollen of another plant of
the same species. We regarded these gradations of sexual sensitiveness
as adaptations to the perfectly or only moderately well-assured visits
of insects, or to their entire absence. I wish to cite these cases as
well as the heterostylism of some flowers as evidence in support of
the conception of the mutual sterility of species which I have just
outlined. But this only in passing. The point to which I chiefly wish
to direct attention is the mutual fertility of many plant-species.
In lower as well as in higher plants fertile hybrids occur not
infrequently under natural conditions, and cultivated hybrids, such
as the new _Medicago media_, a form made by mingling two species of
clover, may go on reproducing with its own kind for a considerable
time. A number of Phanerogams yield fertile hybrids, and in Orchids
even species of different genera have been crossed and have yielded
offspring which was in some cases successfully crossed with a third
genus.
If these facts prove anything it is that the factors which determine
the mutual sterility of species are quite distinct from their
morphological differences, in other words, from the diagnostic
characters of the specific type. For a long time the verdict on this
matter was too entirely based on observations made on animals, among
which mutual sterility arises relatively easily, even where it was not
intended (_sit venia verbo!_). Even the pairing, but still more the
period of maturity, the relations of maturity in ovum and sperm, and
even the most minute details in the structure of the sperm-cell, the
egg-shell, envelope, &c., have to be taken into account, and these may
bring about mutual or, as Born has shown, one-sided sterility. We know,
through the researches of Strasburger, that a great many Phanerogams,
when pollinated artificially from widely separated species of different
genera and families, will at least allow the pollen-tube to penetrate
down to the ovule, and that in many cases amphimixis actually results.
It follows that we must not lay too great stress upon the mutual
sterility which occurs almost without exception among the higher
animals, but must turn to the plants with greater confidence.
Among plants there is very widely distributed mutual fertility between
species. I doubt, however, whether the observations on this point
are sufficient to warrant any certain conclusion in regard to the
importance of the phenomena in the formation of species. At least it
is not easy to see why the mutual sterility of many species of plants
should not have been necessary or useful in separating species, and why
it was not therefore evolved. We may point to the fact that animals
can move from place to place as the chief reason, and this factor
does undoubtedly play a part, but the widespread crossing of plants
by insects makes up to some extent, as far as sexual intermingling
is concerned, for their inability to move from place to place. I do
not know whether the species of orchid which are fertile with one
another belong to different countries, so that we may assume that they
originated in isolation, or whether fertile orchids from the same area
are fertilized by different insects and are thus sexually isolated.
This and many other things must be taken into consideration. Probably
these relations have not yet been adequately investigated; probably
what is known by some experts has not yet been made available to
all. Future investigations and studies must throw more light upon the
problem.
In any case, however, we can see from the frequency of mutual fertility
among plants that mutual sterility is not a _conditio sine qua non_
of the splitting up of species, and we must beware of laying too
great stress upon it even among animals. Germinal selection is a
process which not only forms the basis of all personal selection, but
which is also able to give rise of itself, without the usual aid of
sexual intermingling, to a new specific type. And we cannot with any
confidence dispute that, even without amphigony, a certain degree of
personal selection may not ensue solely on the basis of the favourable
variational departures originating in the germ-plasm. It would be
premature to express any definite views on this point as yet, but the
diverse cases of purely asexual or parthenogenetic reproduction in
groups of plants rich in species make this hypothesis seem probable.
The most remarkable example of this is probably to be found in the
Lichens, the symbiotic nature of which we have already discussed,
in which--now at least--neither the Fungus nor the Alga associated
with it is known to exhibit sexual reproduction. If this is really
the case, then the existence of numerous and well-marked species of
lichens leads us to the hypothesis just expressed, and we must suppose
that the unity of the specific type is attained in this case solely
by a continual sifting of the useful from the useless variations of
the determinants, and through purely germinal intensification of the
surviving variational tendencies.
Of course it is possible that the mutual adaptation of the Algæ and
Fungi in the evolution of species of Lichens took place very long ago,
at a time when sexual reproduction still existed, at least in one of
the associated organisms, the Fungus. The Ascomycetes, to which most
of the Lichens belong, do not at present usually exhibit the process
of amphimixis, as I have already noted; but it may perhaps be still
possible to decide whether they must have exhibited it, or at least
could have exhibited it at an earlier stage in their evolution. As the
group of Thallophytes is a very ancient one, it is not inconceivable
that the modern species of Lichens have existed for a long time, and
that they had their origin in the remote past with the assistance of
amphimixis.
Nor need it be objected to this supposition that it has been found
possible to _make new Lichens_ by bringing together Fungi and Algæ
which had not previously been associated with one another; for in
the first place both were already adapted to partnership with other
species, and, moreover, so far no one has succeeded in rearing these
artificial lichens for any length of time, still less in seeing them
evolve into specific forms persistent in natural conditions.
But if this supposition should prove to be not only improbable, but
actually erroneous, then the existence of Lichens would afford a clear
proof that the 'type' of the species does not depend essentially upon
the constant intermingling of individuals, but upon a process which
we may best designate _uniformity of adaptation_. We have simply to
suppose that under similar external influences similar variational
tendencies were started by germinal selection in all the individuals
of the two parent species of a lichen, and set a-going by germinal
selection, just as a warmer climate gives rise to a black variety
in the butterfly _Polyommatus phlæas_, because similar determinants
of the germ-plasm of all the individuals were impelled to vary in
the same manner and direction. This would then give rise to quite
definite variations, and since only the suitable variational tendencies
could survive, primitive though never complicated adaptations would
arise. But we cannot assume that the lichens are not adapted to the
conditions of their life as well as all other organisms. We cannot
judge how far even their shape is to be regarded as an adaptation,
whether the formation of encrusting growths, of tree-like forms, of
cup or bush-lichens, may not be regarded as adaptations towards a full
utilization of the conditions of their life--but even if this is not
the case, the formation of soredia remains an undoubted adaptation to
the symbiosis of those lichens which exhibit them. The soredia cannot
depend upon the direct effect of the conditions of life, for they are
reproductive bodies which did not exist before the existence of the
lichen, and only originated to facilitate their distribution.
Thus there is still a great deal that is doubtful in our theories as to
the transformations of organisms, and much remains still to be done.
But even though we may doubt whether adaptations could come about in
multicellular organisms without amphigony, we may be quite certain of
the converse, that is, that the specific type can be changed in every
individual feature by natural selection on the basis of amphigony,
even as regards invisible features which only express themselves in
altered periods of growth. Even when there is no isolation whatever
and no mutual sterility, and when a mobile species is uniformly
distributed over a large area, a splitting up into races in regard to
one particular character may occur, _simply through adaptation_ to the
spatially different climatic conditions of the area inhabited.
Early in these lectures we discussed the twofold protective value of
the coloration of the 'variable hare' (_Lepus variabilis_), which is
distributed over the Arctic zone of the Old and New World, and also
occurs in the higher regions of the Alps. Wherever there is a sharp
contrast between winter and summer the variable hare exhibits the
same specific type, being brown in summer and white in winter, but in
regard to this very character of colour-change it forms races to some
extent, for it is white for a longer or shorter time according to the
length of the winter--in Greenland for the whole twelve months of the
year, in Northern Norway only for eight or nine months, in the Alps
for six or seven months, but in the south of Sweden and in Ireland
not at all. There it remains brown in winter like our common hare
(_Lepus timidus_). This is not a question of the direct effect of cold;
if it were the species would become white in Southern Sweden also,
for there is no lack of severe cold there, but the ground is not so
uninterruptedly covered with snow, and so the white colour of the hare
would be as often, probably oftener, a danger than a safeguard, and
the more primitive double coloration has therefore been done away with
by natural selection. The change of colour is thus hereditarily fixed,
as is proved by the fact that the Alpine hare, if caught and kept in
the valleys below, puts on a white dress at the usual time, which the
common hare never does.
As in Southern Sweden the winter coloration has been wholly eliminated,
so, conversely, from there to the Arctic zone the summer colouring
has been more and more crowded out, and in the Farthest North it has
totally disappeared from the characters of the species. We thus see
that wherever the species lives the double colouring is regulated,
as regards the duration of the winter coat, in exact harmony with
the external conditions. There is a pure white, a pure brown, and
a colour-changing race, and the latter is subdivided into two--one
wearing the winter dress for six, the other for eight months. Probably
these could be still further subdivided, if the different regions of
the Scandinavian Peninsula were investigated individually from south
to north. That the duration of the winter dress has its roots in
the germ-plasm, and does not depend solely on the earlier or later
period at which the cold sets in, is made clear by the two extreme
forms, the white and the brown _Lepus variabilis_, as well as by the
behaviour of captive animals. The familiar case of Ross's lemming,
which remained brown in the warm cabin, and then suddenly became
white when it was exposed to the cold of winter, only shows that the
cold acts as a liberating stimulus. The preparatory changes in the
pellage are already present, and the stimulus of cold brings them
rapidly to a climax. Here, therefore, the necessary variations of the
relevant germinal parts must have continually presented themselves for
selection, which is intelligible enough, since it is merely a question
of plus- or minus-variations. The fact that the six-months' dress can
be transformed into an eight-months' dress must have its cause in some
minute biological units of the germ-plasm; the determinants of the fur
must be able to vary in such a way that a longer or shorter duration of
the winter's coat is the result. The possibility of the whole variation
depends upon the continual fluctuations of all determinants, now
towards plus, now towards minus, and the necessity and inevitableness
of each adaptation to the duration of the winter lies in the unceasing
personal selection--the inexorable preferring of the better adapted.
LECTURE XXXV
THE ORIGIN AND THE EXTINCTION OF SPECIES
Adaptation does not depend upon chance--The case of eyes--Of
leaf-mimicry--All persistent change depends ultimately on
selection--Mutual sterility without great significance--Relative
isolation (_Lepus variabilis_)--Influence of hybridization--Decadence
of species--Differences in the duration of decadence--Natural
death of individuals--Extinction due to excessive variability
(Emery)?--_Machairodus_ as interpreted by Brandes--Lower types more
capable of adaptation than higher--Flightless birds--Disturbance of
insular fauna and flora by cultivation--The big game of Central Europe.
In the polar hare we have a case in which the adaptations to the life
conditions both of time and space are recognizable as the effect of
definite causes, and thus as a necessity; but the same must be true
everywhere even in regard to the most complex adaptations which seem
to depend entirely upon chance; everywhere adaptation results of
necessity--if it is possible at all with the given organization of the
species--as certainly as the adaptation dress of the hare depends on
the length of the winter, and in point of fact not less certainly than
the blue colour of starch on the addition of iodine. The most delicate
adaptations of the vertebrate eye to the task set for it by life in
various groups have been gradually brought about as the necessary
results of definite causes, just in the same way as the complex
protective markings and colouring on the wing of the _Kallima_ and
other leaf-mimicking butterflies.
That adaptations can be regarded as mechanically necessitated is due
to the fact that in every process of adaptation the same direction of
variation on the part of the determinants concerned is guaranteed,
since personal selection eliminates those which vary in a wrong
direction, so that only those varying in a suitable direction survive,
and they then continue to vary in the same direction. But the greatest
difference between our conception of natural selection and that of
Darwin lies in this: that Darwin regarded its intervention as dependent
upon chance, while we consider it as necessary and conditioned by the
upward and downward intra-germinal fluctuation of the determinants.
Appropriate variational tendencies not only _may_ present themselves,
they _must_ do so, if the germ-plasm contains determinants at all
by whose fluctuations in a plus or minus direction the appropriate
variation is attainable.
That a horse should grow wings is beyond the limits of the
possibilities of equine variation--there are no determinants which
could present variations directed towards this goal; but that any
multicellular animal which lives in the light should develop eyes lies
within the variational possibilities of its ectoderm determinants, and
in point of fact almost all such animals do possess eyes, and eyes,
too, whose functional capacity may be increased in any direction, and
which are adaptable and modifiable in any manner in accordance with
the requirements of the case. As soon as the determinants of the most
primitive eye came into existence, they formed the fundamental material
by whose plus- or minus-variations all the marvellous eye structures
might be brought about, which we find in the different groups of the
Metazoa, from a mere spot sensitive to light to a shadowy perception
of a moving body, and from that again to the distinct recognition
of a clear image, which we are aware of in our own eyes. And what
wonderful special adaptations of the eye to near and to distant vision,
to vision in the dusk and at night, or in the great ocean-depths, to
recognition of mere movement or the focussing of a clear image, have
been interpolated in the course of this evolution!
All such adaptations are possible, because they can proceed from
variations of determinants which are in existence; and in the same
way it is possible, at every stage of the evolution of organisms, for
eyes to degenerate again, whether they have been high up or low down
in the scale of gradations of this perhaps the most delicate of all
our sense-organs. As soon as a species migrated permanently from the
light into perfect darkness its eyes began to degenerate. We know
blind flat worms, blind water-fleas and Isopods, also blind insects
and higher Crustaceans, and even blind fishes and amphibians, the eyes
of which are now to be found at very different levels of degeneration,
as Eigenmann has recently shown in regard to several species of
cave-dwelling salamanders of the State of Ohio. In all these cases
it is only necessary for the determinants of the eye to continue to
vary in the minus direction, and the disappearance of the eye must be
gradually brought about.
We must picture upward development in quite a similar way. The forest
butterflies of the Tropics could not possibly all have their under
surfaces coloured like a leaf if the protective pattern depended solely
upon the chance of a useful variation presenting itself. It always
presented itself through the fluctuations of the determinants, and
thus the appropriate colourings were not merely able to develop, but
of necessity did so in gradually increasing perfection. If chance
played any part in the matter, it would be quite unintelligible why the
protective colouring should occur only where it acts as a protection,
and why, for instance, it should not appear sometimes upon the upper
surface of the butterfly wing, or upon the posterior wings which are
covered when the butterfly is at rest. We have already studied in
detail the precision with which the coloration is localized on minute
points and corners of the wing: this can only be understood if natural
selection works with the certainty of a perfect mechanism. Chance only
comes into the matter in so far as it depends upon chance whether the
relevant determinants in one id or another are to vary in the direction
of plus or minus; but as the germ-plasm contains many ids, and chance
may decide it differently in each of these, the presence of a majority
of determinants varying in a desirable direction does not depend upon
chance, for if they are not contained in one individual they are in
another. It is only necessary that they should be present in some, and
that these should be selected for reproduction.
We must therefore regard natural selection, that is to say, _personal
selection_, as a mechanical process of development, which begins
with the same certainty and works 'in a straight line' towards its
'goal,' just as any principle of development might be supposed to do.
Fundamentally it is after all a purely internal force which gives
rise to evolution, the power of the most minute vital units to vary
under changing influences, and it is only the guidance of evolution
along particular paths that is essentially left to personal selection,
which brings together what is useful and thus determines the direction
of further evolution. If we bear in mind that even the minutest
variations of the biophors and determinants express nothing more or
less than reactions to changed external conditions in the direction of
adaptation, and that the same is true of each of the higher categories
of vital units, whether they be called cell, tissue, organ, person,
or corm, we see that the whole evolution of the forms of life upon
the earth depends upon adaptations following each other in unbroken
succession, and fitting into each other in the most complex way. The
whole evolution is made possible by the power of variation of the
living units of every grade, and called forth and directed by the
ceaseless changes of the external influences. I said years ago that
_everything_ in organic evolution depended upon selection, for every
lasting change in a vital unit means adaptation to changed external
influences, and implies a preference in favour of the parts of the unit
concerned, which are thereby more fitly disposed.
In this sense we can also say that the species is a complex of
adaptations, for we have seen that it depends upon the co-operation
of different grades of selective processes, that in many cases it is
produced solely by germinal selection, but that in very many more
personal selection plays the chief part, whether in bringing about
sexual adaptations, or adaptations to the conditions of existence.
When we have thus recognized that the origin of a variation in a
definite direction results as inevitably when it is called forth by the
indirect influence of conditions, that is, through the need for a new
adaptation, as when it is induced in the germ-plasm by direct causes
such as those of climate, we shall not be disposed to estimate very
highly the part played by mutual sterility in the origin of species. We
shall rather be inclined to assign it a rôle at a later stage, after
the separation of the forms has taken place, and this view is supported
by the fact of the mutual sterility of most nearly related species, and
by the theoretical consideration that the frequency of hybrids, even if
these are always eliminated in the struggle for existence, must signify
a loss for both the parent species. But no certain conclusion can be
based upon either of these arguments--not upon the theoretical one,
because here again we are unable to estimate the extent of this loss;
and not upon the argument from fact, because the results of experiments
in crossing animals have generally been overestimated, since we are apt
to regard the most nearly related animals that are at our disposal as
being very closely related. Thus, for instance, horse and ass, horse
and zebra are undoubtedly rightly included within the same genus, but
the fact that there are several species of zebra in Africa gives us an
idea of the number of transition stages that may have existed between
the horse and the zebra. Entomologists have sometimes reared hybrids
between the most nearly related indigenous species of hawk-moth of
the genus _Smerinthus_--hybrids of _Smerinthus ocellata_, the eyed
hawk-moth, and _Smerinthus populi_, the poplar hawk-moth. I have myself
made many experiments of this kind, and have often succeeded in getting
the two species to pair and even to deposit eggs, but I have never
seen a caterpillar emerge from them. The hybrids do occur, however,
and they have been repeatedly obtained by Standfuss. In external
appearance they are intermediate between the parent forms, but with
marked divergences, thus, for instance, the beautiful blue eye on the
posterior wing of _S. ocellata_ (Fig. 5, vol. i. p. 69) may have almost
disappeared or be only indicated. They are sterile. But we know three
species of _Smerinthus_ in North America, which are all much nearer to
_S. ocellata_ than _S. populi_ is, for they all possess the eye-spot
referred to, although it is less well developed. The proof that the
most nearly related species do not yield fertile descendants should be
sought for by crossing _Smerinthus ocellata_ with one of these American
species if it is to have any decisive value.
Experiments of the same kind have been made by Standfuss with different
species of indigenous _Saturnia_, and these have shown not only that
crossing is possible, but that the hybrids are fertile in their turn.
These results are to be valued the more highly because it is well known
that Lepidoptera, and even the usually prolific silk-moths, do not
readily reproduce in captivity, even within the same species. We have
in _Saturnia pyri_, _spini_, and _carpini_ three well-marked distinct
species with no intermediate forms in nature, and with quite different
colouring in the caterpillars. That these should have been successfully
combined in a triple hybrid proves at least that sexual alienation
cannot have advanced far in this case.
We must beware, however, of attributing too much to the constant
mutual crossing which occurs in a species living on a connected area
and of regarding its influence as irresistible. Undoubtedly it must
go far towards securing the uniformity of individuals, but not only
is it unable to achieve this, but it cannot successfully resist the
stronger influences making for variation which may be exerted upon a
part of the area of the species. We have already seen that it is quite
erroneous to suppose that every new adaptation must be lost sight of
again because of the continual crossing with other members of the
species upon the same area. Other things being equal, this depends
entirely upon the importance of the adaptation in question. Just as
climatic influences may be so strong that they entirely overcome the
influence of crossing, and give rise to a local race notwithstanding
imperfect geographical isolation, so the same may happen in the case of
adaptations. It is quite conceivable that the polar hare of Scandinavia
may have evolved a whole series of races, each of which is adapted
to the duration of the snow in its geographical range, although a
crossing of these quick-footed animals must frequently occur in the
course of time, even as regards forms from widely separated areas, and
although the whole region is inhabited without a break by the species,
so that a 'mingling' of the hares of all regions from south to north,
and conversely, may take place, and indeed must be continually taking
place, though of course very slowly.
It is precisely this extreme slowness with which the intermingling
of racial characters take place that seems to me essential for the
production of local or, as in this case, regional races. It is not
difficult to calculate the rate of 'blood-distribution' if we assume
that the conditions for a rapid dissemination are as favourable as
possible. Let us assume that it takes place along a certain line--in
this case from south to north--and that the numerical strength of
the species remains constant, each pair of hares yielding a pair of
surviving offspring, which will attain to reproduction. Let us suppose
that one of these hares moves his home northwards to the extent of his
range, that is, as far as a hare is accustomed to range from his head
quarters, and that he pairs with one of the descendants on the next
stretch.
Let us further suppose that this stretch is ten kilometres in extent,
and that the change of quarters take place once in each year, then the
blood of a South Scandinavian hare would have extended ten kilometres
further north in ten years, and in a hundred years 100 kilometres; it
would not, however, be quite pure, but mixed and thinned by crossing
with a hundred mates of different individual bloods, that is, thinned
to the extent of 2 to the 100^{th} power, that is, to less than a
millionth part. Thus even with these much too favourable assumptions
the influence of a region of hares 100 kilometres distant would be
actually _nil_ upon the inhabitants of a region which was in process
of new adaptation. That the assumptions are too favourable is quite
obvious, since every surviving hare would not be likely to move his
home, and probably the majority would remain in the old quarters
and find mates there. The blood-mingling would therefore take place
much more rarely, perhaps only once in ten years, and the wandering
descendants of the second generation might move southward, and so
neutralize the previous blood-mingling, and so on. But let us keep
to our favourable assumptions, and attempt to determine how strong
the assimilating influence of the blood-mingling from south to north
would be upon a point _A_. The blood of the nearest stretch diluted
to a half would affect the inhabitants of _A_ once in each year; the
second stretch would only contribute blood of 1/4 strength, the third
of 1/8, the fourth of 1/16, and the blood of the tenth would be diluted
to 1/1024. A region _B_, extending over twenty such stretches, or 200
kilometres, would thus shelter within it a hare population of which
the centre would only be influenced from the periphery in vanishing
proportions. If the winter were of equal length over the whole area of
_B_, all the inhabitants would be tending to vary the period for which
the winter dress was worn in correspondence with the length of the
winter, and the centre of the region would be the less impeded in this
process because the more peripheral areas would also be approximating
to the same adaptation. But since even the admixture of 1/32 of strange
blood could have no hindering influence upon a variation, there would
remain a region of 2 × 5 = 10 stretches upon which the influence of
the non-varying regions would be without effect. There would therefore
arise a new race in relation to the duration of the winter dress, and
this would not cease abruptly, but would gradually pass over into the
neighbouring regions, which however would be pure at their centre, just
as is probably the case in reality, if we regard _B_ as any point in
the line of distribution from south to north.
The harmony of the individuals within a species will therefore depend
in part upon the mingling of hereditary primary constituents associated
with reproduction, but in greater part upon adaptation to the same
conditions; it is _a similarity of adaptation_, and the strongest
influence which sexual reproduction exerts lies not in the mingling of
these hereditary constituents alone, but above all in the reduction
in the germ-plasm of the two parental hereditary contributions--a
reduction which results from and through the sexual intermingling. It
is only this that prevents these primary constituents from varying at
too unequal a rate in the transformations of species, and causes them
ultimately to resemble each other closely again.
But while mutual sterility is not an absolutely necessary condition in
the separation of species, it would be going too far in the opposite
direction to regard mutual fertility as something general, or to
attribute to it a rôle in the origination of new species.
Certain botanists, like Kerner von Marilaun, regard the mingling of
species as a means of forming new species with better adaptations;
they suppose that fertile hybrids may, in certain circumstances, crowd
out the parent species, and themselves become new species. It will be
admitted that such cases do occur, that, for instance, in the north of
Europe the hybrid between the large and the small water-lily, _Nuphar
luteum_ and _Nuphar pumilum_, to which the name _Nuphar intermedium_
has been given, has driven both the parent species from the field,
because its seeds mature earlier, and it is therefore better adapted
to the short vegetative period of the north, but nevertheless we must
maintain that the evolution of species on the whole does not take
place through hybridization. Such cases are probably nothing more than
rare exceptions. This is corroborated by the entire insignificance of
hybridization in animals, among which species appear in the same way as
they do in plants, and where the mingling of two species occurs only
sporadically and in a few species, never to any very great extent.
If species are complexes of adaptations, based in each case on the
given physical constitution of the parent species, then we can readily
understand the fact that they are in our experience not fixed or
eternal, but that they change in the course of the earth's history.
The numerous fossil remains in the various strata of the earth's crust
prove that this is true in a high degree, that in almost every one of
the more important geological strata new species occur, and that not
only species and genera, but families, orders, indeed whole classes
of animals, which lived at one time, have now completely disappeared
from the face of the earth. We can understand this phenomenon when we
reflect that the conditions of life have also been slowly changing
through the course of the earth's history, so that the old species had
only the alternative of dying out, or of becoming transformed into new
species.
But simple as this conclusion is, it can hardly be deduced with
certainty from the occurrence and succession of the fossil species
alone. For instance, we should strive in vain to recognize the
cause which led one of those regularly arranged snail-species of
the Steinheim lake basin to become transformed into one or two new
species at a particular time, or to find the cause which moved those
curious tripartite Crustaceans of primitive times, the Trilobites,
which peopled the Silurian seas with such a wealth of forms, to
become suddenly scarce towards the end of the Silurian period, and
to disappear altogether in the succeeding period, the Devonian. The
famous geologist Neumayr sought to refer this striking phenomenon to
the fact that just at that time the Cephalopods, 'the most formidable
and savage marauders among the invertebrate marine fauna,' gained the
ascendancy, and it is quite possible that he was right in his surmise,
but who is to prove it? Can we decide even in the case of animals now
living whether the losses inflicted on a much persecuted species by an
abundant and greedy persecutor exceed the numbers of progeny, and are
therefore driving the species gradually towards extermination? Probable
as such a supposition appears, it cannot be accepted as proven.
Since in many cases of the extinction of great animal-groups we cannot
even prove that there was a simultaneous ascendancy of powerful
enemies, other factors must be discovered to which the apparently
sudden disappearance may be attributed. Many naturalists have tried
to guess at internal reasons for extinction, and have adopted the
theory--associated with the tendency to assume mystical principles of
evolution--that species in dying out are obeying an internal necessity,
as if their birth and death were predestined, as it is in the case of
multicellular individuals, as if there were a _physiological death of
the species_ as there is of the multicellular individual.
Neumayr showed, however, that the facts of palæontology afford no
support for this view. I need not repeat his arguments, but will simply
refer to his clear and concise exposition of the problem. It is obvious
that our theory of the extinction of species as due to external causes
cannot be rejected on the ground that our knowledge of the struggle
that species had to maintain for their existence in past times is
even mere imperfect than our knowledge of the struggle nowadays, and
that we are frequently unable to judge of it at all. But the facts of
geology are of value in another, quite different way. They reveal such
an extraordinary dissimilarity in the duration of species, and also
of the great groups of organisms, that the dissimilarity of itself
is sufficient to prevent our regarding the extinction of species as
regulated by _internal_ causes. Certain genera of Echinoderms, such as
starfish (_Astropecten_), lived in the Silurian times, and they are
represented nowadays in our seas by a number of species: and in the
same way the Cephalopod genus _Nautilus_ has maintained itself among
the living all through the enormous period from the Silurian sea to our
own day. Formerly the Nautilids formed a predatory horde that peopled
the seas, and, as we have seen, we may perhaps attribute to their
dominance the disappearance of an order of Crustaceans, the Trilobites,
which were equally abundant at that period. Now only a single species
of nautilus (_Nautilus pompilius_) lives on the coral reefs of the
southern seas. Similarly, the genus _Lingula_ of the nearly extinct
class of Brachiopods, somewhat mussel-like sessile marine animals, has
been preserved from the grey dawn of primitive times, with its records
in the oldest deposits, and is represented in the living world of
to-day by the so-called 'barnacle-goose' mussel, _Lingula anatina_.
On the other hand, we know of numerous species which lasted for quite
a short time, such as, for instance, the individual members of the
series of Steinheim _Planorbis_ species, or of the Slavonic _Paludina_
species. Not infrequently, too, genera make their appearance and
disappear again within the period of one and the same geological
stratum.
These facts not only tell against an unknown vitalistic principle of
evolution, but in general against the idea of the determination of
the great paths of evolution by purely internal causes. If there were
a principle of evolution the dissimilarity in the duration of life
could not be so excessive; if there were a 'senile stage' of species
and a natural death of species comparable to the natural death of
individuals, it would not have been possible for most of the Nautilidæ
to have been restricted to the Silurian epoch, and yet for one species
to have continued to live till now; and if there were a 'tendency'
of species to vary persistently onwards, and to 'become further and
further removed from the primitive type,' as has been maintained, then
such ancient and primitive Cephalopod forms like the _Nautilus_-species
could not have persisted until now, but must long ago have Wen
transmuted into higher forms. The converse, however, is conceivable
enough, namely, that the great mass of the species of a group such as
the Nautilidæ were crowded out by superior rivals in the struggle for
existence, but that certain species were able to survive on specially
protected or otherwise favoured areas. We have a fine example of this
in the few still living species of the otherwise extinct class of
Ganoid fishes. During the Primary and Secondary epochs these Ganoids
peopled all the seas, but at the boundary between the Cretaceous and
the Tertiary period they retrograded considerably, simultaneously with
the great development of bony fishes or Teleosteans, and now they are
only represented by a dozen species distributed over the earth, and
most of these are purely river forms, while the others at least ascend
the rivers during the spawning season to secure the safety of their
progeny. For the rivers are sheltered areas as compared with the seas,
and large fishes like the Ganoids will be able there to hold their
own in the struggle better than they could in the incomparably more
abundantly peopled sea.
Thus I can only regard it as playing with ideas to speak of birth,
blossoming, standstill, decay, and death of species in any other than a
figurative sense. Undoubtedly the life of the species may be compared
with that of the individual, and if the comparison be used only to make
clear the difference between the causes of the two kinds of phenomena,
there can be no objection to it, only we must beware of thinking we
have explained anything we do not know by comparing it with something
else that is also unknown.
We have already shown that the natural death of multicellular organisms
is a phenomenon which first made its appearance with the separation
of the organism into somatic or body cells and reproductive or germ
cells, and that death is not an inevitable Nemesis of every life, for
unicellular organisms do not necessarily die, though they may be killed
by violence. These unicellular organisms have thus no natural death,
and we have to explain its occurrence among multicellular organisms
as an adaptation to the cellular differentiation, which makes the
unlimited continuance of the life of the whole organism unnecessary and
purposeless, and even prejudicial to the continuance of the species.
For the species it is enough if the germ-cells alone retain the
potential immortality of the unicellulars, while, on the other hand,
the high differentiation of the somatic cells necessarily involves
that they should wear themselves away in the performance of their
functions, and so become subject to death, or at least that they should
undergo such changes that they are no longer capable of functioning
properly, so that thus the organism as a whole loses the power of life.
There can be no doubt whatever that death is virtually implied in
the very constitution of a multicellular organism, and is thus, so
to speak, a foreseen occurrence, the inevitable end of a development
which begins with the egg-cell and reaches its highest point with the
liberation of the germ-cells, that is, with reproduction, and then
enters on a longer or shorter period of decadence, leading to the
natural death of the individual.
It is only by straining the analogy that this course of development
can be compared with the origin and transformation or extinction of
species. Not even the entirely external analogy of the blossoming
from a small beginning and the subsequent decay is always correct;
for in the fresh-water snails of Steinheim, at any rate, almost the
whole of the members of the species underwent a transformation at a
particular time, and became a new species, which was after a long
time retransformed without any appreciable decrease in the number of
individuals being observable. To speak of a 'senile stage' of the
species, of a stiffening of its form, of an incapacity for further
transformation, is to indulge in a play of fancy quite inadmissible in
the domain of natural science.
It is admitted, however, that there is a correct idea at the base of
all this, for many species have not passed over into new forms, but
have simply died out because they were unable to adapt themselves
to changed conditions. This did not happen because they had become
incapable of variation, but because they could not produce variations
of sufficient magnitude, or variations of the kind required to enable
the species to remain an active competitor in the struggle for
existence.
It obviously depends upon the coincidence of manifold circumstances,
whether an adaptation can be successfully effected or not. Above all,
it must be able to keep pace with the changes in the conditions of
life, for if these advance at a more rapid rate the organisms will
succumb in the midst of the attempt at adaptation. It is probably
in this way that the striking disappearance of the Trilobites is to
be explained, as Neumayr has pointed out, for the Nautilidæ, a new
group of enemies, multiplied so quickly at their expense that they
had not time to evolve any effective means of protection. It cannot
be maintained for a moment that every species is able to protect
itself against extermination by any other; the increased fertility,
the increased rapidity of locomotion, the increased intelligence
and similar qualities, may all be insufficient, and then extinction
follows; not, however, because the species has become 'senile,' but
because the variations possible to its organization do not suffice to
maintain it in the struggle.
In discussing germinal selection I mentioned the view expressed
by Emery, that excessive variation in the same direction from
intra-germinal causes has not rarely been the cause of the extinction
of species. I also mentioned the very similar view of Döderlein,
who could not refer at that time to germinal selection, but assumed
internal compelling forces, which pressed a variation irresistibly
forward in the direction in which it had started, even beyond the
bounds of what is useful for the desired end, and which might thus
bring about the extinction of the species. I cannot entirely agree
with these views, as I have already indicated, because I do not
believe that the impulse to variation can ever become irresistible
and uncontrollable. If it could, then we should not see, as we do,
innumerable cases in which the augmentation or diminution of a part has
gone on precisely to the point at which it ceases to be purposeful.
Even the degeneration of organs only proceeds as far as is necessary
to accomplish a particular end, as we see plainly from the parasitic
Crustaceans of different orders. In many of these parasitic forms the
swimming legs degenerate, but in the female only, because these attach
themselves by suckers or in some other manner to their host, so that
they cannot let go again. But the males need their swimming legs to
seek out the females. The females too require them in their youth,
in order to seek out the fish from which they are to obtain their
food-supply, and thus the degeneration of the swimming legs has come to
a full stop exactly at the point where they cease to be of use; they
develop in early youth and degenerate later, when the animal becomes
sessile. In accordance with the law of biogenesis we may say that
while the degeneration is complete in the final stages of ontogeny,
its retrogression was not continued back to the germ, but only to the
young stages. From this it follows that the progress of a variation
may at any time have a goal fixed for it, and we have seen that this
is possible by means of personal selection, which accumulates the
never-failing fluctuations of the variation in the direction of plus or
of minus. In the individual id a determinant _X_ may perhaps decrease
and possibly also increase without limit, although we have no certain
knowledge in regard to the latter point, but as this determinant is
contained in all the ids, there are always plus and minus fluctuations
by means of which personal selection can operate.
But of course it requires a certain amount of time for this, and in
the fact that this time is often not available lies, I think, the
reason why excessive differentiations have often led to the extinction
of a species, not because the increase of the excessive organ must go
on irresistibly, but because changes in the conditions have made the
exuberant organ inappropriate, and it could not degenerate quickly
enough to save the species from extinction.
Brandes has recently given a beautiful illustration of this by
associating the existence of the remarkable sabre-toothed tigers
(_Machairodus_) with enormously long canine teeth, which lived in the
Diluvial period in South America, with the gigantic Armadillos which
lived there at the same time, whose bony armature two yards in height
now excites our admiration. He rightly points out that the dentition of
_Machairodus neogæus_ is by no means a typically perfect dentition for
a beast of prey, like that of the Indian tiger or the lion; as far as
incisors and molars are concerned it was much less effective than that
of these predatory animals, and the great length of the dagger-like
flattened canines, which protruded far beyond the mouth, entirely
prevented the bringing together of the teeth of the upper and lower jaw
after the fashion of a pair of pincers. He rightly infers from this
that this dentition was adapted to a specialized mode of nutrition, and
he regards the great mailed Armadillos, such as the three-yards-long
heavy Glyptodont of the Pampas, as the victims into which they were
wont to thrust their sabre-teeth in the region of the unprotected
neck, and thus to master the almost invulnerable creature, which was
invincible as far as all other predatory animals were concerned. Thus
the remarkable dentition is explained on the one hand, and on the other
the amazing extent and hardness of the victim's coat of mail. Thus,
too, we can understand why there should have been at that time a whole
series of cat-like animals with sabre-like teeth, in which the length
and sharpness of the teeth increased with the bodily size, for these
predatory animals corresponded to a whole series of Armadillos, whose
size was increasing, as was also the strength of their armour.
Of course this interpretation is hypothetical, but it contains much
internal probability, so that it may be taken as a good illustration
of the reciprocal increase of adaptations between two animal groups.
We understand now why, on the one hand, this colossal tortoise-like
armour should have developed in a mammal, and, on the other hand,
why these enormously long sabre-teeth should have been evolved; we
also understand--and this is the point with which we are here chiefly
concerned--why these two 'excessive' developments should ultimately
lead to the destruction of their possessors. For a long period
the Armadillos were able to save themselves from extermination by
increasing their bodily size and the strength of their armour, and
they thus saved themselves from persecution on the part of beasts of
prey with smaller and weaker teeth. But the predatory animals followed
suit and lengthened their teeth and increased their bodily size,
until ultimately even the strongest armour of the victim afforded
no efficient protection, and the mighty Glyptodonts were by degrees
utterly exterminated. But then the death-knell of the _Machairodus_
had also sounded, for he was so exactly adapted to this one kind of
diet that he could no longer overpower other victims and feed on their
flesh; the sabre-teeth prevented him from tearing his prey like other
predatory animals, he could probably only suck them.
Even if this is a supposititious case, it serves to show that it was
not an internal principle of variation that caused the teeth of these
carnivores and the armour of their victims to increase so unlimitedly;
it was the necessity of adaptation. They did not perish because armour
and teeth increased so excessively, but because neither of these
adaptations could be neutralized all at once, and small variations were
of no use to them in their final struggle for survival.
In a certain sense we may say that simpler, more lowly organisms are
more capable of adaptation than those which are highly differentiated
and adapted to specialized conditions in all parts of their bodies,
since from the former much that is new may arise in the course of time,
while very little and nothing very novel can spring from the latter.
From the simplest Protozoan the whole world of unicellular organisms
could arise, and also the much more diverse Metazoa; from the lower
marine worms there could arise not only many kinds of higher marine
worms--the segmented worms or Annelids--but also quite new groups of
animals, the Arthropods and the Vertebrates. It is hardly likely that a
new class of animals will evolve from our modern birds, because these
are already so perfectly adapted to their aerial life that they could
hardly adapt themselves to life on land or in the water sufficiently
well to be able to hold their own in regard to all the possibilities of
life with the rest of the dwellers on land or in water. We do indeed
know of birds which have returned entirely to a purely terrestrial
life--the ostriches, for instance--and of others which have adapted
themselves to a purely aquatic life, such as the penguins, but these
are small groups of species, and are hardly likely to increase. On
the contrary, we can prove that many have already succumbed in the
struggle with man, and we anticipate the extermination of others. But
the reason why they are so readily exterminated obviously lies in the
fact that they have surrendered the advantage given to them by their
bird-nature, by adapting themselves to terrestrial life, and that they
are not able to regain it, at least not in the short time that is at
their disposal if they are to be saved from extermination. The best
example of this is the Dodo (_Didus ineptus_). This remarkable-looking
bird, of about the size of a swan, lived in flocks upon the island of
Mauritius until about the end of the seventeenth century. It had small
wings with short quills which were useless for flight. As it could
neither escape by flight nor through the water, and could only move
clumsily and awkwardly upon land with its short legs and heavy body, it
was hopelessly doomed as soon as a stronger enemy made his appearance.
It fell a victim to the sailors who first landed on the island and
clubbed it with sticks in huge numbers. Until that event it was without
doubt excellently adapted to life on that fertile island, for on a
volcanic island in the middle of the ocean there were no large enemies,
and it was therefore not dependent on the power of flight for safety,
and could pick up abundant food from the ground. But when man suddenly
appeared on the scene and began to persecute it, it was not the 'senile
rigidity' of its organization that prevented it from making use of
its wings again; it was the slowness of variation and consequently
of selection, which is common to all species, which impelled it to
extinction. The same fate will probably overtake the Kiwi of New
Zealand (_Apteryx australis_) in the near future, for though it has
so far escaped the arrows of the aborigines, it is not likely in its
wingless condition to be able to hold out long against European guns,
unless close times and preserved forests are instituted for it, as they
have been for our chamois.
Even sadder from the biologist's point of view than such extermination
of individual species through the vandalism and greed of our own
race is the disturbance of whole societies of animals and plants by
man that is going on or has been accomplished on most of the oceanic
islands, and we must briefly notice these cases while we are dealing
with the decadence of species. I refer to the crowding out of the
usually endemic animal and plant population on such islands through
cultivation. The first step in this work of 'cultivation' is always the
cutting down of the forests which for thousands of years have clothed
these islands as with a mantle of green, have regulated their rainfall,
secured their fertility, and allowed a medley of indigenous animals,
usually peculiar to the spot, to arise. We have already spoken of St.
Helena. The original and remarkable fauna and flora of this island had
for the most part disappeared 200 years ago, through the cutting down
of trees in the forests, and these were later wholly destroyed by the
introduction of goats, which devoured all the young trees as they grew.
But with the forests most of the indigenous insects and birds were
doomed to destruction, so that now there is not an indigenous bird or
butterfly to be found there; only a few terrestrial snails and beetles
of the original fauna still survive.
But it is not only on islands that a large number of species have
been decimated or entirely exterminated by deforestation, by the
introduction of plants cultivated by man and of the 'weeds' associated
with these, and by the importation of domesticated animals. In Central
Europe not only have all the larger beasts of prey, like the bear, the
lynx, and the wolf, almost completely disappeared, but the reindeer,
the bison, the wild ox (Aurochs), and the elk have been exterminated
as wild animals, and in North America the buffalo will soon only exist
in preserved herds, if that is not already the case. Here, of course,
the direct interference of the all-too-powerful enemy, man, has played
the largest part in causing the disappearance of the species referred
to, but the process may give us an idea of the way in which a superior
animal enemy may be able gradually to exterminate a weaker species
where there is no attainable or even conceivable variation which
might preserve them from such a fate. Several of the mammals which I
have mentioned are not yet entirely exterminated; even the Aurochs
perhaps still exists in the pure white herds preserved in some British
parks; but there are more instances than that of the Dodo of the utter
extermination of a species through human agency within historic times.
It may be doubtful whether the sea-otter (_Enhydris marina_) has not
been already quite exterminated because of its precious fur, but it is
quite certain that the huge sea-cow (_Rhytina stelleri_), which lived
in large numbers in the Behring Straits at the end of the eighteenth
and the beginning of the nineteenth centuries, was completely
exterminated by sailors within a few decades.
We may therefore gain from what is going on before our eyes, so to
speak, some sort of idea of the way in which the extermination of
species may go on even independently of man at the present time, and
how it must have gone on also in past ages of the earth's history.
Migrations of species have taken place ceaselessly, although very
slowly, for every species is endeavouring slowly to extend its range
and to take possession of new territories, and thus the fauna and
flora of any region must have changed in the course of time, new
species must have settled in it from time to time, and the conditions
of life must have changed, and in many cases this must have led to the
extermination of species, in the same way, though not so quickly, as
human interference now brings about their doom.
This is true for plants as for animals. A good example, not indeed of
complete extermination, but of very considerable diminution in the
numbers of individuals of a plant-species by the advent of a species of
mammal, is communicated to us by Chun in regard to Kerguelen Land. A
flowering plant, the Kerguelen cabbage (_Pringlea antiscorbutica_), has
been greatly reduced in numbers since the thoughtless introduction of
rabbits to this uninhabited island (1874). While, in 1840, Captain Ross
used this plant in great quantities as a preventative against scurvy in
his crew, and even carried away stores to last for months, the Valdivia
Expedition in 1898 found rabbits in abundance, but the Kerguelen
cabbage had been entirely exterminated at every spot accessible to
these prolific and voracious rodents. It was only found growing upon
perpendicular cliffs or upon the islands lying out in the fiords.
An avoidance of the threatened destruction of a species by its
adaptation to the new circumstances can only be possible when the
changes occur very slowly, and will therefore be more likely to be
achieved in the case of physical changes in the conditions of life,
such as climatic changes, a change in the mutual relations of land
and sea, and so on. But it appears that even climatic changes do
not evoke any variation and new adaptation as long as the species
can avoid the changes by migrating. The often quoted case of Alpine
and Arctic plants proves this at any rate, that those species which
inhabited the plateaus and highlands of Europe did not all vary to
suit the change when a warmer climate prevailed, but that in part at
least they followed the climate to which they were already adapted,
that is, that they migrated towards the north on the one hand and
higher up the Alps on the other. It cannot be denied that many of the
insects and plants did adapt themselves at that time to the warmer
climate, and became the modern species which now inhabit the plains,
for many related species occur on the Alps and in the plains, but
apparently many others simply made their escape from a climate which
no longer suited their requirements. Thus, as far as I am aware, there
is no species of _Primula_ in South or Central Germany which could be
derived from the beautiful red _Primula farinosa_ of the Alps, but this
species occurs also upon the old glacier-soil at the northern base of
the Alps, and in similar soil again in the north of Germany and on the
meadows of Holstein. Similar examples might be cited in regard to the
Alpine-Arctic butterflies.
It is intelligible enough that we are still very far from being able
to give a precise account of the main changes in the plant and animal
world during the history of the earth in regard to the special causes
which have produced them. Possibly the future will throw more light
upon this subject by extending our knowledge of the fossil remains
of all countries. But so much at least we can say at present, that
there is no reason to refer the dying out of the earlier forms to
anything else than the changes in the conditions of life, the struggle
for existence, and the limitation of the power of transformation and
adaptation due to the organization which the species has already
attained; there is no trace of any such thing as a phyletic principle
of life in the vitalistic sense, as far as the decadence of species is
concerned.
LECTURE XXXVI
SPONTANEOUS GENERATION AND EVOLUTION: CONCLUSION
Spontaneous generation--Experimental tests impossible--Only the
lowest and smallest forms of life can be referred to spontaneous
generation--Chemical postulates for spontaneous generation--Empedocles
modernized--The locality of spontaneous generation--Progress of
organization--Direct and indirect influences causing variation--The
various modes of selection--Everything depends upon selection--Sinking
from heights of organization already attained--Paths of evolution--The
forces effecting it--Plasticity of living matter--Predetermination
of the animate world--Many-sided adaptation of each group--Aquatic
mammals and insects, parasites--Nägeli's variation in a definite
direction--Analogy of the traveller--Genealogical trees--The
diversity of forms of life is unlimited--The origin of the purposeful
apart from purposive forces working towards an end--The limits of
knowledge--Limitation of the human intelligence by selection--Human
genius--Conclusion.
We have now reached the end of our studies, and they have given us
satisfaction, at least in so far that they have brought us certainty
in regard to the chief and fundamental question which can be asked in
reference to the origin of the modern animate world of organisms. There
remains no doubt in our minds that the theory of descent is justified;
we know, just as surely as that the earth goes round the sun, that the
living world upon our earth was not created all at once and in the
state in which we know it, but that it has gradually evolved through
what, to our human estimate, seem enormously long periods of time.
This conclusion is now firmly established and will never again become
doubtful. The assumption, too, that the more lowly organisms formed
the beginnings of life, and that an ascent has taken place from the
lowest to the higher and highest, has become to our minds a probability
verging upon certainty. But there remains one point which we have not
yet touched upon--the problem of the origin of these first organisms.
There are only two possibilities: either that they have been borne to
our earth from outside, from somewhere else in the universe, or that
they have originated upon our earth itself through what is called
'spontaneous generation'--_generatio spontanea_.
The idea that very lowly living organisms might have been concealed
within the clefts and crevices of meteorites, and might thus have
fallen upon our earth and so have formed the first germs of life, was
first formulated by that chemical genius, Justus Liebig. It seems
certain that the state of glowing heat in which meteorites are, when
they come into our atmosphere, only affects the outer crust of these
cosmic fragments, and that living germs, which might be concealed in
the depths of their crevices and fissures, might therefore remain
alive, but nevertheless it is undoubtedly impossible that any germ
should reach us alive in this way, because it could neither endure
the excessive cold nor the absolute desiccation to which it would be
exposed in cosmic space, which contains absolutely no water. This could
not be endured even for a few days, much less for immeasurable periods
of time.
But we have to take account, too, of an entirely general reason,
which lies in the fact that all life is transient, that it can be
annihilated, and is not merely mortal! Everything that is distinctively
organic may be destroyed to the extent of becoming inorganic. Not
only may the phenomena of life disappear, and the living body as such
cease to be, but the organic compounds which form the physical basis
of all life are ceaselessly breaking up, and they fall back by stages
to the level of the inorganic. It seems to me that we must necessarily
conclude from this that the basis of Liebig's idea was incorrect, that
is, the assumption that 'organic substances are everlasting and have
existed from the first just in the same way as inorganic substances.'
This is obviously not the case, for a thing that has an end cannot be
everlasting; it must have had a beginning too, and consequently organic
combinations are not everlasting, but are transitory; they come and
go, they arise wherever the conditions suitable for them occur, and
they break up into simpler combinations when these conditions cease
to be present. It is only the elements which are eternal, not their
combinations, for these are subject to more or less rapid continual
change, whether they have arisen outside of organisms or within them.
It seems to me that these considerations destroy the foundations of
the hypothesis of the cosmic origin of life on our earth; in any case
they leave the hypothesis without great significance; for if we could
even admit the possibility of a transference of living organisms from
space, the question would only be pushed a little further back by the
assumption, and not solved, for the organisms thus brought in must have
had their origin on some other planet, since they are, _ex hypothesi_,
not everlasting.
Thus we are directed to our earth itself as the place of the origin
of the tellurian world of life, and I see no possibility of avoiding
the assumption of _spontaneous generation_. It is for me a logical
necessity.
Even about the middle of the nineteenth century there was acute
discussion in regard to the occurrence of spontaneous generation. In
the French Academy especially Pouchet brought forward arguments in
favour of it, and Pasteur against it. Pouchet observed that living
organisms made their appearance in infusions of hay and other vegetable
material in which any possible living germs had presumably been
destroyed by prolonged boiling. Living organisms, Algæ, and Infusorians
appeared, notwithstanding the fact that the glass bottles in which
they were kept were hermetically sealed. But Pasteur showed that the
air contains numerous living germs of lowly organisms in its so-called
motes, and that, if these were first removed, Pouchet's infusion would
not exhibit any signs of life. He caused the air, which was continually
passed through the tubes, to stream first along the heated barrel of
a gun, and so destroyed these germs, and no organisms were obtained
in the infusions. He showed that the air is teeming with germs by an
experiment with boiled infusions which were allowed to lie undisturbed
for a considerable time in bottles with open necks, one on the roof
of the Institute at Paris, the other on the top of the Puy de Dôme in
Auvergne, which was at that time still the highest mountain in France.
In the Parisian experiment, organisms appeared in the bottles in a very
few days, while in those exposed to the pure air at the mountain-top
none were seen, even after months had elapsed.
Strangely enough, these and similar experiments were at the time
regarded as conclusive proof against the existence of spontaneous
generation, though it is obvious enough that the first living being
on this earth cannot have sprung from hay, or from any other organic
substance, since that would presuppose what we are attempting to
explain. After the fiery earth had so far cooled that its outermost
layer had hardened to a firm crust, and after water had condensed to a
liquid form, there could at first only have been inorganic substances
in existence. In order to prove spontaneous generation, therefore,
it would be necessary to try to find out from what mingling of
inorganic combinations organisms could arise; to prove that spontaneous
generation could never have been possible is out of the question.
It would be impossible to prove by experiment that spontaneous
generation could _never_ have taken place; because each negative
experiment would only prove that life does not arise _under the
conditions of the experiment_. But this by no means excludes the
possibility that it might arise under other conditions.
Up till now all attempts to discover these conditions have been futile,
and I do not believe that they will ever be successful, not because the
conditions must be so peculiar in nature that we cannot reproduce them,
but above all, because we should not be able to perceive the results
of a successful experiment. I shall be able to prove this convincingly
without difficulty.
If we ask ourselves the question how the living beings which might have
arisen through spontaneous generation must be constituted, and on the
other hand, in regard to what kinds of living forms we can maintain
with certainty that they _could not_ have arisen thus, it is obvious
that we must place on the latter list all organisms which presuppose
the existence of others, from which they have been derived. But to
this category belong all the organisms which possess a germ-plasm,
an idioplasm that we conceive of as composed of primary constituents
(_Anlagen_) which have gradually been evolved and accumulated through
a long series of ancestors. Thus not only all multicellular animals
and plants which reproduce by means of germ-cells, buds, and so forth,
but also all unicellular organisms, must be placed in this class. For
these last--as we have seen--possess in their nucleus a substance made
up of primary constituents, without which the mutilated body is unable
to make good its loss, in short, an idioplasm. That this plays the same
rôle in unicellular as in multicellular organisms we can infer with
the greatest certainty from the process of amphimixis, which runs its
course in an analogous way in both cases.
Thus, even though we did not know what Ehrenberg demonstrated in the
third decade of last century, that Infusorians in an encapsuled state
can be blown about everywhere, and can even be carried across the
ocean in the dust of the trade-winds, to re-awaken to life wherever
they fall into fresh water, we should still not have remained at the
standpoint of Leuwenhoek, who regarded Infusorians as having arisen
through spontaneous generation. They cannot arise in this way, nor can
they have done so at any time, because they contain a substance made
up of primary constituents, which can only be of historic origin, and
cannot therefore have arisen suddenly after the manner of a chemical
combination.
The same is true of all the unicellular organisms, even of those
which are much more simple in structure than the Infusorians, whose
differentiation into cortical and medullary substances, oral and anal
openings, complex arrangements of cilia and much else, betokens a high
degree of differentiation in the cell. But even the Amœba is only
apparently simple, for otherwise it could not send out processes and
retract them again, creep in a particular direction, encyst itself,
and so on, for all this presupposes a differentiation of its particles
in different directions, and a definite arrangement of them; and
there is in addition the marvellous dividing-apparatus of the nucleus
which is not wanting even in the Amœba. All this again points to a
historic evolution, a gradual acquiring and an orderly arrangement of
differentiations, and such an organism cannot have arisen suddenly like
a crystal or a chemical combination.
Thus we are driven back to the lowest known organisms, and the question
now before us is whether these smallest living organisms, which are
only visible under the highest powers of the microscope, may be
referred to spontaneous generation. But here too the answer is, No; for
although there is no nucleus to be found, and no substance which we
can affirm with any certainty to be composed of primary constituents
or idioplasm, we do find distinct traces of a previous history, and
not the absolutely simple structure of homogeneous living particles,
unarranged in any orderly way, which is all that could be derived from
spontaneous generation. It has been shown quite recently that the
typhus bacillus possesses an extremely delicate much-branched tuft
of flagella, which gives it a tremulous motion, and in the cholera
bacillus cortical and medullary substances can be distinguished. Thus
even here there is differentiation according to the principle of
division of labour, and how numerous must be the minute vital particles
of which a substance consists when it can form such fine threads as the
flagella just mentioned! Nägeli, who elaborated an analogous train of
thought in regard to spontaneous generation, calculated the number of
these smallest vital particles (his 'micellæ') which must be contained
in a 'moneron' of 0.6 mm. diameter, if we take its dry substance at
10 per cent., and he arrived at the amazing figure of 100 billions of
vital particles. Even if we suppose the diameter of such an organism
to be 0.0006 mm., it would still be composed, according to this
calculation, of a million of these vital particles.
We have reached, in the course of these lectures, the conviction
that minute living units form the basis of all organisms, namely,
our 'life-bearers' or 'biophors.' These must be present in countless
multitudes, and in a great number of varieties in the different forms
of life, but all agree in this, that they are simple, that is, they are
not composed in their turn of living particles, but only of molecules,
whose chemical constitution, combination, and arrangement are such as
to give rise to the phenomena of life. But they may vary, and on this
power depends the possibility of their differentiation, which has
taken place in more and more diverse ways in the course of phylogeny.
They, too, arise in the existing organism, like all vital units, only
by multiplication of the biophors already present, but they do not
necessarily presuppose a historic origin; it is conceivable of them,
at least as far as their first and simplest forms are concerned, that
they may have arisen some time or other through spontaneous generation.
In regard to them alone is the possibility of origin through purely
chemico-physical causes, without the co-operation of life already
existing, admissible. It is only in regard to them that spontaneous
generation is not inconceivable.
We must, therefore, assume that, at some time or other in the history
of the earth, the conditions necessary to the development of these
invisible little living particles must have existed, and that the whole
subsequent development of the organic world must have depended upon an
aggregation of these biophors into larger complexes, and upon their
differentiation within these complexes.
We shall never be able, then, directly to observe spontaneous
generation, for the simple reason that the smallest and lowest living
particles which could arise through it, the Biophoridæ, are so
extremely far below the limits of visibility, that there is no hope
of our ever being able to perceive them, even if we should succeed in
producing them by spontaneous generation.
I do not propose to discuss the chemical problem raised by the possible
occurrence of spontaneous generation. We have already seen that dead
protoplasm, in addition to water, salts, phosphorus, sulphur, and some
other elements, chiefly and invariably contains albumen; an albuminoid
substance must, therefore, have arisen from inorganic combinations.
No one will maintain that this is impossible, for we continually see
albuminoid substances produced in plants from inorganic substances,
compounds of carbon and nitrogen; but under what conditions this would
be possible in free nature, that is, outside of organisms, cannot as
yet be determined. Possibly we may some time succeed in procuring
albumen from inorganic substances in the laboratory, and if that
happens the theory of spontaneous generation will rest upon a firmer
basis, but it will not have been experimentally proved even then.
For while dead albumen is certainly nearly allied to living matter,
it is precisely _life_ that it lacks, and as yet we do not know what
kinds of chemical difference prevail between the dead proteid and the
living; indeed we must honestly confess that it is a mere assumption
when we take for granted that there are only chemico-physical
differences between the two. It cannot be proved, in the meantime,
that there is not another unknown power in the living protoplasm,
a 'vitalistic principle,' a 'life-force,' on the activity of which
these specific phenomena of life, and particularly the continually
repeated alternation of disruption and reconstruction of the living
substance, dissimilation and assimilation, growth and multiplication,
depend. It is just as difficult to prove the converse, that it is
impossible that chemico-physical forces alone should have called forth
life in a chemical substance of very special composition. Although
no one has ever succeeded, in spite of many attempts, in thinking
out a combination of chemical substances which--as this wonderful
living substance does--on the one hand undergoes combustion with
oxygen and, on the other hand, renews itself again with 'nutritive'
material, yet we cannot infer from this the impossibility of a purely
chemico-physical basis of life, but must rather hold fast to it until
it is shown that it is not sufficient to explain the facts, thus
following the fundamental rule that natural science must not assume
unknown forces until the known ones are _proved_ insufficient. If
we were to do otherwise we should have to renounce all hope of ever
penetrating deeper into the phenomena. And we have no need to do
this, for in a general way we can quite well believe that an organic
substance of exactly proportioned composition exists, in which the
fundamental phenomena of all life--combustion with simultaneous
renewal--must take place under certain conditions by virtue of its
composition.
How, and under what external conditions, such a substance first arose
upon the earth, from and of what materials it was formed, cannot
be answered with any certainty in the meantime. Who knows whether
the fantastic ideas of Empedocles in an altered form would not be
justified here? I mean that, at the time of the first origin of
life, the conditions necessary for many kinds of complex chemical
combinations may have been present simultaneously on the earth, and
that, out of a manifold variety of such substances, only those survived
which possessed that marvellous composition which conditioned their
continual combustion, but also their ceaseless reconstruction by
multiplication. According to Empedocles, there arose from chaos only
parts of animals--heads without bodies, arms without trunks, eyes
without faces, and so on--and these whirled about in wild confusion
and flew together as chance directed them. But those only survived
which had united rightly with others so as to form a whole, capable of
life. Translated into the language of our time, that would mean what I
have just said--that, of a large number of organic combinations which
arose, only a few, perhaps one, would possess the marvellously adjusted
composition which resulted in life, and with it self-maintenance and
multiplication; and that would be the first instance of selection!
But let us leave these imaginings, and wait to see whether the chemists
will not possibly be able to furnish us with a starting-point for a
more concrete picture of the first origin of life. In the meantime, we
must confess that we find ourselves confronted with deep darkness.
The question as to the 'Where' of spontaneous generation must also be
left without any definite answer. Some have supposed that life began
in the depths of the sea, others on the shore, and others in the air.
But who is to divine this, when we cannot even name theoretically the
conditions and the materials out of which albuminoid-like substances
might be built up in the laboratory? Nägeli's hypothesis still seems
to me to have the greatest probability. According to his theory, the
first living particles originated not in a free mass of water, but in
the reticulated superficial layer of a fine porous substance (clay or
sand), where the molecular forces of solid, fluid, and gaseous bodies
were able to co-operate.
Only so much is certain, that wherever life may first have arisen upon
this earth, it can have done so only in the form of the very simple
and very minute vital units, which even now we only infer to be parts
of the living body, but which must first have arisen as independent
organisms, the 'Biophoridæ.' As these, according to our theory,
possessed the character of life, they must have possessed above all the
capacity of assimilating in the sense in which the plants assimilate,
that is, of renewing their bodily substance continually from inorganic
substances, of growing, and of reproducing. They need not on that
account have possessed the chemical constitution of chlorophyll,
although the capacity of assimilation in green plants depends upon this
substance, for we know colourless fungi, which, notwithstanding the
absence of chlorophyll, are able to build up the substance of their
body from compounds of carbon and nitrogen.
The first advance to a higher stage of life must have been brought
about by multiplication, since accumulations of Biophoridæ,
unintegrated but connected masses, would be formed.
In this way the threshold of microscopical visibility would gradually
be reached and crossed, but--to argue from the modern Baccilli--long
before that time a differentiation of the biophors on the principle
of division of labour would have taken place within a colony of
Biophoridæ. This first step towards higher organization must probably
have taken enormous periods of time, for before any differentiation
could occur and bring any advantage the unintegrated aggregates of
Biophors must first have become orderly, and have formed themselves
into a stable association with definite form and definite structure,
somewhat analogous to the spherical cell-colonies of _Magosphæra_ or
_Pandorina_. Only then was the further step made of a differentiation
of the individual biophors forming the colony, and this is comparable
to the species of _Volvox_ among the lower Algæ. The gradual ascent of
these colonies of biophors must, then, be referred to the principles
to which we attribute the ascent of the higher forms of life to
ever-higher and ever-new differentiations; the principles of division
of labour and selection.
These differentiated colonies of biophors have brought us nearer to the
lowest known organisms, among which there are some whose existence we
can only infer from their pathological effects, since we have not been
able to make them visible. The bacillus of measles has never yet been
seen, but we cannot doubt its existence, and we must assume that there
are bacilli of such exceeding smallness that we shall never be able
to see them, even with the most improved methods of staining and the
strongest lenses.
These non-nucleated Monera lead on to the stage of nucleus-formation,
and this at once implies the cell. As, on our view, the nucleus is
primarily a storehouse of 'primary constituents' (_Anlagen_), its
origin must have begun at the moment at which the differentiation of
the cell-body reached such a degree of differentiation of its parts
that a mechanical division into two halves was no longer possible, and
that the two products of division, if they were each to develop to
a new and intact whole, required a reserve of primordia (_Anlagen_)
to give rise to the missing parts. As this higher differentiation
would bring about a superiority over the lower forms of life, in that
they would make possible the utilization of new conditions of life,
but on the other hand could only survive if the differentiation of a
reserve of primary constituents, that is, a nucleus, were introduced
at the same time, the development of the nucleus can be ranged under
the principle of utility to which we traced back the evolution of all
higher and more differentiated forms of life. But it would scarcely
be profitable to try to follow out in detail the first steps in the
progress of organization under the control of selective processes,
since we know far too little about the life of the simplest organisms
to be able to judge how far their differentiations are of use in
improving their capacity for life.
That would be a bold undertaking even in regard to unicellular
organisms, and it is only in the case of multicellular organisms that
we can speak with greater certainty and really recognize the changing
of the external conditions, in the most general and comprehensive
sense, as the fundamental cause of the lasting variations of organic
forms. We can here distinguish with certainty between the direct and
the indirect effect of external influences, and we see how these
sources of variation interact upon each other. The lowest and deepest
root of variation is without doubt the direct effect of changed
conditions. Without this the indirect effect would have had no lever
with which to work, for the primitive beginnings of variation would be
absent, and an accumulation of these through personal selection could
not take place. It is a primitive character of living substance to be
variable, that is, to be able to respond to some extent to changed
external conditions, and to vary in accordance with them, or--as we
might also say--to be able to exist in many very similar but not
identical combinations of substances, and we must imagine that even
the first biophors which arose through spontaneous generation were
different according to the conditions under which, and the substances
from which, they originated. And from each of these slightly different
beginnings there must, in the course of multiplication by fission,
have been produced a whole genealogical tree of divergent variations
of the primitive Biophoridæ, since it is inconceivable that all the
descendants would remain constantly under the same conditions of
life under which they originated. For every persistent change in
the conditions of existence, and especially of nutrition, must have
involved a variation in the constitution of the organism, whose vital
processes, and especially the repair of its body, depended on these
conditions.
But the external influences to which the descendants of a particular
form of life were subject never remained permanently the same. Not
only did the surface of the earth and its climatic conditions change
in the course of time with the cooling of the earth, but mountains
arose and were levelled again, old land-surfaces sank out of sight or
emerged again, and so on; all that, of course, played its part in the
transformation of the forms of life, but did so to any considerable
extent only at a later stage, when there were already highly
differentiated organisms. These unknown primitive beginnings of life
must have been forced to diverge into different variations through the
different conditions of the same place in which they lived.
Let us think of the simplest microscopic Monera on the mud of the
sea-coast, equipped with the faculty of plant-like assimilation, and we
shall see that their unlimited multiplication would cause differences
in nutrition, for those lying uppermost would be in a stronger light
than those below, and would, therefore, be better nourished, and,
consequently, would transmit the variations thus induced to their
progeny which arose by fission. Thus it is conceivable that even the
more or less favourable position as regards light would bring about the
origin of two different races from the same parent form, and as it is
conceivable in the case of light, so is it also in regard to all the
influences which cause variation in the organism.
We have already seen that variations in the lowest (non-nucleated)
forms of life caused by the direct influence of the vital processes
may be directly transmitted to the descendants, but that in all
those whose bodies have already differentiated into a germ- or
idioplasmic-substance, in contrast to a somatic substance in the more
restricted sense, this hereditary transmission is only possible in the
case of the variations of the germ-plasm, and _hereditary_ variations
of the species can only arise by the circuitous route of influencing
the germ-plasm. The body (soma) can be caused to change by external
influences, by the use or disuse of an organ, but variations of this
kind are not transmitted; they do not become a lasting possession of
the species, but cease with the individual; they are transient changes.
Thus it was only through those external influences--including
those from the soma of the organism itself--which affected the
germ-substance, either as a whole or in certain of its primary
constituents, that hereditarily transmissible variations of the
organism arose, and we have already discussed in detail how particular
variational tendencies may arise through the struggle of the parts
within the germ-plasm, which may give an advantage to certain groups of
primary constituents. And these tendencies are of themselves sufficient
to cause the specific type to vary further and further in given
directions.
Nevertheless, the infinite diversity of the forms of life could never
have been brought about in this way alone, if there had not been
another--the _indirect_--effect of the changeful external influences.
This is due to the fact that the variations of direct origin sooner
or later obtain an influence in determining the viability of their
possessors, either increasing or diminishing it. It is this, in
association with the unlimited multiplication of individuals, which
gives a basis to the principle of transformation, which it is the
immortal merit of Charles Darwin and Alfred Russel Wallace to have
introduced into science: _the principle of selection_. We have seen
that this principle may have a much more comprehensive meaning than
was attributed to it by either of these two naturalists; that there
is not merely a struggle between individuals which brings about their
adaptation to their environment, by preserving those which vary in
the most favourable way and rejecting those which vary unfavourably,
but that there is an analogous struggle between the parts of these
individuals, which, as Wilhelm Roux showed, effects the adaptation
of the parts to their functions, and that this struggle must be
assumed to occur even between the determinants and biophors of the
germ-plasm. There is thus a germinal selection, a competition between
the smaller and larger particles of the germ-plasm for space and
food, and that it is through this struggle that there arise those
definitely and purposefully directed variations of the individual,
which are transmissible because they have their seat in the immortal
germ-plasm, and without which an adaptation of individuals in the sense
and to the extent in which we actually observe it would be altogether
inconceivable. I have endeavoured to show that the whole evolution
of the living world is guided essentially by processes of selection,
in as far as adaptations of the parts to one another, and of the
whole to the conditions of life, cannot be conceived of as possible
except through these, and that all fluctuations of the organism,
from the very lowest up to the highest, are forced into particular
paths by this principle, by 'the survival of the fittest.' This ends
the whole dispute as to whether there are indifferent 'characters'
which have no influence on the existence of the species, for even the
characters most indifferent for the 'person' would not exist unless
the germinal constituents (determinants) which condition them had
been victorious in the struggle for existence over others of their
kind, and even the 'indifferent' characters, which depend solely
upon climatic or other external influences, owe their existence to
processes of germinal selection, for those elements of the determinants
concerned were victorious which throve best under such influences.
But should variations thus produced by external influence increase so
far that they become prejudicial to the survival of their bearers,
then they are either set aside by personal selection or, if that be
no longer possible, they lead to the extinction of the species. Thus
the multitude of small individual variations, which probably occur in
every species, but which strike us most in Man--the differences in the
development of mouth, nose, and eyes, in the hair, in the colour of
skin, &c., as far as they are without significance in the struggle for
existence--depend upon processes of germinal selection, which permitted
the greater development of one group of determinants, or of one kind of
biophor in one case, of another in another. The proportionate strength
of the elements of the germ-plasm is not readily lost at once, but is
handed on to successive generations, and thus even these 'indifferent'
characters are transmitted.
It is obvious that, if the principle of selection operates in nature
at all, it must do so wherever living units struggle together for the
same requirements of life, for space and food, and these units need not
be persons, but may represent every category of vital units, from the
smallest invisible units up to the largest. For in all these cases the
conditions of the selection-process are given: individual variability,
nutrition, and multiplication, transmission of the advantage
attained, and, on the other hand, limitation of the conditions of
existence--especially food and space. The resulting struggle for
existence must, in every category of vital units, be most acute
between the individual members of each category, as Darwin emphasized
in the case of species from the very first, and persistent variations
of a species of living units can only be brought about by this kind
of struggle. Strictly speaking, therefore, we should distinguish as
many kinds of selection-processes as there are categories of living
units, and these could not be sharply separated from one another,
apart from the fact that we have to infer many of them, and cannot
recognize their gradations. Here, as everywhere else, we must break up
the continuity of nature into artificial groups, and it seems best to
assume and distinguish between four main grades of selective processes
corresponding to the most outstanding and significant categories of
vital units, namely: Germinal, Histonal, Personal, and Cormal Selection.
Histonal Selection includes all the processes of selection which take
place between the elements of the body (soma), as distinguished from
the germ-plasm, of the Metazoa and Metaphyta, not only between the
'tissues' in the stricter sense, but also between the parts of the
tissues, that is, the lower vital units of which they are composed, and
which Wilhelm Roux, when he published his _Kampf der Teile_ ('Struggle
of the Parts'), called 'molecules.' It occurs between all the parts
of the tissues down to the lowest vital units, the biophors. We must
also reckon under histonal selection the processes of selection
which take place between the elements of the simplest organisms, and
through which these have gradually attained to greater complexity of
structure and increased functional capacity. As long as no special
hereditary substance had been differentiated, variations which arose
in the simplest organisms through selection-processes of this kind
were necessarily transmitted to the descendants, but after this
differentiation had taken place this could no longer occur--'acquired'
modifications of the soma were no longer transmitted, and the
importance of histonal selection was limited to the individual. But
this form of selection must be of the greatest importance in regard to
the adaptations of the parts which develop from the ovum, especially
during the course of development, and it is also indispensable all
through life in maintaining the equilibrium of the parts, and their
adaptation to the varying degree of function required from them (use
and disuse). But its influence does not reach directly beyond the
life of the individual, since it can only give rise to 'transient'
modifications, that is, to changes which cease with the individual life.
In contrast to this is Germinal Selection, which depends upon the
struggle of the parts of the germ-plasm, and thus only occurs in
organisms with differentiation of somatoplasm and germ-plasm,
especially in all Metazoa and Metaphyta--forming in these the basis of
all hereditary variations. But not every individual variation to which
germinal selection gives rise persists and spreads gradually over the
whole species, for, apart from the cases we have already mentioned, in
which indifferent variations favoured by external circumstances gain
the victory, this happens only if the variations in question are of use
to their bearer, the individual. Any variation whatever may arise in a
particular individual purely through germinal selection, but it is only
the higher form of selection--Personal Selection--that decides whether
the variation is to persist and to spread to many descendants so that
it ultimately becomes the common property of the species. Germinal and
personal selection are thus continually interacting, so that germinal
selection continually presents hereditary variations, and personal
selection rejects those that are detrimental and accepts those that
are useful. I will not repeat any exposition of the marvellous way in
which personal selection reacts upon germinal selection, and prevents
it from continuing to offer unfavourable variations, and compels it to
give rise to what is favourable in ever-increasing potency. Although
it apparently selects only the best-adapted _persons_ for breeding,
it really selects the favourable id-combinations of the germ-plasm,
that is, those which contain the greatest number of favourably varying
determinants. We saw that this depends upon the multiplicity of ids
in the germ-plasm, since every primary constituent of the body is
represented in the germ-plasm, not once only, but many times, and it is
always half of the homologous determinants contained in the germ-plasm
of an individual which reach each of its germ-cells, always, moreover,
in a different combination. Thus, with the rejection of an individual
by personal selection, a particular combination of ids, a particular
kind of germ-plasm is in reality removed, and thus prevented from
having any further influence upon the evolution of the species. By
this means germinal selection itself is ultimately influenced, because
only those ids remain unrejected in the germ-plasm whose determinants
are varying in directions useful to the species. Thus there comes about
what until recently was believed to be impossible: the conditions
of life give rise to useful directions of variation, not directly,
certainly, but indirectly.
We may distinguish as a fourth grade of selection Cormal Selection,
that is, the process of selection which effects the adaptation of
animal and plant stocks or corms, and which depends on the struggle of
the colonies among themselves. This differs from personal selection
only in that it decides, not the fitness of the individual person, but
that of the stock as a whole. It is a matter of indifference whether
the stocks concerned are stocks in the actual material sense, or only
in the metaphorical sense of sharing the common life of a large family
separated by division of labour. In both cases, in the polyp-stock as
well as in the termite or ant-colony, the collective germ-plasm, with
all its different personal forms, is what is rejected or accepted. The
distinction between this cormal selection and personal selection is,
therefore, no very deep one, because here too it is in the long run the
two sexual animals which are selected, not indeed only in reference
to their visible features, but also in reference to their invisible
characters, those, namely, which determine in their germ-plasm the
constitution of their neuter progeny or, in the case of polyps, their
asexually reproducing descendants.
We venture to maintain that everything in the world of organisms that
has permanence and significance depends upon adaptation, and has arisen
through a sifting of the variations which presented themselves, that
is, through selection. Everything is adaptation, from the smallest
and simplest up to the largest and most complex, for if it were not
it could not endure, but would perish. The principle which Empedocles
announced, in his own peculiar and fantastic way, is the dominating
one, and I must insist upon what has so often been objected to as an
exaggeration--that everything depends upon adaptation and is governed
by processes of selection. From the first beginnings of life, up to
its highest point, only what is purposeful has arisen, because the
living units at every grade are continually being sifted according to
their utility, and the ceaseless struggle for existence is continually
producing and favouring the fittest. Upon this depends not only the
infinite diversity of the forms of life, but also, and chiefly, the
closely associated progress of organization.
It cannot be proved in regard to each individual case, but it can be
shown in the main that attaining a higher stage in organization also
implies a predominance in the struggle for existence, because it opens
up new possibilities of life, adaptations to situations not previously
utilizable, sources of food, or places of refuge. Thus a number of the
lower vertebrates ascended from the water to the land, and adapted
themselves to life on dry land or in the air, first as clumsily moving
salamanders, but later as actively leaping frogs; thus, too, other
descendants of the fishes gained a sufficient carrying power of limbs
to raise the lightened body from the ground, and so attained to the
rapid walk of the lizards, the lightning-like leaps of the arboreal
agamas, the brief swooping of the flying-dragons, and ultimately
the continuous flight which we find in the flying Saurians and the
primitive birds of the Jurassic period, and in the birds and bats of
our own day.
It is obvious that each of these groups, as it originated, conquered
a new domain of life, and in many cases this was such a vast one, and
contained so many special possibilities, that numerous subordinate
adaptations took place, and the group broke up into many species and
genera, even into families and orders. All this did not come about
because of some definitely directed principle of evolution of a
mysterious nature, which impelled them to vary in this direction and in
no other, but solely through the rivalry of all the forms of life and
living units, with their enormous and ceaseless multiplication, in the
struggle for existence. They were, and they are still, forced to adapt
themselves to every new possibility of life attainable to them; they
are able to do this because of the power of the lowest vital units of
the germ to develop numerous variations; and they are obliged to do it
because, of the endless number of descendants from every grade of vital
unit, it is only the fittest which survive.
Thus higher types branched off from the lower from time to time,
although the parent type did not necessarily disappear; indeed it
could not have disappeared as long as the conditions of its life
endured; it was only the superfluous members of the parent form that
adapted themselves to new conditions, and as, in many cases, these
required a higher organization, there arose a semblance of general
upward development which simulated a principle of evolution always
upwards. But we know that, at many points on this long road, there
were stations where individual groups stopped short and dropped back
again to lower stages of organization. This kind of retreat was almost
invariably caused by a parasitic habit of life, and in many cases this
degeneration has gone so far that it is difficult to recognize the
relationship of the parasite to the free-living ancestors and nearest
relatives. Many parasitic Crustaceans, such as the Rhizocephalids,
lack almost all the typical characteristics of the crustacean body,
and dispense not only with segmentation, with head and limbs, but also
with stomach and intestine. As we have seen, they feed like the lower
fungi, by sucking up the juices of their hosts, by means of root-like
outgrowths from the place where the mouth used to be. Nevertheless,
their relationship to the Cirrhipedes can be proved from their larval
stages. There are, however, parasites in the kidneys of cuttlefish--the
Dicyemidæ--in regard to which naturalists are even now undecided
whether they ought to form a lowly class by themselves between
unicellular animals and Metazoa, or whether they have degenerated, by
reason of their parasitism, from the flat worms to a simplicity of
structure elsewhere unknown. They consist only of a few external cells,
which enclose a single large internal cell, possess no organs of any
kind, neither mouth nor intestine, neither nervous system nor special
reproductive organs. But although degeneration cannot be proved in this
case, it can be in hundreds of other cases with absolute certainty,
as, for instance, in the Crustacea belonging to the order of Copepods,
which are parasitic upon fishes, in which we find all possible stages
of degeneration, according to the degree of parasitism, that is, to
the greater or less degree of dependence upon the host; for organs
degenerate and disappear in exact proportion to the need for them, and
they thus show us that degeneration also is under the domination of
adaptation.
Thus retrogressive evolution also is based upon the power of the living
units to respond to changing influences by variation, and upon the
survival of the fittest.
The roots of all the transformations of organisms, then, lie in changes
of external conditions. Let us suppose for a moment that these might
have remained absolutely alike from the epoch of spontaneous generation
onwards, then no variation of any kind and no evolution would have
taken place. But as this is inconceivable, since even the mere growth
of the first living substance must have exposed the different kinds
of biophors composing it to different influences, variation was
inevitable, and so also was its result--the evolution of an animate
world of organisms.
External influences had a twofold effect at every stage upon every
grade of vital unit, namely, that of directly causing variation and
that of selecting or eliminating. Not only the biophors, but every
stage of their combinations, the histological elements, chlorophyll
bodies, muscle-disks, cells, organs, individuals, and colonies, can
not only be caused to vary by the external influences to which they
are subjected, but can be guided by these along particular paths of
variation, so that among the variations which crop up some are better
adapted to the conditions than others, and these thrive better, and
thus alone form the basis of further evolution. In this way definite
tendencies of evolution are produced, which do not move blindly and
rigidly onwards like a locomotive which is bound once for all to the
railroad, but rather in exact response to the external conditions,
like an untrammelled pedestrian who makes his way, over hill and dale,
wherever it suits him best.
The ultimate forces operative in bringing about this many-sided
evolution are the known--and although we do not recognize it as yet,
perhaps the unknown--chemico-physical forces which certainly work only
according to laws; and that they are able to accomplish such marvellous
results is due to the fact that they are associated in peculiar and
often very complex different kinds of combinations, and thus conform
to the same sort of regulated arrangements as those which condition
the operations of any machine made by man. All complex effects depend
upon a co-operation of forces. This is seen, to begin with, in the
chemical combinations whose characters depend entirely upon the number
and arrangement of the elementary substances of which they consist; the
atoms of carbon, hydrogen, and oxygen, which compose sugar, can also
combine to form carbonic acid gas and water, or alcohol and carbonic
acid gas; and the same thing is true if we ascend from the most complex
but still inanimate organic molecules to those chemical combinations
which, in a still higher form, condition the phenomena of life, to the
lowest living units, the biophors. Not only do these last differ in
having life, but they themselves may appear in numerous combinations,
and can combine among themselves to form higher units, whose characters
and effectiveness will depend upon these combinations. Just as man may
adjust various metallic structures, such as wheels, plates, cylinders,
and mainsprings in the combination which we call a watch, and which
measures time for us, so the biophors of different kinds in the living
body may form combinations of a second, third, &c., degree, which
perform the different functions essential to life, and by virtue of
their specific, definite combination of elementary forces.
But if it be asked, what replaces human intelligence in these
purposeful combinations of primary forces, we can only answer that
there is here a self-regulation depending upon the characters of the
primary vital parts, and this means that these last are caused to vary
by external influences and are selected by external influences, that
is, are chosen for survival or excluded from it. Thus combinations of
living units must always result which are appropriate to the situation
at the moment, for no others can survive, although, as we have seen,
they must arise. This is our view of the causes of the evolution of the
world of organisms; the living substance may be compared to a plastic
mass which is poured out over a wide plain, and in its ceaseless
flowing adapts itself to every unevenness, flows into every hole,
covers every stone or post, leaving an exact model of it, and all this
simply by virtue of its constitution, which is at first fluid and then
becomes solid, and of the form of the surface over which it flows.
But it is not merely the surface in our analogy which determines
the form of the organic world; we must take account not only of the
external conditions of existence, but also of the constitution of
the flowing mass, the living substance itself, at every stage of its
evolution. The combination of living units which forms the organism is
different at each stage, and it is upon this that its further evolution
depends; this difference determines what its further evolution _may_
be, but the conditions of life determine what it _must_ be in a
particular case. Thus, in a certain sense, it was with the first
biophors, originating through spontaneous generation, that the whole
of the organic world was determined, for their origin involved not
only the physical constitution by which the variations of the organism
were limited, but also the external conditions, with their changes up
till now, to which organisms had to adapt themselves. There can be
no doubt that on another planet with other conditions of life other
organisms would have arisen, and would have succeeded each other in
diverse series. On the planet Mars, for instance, with its entirely
different conditions as regards the proportions in weight and volume of
the chemical elements and their combinations, living substance, if it
could arise at all, would occur in a different chemical composition,
and thus be equipped with different characters, and without doubt
also with quite different possibilities of further development and
transformation. The highly evolved world of organisms which we may
suppose to exist upon Mars, chiefly on the ground of the presence
of the remarkable straight canals discovered by Schiaparelli, must
therefore be thought of as very different from the terrestrial living
world.
But upon the earth things could not have been very different from
what they actually are, even if we allow a good deal to chance and
assume that the form of seas and continents might have been quite
different, the folding of the surface into mountains and valleys, and
the formation of rents and fissures, with the volcanoes that burst
from them, need not have turned out exactly as it has done. In that
case many species would never have arisen, but others would have taken
their place; on the whole, the same types of species-groups would
have succeeded each other in the history of the earth. Let us suppose
that the Sandwich Islands, like many other submarine volcanoes, had
never risen above the surface of the sea, then the endemic species of
snails, birds, and plants which now live there could not have arisen,
and if the volcanic group of the Galapagos Islands had arisen from
the sea not in their actual situation, but forty degrees further
south or north, or 1,000 kilometres further west, then it would have
received other colonists, and probably fewer of them, and a different
company of endemic species would be found there now. But there would
be terrestrial snails and land-birds none the less, and on the whole
we may say that both the extinct and the living groups of organisms
would have arisen even with different formations of land and sea, of
heights and depths, of climatic changes, of elevations and depressions
of the earth's crust, at least in so far as they are adaptations to
the more general conditions of life and not to specialized ones. The
great adaptation to swimming in the sea, for instance, must have taken
place in any case; swimming worms, swimming polyps (Medusæ), swimming
vertebrates, would have arisen; terrestrial animals would have evolved
also, on the one hand from an ancestry of worms in the form of jointed
animals and land or freshwater worms, and again from an ancestry of
fishes. Aerial animals would also undoubtedly have evolved even if the
lands had been quite differently formed and bounded, and I know of no
reason why the adaptation to flight should not have been attempted in
as many different ways as it has actually been by so many different
groups--the insects, the reptiles (the flying Saurians of the Jurassic
period), the extinct _Archæopteryx_, the birds, and the bats among
mammals.
We can trace plainly in every group the attempt not only to spread
itself out as far as possible over as much of the surface of the earth
as is accessible to it, but also to adapt itself to all possible
conditions of life, as far as the capacity for adaptation suffices.
This is very obvious from the fact that such varied groups have striven
to rise from life on the earth to life in the air, and have succeeded
more or less perfectly, and we can see the same thing in all manner of
groups. Almost everywhere we find species and groups of species which
emancipate themselves from the general conditions of life in their
class, and adapt themselves to very different conditions, to which
the structure of the class as a whole does not seem in the least
suited. Thus the mammals are lung-breathers, and their extremities
are obviously adapted for locomotion on the solid earth, yet several
groups have returned to aquatic life, as, for instance, the family of
otters and the orders of seals and whales. Thus among insects which
are adapted for direct air-breathing, certain families and stages
of development have returned to aquatic life, and have developed
breathing-tubes by means of which they can suck in air from the surface
of the water into their tracheal system, or so-called tracheal gills,
into which the air from the water diffuses. But the most convincing
proof of the organism's power of adaptation is to be found in the
fact that the possibility of living parasitically within other
animals is taken advantage of in the fullest manner, and by the most
diverse groups, and that their bodies exhibit the most marvellous and
far-reaching adaptations to the special conditions prevailing within
the bodies of other animals. We have already referred to the high
degree reached by these adaptive changes, how the parasite may depart
entirely from the type of its family or order, so that its relationship
is difficult to recognize. Not only have numerous species of flat worms
and round worms done this, but we find numerous parasites among the
great class of Crustaceans; there are some among spiders, insects,
medusoids, and snails, and there are even isolated cases among fishes.
If we consider the number of obstacles that have to be overcome in
existence within other animals, and how difficult and how much a matter
of chance it must be even to reach to such a place as, for instance,
the intestine, the liver, the lungs, or even the brain or the blood of
another animal, and when, on the other hand, we know how exactly things
are now regulated for every parasitic species so that its existence is
secured notwithstanding its dependence upon chance, we must undoubtedly
form a high estimate of the plasticity of the forms of life and their
adaptability. And this impression will only be strengthened when we
remember that the majority of internal parasites do not pass directly
from one host to another, but do so only through their descendants, and
that these descendants, too, must undergo the most far-reaching and
often unexpected adaptations in relation to their distribution, their
penetration into a new host, and their migrations and change of form
within it, if the existence of the species is to be secured.
We are tempted to study these relations more closely; but it is now
time to sum up, and we must no longer lose ourselves in wealth of
detail. Moreover, the life-history of many parasites, and of the
tape-worm in particular, is widely known, and any one can easily fill
up the story, of which we have given a mere outline. I simply wish to
point out that in parasitic animals there is a vast range of forms of
life in which the most precise adaptation to the conditions occurs
in almost every organ, and certainly at every stage of life, in the
most conspicuous and distinct manner. In the earlier part of these
lectures we gained from the study of the diverse protective means by
which plants and animals secure their existence the impression that
whatever is suited to its end (_Das Zweckmässige_) does not depend
upon chance for its origin, but that every adaptation which lies at
all within the possibilities of a species will arise if there is any
occasion for it. This impression is notably strengthened when we think
of the life-history of parasites, and we shall find that our view
of adaptations as arising, not through the selection of indefinite
variations, but through that of variations in a definite direction,
will be confirmed. Adaptations so diverse, and succeeding one another
in such an unfailing order as those in the life-history of a tape-worm,
a liver-fluke, or a _Sacculina_, cannot possibly depend upon pure
chance.
Nevertheless, chance does play a part in adaptations and
species-transformations, and that not only in relation to the
fundamental processes within the germ-plasm, but also in connexion
with the higher stages of the processes of selection, as I have
already briefly indicated. After the publication of my hypothesis of
germinal selection it was triumphantly pointed out that I had at last
been obliged to admit a phyletic evolutionary force, the 'definitely
directed' variation of Nägeli and Askenazy. This reproach--if to
allow oneself to be convinced be a reproach--is based upon a serious
misunderstanding. My 'variation in a definite direction' does not refer
to the evolution of the organic world as a whole. I do not suppose,
as Nägeli did, that this would have turned out essentially as it has
actually done, even although the conditions of life or their succession
upon the earth had been totally different; I believe that the organic
world, its classes and orders, its families and species, would have
differed from those that have actually existed, both in succession and
appearance, in proportion as the conditions of life were different.
My 'variation in a definite direction' is not predetermined from the
beginning, is not, so to speak, exclusive, but is many-sided; each
determinant of a germ-plasm may vary in a plus or minus direction, and
may continue under certain circumstances in the direction once begun,
but its components, the different biophors, may do the same, and so
likewise may the groups, larger and smaller, of biophors which form
the primordia (_Anlagen_) of the organs within the germ-plasm. Thus
an enormously large number of variational tendencies is available for
every part of the complete organism, and as soon as a variation would
be of advantage it arises--given that it is within the possibilities
of the physical constitution of the species. It occurs because its
potentialities are already present, but it persists and follows a
definite course because this is the one that is favoured. In other
words, it is primarily fixed by germinal selection alone, but is then
preferred by personal selection above the variants running parallel
with it. In my opinion the definite direction of the chance germinal
variations is determined only by the advantage which it affords to
the species with regard to its capacity for existence. But according
to Nägeli the direction of a variation is quite independent of its
utility, which may or may not exist. From Nägeli's point of view we
could never understand the all-prevailing adaptation, but if the
utility of a variant is itself sufficient to raise it to the level of a
persistent variational tendency, then we understand it.
Years ago (1883) I compared the species to a wanderer who has before
him a vast immeasurable land, through which he is at liberty to choose
whatever path he prefers, and in which he may sojourn wherever and
for as long as he pleases. But although he may go or stay entirely
of his own free will, yet at all times his going or staying will be
determined--it must be so and cannot be otherwise--by two factors:
first, by the paths available at each place--the variations which crop
up--and secondly, by the prospects each of these available paths open
up to him. He is striving after a restful place of abode which shall
afford him comfortable subsistence, his former home having been spoilt
for him by increasing expensiveness or too great competition. Even the
direction of his first journey will not depend upon chance, since of
the many paths available he will, and must, choose that which leads to
a habitable and not too crowded spot. If this has been reached--that is
to say, if the species has adapted itself to the new conditions--the
colonist sets up his abode there, and remains as long as a comfortable
existence and a competence are secure; but if these fail him, if grain
becomes scarce, or if prices rise, or if a dangerous epidemic breaks
out, then he makes up his mind to wander anew, and once more he will
choose, among the many available paths, that which offers him the
prospect of the speediest and most certain exit from the threatened
region, and leads him to another where he may live without risk. There,
too, he will remain as long as he is comfortable and not exposed to
want or danger, for the species as a whole only becomes transformed
when it must. And so it will go on _ad infinitum_; the traveller will,
when he is scared away from one dwelling-place, be able to continue his
journey in many directions, but he will always select the one path
which offers him the best prospects of a comfortable settlement, and
will follow it only to the nearest suitable place of abode, and never
further. The transformation of a species only goes on until it has
again completely adapted itself. In this way he will in the course of
years have traversed a large number of different places which, taken
together, may lie in a strange and unintelligible course, but this
course has nevertheless not arisen through a mere whim, but through
the twofold necessity of starting from a given spot--that in which he
had previously lived--the constitution of the species, and secondly of
choosing the most promising among the many available paths.
But chance does play a part in determining the route of the traveller,
for on it depends the nature of the conditions in the surroundings of
his previous dwelling-place, when he is forced to make another move;
for these conditions change, colonies are extended or depopulated,
a town previously cheap becomes dear, competition increases or
decreases, disease breaks out or disappears; in short, the chances of a
pleasureable sojourn in a particular place may alter and determine the
wanderer who is on the point of leaving his place of abode to take a
different direction from that which he would probably have chosen, say,
ten years earlier.
The analogy might be carried further, as, for instance, to illustrate
the possibility of a splitting up of the species; we may suppose that
instead of one wanderer there is a pair, who found a family at their
first halting-place. Children and grandchildren grow up in numbers and
food becomes scarce. One part of the descendants still finds enough
to live upon, but the rest set out to look for a new habitation. In
this case, too, many paths, sidewards or backwards, stand open to the
wanderers, but only those paths will be actually and successfully
followed by any company of them which will lead to a habitable place
where settlement is possible. If some of the descendants follow paths
with no such prospect they will soon turn back or will succumb to the
perils of the journey.
It seems to me that the contrast between this and Nägeli's view of
the transmutation of species is obvious enough. According to him the
wanderer is not free to choose his path, but goes on and on along a
definite railway-line that only diverges here and there, and it cannot
be foreseen whether the track leads to paradisaic dwellings or to
barren wastes--the travellers must just make the best of what they
find. They carry a marvellous travelling outfit with them--a sort of
_Tischlein, deck' dich_--the Lamarckian principle, but the magic power
of this is very doubtful, and it will hardly suffice to guard them
against the heat of the deserts, the frost of the Arctic regions, or
the malaria of the marshes into which their locomotive blindly carries
them.
According to my view, the traveler--that is, the species--has always
a large choice of paths, and is able, even while he is on the way,
to discern whether he has chosen a right or a wrong one; moreover,
in most cases, one or, it may be, a number of the paths lead to the
desired dwelling-place. But it also undoubtedly happens that, after
long wandering and when many regions have been traversed, a company
may finally arrive at a place which is quite habitable and inviting at
first sight, but which is surrounded on several sides by the sea or by
a rushing stream. As long as the soil remains fertile and the climate
healthy all goes well, but when matters change in this respect, and
perhaps the only way back lies through marshes and desert land and is
therefore impassable, then the colony will gradually die out--that is
the death of the species.
But let us now leave our parable and inquire what paths the organic
world has actually taken in its transformations, in what succession
the individual forms of life have evolved from one another; in short,
how the actual genealogical tree of this earth's animate population is
really constructed in detail. To this I can only reply that we have
many well-grounded suppositions, but only real certainty in regard
to isolated cases. Thus the genealogical tree of the horse has been
traced far back, and a great deal is known of the phylogeny of several
Gastropods and Cephalopods, but in regard to the genealogical tree
of organisms as a whole we can only make guesses, many of which are
probable, but are never quite certain. The palæontological records
which the earth's crust has preserved for us for all the ages are much
too incomplete to admit of any certainty. Many naturalists, notably
Ernst Haeckel, have done good service in this direction, for from
what we know of palæontology, embryology, and morphology, they have
constructed genealogical trees of the different groups of organisms,
which are intended to show us the actual succession of animal and plant
forms. But, interesting as these attempts are, they cannot for the
most part be anything more than guesswork, and I need not, therefore,
state or discuss them here in any detail, since they can afford us no
aid in regard to the problem of the origin of species with which these
lectures are concerned. In regard to the animal world at least--and the
case of plants is probably very similar--the record of fossil forms
fails us at an early stage. Thus the oldest and deepest strata in which
fossils can be demonstrated, the Cambrian formation, already contains
Crustaceans, animals at a relatively high stage of organization, which
must have been preceded by a very long series of ancestors of which
no trace has been preserved. The whole basal portion of the animal
genealogical tree, from the lowest forms of life at least up to these
primitive Crustaceans, the Trilobites, lies buried in the deepest
sedimentary rocks raised from the sea-floor, the crystalline schists,
in which it is unrecognizable. Enormous pressure and, probably also,
high temperature have destroyed the solid parts as far as there were
any, and the soft parts have only left an occasional impression even in
the higher strata.
Thus enormous periods of time must have elapsed from the beginning
of life to the laying down of that deepest 'Palæozoic' formation,
the Cambrian, for not only does the whole chain which leads from the
Biophoridæ to the origin of the first unicellulars fall within this
period, as well as the evolution of these unicellulars themselves
into their different classes, and their integration into the first
multicellulars, but also the evolution of these last into all the main
branches of the animal kingdom as it is now, into Sponges, Starfishes,
and their allies, Molluscs, Brachiopods, and Crustaceans, for all these
branches appear even in the Cambrian formation, and we may conclude
that the worms also, most of which are soft and not likely to be
preserved, were abundantly present at that time, since jointed animals
like the Crustaceans can only have arisen from worms. Moreover, we have
every reason for the assumption that Cœlenterates also, that is to say
polyps and medusoids, lived in the Cambrian seas, because their near
relatives with a solid skeleton, the corals, are represented in the
formation next above, the Silurian. The same is true of the fishes,
of which the first undoubtedly recognizable remains, the spines of
sharks, have been found in the Silurian. These two presuppose a long
preparatory history, and thus we come to the conclusion already stated,
that all the branches of the animal kingdom were already in existence
when the earth's crust shut up within itself the first records
available for us of the ancestors of our modern world of organisms.
Of course at that time the higher branches had only been represented
by their lower classes, and this is true especially of vertebrates,
so that, from the laying down of the Cambrian strata to the modern
world of organisms, a very considerable increase of complexity in
structure and an infinite diversifying of new groups must have taken
place. Amphibians do not appear to have been present in Cambrian times;
reptiles are represented in the Carboniferous strata, but only appear
in abundance in Secondary times; birds appear first in the Jurassic,
but in a very different guise (_Archæopteryx_) from the modern forms,
covered indeed with feathers, but still possessing a reptilian tail;
later they occur as toothed birds in the Cretaceous, and in Tertiary
times they have their present form. The development of mammals must
have run almost parallel with that of birds, that is, from the
beginning of Secondary times onwards, and their highest and last member
appears, as far as is known to research, only in post-Glacial times, in
the Diluvial deposits.
To the types which have arisen since the Cambrian period belongs the
class of Insects with its twelve orders and its enormous wealth of
known species, now reckoned at 200,000. They are demonstrable first in
the Devonian, and then in the Carboniferous period, in forms, just as
our theory requires, with _biting_ mouth-organs; it is not until the
Cretaceous strata that insects with purely suctorial mouth-organs--bees
and butterflies--occur, as it was also at that time that the flowers,
which have evolved in mutual adaptation with insects, first appeared.
The number of fossil species hitherto described is reckoned at about
80,000--certainly only a mere fragment of the wealth of forms of
life which have arisen on our earth throughout this long period, and
which must have passed away again; for very few _species_ outlive a
geological epoch, and even genera appear only for a longer or shorter
time, and then disappear for ever. But even of many of the older
classes, such, for instance, as the Cystoids among the Echinoderms of
the Silurian seas, no living representative remains; and in the same
way, the Ichthyosaurs or fish-lizards of the Secondary times have
completely disappeared from our modern fauna, and many other animal
types, like the class of Brachiopods and the hard-scaled Ganoid fishes,
have almost died out and are represented only by a few species in
specially sheltered places, such as the great depths of the sea, or in
rivers.
Thus an incredible wealth of animal and plant species was potentially
contained in these simplest and lowest 'Biophorids' which lay far
below the limits of microscopic visibility--an indefinitely greater
wealth than has actually arisen, for that is only a small part of
what was possible, and of what would have arisen had the changes of
life-conditions and life-possibilities followed a different course. The
greater the complexity of the structure of an organism is, the more
numerous are the parts of it which are capable of variation, and the
different directions in which it can adapt itself to new conditions;
and it will hardly be disputed that _potentially_ the first Biophorids
contained an absolutely inexhaustible wealth of forms of life, and
not merely those which have actually been evolved. If this were not
so, Man could not still call forth new animal and plant forms, as he
is continually doing among our domesticated animals and cultivated
plants, just as the chemist is continually 'creating' new combinations
in the laboratory which have probably never yet occurred or been
formed on the earth. But just as the chemist does not really 'create'
these combinations, but only brings the necessary elements and their
forces together in such a combination that they must unite to form the
desired new body, so the breeder only guides the variational tendencies
contained in the germ-plasm, and consciously combines them to procure a
new race. And what the breeder does within the narrow limits of human
power is being accomplished in free nature, through the conditions
which allow only what is fit to survive and reproduce, and thus bring
about the wonderful result--as though it were guided by a superior
intelligence--the adaptation of species to their environment.
Thus in our time the great riddle has been solved--the riddle of the
origin of what is suited to its purpose, without the co-operation of
purposive forces. Although we cannot demonstrate and follow out the
particular processes of transformation and adaptation in all their
phases with mathematical certainty, we can understand the principle,
and we see the factors through the co-operation of which the result
must be brought about. It has lately become the fashion, at least among
the younger school of biologists, to attach small value to natural
selection, if not, indeed, to regard it as a superseded formula;
mathematical proofs are demanded or, at any rate, desired. I do not
believe that we shall ever arrive at giving such proofs, but we shall
undoubtedly succeed in clearing up much that now remains obscure, and
in essentially modifying and correcting many of the theories we have
formed in regard to this question. But what has been already gained
must certainly be regarded as an enormous advance on the knowledge of
fifty years ago. We now _know_ that the modern world of organisms has
been evolved, and we can form an idea, though still only an imperfect
one, how and through the co-operation of what factors it could and must
have evolved.
When I say _must_, this refers only to the course of evolution from
a given beginning; but as to this beginning itself, the spontaneous
generation of the lowest Biophorids from inorganic material, we are far
from having understood it as a necessary outcome of its causes. And if
we have assumed it as a reasonable postulate, we by no means seek to
conceal that this assumption is far from implying an understanding of
what the process of biogenesis was. I do not merely mean that we do
not know under what external conditions the origin of living matter,
even in the smallest quantity, can take place; I mean, especially,
that we do not understand how this one substance should suddenly
reveal qualities which have never been detected in any other chemical
combination whatever--the circulation of matter, metabolism, growth,
sensation, will, and movement. But we may confidently say that we shall
never be able fully to understand these specific phenomena of life, as
indeed how should we, since nothing analogous to them is known to us,
and since understanding always presupposes a comparison with something
known. Even although we assume that we might succeed in understanding
the mere chemistry of life, as is not inconceivable, I mean the
_perpetuum mobile_ of dissimilation and assimilation, the so-called
'animal' functions of the living substance would remain uncomprehended:
Sensation, Will, Thought. We understand in some measure how the kidneys
secrete urine, or the liver bile; we can also--given the sensitiveness
to stimulus of the living substance--understand how a sense-impression
may be conveyed by the nerves to the brain, carried along certain
reflex paths to motor nerves and give rise to movement of the muscles,
but how the activity of certain brain-elements can give rise to a
thought _which cannot be compared with anything material_, which is
nevertheless able to react upon the material parts of our body, and, as
Will, to give rise to movement--that we attempt in vain to understand.
Of course the dependence of thinking and willing upon a material
substratum is clear enough, and it can be demonstrated with certainty
in many directions, and thus materialism is so far justified in drawing
parallels between the brain and thought on the one hand, and the
kidneys and urine on the other, but this is by no means to say that we
have understood how Thought and Will have come to be. In recent times
it has often been pointed out that the physical functions of the body
increase very gradually with the successive stages of the organization,
and from the lowest beginnings ascend slowly to the intelligence of
Man, in exact correspondence with the height of organization that has
been reached by the species; that they begin so imperceptibly among
the lower animal forms that we cannot tell exactly where the beginning
is; and it has been rightly concluded from this that the elements of
the Psyche do not originate in the histological parts of the nervous
system, but are peculiar to all living matter, and it has further been
inferred that even inorganic material may contain them, although in an
unrecognizable expression, and that their emergence in living matter
is, so to speak, only a phenomenon of summation. If we are right in our
assumption of a spontaneous generation it can hardly be otherwise, but
saying this does not mean that we have understood Spirit, but at most
secures us the advantage and the right of looking at this world, as far
as we know it, as a unity. This is the standpoint of Monism.
The psychical phenomena, which we know from ourselves, and can assume
among animals with greater certainty the nearer they stand to us,
occupy a domain by themselves, and such a vast and complex one that
there can be no question of bringing it within the scope of our present
studies, and the same is true of the phyletic development of Man. But
we must at least take up a position in regard to these problems, and
there can be no question that Man has evolved from animal ancestors,
whose nearest relatives were the Anthropoid Apes. Not many years ago
bony remains of a human skeleton, or at least of some form very near
to modern Man, were found in the Diluvial deposits of Java, and this
has been designated _Pithecanthropus erectus_, and perhaps rightly
regarded as a transition form between Apes and Man. It is possible that
more may yet be discovered; but even if that is not so, the conclusion
that Man had his origin from animal forefathers must be regarded as
inevitable and fully established. We do not draw conclusions with our
eyes, but with our reasoning powers, and if the whole of the rest of
living nature proclaims with one accord from all sides the evolution
of the world of organisms, we cannot assume that the process stopped
short of Man. But it follows also that the _factors_ which brought
about the development of Man from his Simian ancestry must be the same
as those which have brought about the whole of evolution: change of
external influences in its direct and indirect effects, and, besides
this, germinal variational tendencies and their selection. And in
this connexion I should like to draw attention to a point which has,
perhaps, as yet received too little attention.
Selection only gives rise to what is suited to its end; _beyond
that it can call forth nothing_, as we have already emphasized on
several occasions. I need only recall the protective leaf-marking of
butterflies, which is never a botanically exact copy of a leaf, with
all its lateral veins, but is comparable rather to an impressionist
painting, in which it is not the reproduction of every detail that is
of importance, but the total impression which it makes at a certain
distance. If we apply this to the organs and capacities of Man, we
shall only expect to find these developed as far as their development
is of value for the preservation of his existence and no further. But
this may perhaps seem a contradiction of what observation teaches us,
that, for instance, our eyes can see to the infinite distance of the
fixed stars, although this can be of no importance in relation to
the struggle for existence. But this intensity of the power of vision
has obviously not been acquired for the investigation of the starry
heavens, but was of the greatest value in securing the existence of
many of our animal ancestors, and was not less important for our own.
In the same way our finely evolved musical ear might be regarded as a
perfecting of the hearing apparatus far beyond the degree necessary to
existence, but this is not really the case: our musical ear, too, has
been inherited from our animal ancestors, and to them, as to primitive
Man, it was a necessity of existence. It was quite necessary for the
animals to distinguish the higher and lower notes of a long scale,
sharply and certainly, in order to be able to evade an approaching
enemy, or to recognize prey from afar. That we are able to make music
is, so to speak, only an unintentional accessory power of the hearing
organs, which were originally developed only for the preservation of
existence, just as the human hand did not become what it is _in order
to play the piano_, but to touch and seize, to make tools, and so on.
_Must this, then, be true also of the human mind?_ Can it, too, only
be developed as far as its development is of advantage to Man's
power of survival? I believe that this is certainly the case in a
general way; the intellectual powers which are the common property
of the human race will never rise beyond these limits, but this is
not to say that certain individuals may not be more highly endowed.
The possibility of a higher development of certain mental powers or
of their combinations--whether it be intelligence, will, feeling,
inventive power, or a talent for mathematics, music or painting--may
be inferred with certainty from our own principles; for not only may
the variational tendencies of individual groups of determinants in the
germ-plasm be continued for a series of generations without becoming
injurious, that is to say, without being put a stop to by personal
selection, but sexual intermingling always opens up the possibility
that some predominantly developed intellectual tendencies (_Anlagen_)
may combine in one way or another, and so give rise to individuals of
great mental superiority, in whatever direction. In this way, it seems
to me, the geniuses of humanity have arisen--a Plato, a Shakespeare, a
Goethe, a Beethoven. But they do not last; they do not transmit their
greatness; if they leave descendants at all, these never inherit the
_whole_ greatness of their father, and we can easily understand this,
since the greatness does not depend upon a single character, but upon
a particular combination of many high mental qualities (_Anlagen_).
Geniuses, therefore, probably never raise the average of the race
through their descendants; they raise the intellectual average only
through their own performances, by increasing the knowledge and power
handed on by tradition from generation to generation. But the raising
of the average of mental capacity, which has undoubtedly taken place
to a considerable degree from the Australasian aborigines to the
civilized peoples of antiquity and of our own day, can only depend on
the struggle for existence between individuals and races.
But if the human mind has been raised to its present level through
the same slow process of selection by means of which all evolution
has been directed and raised to the height necessary for the 'desired
end,' we must see in this a definite indication that even the greatest
mind among us can never see beyond the conditions which limit our
capacity for existence, and that now and for all time we cannot hope
to understand what is supernatural. We can recognize the stars in the
heavens, it is true, and after thousands of years of work we have
succeeded in determining their distance, their size, and gravity, as
well as their movements and the materials of which they are composed,
but we have been able to do all this with a thinking power created
for the conditions of human existence upon the earth, that is to say,
developed by them, just as we do not only grasp with our hands, but
may also play the piano with them. But all that involves a higher
thinking power that would enable us to recognize the pseudo-ideas of
everlastingness and infinity, the limits of causality, in short, all
that we do not know but regard as at best a riddle, will always remain
sealed to us, because our intelligence did not, and does not, require
this power to maintain our capacity for existence.
I say this in particular to those who imagine they have summed up the
whole situation when they admit that much is still lacking to complete
knowledge, say, to a true understanding of the powers of Nature or
of the Psyche, but who do not feel that in spite of all our very
considerably increased knowledge we stand before the world as a whole
as before a great riddle. But I say it also to those who fear that the
doctrine of evolution will be the overthrow of their faith. Let them
not forget that truth can only be harmful, and may even be destructive,
when we have only half grasped it, or when we try to evade it. If
we follow it unafraid, we shall come now and in the future to the
conclusion that a limit is set to our knowledge by our own minds, and
that beyond this limit begins the region of faith, and this each must
fashion for himself as suits his nature. In regard to ultimate things
Goethe has given us the true formula, when the 'Nature-spirit' calls
to Faust, 'Du gleichst dem Geist, den Du begreifst, nicht mir!' For all
time Man must repeat this to himself, but the need for an ethical view
of the world, a religion, will remain, though even this must change in
its expression according to the advance of our knowledge of the world.
But we must not conclude these lectures in a spirit of mere
resignation. Although we must content ourselves without being able to
penetrate the arcana of this wonderful world, we must remain conscious,
at the same time, that these unfathomable depths exist, and that we
may 'still verehren was unerforschlich ist' (Goethe). But the other
half of the world, I mean the part which is accessible to us, discloses
to us such an inexhaustible wealth of phenomena, and such a deep and
unfailing enjoyment in its beauty and the harmonious interaction of the
innumerable wheels of its marvellous mechanism, that the investigation
of it is quite worthy to fill our lives. And we need have no fear
that there will ever be any lack of new questions and new problems to
solve. Even if Mankind could continue for centuries quietly working
on in the manifold and restless manner that has, for the first time
in the history of human thought, characterized the century just gone,
each new solution would raise new questions above and below, in the
immeasurable space of the firmament, as in the world of microscopical
or ultramicroscopical minuteness, new insight would be gained,
new satisfaction won, and our enthusiasm over the marvel of this
world-mechanism, so extraordinarily complex yet so beautifully simple
in its operation, will never be extinguished, but will always flame up
anew to warm and illumine our lives.
INDEX
[References to vol. ii have the volume prefixed.]
Accessory idioplasm, 383.
Acræides, immunity of, 100.
Adaptation, in leaf butterflies, ii. 346;
of the sperm-cells to fertilization, 278, 279;
facultative, ii. 278;
functional, 244;
harmonious, ii. 80, 197;
not chance but necessity, ii. 346;
all evolution depends upon, ii. 347.
Affinities, vital, within the 'person,' ii. 36;
within the id, 374.
Agassiz, L., immutability of the species, 16.
Alcoholism, ii. 68.
Aldrovandi, 13.
Amixia, ii. 285, 286.
Ammon, O., the variation-playground, ii. 199, 202.
Amœba 'nests,' ii. 219.
Amphigony, 267;
as a factor in maintaining species, ii. 204.
Amphimixis, general significance of, ii. 192;
antiquity of, ii. 202;
Ammon's playground of variations, ii. 206;
Amœba 'nests' as a preliminary stage, ii. 219;
beginnings of, ii. 213;
parthenogenesis as self-fertilization, ii. 233;
in Coccidium, ii. 214, 216;
chromosomes in Protozoa, ii. 216;
the 'cycle' idea, 326;
increased stability due to, ii. 200;
continued inbreeding, ii. 231;
'formative' stimulus, ii. 229;
Galton's curves of frequency, ii. 206;
in relation to rudimentary organs, ii. 226;
immediate consequences of, ii. 224;
plastogamy as a preliminary stage of, ii. 222;
alters individuality, ii. 192;
and natural death, 335;
direct advantages of, ii. 198;
origin of, ii. 211;
association of, with reproduction, ii. 210;
increases power of adaptation, ii. 223;
preliminary stages of, ii. 213;
not a rejuvenescence in the sense of preserving life, ii. 221.
Ancestral plasm, ids of, ii. 38.
Ants, several kinds of ids in the germ-plasm of, 390;
harmonious adaptation of sterile forms, ii. 89;
degeneration of wings and ovaries in the workers, ii. 90;
transition forms between females and workers, ii. 92;
Wasmann's explanation of these, ii. 93;
_Polyergus rufescens_, ii. 95;
dimorphism of workers, ii. 96;
number of queens, ii. 98.
Apes, furred, in Tibet, ii. 269.
Arctic animals, sympathetic colouring in, 62.
Aristotle, 10.
Assimilation, ii. 371.
Auerbach, spindle-figure of the dividing cell-nucleus, 289.
Autotomy, self-amputation, ii. 18.
Baer, K. E. von, development of the chick in the egg, 25.
Barfurth, on the segmentation of the egg in the sea-urchin, 408.
Bates, discovery of mimicry, 91;
on the Sauba ant, ii. 96.
Beccari, _Amblyornis inornata_, 223.
Bees, harmonious adaptation in the workers, ii. 89;
influence of nutrition on the degeneration of the ovaries, ii. 92;
importance of the fact that there is only one queen, ii. 97.
Belt, plants and ants, 171.
Beneden, E. van, fertilization of the ovum of _Ascaris_, 295;
deutoplasm, 282;
theory of mitotic cell-division, 291.
Bickford, Elizabeth, experiments on regeneration, ii. 90.
Binswanger, on artificial epilepsy in guinea-pigs, ii. 68.
Biogenetic Law, Fritz Müller's view, ii. 160;
crustacean larvæ, ii. 161;
Haeckel's views, ii. 173;
markings of the caterpillars of the Sphingidæ, ii. 177;
shunting back of the stages in the ontogeny, ii. 177.
Biophors, the smallest vital units, 369;
struggle of the, ii. 52;
spontaneous generation of, 369.
Birds, adaptation in, ii. 315.
Blochmann, on the directive corpuscles in parthenogenetic ova, 304;
on the development of the ovum of the bee, 336;
on chromosomes in unicellulars, ii. 217.
Blumenbach, 'nisus formativus,' 352;
inheritance of mutilations, ii. 66.
Bois-Reymond, doubts as to the inheritance of functional
modifications, 242.
Bonnet, preformation theory, 350, 351.
Bordage, regeneration, ii. 20.
Borgert, proof of the splitting of the chromosomes in the division of
unicellulars, ii. 216.
Boveri, fertilization of non-nucleated pieces of ovum with nucleus of
another species, 341.
Brandes, on the extinction of _Machairodus_ species and the giant
armadillos, ii. 358, 359;
on the supposed transformation of the stomach in birds as a result
of nutrition, 267.
Brown-Séquard, artificial epilepsy in guinea-pigs, ii. 67.
Brücke, Ernst, organization of the living substance, 368.
Budding and division, ii. 1.
Bütschli, theories of amphimixis, 330;
discovery of the spindle-figure in nuclear division, 289.
Burdach, inheritance of mutilations, ii. 65.
Buttel-Reepen, Hugo von, on fertilization in the bee ovum, 306.
Butterflies, their enemies, 98;
aggressive colourings, 68, 70;
aberrations due to cold, ii. 274;
transmissibility of these, 275;
endemic species, 285;
polar and Alpine species, 285;
species of the Malay region, 291.
Butterflies, protective coloration in, 74.
Cænogenesis, ii. 173.
Calkins, conjugation of infusorians, 329.
Caterpillars, protective coloration in, 67.
_Catocala_, adaptive coloration in the various species, ii. 310.
Cell-division, integral and differential, 374;
differential in Ctenophores, 408;
proofs of differential, 377.
Centrospheres, 289, 309.
Ceratium, ii. 326.
Chance, elimination sometimes due to, 44, 47.
Characters, purely morphological, ii. 133.
Child, determination of, at fertilization, ii. 46.
Chromatin, the hereditary substance, 287;
grounds for the belief, 337-43.
Chromosomes, their occurrence in unicellulars, ii. 217;
simple and plurivalent (-idants), 349, 350;
individuality of, 349;
number of, in different species, 291;
indications of complexity of their structure, 292;
reasons for their existence, 303.
Chun, segmentation of the ovum in Ctenophores, 408;
Kerguelen cabbage and rabbits, ii. 362;
deep-sea investigation, ii. 322.
Cirrhipeds, ii. 241.
Climate, influence of, in causing variation, ii. 269.
Climatic varieties, ii. 269, 272.
Coadaptation, ii. 80;
in crustaceans, ii. 81;
in the markings of butterflies, ii. 87;
in the forelegs of the mole-cricket, ii. 86.
Cold aberrations in butterflies, transmissibility of, ii. 275.
Coloration, animal, its biological import, 58;
sympathetic in butterflies, 74;
in moths, 76;
of animals in green surrounding, 64;
of eggs, 60;
of nocturnal animals, of polar animals, 64;
water animals, 63.
Coloration, shunting backwards of, in the ontogeny, 73.
Colour-adaptation, double, 64, 73;
colour change in fishes, amphibians, reptiles and
Cephalopoda, ii. 278.
Combinations of determinants, ii. 40.
Conjugation, in Protozoa, 317;
in _Paramæcium_, 319.
Conklin, on the behaviour of the centrosphere in the ovum
of _Crepidula_, 309, ii. 41.
Connective tissue of vertebrates, 386.
Constancy and variability, periods of, ii. 294, 295;
degree of constancy of a character increases with its age, ii. 200.
Convergence, ii. 323.
Cope, supposed palæontological proofs for the Lamarckian
principle, ii. 77.
Copernicus, 13.
Copulation of _Coccidium proprium_, ii. 217.
Correlation of the parts of the body, 41;
of determinants of the germ-plasm, ii. 153.
Correns on Xenia, ii. 59.
Corsica, endemic butterflies of, ii. 285.
Crampton, segmentation in a marine snail, _Ilyanassa_, 409.
Crystal animals, sympathetic colouring, 63.
Cultivated plants, asexual reproduction in, ii. 261.
Cuvier, 16;
his dispute with St.-Hilaire, 24.
Dahl, the ants of the Bismarck Archipelago, ii. 101.
Danaides, immune butterflies, 94.
_Danais erippus_ and _Limenitis archippus_ (mimicry), 113, 114.
Darwin, Charles, first appearance of _The Origin of Species_, 28;
story of his life, 29.
Darwin, Erasmus, theory of evolution, 17.
Darwin and Nägeli, ii. 322.
Darwinian theory, dependence of the frequency of species on
enemies, 47;
on external circumstances, 45;
correlation of parts, 41;
races of pigeons, 34;
of domesticated animals, 31;
geometrical ratio of increase, 46;
struggle for existence, 47;
struggle between individuals of the same species, 52;
artificial selection, 39;
natural selection, 42;
affects all parts and stages, 54;
variation, 43;
summary, 55;
origin of flowers, 182;
pangenesis, ii. 62.
Death, natural, 260.
Degeneration of a typical organ not an ontogenetic but a phylogenetic
process, ii. 91;
of disused parts, ii. 116.
Delage, the germ-substance, 401;
'a portmanteau theory,' ii. 3;
experiments with sea-urchins, 342.
Desert animals, sympathetic colouring in, 62.
Determinants, active and passive state, 380;
controlling the cells, 381;
proofs of their existence, 361, 371, 408;
in limbs of Arthropods, 361;
liberation of, 382;
size and number, 369.
Determinates, 355.
Deutoplasm, 280.
Dewitz, degeneration of wings in the ontogeny of worker-ants, ii. 90.
Diatoms, ii. 324.
Dimorphism, sexual, its idioplasmatic cause, 388.
Disappearance of disused parts, ii. 135;
unequal rate of, ii. 129.
Dividing apparatus of the ovum, 288, 308.
Division, proof of differential nuclear division (_Phylloxera_), 377;
multiplication by division, ii. 1.
Dixon, isolation as a condition of species formation, ii. 284.
Döderlein, increase of characters in diluvial forms, ii. 139.
Dog, breeds of, 31;
attachment to man, ii. 73.
Driesch, 'prospective' importance of a cell, 378, 408.
Dzierzon, discovery of parthenogenesis in bees, 303.
Echinoderms, mesoderm cells of, 386, 387.
Ectocarpus, 334.
Egg-cell, form and structure, 280;
its migrations, 281.
Ehrlich, experiments with ricin and abrin, ii. 106.
Eigenmann, on blind cave-salamanders, ii. 347;
on species of Leptocephalus, ii. 133.
Eisig, on symbiosis, 162.
Elimination, ratio of, 47.
_Elymnias_, a genus of mimetic butterflies, 103.
Emery, on extinction of species, ii. 357;
on _Colobopsis truncata_, ii. 96;
on germinal selection, ii. 139;
'mixed' forms in ants, ii. 93;
variation of homologous parts, ii. 189.
Empedocles, 9;
ii. 370, 378.
Endemic species, ii. 283.
Endres, 'prospective' significance of the blastomeres of the ovum of
the frog, 407.
Epigenesis and evolution, 350.
Epilepsy, artificial, in guinea-pigs, ii. 67.
Equilibrium between species of a region, 49.
Evolution, phyletic, ii. 332;
paths of, ii. 381;
forces of, ii. 381;
mechanism of, 353;
facts of, 406.
Evolution, progressive, attempt of species to extend its
range, ii. 383;
unlimited diversity of forms of life, ii. 391;
parable of the traveller, ii. 386.
Evolution theory, general meaning of, 6;
'prospective' import of the cell, 378.
Exner, electric adaptation of the fur of mammals and feathers of
birds, ii. 316;
vision of insects, 216.
Eye-spots, 69;
ii. 179.
Falkland Islands, influence of climate on cattle and horses, ii. 268.
Feathers, regarded as an adaptation, ii. 316.
Fertilization, process of, 286;
in lichens, 313;
in _Ascaris_, 296;
in the sea-urchin ovum, 293;
in Phanerogams, 313;
in higher plants, ii. 251;
importance of the chromatin, 290;
conjugation, 317;
the centrosphere the dividing apparatus of the cell, 289;
chromatin the hereditary substance, 287;
differentiation of individuals among the Protozoa, 322;
number of chromosomes reduced to half, 297;
rôle of the centrosphere, 308;
summary of process of fertilization, 343.
Fischel, segmentation, of the Ctenophore ovum, 408;
regeneration of the lens in Triton, ii. 20.
Fischer, E., experiments with butterfly pupæ in low
temperature, ii. 275.
Flowers, origin of, 179;
adaptation to insects, 189;
in _Aristolochia_, _Pinguicula_, and _Daphne_, 186;
colour as an attraction to insects, 195;
collecting apparatus of bee, 193;
cross-fertilization, means for securing, 182;
in _Salvia_, 183;
lousewort, 184;
flowers adapted to fly-visits, 185;
orchids, 187;
deceptive flowers, _Cypripedium_, 200;
fertilization of Yucca, 202;
imperfection of adaptation a proof of origin through selection, 204;
mouth-parts of insects, 189;
bee, 172;
butterfly, 193;
cockroach, 191;
wind-pollination, 182.
Forel, Auguste, alarm-signals in ants, ii. 83.
Fraisse, on regeneration, ii. 30.
Function, passively functioning parts in relation to the Lamarckian
principle, ii. 77;
harmonious adaptation in these, ii. 81.
Fungi, reproduction of, ii. 267.
Fur of mammals, adaptation to the conditions of life, ii. 269.
Galapagos Islands, fauna of, ii. 283, 292.
Galileo, Galilei, 13.
Galls, plant, 385;
ii. 271.
Gall-wasps, reproduction of, ii. 245.
Galton, Francis, on continuity of the germ-plasm, 411;
on inheritance of talents, ii. 150;
curves of frequency, ii. 206;
doubt of the Lamarckian principle, 242.
Genius, human, ii. 394.
Germ-cells, and somatic cells, 411;
development of, 410;
their mutual attraction, ii. 230.
Germinal infection, ii. 69.
Germinal Selection, ii. 113;
influenced by personal selection, ii. 155;
relation of determinants to determinates, ii. 153;
combination of mental gifts, ii. 150;
influence of amphimixis, ii. 125;
influence of the multiplicity of ids, ii. 124;
objections on the score of smallness of the substance of the
germ-plasm, ii. 156;
degeneration of a species through cultivation, ii. 144;
there are only plus and minus variations, ii. 151;
excessive increase of variations, ii. 139;
basis of sexual characters, ii. 135;
its sphere of operation, ii. 127;
small hands and feet in the higher classes, ii. 147;
climatic forms, ii. 134;
bud-variations, ii. 141;
play of forces in the determinant system, ii. 154;
artificial selection, ii. 123;
short-sight, ii. 146;
milk-glands, ii. 147;
deformities, ii. 137;
muscular weakness in the higher classes of men, ii. 147;
positive variation, ii. 122;
regulated by personal selection, ii. 131;
source of purely morphological characters, ii. 132;
disappearance of disused parts, ii. 119, 129;
self-regulation of the germ-plasm, ii. 128;
specific talents, ii. 149;
sport-variations, ii. 140;
spontaneous and induced, ii. 137;
excessive increase of a variation tendency, ii. 130;
preponderance of panmixia, ii. 120;
origin of secondary sexual characters, ii. 143.
Germinal vesicle, 295.
Germ-plasm, conception of, 410;
continuity of, 411;
at once variable and persistent, ii. 220;
disintegration of, in ontogeny, 379;
nutritive variations within the, 379;
structure of the, 373;
variation of, due to environment, ii. 267;
to nutrition, ii. 268.
Germ-plasm theory, 345;
accessory idioplasm, 383;
active and passive state of determinants, 379;
connective tissue-cells, 386;
determinants and determinates, 355;
lithium-larvæ, 383;
ids, conception of, 349;
idants, 349;
male end female ids, 389;
mesoderm cells of sea-urchin, 387;
plant-galls, 385;
polymorphism, 390;
proofs of existence of determinants (_Lycæna agestis_, insect
metamorphosis, &c.), 356;
sexual dimorphism, 388.
Germ-tracks, 411.
Gesner's _Book of Animals_, 13.
Godelmann, regeneration of Phasmids, ii. 28 _n._
Goebel, 269.
Goethe, archetypal animal and plant, 18.
Green animals, 64.
Gruber, A., regeneration experiments on the Protozoa, 340.
Guignard, fertilization of Phanerogams, 315.
Gulick, snails in the Sandwich Islands. ii. 329.
Haase, Erich, on Pharmacopagæ, 101;
on mimicry, 104.
Haberlandt, protection of leaves, ii. 133;
Auxo-spores, ii. 221.
Haeckel, Ernst, fundamental biogenetic law, ii. 173;
monogony and amphigony, 267;
palingenesis and cœnogenesis, ii. 173;
genealogical trees, ii. 388.
Häcker, Valentin, importance of the nucleolus, 287;
separateness of paternal and maternal nuclear substance during
development, ii. 42;
process of nuclear division, 291.
Hahnel, observations on the enemies of butterflies, 154;
lizards and birds as enemies of butterflies, 97, 98.
Haller, 267.
Harmony, pre-established, apparently existing in development, ii. 309.
Hartog, views on amphimixis, 334;
ii. 194.
Haycraft, on the equalizing effect of amphigony, ii. 203.
Heidenhain, theory of mitotic division, 291.
Heider, on the intimate processes of segmentation of the ovum,
'regulation' and 'mosaic' ova, 409.
Heliconiidæ, first example of immune butterflies, 91.
Henslow, on purely morphological specific differences, ii. 308.
Herbst, lithium-larvæ, 383;
ii. 277.
Hereditary sequence, alternation of, ii. 50.
Hering, his reasons for assuming the inheritance of functional
modifications, ii. 110.
Hermaphroditism in flowers, ii. 250;
in animals, ii. 239;
advantages of, ii. 239.
Herrich-Schäfer, on mimicry, 105.
Hertwig, O., fertilization of sea-urchin eggs, 293;
theory of development, 354;
differential cell-division, 376;
inheritance of functional modifications, ii. 106;
maturation divisions of the sperm-cells, 300.
Hertwig, R., chromosomes in Actinosphærium, ii. 216.
Heterogony, ii. 244.
Heteromorphosis, Loeb on, ii. 7.
Heterostylism, ii. 254.
Heterotopia, 365, 367.
Hirasé, fertilization of Phanerogams, 313.
Histonal selection, 240;
and personal selection, 280.
Hübner, O., experiments on regeneration in _Volvox_, ii. 4.
Humming-birds, species fixed by isolation, ii. 290.
Hyatt, Alpheus, the snail-strata of Steinheim, ii. 305.
Hybrids, ii. 60;
of pigeons, 34;
plant, ii. 57.
Hydra, regeneration in, ii. 4.
Hydroid polyps, development of germ-cells in, 411.
Idants, 349.
Ids, 349;
male and female, 389;
mimicry a proof of the existence of, 390.
Immortality, potential, of the Protozoa, 260.
Immunity of butterflies, 99.
Imperfection of adaptation, 203.
Inbreeding, evil consequences of, ii. 231.
Infection of the germ, ii. 69.
Infusorians, experiments of Maupas on, 328;
Calkins on, 329;
differentiation of nucleus into macro-and micro-nucleus a means of
compelling conjugation, 334.
Inheritance, of acquired characters, ii. 62 (_see_ also Lamarckian
principle);
of functional modifications, ii. 64;
of mutilations disproved, ii. 65;
from parent to child, ii. 38;
hereditary substance, 288, 341;
preponderance of one parent, ii. 47;
alternation in ontogeny, ii. 48.
Instinct, 141, ii. 70;
will and, 152.
Instincts, aberrant, 149;
attachment of dog, ii. 73;
change of, in _Eristalis_, &c., 150;
egg-laying of butterfly, 159;
exercised only once, 155;
ii. 75;
'feigning death,' 145;
imperfectly adapted, 152;
inheritance of, ii. 72;
masking of crabs, 145;
material basis of, 142;
monophagy of caterpillars, 146;
new in domesticated animals, ii. 73;
nutritive, 146;
in Ephemerids and sea-cucumbers, 148;
in predatory fishes, 149;
origin of, ii. 70;
pupation of butterflies, 156;
self-preservation of, 144;
wild animals on lonely islands, ii. 73.
Intra-selection (histonal selection), 240.
Ischikawa, on chromosomes in unicellulars, ii. 216;
on the conjugation of _Noctiluca_, 317;
ii. 42.
Island faunas, ii. 283.
Isolated regions, ii. 284.
Isolation, favours species-formation, ii. 383;
relative, ii. 350;
snails on the Sandwich Islands, ii. 292.
Jäger, G., on the continuity of the germ-plasm, 411.
Japanese cock, 356.
Kaleidoscope, transformation resembles a, ii. 307.
Kallima, mimicry of leaf, 83, 236, 237.
Karyokinesis, 290.
Kathariner, birds as enemies of butterflies, 97.
Kennel, birds as enemies of butterflies, 97.
Kerner von Marilaun, Alpine plants, 122;
influence of hybridization on the formation of new species, ii. 352.
Knowledge, limits of, ii. 392.
Köhler, on scent-scales in the Lycænidæ, 370.
Koshewnikow, on the influence of royal food on drone-larvæ, ii. 92.
Kükenthal, on the fur of aquatic mammals, ii. 270.
Lamarck, theory of development, 21;
on limits of genera and species, ii. 306.
Lamarckian principle, ii. 62;
Lamarck regarded inheritance of functional modifications as a matter
of course, 241;
cleaning apparatus of bees, ii. 84;
claw of crustacean, ii. 85;
Darwin's attitude to, 242;
facts (foreleg of mole, cricket, &c.), ii. 86;
Galton's attitude to, 242;
Hering's view, ii. 109;
O. Hertwig's view, ii. 106;
neuters among ants and bees, ii. 89;
phyletic development, ii. 77;
skeleton of Arthropods, ii. 82;
stridulating organs, ii. 83;
theoretical impossibility of, ii. 107;
variation of passive parts, ii. 77;
venation of butterfly's wing, ii. 87;
Zehnder's defence of, ii. 99.
_Lathræa_, 135.
Lauterborn, on amphimixis in diatoms, ii. 216.
Leaf-imitation, in Locustidæ, 88;
in moths, 87;
in butterflies, 83, 357-61;
in Anæa species, ii. 310.
_Lepus variabilis_, 62;
ii. 344, 350.
Leeuwenhoek, first use of the microscope, 14.
Leuckart, _Trichosomum crassicauda_, with dwarf males, 227;
structure of snails, ii. 301.
Leuckart and von Siebold, 333.
Leydig, regeneration of the lizard's tail, ii. 30.
Liberation of the determinants in ontogeny, 382-6;
quality of nutrition as a liberating stimulus in bees and
ants, ii. 92.
Liebig, theory of the origin of life, ii. 365.
Limits of knowledge determined by selection, ii. 394.
Linné, conception of species, 14.
Lloyd Morgan, artificially induced instincts, ii. 72.
Loeb, experiments on regeneration, ii. 6, 7;
the cell-nucleus as an organ for oxidation, ii. 31.
Luminous organs in deep-sea animals, ii. 321.
MacCullock, autotomy, ii. 19.
_Machairodus_, ii. 358.
Mammals, adaptation to aquatic life, ii. 333.
Maturation divisions, ii. 40;
in plants, 315;
in the ovum, 298;
in the sperm, 301;
influence of, ii. 44.
Maupas, intimate processes of conjugation, 319;
conjugation of Infusorians, 329.
Medium, influence of, ii. 267.
Mendel's Law, ii. 57.
Merogony, fertilization of non-nucleated pieces of ovum, 343.
Merrifield, temperature-experiments with _Polyommatus
phlæas_, ii. 273;
cold experiments with _Vanessa_, ii. 274.
Meyer, Hermann, architecture of the bone spongiosa, 246.
Mimicry, 91;
in beetles, bees, ants, &c., 116;
in butterflies does not affect caterpillar or pupa, 104;
in both sexes, 96;
in vertebrates, 117;
degree of resemblance to model, 104;
_Elymnias undularis_, 106;
_Papilio merope_, 108;
_P. turnus_, 110;
same effect produced in different ways, 105;
several imitators of one immune species, 101;
species of genera which need protection imitate different immune
models, 102;
'rings' of mimetic species, 112;
rarity of mimetic species, 108;
wide divergence of mimetic species from their congeners, 115.
Mitosis, 288.
Möbius, 296.
Monism, 393.
Monogony, 266.
Montgomery, on reduction of the chromosomes, ii. 43.
Morgan, experiments on regeneration, ii. 15.
Morphological characters, dependent on germinal selection, ii. 132;
discussion as to indifferent characters, ii. 132, 309.
Mortality of multicellular organisms, 260;
causes of this, 263.
Morton, Thomas, on degeneration in the children of alcoholics, ii. 69.
Moths, protective coloration in, 80.
Müller, Fritz, scent-scales, 217;
on mimicry, 111;
plants and ants, 171;
relation between ontogeny and phylogeny, ii. 160.
Müller, Johannes, the vision of insects, 216.
Musical sense in man, ii. 148.
Mutation theory of de Vries, ii. 317.
Mutilations, supposed inheritance of, ii. 65.
Mutual sterility, of no great importance in connexion with lasting
variation, 349.
Nägeli, Carl von, on the definite directions of
variations, ii. 306, 385;
objection to origin of flowers through selection, 198;
on the difference in size between egg and sperm, 337;
his _Hieracium_ experiments, ii. 272;
Nägeli's view and Darwin's reconciled through germinal
selection, ii. 334;
number of smallest vital units in a 'moneron,' ii. 368.
Nathusius, inbreeding experiments, ii. 231.
Natural Selection, not directly observable, 58;
under the influence of isolation, ii. 292.
Neo-Lamarckism, 243.
Neotaxis, ii. 40.
Nerve-tracks in relation to instincts, ii. 71.
Normal number of a species, 45.
_Notodonta_, protective coloration in, 80.
Nuclear division, process of, 289;
integral and differential, 374, 377.
Nussbaum, M., regeneration-experiments in Protozoa, 340;
on the continuity of the germ-cells, 411;
infection of the ovum in Hydra, ii. 68.
Nutrition, influence of, on variation, ii. 267;
relation between nutrition and the number in a species, 45.
Oken's 'Naturphilosophie,' 21.
Omnipotence of selection, ii. 348.
Ontogenesis, relation to phylogenesis, ii. 159;
shunting back of the phyletic stages in embryogenesis, ii. 176;
condensation of phylogeny in ontogeny, ii. 186.
Orchids, fertilization of, ii. 256.
Organs, rudimentary, ii. 226.
Origin of flowers, _see_ Flowers.
Osborn, supposed palæontological proofs for the Lamarckian
principle, ii. 77.
Ovaries, 282.
Ovogenic determinants, 388.
Ovum, maturation of, 295.
Packard, disappearance of useless parts, 129.
Palingenesis, ii. 173.
_Pandorina_, reproduction of, 257, 293.
Pangenesis, ii. 62.
Panmixia, ii. 114.
_Papilio meriones_, 108, 427;
_P. turnus_, 110.
Parasites, power of adaptation in, ii. 384.
Parthenogenesis, discovery of, 303;
exceptional and artificial, 307;
facultative in bees, ii. 235;
receptaculum seminis in Cypris-species without males, 326, ii. 234;
advantages of, ii. 243;
its effects compared with those of inbreeding, ii. 233;
alternation of, with bisexual generations (heterogony), ii. 243.
Personal selection, indirect effects of, ii. 200.
Petrunkewitsch, A., maturing divisions in the ovum of the
bee, 306, 336.
Pfeffer, rôle of malic acid in the fertilization of ferns, 273.
Pflüger and Born, experiments in hybridization, ii. 232.
Phasmids, regeneration in, ii. 17.
_Phylloxera_, reproduction in, ii. 249.
Phylogenetic variation of butterfly and caterpillar independent of
each other, 362.
Phylogeny, condensation of, in ontogeny, ii. 186.
_Physiologus_, 11.
Pictet, turban eyes in male Ephemerids, 229.
Pigeons, breeds of, 34.
Plants, fertilization of the higher, ii. 250;
carnivorous, 132;
_Aldrovandia_, 138;
_Dionæa_, 138;
_Drosera_, 136;
_Lathræa_, 135;
_Nepenthes_, 134;
_Pinguicula_, 135;
_Utricularia_, 133.
Plant-galls, ii. 270.
Plastogamy a preliminary stage to fertilization, ii. 220.
Pliny, 11.
Polar bodies, 294.
Polymorphism, its idioplasmic roots, 390.
_Polyommatus phlæas_, dimorphism of caterpillars, 363;
climatic varieties, ii. 272.
Postgeneration (Roux), 407.
Pouchet, spontaneous generation, ii. 366.
Poulton, on facultative colour adaptation in caterpillars, ii. 278;
on mimicry, 105.
Prediction on the basis of the evolution theory, 3.
Preformation and Epigenesis, 351.
Primordial males among Cirrhipeds, ii. 242.
Protective arrangements in plants, 119;
Alpine plants, 126;
chemical substances, 128;
ethereal oils, 128;
hairs, 122;
poisons, 120;
Raphides, 129;
'Prigana scrub,' 126;
against small enemies, 127;
Tragacanth, 124.
Protective colouring, rôle of light in, 78;
_Kallima_, 83;
_Notodonta_, 80;
_Xylina_, 82.
Protective marking in caterpillars, 67.
Protozoa, chromosomes in, ii. 216.
Quetelet, amphigony preserves the mean of the species, ii. 204.
Races, development of, depending on adaptation, ii. 335;
dependent on germinal selection, ii. 144.
Radiolarians, skeleton of, ii. 324.
Rand, experiments on regeneration in Hydra, ii. 5.
Rath, O. von, on the influence of royal food on drone-larvæ, ii. 91.
Ray, John, conception of 'species,' 14.
Reactions, primary and secondary, ii. 277.
Reducing divisions, _see_ Maturation divisions.
Regeneration, ii. 1;
atavistic, ii. 30;
autotomy, ii. 16;
in birds, ii. 14;
in Hydra, ii. 4;
in Hydroid polyps, ii. 9;
in plants, ii. 9, 32;
in Planarians, ii. 6, 13;
in starfishes, ii. 30;
in Vertebrates, ii. 10;
of the lens in Triton, ii. 19;
a phenomenon of adaptation, ii. 9;
nuclear substance the first organ of, ii. 31;
phyletic origin of, ii. 23;
disappearance of the power of, ii. 16;
and budding, ii. 31;
relation of, to liability of part to injury, ii. 7;
not always purposive, ii. 25.
Reinke, objections to the 'machine theory' of life, 402;
on regeneration, 32.
Rejuvenescence, theory of, 325-8.
Reproduction, adaptation of the germ-cells, 277;
asexual, ii. 259;
structure of the ovum, 280;
of the bird's egg, 285;
zoosperm, 273;
in Amœbæ, 253;
in Infusorians, 254;
in _Pandorina morum_, 257, 269;
in fungi, 267;
by means of germ-cells, 266;
differentiation of germ-cells into male and female, 267;
by division, 264;
two kinds of eggs in same species, 282;
nutritive ovum cells, 283;
introduction of death into the living world, 261;
contrast between reproductive and body cells in the Metazoa, 256;
budding and division in the Metazoa, 264;
potential immortality of the Protozoa, 260;
sperm and ovum in Algæ, 272;
in _Volvox_, 265, 271;
zoosperms of Ostracods, 275;
different kinds of spermatozoa, 278.
Reproductive cells, development of, 410;
in Diptera, 411;
in Hydroid polyps, 413.
Reversion, ii. 53;
in doves, ii. 55;
in the horse, ii. 55.
Riley, fertilization of the Yucca by a moth, 202.
Ritzema Bos, experiments on mice, ii. 65, 66.
Romanes, isolation theory, ii. 284;
physiological selection, ii. 337;
panmixia, ii. 115.
Rosenthal, experiments with mice, ii. 65, 66.
Roux, Wilhelm, Mosaic theory, 379;
struggle of the parts, 244;
postgeneration, 407.
Rückert, the nuclear substances in Copepods, ii. 42.
Rudimentary organs in man, ii. 226.
St.-Hilaire, unity of type, 18.
Samassa, segmentation of the frog's egg, 407.
Sarasin, snails of Celebes, ii. 299.
_Saturnia_, pupation of, 158.
Schaudinn, fertilization in Coccidia, ii. 214;
maturing division in Sun-animalcule, 318.
Schimper, plants and ants, 171.
Schleiden and Schwann, discovery of the cell, 26.
Schmankewitsch, experiments with _Artemia_, ii. 277.
Schmidt, Oscar, ii. 324.
Schneider, discovery of the 'spindle-figure' of nuclear division, 289.
Schütt, Diatoms, ii. 325.
Schwarz, Ostracods, 276.
Segmentation-cells in animal ova, their prospective importance, 406.
Seitz, a case of mimicry, 114.
Selection-processes, grades of, ii. 265;
evolution guided by, ii. 298.
Selection, sexual, 210-39;
absence of secondary sexual characters in the lower animals, 231;
adaptations for seizing the females, 229;
choice on the part of the females, 214;
odours and scent-scales, 217;
song of cicadas and birds, 221;
superfluity of males, 213;
weapons for the struggle for mates, 228;
summary, 238.
Selection value, ii. 132, 311.
Self-fertilization in plants, ii. 252;
continued influence of, ii. 257;
alternation of self- with cross-fertilization, ii. 241.
Self-preservation, instinct of, 144.
Sex-cells, mutual attraction of, ii. 228.
Sex, determination of, 377;
ii. 44.
Sexual characters, secondary, have their roots in germinal
selection, ii. 130, 143, 289-91, 378.
Sexual selection, _see_ Selection, sexual.
Sexual selection through isolation, ii. 289.
Short-sight, ii. 146.
Siedlecky, copulation in _Coccidium proprium_, ii. 218.
Simroth, ii. 302.
Slevogt, on birds as enemies of butterflies, 97.
Sluiter, on symbiosis, 167.
Smerinthus, markings of the caterpillars, ii. 177, 184.
Snail-strata of Steinheim, ii. 305.
Sommer, on artificial epilepsy in guinea-pigs, ii. 68.
Special investigation, period of, 25.
Species, the, a complex of adaptations and variations, ii. 307.
Species-colonies, ii. 280.
Species, extinction of, ii. 357;
dying out of the large animals of Central Europe, ii. 361;
extinction due to cultivation, ii. 360;
to unlimited variation, ii. 357;
_Machairodus_, ii. 358;
lower types more capable of adaptation than higher, ii. 359;
extinction of flightless birds, ii. 360.
Species-formation, ii. 299;
favoured by isolation, ii. 284;
snails of Celebes, ii. 219;
without amphigony in lichens, ii. 343;
without isolation in _Lepus variabilis_, ii. 344;
Peridineæ, ii. 325;
protective coloration in butterflies, ii. 310;
the Steinheim snail-strata, ii. 315;
telescope eyes in deep-sea animals, ii. 323;
typical species, ii. 304;
variation in definite directions, ii. 306;
the bird as a complex of adaptations, ii. 316;
the whale as a complex of adaptations, ii. 313;
mutual fertility between many plant-species, ii. 340.
Species, variable and constant, ii. 286.
Specific type, its occurrence favoured by germinal
variation, ii. 333, 334;
by natural selection, ii. 334;
origin of the, ii. 299, 332-5.
Spencer, Herbert, germinal substance composed of homogeneous
particles, 355;
on 'units,' the smallest vital particles, 369;
protective adaptations in plants to be referred to
selection, ii. 77.
Spermaries, 282.
Spermatozoa, _see_ Zoosperms.
Sperm-cells, 272.
Spermogenic determinants, 388.
Sphingidæ, caterpillars of the, biological value of their
markings, 73;
ontogeny and phylogeny of the markings, ii. 177.
_Sphinx convolvuli_, double adaptation of the caterpillar, 71, 72;
_S. euphorbiæ_, var. _Nicæa_, purely local form of caterpillar, 362.
Spontaneous generation, 410;
conditions necessary, ii. 370;
only possible as regards invisible minute organisms, ii. 369;
the 'where' of, ii. 371;
impossibility of proving or disproving it experimentally, ii. 366.
Sprengel, fertilization of flowers, 180.
Standfuss, cold experiments with butterfly pupæ, ii. 275.
Steinheim snail-strata, ii. 305.
Steller's sea-cow (_Rhytina stelleri_), ii. 74.
Stick-insects, 88.
Strasburger, fertilization of Phanerogams, 314.
Stuhlmann, zoosperms in Ostracods, 276.
Swammerdam, 14.
Symbiosis, candelabra trees and ants, 171;
hermit-crabs and Hydroid polyps, 163;
hermit-crabs and sea-anemones, 162;
origin of symbiosis, 176;
lichens, 173;
fishes and sea-anemones, 167;
green Amœbæ, 170;
green fresh-water polyp (_Hydra viridis_), 168;
_Nostoc_ and _Azolla_, 177;
sea-anemones and yellow Algæ, 171;
root-fungi, 175.
Talents, specific, of man referred to germinal selection, ii. 149;
depend on a combination of mental gifts, ii. 150.
Tichomiroff, artificial parthenogenesis, 307, 333.
Thorn-bugs, 89.
Transparent winged butterflies, 106.
Treviranus, as founder of the evolution theory, 18;
on generic differences, ii. 306.
Trimen, observations on the immunity of the Acræidæ, 100.
Tropism in plants, ii. 276.
Twins, identical, ii. 44.
_Vanessa_, endemic species of, with protective colouring, 75.
Variability, fluctuating, ii. 327.
Variation, all ultimately quantitative, ii. 151;
in a definite direction, ii. 118;
double roots of, ii. 195;
ascending, ii. 122;
sports or saltatory variations, ii. 140;
roots of hereditary, ii. 118.
Variation of individual characters, ii. 336;
not always due to adaptation, ii. 197.
Variation, periods of, ii. 294.
Vital force, ii. 369.
Vitalism, ii. 369.
Virchow, Rudolf, on the inheritance of mutilations, ii. 65.
Vöchting, influence of light on the production of flowers, ii. 276;
on regeneration, ii. 32.
Voigt, Walter, experiments in regeneration, ii. 6;
on Planarians, ii. 25.
Voit, Carl von, influence of nutrition on bodily size, ii. 268.
Volvocineæ, reproduction in, 257.
Vries, de, asymmetrical curves of frequency, ii. 234;
theory of mutations, ii. 317;
Pangen theory, 380.
Wagner, Franz von, regeneration in _Lumbriculus_, ii. 27.
Wagner, Moriz, on the influence of isolation, ii. 284.
Wahl, Bruno, on the development of _Eristalis_, 399.
Wallace, on the immunity of Heliconiidæ, 99;
on the causes of the coloration of butterflies, 211.
Wasmann, Erich, on transition forms in ants, ii. 93;
on sounds produced by ants, ii. 83.
Weaver birds, ii. 290.
Whales, their origin through adaptation, ii. 313.
Wheeler, rôle of the centrosphere in the ovum, 309.
Wiedersheim, rudimentary organs in man, ii. 226.
Wiesner, the smallest vital particles, 369.
Wing-primordia in insects, 364.
Winkler, Hans, experiments on artificial parthenogenesis, 307, 333;
on merogony, 343.
Wolff, G., regeneration of the lens in Triton, ii. 19.
Wolff, K. v., the founder of the epigenetic theory of evolution, 352.
Wroughton, Robert, production of sounds by Indian ants, ii. 95.
Würtemberger, form-series of ammonites, ii. 176.
Xenia, ii. 58.
_Xylina_, protective colouring of, 82.
Yolk of egg, 282.
Zehnder, the living substance made up of fistellæ, ii. 217;
polymorphism in ants, ii. 99;
on the Lamarckian principle, ii. 99-106;
on the skeleton of Arthropods, ii. 103;
effect of amphimixis, ii. 223.
Ziegler, Ernst, on deformities, ii. 138.
Ziegler, H. E., experiments on merogony in sea-urchin ova, 342.
Zoja, experiments with the ova of Medusæ, 407.
Zoosperms, 273, 278, 279.
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Transcriber's Notes:
Italics are shown thus: _sloping_.
Punctuation has been retained as published.
Variations in spelling and hyphenation are retained.
Illustrations have been moved out of mid-paragraph.
Figures repeated from Volume I have been added to the
Table of Illustrations.
On page 246 (Fig. 125, B and C) has been corrected to
Fig. 124, B and C).
On page 216 Fig. 118, D, has been corrected to Fig. 122,D.
In the index, Reproductive cells, development of, in Diptera,
has been corrected from 471 to 411.
In the index, Spontaneous generation, ii. has been corrected
to Spontaneous generation, and thus refers to volume I.
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