Title: Studies in the Theory of Descent, Volume II
Author: Weismann, August
Language: English
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Pages 1-400, Plates I-II, and some referenced footnotes are in Volume I.



  STUDIES IN THE THEORY
  OF DESCENT

  BY

  DR. AUGUST WEISMANN
  PROFESSOR IN THE UNIVERSITY OF FREIBURG

  _WITH NOTES AND ADDITIONS BY THE AUTHOR_

  TRANSLATED AND EDITED, WITH NOTES, BY

  RAPHAEL MELDOLA, F.C.S.
  LATE VICE-PRESIDENT OF THE ENTOMOLOGICAL SOCIETY OF LONDON

  WITH A PREFATORY NOTICE BY

  CHARLES DARWIN, LL.D., F.R.S.
  _Author of “The Origin of Species,” &c._

  IN TWO VOLUMES
  VOL. II.

  WITH EIGHT COLOURED PLATES

  London:

  SAMPSON LOW, MARSTON, SEARLE, & RIVINGTON
  CROWN BUILDINGS, 188, FLEET STREET

  1882

  [_All rights reserved_]



I.

LARVA AND IMAGO VARY IN STRUCTURE INDEPENDENTLY OF EACH OTHER.


It would be meaningless to assert that the two stages above mentioned
were _completely_ independent of one another. It is obvious that the
amount of organic and living matter contained in the caterpillar
determines the size of the butterfly, and that the quantity of organic
matter in the egg must determine the size of the emergent larva.
The assertion in the above heading refers only to the structure;
but even for this it cannot be taken as signifying an absolute, but
only a relative independence, which, however, certainly obtains in
a very high degree. Although it is conceivable that every change of
structure in the imago may entail a correlative change of structure
in the larva, no such cases have as yet been proved; on the contrary,
all facts indicate an almost complete independence of the two stages.
It is quite different with cases of _indirect_ dependence, such, for
example, as are brought about by ‘nurse-breeding.’ This phenomenon is
almost completely absent in Lepidoptera, but is found in Diptera, and
especially in Hymenoptera in every degree. The larvæ of ichneumons
which live in other insects, require (not always, but in most
instances) that the female imago should possess a sharp ovipositor,
so that in this case also the structure and mode of life of the larva
influences the perfect insect. This does not depend, however, on
inherent laws of growth (correlation), but on the action of external
influences, to which the organism endeavours to adapt itself by natural
selection.

I will now let the facts speak for themselves.

It is shown by those species in which only one stage is di- or
polymorphic that not every change in the one stage entails a
corresponding change in the other. Thus, in all seasonally dimorphic
species we find that the caterpillars of butterflies which are often
widely different in the colour and marking of their successive
generations are absolutely identical. On the other hand, many species
can be adduced of which the larvæ are dimorphic whilst the imagines
occur only in one form (compare the first and second essays in this
volume).

There are however facts which directly prove that any one stage can
change independently of the others; I refer to the circumstance that
any one stage may become independently variable--that the property of
greater variability or of greater constancy by no means always occurs
in an equal degree in all the three stages of larva, pupa, and imago,
but that sometimes the caterpillar is very variable and the pupa and
imago quite constant. On the other hand, all three stages may be
equally variable or equally constant, although this seldom occurs.

If variability is to be understood as indicating the period of
re-modelling of a living form, whether in its totality or only in
single characters or groups of characters, from the simple fact of the
heterochronic variability of the ontogenetic stages, it follows that
the latter can be modified individually, and that the re-modelling
of one stage by no means necessarily entails that of the others. It
cannot however be doubted that variability, from whatever cause it may
have arisen, is in all cases competent to produce a new form. From the
continued crossing of variable individuals alone, an equalization of
differences must at length take place, and with this a new, although
not always a widely deviating, constant form must arise.

That the different stages of development of a species may actually be
partly variable and partly constant, and that the variable or constant
character of one stage has no influence on the other stages, is shown
by the following cases, which are, at the same time, well adapted to
throw light on the causes of variability, and are thus calculated to
contribute towards the solution of the main problem with which this
investigation is concerned.

When, in the following pages, I speak of _variability_, I do not
refer to the occurrence of local varieties, or to variations which
occur in the course of time, but I mean a high degree of individual
variability--a considerable fluctuation of characters in the
individuals of one and the same district or of the same brood. I
consider a species to be constant, on the other hand, when the
individuals from a small or large district differ from one another only
to a very slight extent. Constant forms are likewise generally, but not
invariably, such as are poor in local varieties, whilst variable forms
are those which are rich in such variations. Since the terms “variable”
and “constant” are but relative, I will confine myself to the most
extreme cases, those in which the individual peculiarities fluctuate
within very wide or very narrow limits.

As no observations upon the degree of variability shown by a species in
the different stages of its development were available, I was obliged
to fall back upon my own, at least so far as relates to the larval and
pupal stages, whilst for the imaginal stage the wide experience of my
esteemed friend Dr. Staudinger has been of essential service to me.

Let us in the first place confine our attention to the three chief
forms which every Lepidopteron presents, viz. larva, pupa, and imago.
With respect to the constancy or variability of these three forms, we
actually find in nature all the combinations which are theoretically
conceivable.

(1.) There are species which possess a high degree of constancy in
all three stages, such, for example, as _Limenitis Camilla_, _Pieris
Brassicæ_,[171] _Sphinx Ligustri_, and _Euchelia Jacobææ_.

(2.) There are species showing a high degree of variability in all
three stages. This case must be of rare occurrence, as I am only able
to adduce _Araschnia Prorsa-Levana_, a fact which arises from the
circumstance that the pupal stage is, as a rule, but seldom variable.

(3.) There are species which are variable in two stages and constant
in the third. To this class, for example, belongs _Smerinthus Tiliæ_,
of which the larva and imago are very variable, whilst the pupa is
quite constant. The same is the case with _Lasiocampa Pini_, the
well-known fir moth. Many butterflies show this same phenomenon in
other combinations, such, for instance, as _Vanessa Urticæ_ and
_Polychloros_, in which the larva and pupa are very variable, and the
imago very constant. In a less degree the same is also the case with
_Vanessa Atalanta_, whilst in _Pieris Napi_ the pupa and imago are
variable, and the caterpillar remarkably constant, this likewise being
the case with the local form _Bryoniæ_, which, according to my theory,
is to be regarded as the parent form of _Napi_ (See Part I. of the
present volume).

(4.) There are species which are constant in two stages, and variable
only in the third. Thus, a few species can be found in which the
larva and pupa are constant and the imago variable. This is the case
with _Saturnia Yamamai_, the imago of which is well known to present
numberless shades of colour, varying from light yellow to greyish
black, whilst the green caterpillar shows only slight individual
differences of marking, and scarcely any differences of colour. The
pupa of this species is quite constant. _Arctia Caja_ and _Hebe_, and
_Chelonia Plantaginis_ belong to this same category.

There are a very large number of species which possess very constant
imagines and pupæ, but extremely variable larvæ. The following are
the cases known to me:--_Macroglossa Stellatarum_, _Fuciformis_
and _Bombyliformis_; _Chærocampa Elpenor_, _Celerio_, and _Nerii_;
_Deilephila Galii_, _Livornica_, Hübn., _Hippophaës_, _Vespertilio_,
and _Zygophylli_; _Sphinx Convolvuli_; _Acherontia Atropos_;
_Smerinthus Ocellatus_ and _Tiliæ_; _Callimorpha Hera_; _Cucullia
Verbasci_ and _Scrophulariæ_.

Cases in which the variability depends entirely upon the pupa, while
the larva and imago are extremely constant, are of great rarity.
_Vanessa Io_ is a case in point, the pupa being light or dark brown, or
bright golden green, whilst in the two other stages scarcely any light
shades of colour or variations in the very complicated marking are to
be met with.

The facts thus justify the above view that the individual stages of
development change independently--that a change occurring in one stage
is without influence on the preceding and succeeding stages. Were this
not the case no one stage could possibly become variable without all
the other stages becoming so. Did there exist a correlation between
larvæ, pupæ, and imagines of such a nature that every change in the
larva entailed a corresponding change in the imago, as soon as a large
number of larval characters became fluctuating (_i.e._ as soon as this
stage became variable), a large number of imaginal characters would
necessarily also become fluctuating (_i.e._ this stage would also
become correspondingly variable).

There is one other interpretation which might perhaps be attempted from
the point of view of the old doctrine of species. It might be said that
it is a special property of certain larval or imaginal markings to be
variable whilst others are constant, and since the larval and imaginal
markings of a species are generally quite distinct, it may easily
happen that a butterfly possessing markings having the property of
constancy may belong to a caterpillar having variable markings.

There is a soul of truth underlying this objection, since it is
true that the various forms of markings which occur in Lepidoptera
apparently reach different degrees of constancy. If we speak of the
constancy or variability of a species, a different meaning is attached
to these expressions according as we are dealing _e.g._ with a species
of _Sphinx_ or a species of _Arctia_. That which in the latter would
be estimated as a high degree of constancy, in the former would be
taken as a considerable amount of variability. It is of interest, in
connection with the question as to the causes of constancy, to note
that the power of any form of marking to attain to a high degree of
constancy is by no means inversely proportional to the complication of
the marking, as would have been expected _à priori_.

Thus, the species of _Sphinx_ and of allied genera possess on their
fore-wings, which are mostly coloured with a mixture of dull grey,
white and black, an exceedingly complicated arrangement of lines
which, in constant species, show a high degree of uniformity: on the
other hand, the checquered fore-wings of our _Arctiidæ_, which are far
more coarsely marked, always show, even in the most constant species,
well-marked individual differences. The different types of marking must
therefore be measured by different standards.

But in granting this, we decidedly refute the statement that constancy
and variability are inherent properties of certain forms of marking.

This reasoning is based on the simple fact that a given type of marking
comprises both species of great constancy and of (relatively) great
variability.

Thus, the fore-wings of _Sphinx Ligustri_ and _S. Convolvuli_ are
extremely constant, whilst the very similarly marked _Anceryx
(Hyloicus) Pinastri_ is exceedingly variable. Similarly _Deilephila
Euphorbiæ_ is known by its great variability of colouring and marking,
whilst _D. Galii_, which resembles this species so closely as to be
sometimes confounded with it, possesses a high degree of constancy,
and further, the Corsican and Sardinian _D. Dahlii_ is very variable.
Among the family _Arctiidæ_, _Callimorpha Hera_ and the Alpine _Arctia
Flavia_ are cases of constancy, whilst _A. Caja_, which is so similar
to the last species, is so generally variable that two perfectly
identical specimens can scarcely be found together.

The same can be shown to hold good for the markings of caterpillars.
Thus, the larva of _D. Dahlii_ shows very considerable variability,
whilst that of _D. Galii_ is very constant in marking (disregarding the
ground-colour). So also the larva of _Vanessa Urticæ_ is very variable
and that of _V. Antiopa_ very constant, &c.

The great differences with respect to constancy or variability which
are displayed by the different stages of one and the same species,
must therefore find their explanation elsewhere than in the type of
the marking itself. The explanation must be found in the circumstance
that each stage changes independently of the others, and at different
periods can enter a new phase of variability.

We are here led in anticipation to the main question:--Are changes
produced by internal or external causes? is it the physical nature of
the organism which is compelled to become remoulded spontaneously after
the lapse of a certain period of time? or does such modification only
occur when produced directly or indirectly by the external conditions
of life?

In the cases before us the facts undoubtedly indicate a complete
dependence of the transformations upon external conditions of life.

The independent appearance of variability in the separate stages of the
metamorphosis might, however, be regarded as only apparent. It might
still be attempted to attribute the changes to a purely inherent cause,
_i.e._, to a phyletic vital force, by assuming that the latter acts
periodically in such a manner that at first one and then the following
stage becomes variable, until finally the entire species is transformed.

There is but little to be said in reply to this if we once take refuge
in entirely unknown forces, the operation of which can be arbitrarily
conceived to be either constant or periodic.

But granting that such a transforming power exists and acts
periodically, the variability must always pass over the different
stages in a fixed direction, like a wave over the surface of
water--imago, pupa, and larva, or larva, pupa, and imago, must
_successively_ become variable. Cases like that of _Araschnia
Prorsa_, in which all three stages are variable, may certainly be
thus explained, but those instances in which the larva and imago
are extremely variable, and the pupa quite constant, are entirely
inexplicable from this point of view.

The latter can, however, be very simply explained if we suppose the
changes to be dependent upon external influences. From this standpoint
we not only see how it is possible that an intermediate stage should
remain uninfluenced by the changes which affect the two other stages,
but we can also understand why it should just be the pupal stage that
plays this part so frequently. If we ask why most pupæ are constant
and are relatively but very slightly variable, the answer will be
found in the facts that all pupæ which remain concealed in the earth
or inside plants (_Sesiidæ_), or which are protected by stout cocoons,
show complete constancy, whilst any considerable amount of variability
occurs only in those pupæ which are suspended or openly exposed. This
is closely connected with a fact to which I have called attention on a
former occasion,[172] viz., that dimorphism occurs in certain pupæ,
but only in those which are openly exposed and which are therefore
visible to their foes. I am only acquainted with such cases among the
pupæ of butterflies, and it is likewise only among these that I have
found any considerable amount of variability.

Facts of this kind indicate that Nature does not uselessly sport with
forms, but that at any rate changes of this sort result from external
influences. The greater frequency of variability among larvæ and its
comparative rarity in imagines is also undoubtedly in favour of this
view.

It has already been shown that species with variable larvæ and constant
imagines are extremely common, but that those with constant larvæ
and variable imagines are very rare. This confirms the conclusions,
already drawn above, first, that the variability of the imago cannot
owe its existence to the variability of the larvæ, and secondly, that
the causes which produce variability affect the larval condition more
commonly than that of the imago.

Where can these causes be otherwise sought than in the external
conditions of life, which are so widely different in the two stages,
and which are much more variable for the larva than for the imago?

Let us take the species of one genus, _e.g._ those of _Deilephila_.
The imagines of our European species--as far as we know--all live in
precisely the same manner; they all fly at twilight,[173] showing a
preference for the same flowers and very often frequenting the same
spots, so that in the haunts of one species the others are almost
always to be met with, supposing them to occur in the same locality.
They conceal themselves by day in similar places, and are attacked by
similar foes.

It is quite different with the caterpillars. These, even in the case of
the most closely allied species, live under different conditions, as
appears from the fact that they feed on different plants. The latter
can, however, produce changes both directly and indirectly. The larvæ
may acquire adaptive colours and markings, and these would vary in
accordance with the colour and structure of the food-plant; or they may
become brightly coloured as a sign of distastefulness in cases where
they are inedible. Then again the colour of the soil on which the larvæ
live would act upon their colours making these adaptive. Certain habits
of the caterpillars may also be dependent upon the nature of their
food-plants. Thus, _e.g._ _Deilephila Hippophaës_ feeds only at night,
and conceals itself by day under moss and among the leaves at the base
of the food-plant; but _D. Euphorbiæ_ could not acquire such a habit,
because _Euphorbia Cyparissias_ generally grows on arid soil which is
poor in vegetation, and which therefore affords no concealment, and
furthermore, because a caterpillar, as long as it continues to feed,
cannot, and as a matter of fact does not, ever wander far from its
food-plant. A habit of concealment by burying in the earth also, such
for example as occurs in _Acherontia Atropos_, could not be acquired by
_D. Euphorbiæ_, because its food-plant generally grows on hard, dry,
and stony ground.

In addition to these considerations, the foes would be different
according as the caterpillar lived on plants which formed dense
thickets covering large extents of the shore (_Hippophae_) or grew
isolated on dry hillocks and declivities where the herbage was scanty
or altogether absent; or again, according as the insect, in conjunction
with such local differences, fed by day or had acquired the habit
of feeding only by night. It must in fact be admitted that new and
improved adaptations, or, in more general terms, that inducements to
change, when depending on the environment, must be more frequently
dissimilar for larvæ than for the imagines. We must accordingly expect
to find actual change, or that condition of variability which may be
regarded as initiative to change, occurring more commonly in larvæ than
in perfect insects.

Since facts are in complete accordance with the results of these _à
priori_ considerations we may also venture to conclude that the basis
of the considerations is likewise correct, viz., the supposition that
the changes of colour and marking in caterpillars, pupæ, and imagines
result from external influences only.

This must not be taken as signifying that the single stages of the
larval development are also only able to change through the action of
external influences. The larval stages are correlated with each other,
as has already been shown (see the previous essay): new characters
arise in the adult caterpillar at the last stage and are then gradually
transferred back to the younger stages quite independently of external
influences, this recession being entirely brought about by the laws of
correlation. Natural selection here only exerts a secondary action,
since it can accelerate or retard this transference, according as the
new characters are advantageous or disadvantageous to the younger
stages.

Now as considerable individual differences appear in the first
acquisition of a new character with respect to the rapidity and
completeness with which the individuals acquire such a character, the
same must obtain for the transference of an improvement acquired in
the last stage to the next younger stage. The new character would be
acquired by different individuals in different degrees and at different
rates--it would have, to a certain extent, to struggle with the older
characters of the stage; in brief, the younger stage would become
variable.

Variability of this kind might well be designated as _secondary_,
in contradistinction to _primary_ variability; the latter (primary)
depends upon an unequal reaction of the individual organisms to
external influences, the former (secondary) results from the unequal
strength and rate of the action of the innate laws of growth governing
the organism. In both cases alike exceeding variability may occur, but
the causes producing this variability are dissimilar.

The different stages of larval development would thus frequently
display independent variability in a manner similar to the pupal or
imaginal stages, since they can show individual variability while
the other stages of development remain constant. This appearance
of independent variability in the different stages of the larval
development, however, is in truth deceptive--we have here in fact
a kind of wave of variability, which passes downwards through the
developmental stages, becoming gradually weaker, and finally dying out
completely.

In accordance with this, we very frequently find that only the last or
two last stages are variable, while the younger stages are constant.
Thus in _Macroglossa Stellatarum_, the larvæ are constant in the first,
second, and third stages, but become variable in the fourth, and in
the fifth stage first show that high degree of variability which has
already been described in detail (See. Pl. III., Figs. 3-12). The
larvæ, of _Vanessa Cardui_ also, according to my notes, are extremely
constant in the first four stages in spite of their complicated
marking, but become variable in the fifth stage, although to no very
great extent.

In _Smerinthus Tiliæ_, _Ocellatus_ and _Populi_ also, the greatest
larval variability is shown only in the last stage, the preceding
stages being very constant. These cases by no means depend upon the
marking of the young stages being simpler and therefore being less
capable of varying. The reverse case also occurs. In a somewhat similar
manner as the young of the tapir and wild hog are striped, while the
adult animals are plainly coloured, the young caterpillars of _Saturnia
Yamamai_ possess longitudinal black lines on a yellow ground, while as
early as in the second stage a simple green colour appears in the place
of this complicated but perfectly constant marking. If the young stages
are so frequently constant, this rather depends upon the fact that the
transference of a new character to these stages not only takes place
gradually, but also with continually diminishing energy, in a manner
somewhat similar to physical motion, which continually diminishes in
speed by the action of resistance till it is completely arrested. This
constancy of the younger stages may further be due to the circumstance
that the characters would only be transferred when they had become
fixed in the last stage, and were consequently no longer variable. The
transferred characters may thus have acquired a greater regularity,
_i.e._ a less degree of variability, than they possessed at their first
origination. Extensive investigations in this special direction must
be made if the precise laws, in accordance with which the backward
transference of new characters takes place, are to be discovered. By
such researches only should we arrive with certainty at the causes
which determine the lesser variability of the young larval stages.

It may also occur that the early stages are variable, whilst the
later stages are constant, although this case appears to happen less
frequently. Thus, the caterpillars of _Gastropacha Quercifolia_ vary
considerably in the second stage but are constant at a later period,
and the same is the case with _Spilosoma Urticæ_, which in the second
stage may be almost considered to be dimorphic, but which subsequently
becomes constant.

Cases in which the first stage is variable appear to be of the least
frequent occurrence. I know of only one such instance, viz., _Anceryx
Pinastri_, of which the newly hatched larvæ (Pl. VI., Fig. 53) show
considerable differences in the brownish-black crescentic spots. The
second (Fig. 54), third, and fourth stages are then tolerably constant,
while the fifth stage again is very variable.

An instance of this kind can be easily explained by two waves of
variation, the first of which now affects only the first stage,
while the second has just commenced to affect the fifth stage. Such
a supposition is not opposed to any theoretical considerations, but
rather has much probability in its favour, since we know that species
are from time to time subject to be remodelled; and further, that the
coalescence of several stages of phyletic development in the ontogeny
of one and the same species (see p. 226, development of the genus
_Deilephila_) shows that during the backward transference of one
character, new characters may appear in the last stage of the ontogeny,
and indeed very frequently at a time when the next youngest character
has not been transferred back so far as to the first stage.

That this secondary variability is to a certain extent brought about by
the conflict between the old and new characters, the latter striving to
suppress the former, is shown by the caterpillar of _Saturnia Carpini_
which I have observed for many years from this point of view, and than
which I do not know a more beautiful illustration.

When these larvæ leave the egg they are black, but in the adult state
are almost bright green--this at least being the case in a local form
which, from the district in the vicinity of Genoa where it is found,
I will designate as the var. _Ligurica_. Now whilst these two extreme
stages of development are relatively constant, the intermediate stages
show a variability which becomes greater the nearer the last stage
is approached, this variation in the marking depending simply on the
struggle between the green colour and the more anciently inherited
black. In this manner there arises, especially in the fourth stage
of the German local form, an incredible mixture of the most diverse
markings, all of which can, however, be very easily explained from the
foregoing point of view.

The simpler and, as I am inclined to believe, the older form of the
transformation is presented to us in the local variety _Ligurica_. In
the last stage, when 7.5 centimeters long, this form is of a beautiful
bright green colour without any trace of black marking[174] (Pl. VIII.,
Fig. 77). The colour of the six orange warts which are situated on
each segment is also similar in all specimens, so that this stage is
perfectly constant.

Our German _S. Carpini_ shows different characters in the fifth stage.
It is true that individual specimens occur which are entirely green
without any black, but these are rare; the majority possess a more or
less broad black ring encircling the middle of each segment (Pl. VIII.,
Figs. 78 and 79). Those specimens in which the black ring has become
broken up into large or small spots surrounding the base of the warts
constitute intermediate forms (Fig. 80). The last stage of the German
local form, unlike that of the Genoese local form, is therefore very
variable.

The two forms, moreover, do not simply differ in being more or less
advanced in phyletic development, but also in several other points. As
it is of great theoretical interest to show that a species can develop
local differences only in the stage of larva, I will here subjoin the
plain facts.

The differences consist in that the Genoese local form goes through
five moults whilst the German local form, like most caterpillars,
has only four moults. Further, in the Genoese form the light green,
which is also possessed by the German form in the fourth stage, when
it once appears, is retained to the end of the larval development,
whilst in the fifth stage of the German form this colour is replaced
by a dull greyish-green (compare Figs. 77 and 78). There is further
a very considerable difference in the earlier stages which shows
that the phyletic transforming process has taken a quite independent
course in the two forms. Since the struggle between the green and
black--retaining this idea--appears to be quite finished in the last
stage of the Genoese form, we should expect that the new colour, green,
would now also have encroached further upon the younger stages than
in the German form. Nevertheless, this is not the case, but quite
the reverse happens, the black maintaining its ground longer in the
Italian than in the German form.

In the Genoese form the two first stages are completely black, and
in the third stage an orange-yellow lateral stripe first appears. In
the German form this stripe appears in the second stage, and there
is not subsequently added, at least on the middle segments, a yellow
border surrounding some of the warts of the median series. In the
third stage, however, the yellow (which is but the precursor of the
later green colour) becomes further extended, so that the caterpillars
often appear of an orange colour, some or all of the warts and certain
spots and stripes only being black (Figs. 66 and 68). The warts are
also often yellow while the ground remains in most part black--in
brief, the bright colour is in full struggle with the black, and an
endless series of variations is the result of this conflict, whilst in
the corresponding stage of the Genoese form almost complete constancy
prevails.

This constancy remains also in the following (fourth) stage, the
caterpillar still being deep black, only the yellow (sulphur-coloured)
lateral stripe, which has now become brighter, indicating the impending
change (Fig. 67). This takes place in the fifth stage, in which the
ground-colour suddenly becomes bright green, the black remaining at
most only in traces on the anterior edges of the segments.

This is the same marking as is shown by the fourth stage of the German
form, only in this case individuals quite destitute of black do not
occur. In many specimens indeed black forms the ground-colour, the
green only appearing in certain spots (Figs. 71 to 75); in others the
green predominates, and these two extremes are connected by innumerable
intermediate forms, so that this stage must be regarded as the most
variable of all.

The sixth stage of the Genoese and the fifth of the German form have
already been compared together. The results may be thus tabulated:--

  _A. German form._                  _B. Genoese form._

  STAGE I. 9 days.                 9 days.
    Black; constant.                   Black; constant.

  STAGE II. 8 days.                11 days.
    Black, with orange-yellow          Black; constant.
      lateral stripe; variable.

  STAGE III. 5 days (in some       12 days.
    cases as much as 16 days).
    Black, with yellow; very           Black, with orange-yellow
      variable.                          lateral stripes; constant.

  STAGE IV. 16 days (in some       6 days.
    cases only 5 days).
    Bright green and black,            Black, with bright yellowish
      mixed; very variable.              lateral stripe; constant.

  STAGE V. 6 days (frequently      6 days.
    longer).
    Dark green, with or without        Bright green, small traces
      black bands; variable.             of black; variable.

  STAGE VI. Pupation.              18 days.
                                       Bright green, without any
                                         black; constant.

                                   STAGE VII. Pupation.

From this comparison we perceive that the process of transformation
has at least become preliminarily concluded in the Genoese form. Why
the backward transference of the newly-acquired character to the young
stages has not yet occurred, or, at least, why it is not in progress,
does not appear; neither can it be stated whether this will take place
later, although we may venture to suppose that such will be the case.
At first sight but a relatively short time appears necessary for the
single stage V., which is still in a state of fluctuation (variable),
to become constant by continued crossing, like all the other stages.

That the transformation is still in full progress in the German form,
is shown by the fact that in this case all the stages are variable with
the exception of the first--the second stage being only variable to a
small extent, the third to a much greater extent, and the fourth to the
highest degree conceivable, whilst the fifth and last stage is again
less variable--so that the greatest struggle between the old and new
characters takes place in the fourth stage.

Among the innumerable variations presented by this last stage a
complete series of transitional forms can be arranged so as to show the
gradual conquest of the black by the green, and thus indicating, step
by step, the course which the latter colour has taken.

In the blackest specimens there is nothing green but the lateral
(infra-spiracular) line which was yellow in the preceding stage, and a
crescent-shaped streak at the base of the middle warts together with
a still smaller crescent at the base of the upper warts (Figs. 71 and
81). These spots become extended in lighter specimens and approximate
so as to leave only narrow black bridges, a third spot being added at
the posterior edge of the warts (Figs. 72 and 82). The three spots
then extend on all sides, still leaving for a long period narrow
black lines at the boundaries where their growth has caused them to
abut. In this manner there frequently arises on the green ground a
true hieroglyphic-like marking (Figs. 85 and 86). Finally the black
disappears from the anterior edge and diminishes on the middle line of
the back where it still partly remains as a T-shaped figure (Figs. 73
and 74), although generally replaced elsewhere by the green with the
exception of small residues.

One point remained for a long time inexplicable to me, viz., the change
of the light green into dark grey-green which appeared in the last
stage in connection with a total change of the black marking.

Supposing that new characters are actually acquired only in the last
stage, and that from this they are transferred to the younger stages,
we should expect to find completely developed in the last stage the
same colouring and markings as are possessed more or less incompletely
in the fourth stage. Now since the developmental tendency to the
removal of black and to the predominance of green--if we may thus
venture to express it--is obvious in the fourth stage, we may expect to
find in the fifth stage a bright green ground-colour, either without
any mixture of black or with such black spots and streaks as were
retained in the fourth stage as residues of the original ground-colour.
But instead of this the fifth stage shows a dark green colour, and a
more or less developed black marking which cannot in any way be derived
from that of the fourth stage.

The Genoese local form observed last year first gave me an explanation
to the extent that in this form the last stage is actually only the
potential penultimate stage, or, more correctly expressed, that
the same characters which at present distinguish the last stage of
this form, are already more or less completely transferred to the
penultimate stage.

The apparently paradoxical behaviour of the German form can be
explained by supposing that before the pure bright green had become
completely transferred to the penultimate stage a further change
appeared in the last stage, the green ground-colour becoming darker,
and black transverse bands being formed. The marking of the last
stage would then be regarded as the reverse of that of the preceding
stage; the absence of black would be the older, simple black spots at
the base of the warts the next in succession, and a connected black
transverse band the most advanced state of the development.

Whether this explanation is correct, and if so, what causes have
produced the second change, may perhaps be learnt at some future
time by a comparison with the ontogeny of other _Saturniidæ_; in the
meantime this explanation receives support from another side by the
behaviour of the Genoese local form. If the last stage of the German
form has actually commenced to be again re-modelled, then this variety
is further advanced in phyletic development than the Genoese form; and
this corresponds entirely with the theory that in the former the light
colour (the orange considered as preliminary to the transformation into
green) has already been carried down into the second stage, whilst in
the Genoese variety even in the fourth stage only the first rudiments
of the colour-transformation show themselves.

The Genoese form is to a certain extent intermediate between the
German form of _Saturnia Carpini_ and the nearly related _S. Spini_,
a species inhabiting East Germany. In this latter the larvæ, even in
the adult state, are completely black with yellow warts. This form of
caterpillar must therefore be regarded as phyletically the oldest, and
this very well agrees with the character of the moth, which differs
essentially from _S. Carpini_ only in not being sexually dimorphic. In
_Carpini_ the male possesses a far more brilliant colouring than the
female, the latter agreeing so completely with the female of _Spini_
that it can hardly be distinguished therefrom, especially in the case
of the somewhat larger South European specimens of the last species.
Now as the more simple colouring of the female must in any case be
regarded as the original form, we must consider _Spini_, both sexes
of which possess this colouring, to be phyletically the older form,
and _Carpini_, the male of which has become differently coloured, must
be considered as the younger type. This completely accords with the
characters of the larvæ.

I must here mention that I have also asked myself the question whether
the variations of the different larval stages are connected together as
cause and effect--whether the lightest specimens of the fifth stage may
perhaps not also have been the lightest individuals of the third and
fourth stages.

Such relationship is only apparent between the third and fourth stages;
the darkest larvæ of the third stage become the darker varieties of the
fourth stage, although it is true that the lighter forms of the third
sometimes also become dark varieties in the fourth stage. Between the
fourth and fifth stages there is scarcely any connection of this kind
to be recognized. Thus, the darkest varieties of the fourth stage
sometimes become the lightest forms of the fifth stage, whilst in other
cases from the lightest individuals of the fourth stage there arise
all the possible modifications of the fifth stage. Further details may
be omitted: the negative result cannot cause any surprise, as it is a
necessary consequence of the continued crossing that must take place.

We thus see that the three chief stages of development (larva, pupa,
and imago) actually change in colour independently of each other,
the single stages of the larval development being however in greater
dependence upon one another, and being connected indeed in such a
manner that a new character cannot be added to the last stage without
being transferred in the course of time to the preceding stage, and at
a later period from this again even to the youngest stage, supposing
it not to be previously delayed in the course of its transference by
unknown opposing forces. On this last point, however, the facts at
present available do not admit of any certain decision.

But why do the individual larval stages behave in this respect so
very differently to the chief stages of the whole development? why
are the former so exactly correlated whilst the latter are not? If
new characters have a general tendency to become transferred to the
younger ontogenetic stages, why are not new imaginal characters first
transferred to the pupa, and finally to the larva?

The answer to these questions is not far to find. The wider two stages
of a species differ in structure, the less does correlation become
possible; the nearer the two stages are morphologically related, the
more powerful does the action of correlation become. It is readily
conceivable that the more widely two succeeding stages deviate
in structure and mode of life, the less possible does it become
for characters to be transferred from one to the other. How is it
possible, for example, that a new character in the proboscis or on the
wings of a butterfly can be transferred to the caterpillar? If such
correlation existed it could only manifest itself by some other part
of the caterpillar changing in correspondence with the change of the
proboscis or wings of the butterfly. That this is not the case has, in
my opinion, been conclusively shown by all the foregoing considerations
respecting the independent variability of the chief stages of the
metamorphosis.

There are, moreover, an endless number of facts which prove the
independence of the individual stages of development--I refer to
the multitudinous phenomena presented by metamorphosis itself.
The existence of that form of development which we designate as
metamorphosis is alone sufficient to prove incontestably that the
single stages are able to change independently of one another to a most
remarkable extent.

If we now ask the question: how has the so-called “complete”
metamorphosis of insects arisen? the answer can only be: through the
gradual adaptation of the different stages of development to conditions
of life which have continually deviated more and more widely from each
other.[175]

But if individual stages of the post-embryonic development can finally
attain to such complete diversity of structure as that of the larva and
imago through gradual adaptations to continually diverging conditions
of life, this shows that the characters acquired by the single stages
are always only transferred to the same stages of the following
generation, whilst the other stages remain uninfluenced thereby. This
depends upon that form of heredity designated by Darwin “inheritance at
corresponding periods of life,” and by Haeckel “homochronic heredity.”



II.

DOES THE FORM-RELATIONSHIP OF THE LARVA COINCIDE WITH THAT OF THE IMAGO?


Having thus established the independence in the variability of
the individual stages of metamorphosis, I will now turn to the
consideration of the question as to how far a parallelism is displayed
in the phyletic development of these stages. Is there a complete
congruence of form-relationship between larvæ on the one hand and
imagines on the other? does the classification founded on the
morphology of the imagines agree with that based on the morphology of
the larvæ or not?

If, according to Claus,[176] we divide the order Lepidoptera into six
great groups of families, it is at once seen that these groups, which
were originally founded exclusively on imaginal characters, cannot by
any means be so clearly and sharply defined by the larval characters.

This is certainly the case with the _Geometræ_, of which the larvæ
possess only ten legs, and on this account progress with that peculiar
“looping” movement which strikes even the uninitiated. This group,
which is very small, is however the only one which can be founded on
the morphology of the larvæ; it comprises only two nearly related
families (_Phytometridæ_ and _Dendrometridæ_), and it is not yet
decided whether these should not be united into one group comprising
the family characters of the whole of the “loopers.”

Neither the group of Micro-lepidoptera, nor those of the _Noctuina_,
_Bombycina_, _Sphingina_, and _Rhopalocera_, can be based
systematically on larval characters. Several of these groups are indeed
but indistinctly defined, and even the imagines present no common
characteristics by which the groups can be sharply distinguished.

This is well shown by the _Rhopalocera_ or butterflies. These insects,
in their large and generally brilliantly coloured wings, which are
usually held erect when at rest, and in their clubbed antennæ, possess
characters which are nowhere else found associated together, and
which thus serve to constitute them a sharply defined group.[177]
The caterpillars, however, show a quite different state of affairs.
Although the larval structure is so characteristic in the individual
families of butterflies, these “larval-families” cannot be united
into a larger group by any common characters, and the “_Rhopalocera_”
would never have been established if only the larvæ had been known.
It is true that they all have sixteen legs, that they never possess a
Sphinx-like horn, and that they are seldom hairy, as is the case with
many _Bombycidæ_,[178] but these common _negative_ characters occur
also in quite distinct groups.

In the butterflies, therefore, a perfect congruence of
form-relationship does not exist, inasmuch as the imagines constitute
one large group of higher order whilst the larvæ can only be formed
into families. If it be admitted that the common characters of
butterflies depend on their derivation from a common ancestor, the
imagines must have retained certain common characters which enable them
to be recognized as allies, whilst the larvæ have preserved no such
characters from the period at which the families diverged.

Without going at present into the causes of these phenomena I will pass
on to the consideration of further facts, and will now proceed to
investigate both the form-relationships within the families. Here there
can be no doubt that in an overwhelmingly large majority of cases the
phyletic development has proceeded with very close parallelism in both
stages; larval and imaginal families agree almost completely.

Thus, under the group _Rhopalocera_ there is a series of families
which equally well permit of their being founded on the structure of
the larva or on that of the imago, and in which the larvæ and imagines
therefore deviate from one another to the same extent. This is the
case, for instance, with the families of the _Pieridæ_, _Papilionidæ_,
_Danaidæ_, and _Lycænidæ_.

But there are also families of which the limits would be very different
if the larvæ were made the basis of the classification instead of the
butterflies as heretofore. To this category belongs the sub-family
_Nymphalinæ_. Here also a very characteristic form of caterpillar
indeed prevails, but it does not occur in all the genera, being
replaced in some by a quite different form of larva.

In the latest catalogue of Diurnal Lepidoptera, that of Kirby (1871),
112 genera are comprised under this family. Of these most of the larvæ
possess one or several rows of spines on most or on all the segments, a
character which, as thus disposed, is not met with in any other family.

This character is noticeable in genera 1 to 90, if, from those
genera of which the larvæ are known, we may draw a conclusion with
reference to their allies. I am acquainted with larvæ of genus 2,
_Agraulis_, Boisd. (_Dione_, Hübn.); of genus 3, _Cethosia_, Fabr.; 10,
_Atella_, Doubl.; 12, _Argynnis_, Fabr.; 13, _Melitæa_,[179] Fabr.;
19, _Araschnia_, Hübn.; 22, _Vanessa_, Fabr.; 23, _Pyrameis_, Hübn.;
24, _Junonia_, Hübn.; 31, _Ergolis_, Boisd.; 65, _Hypolimnas_, Hübn.
(_Diadema_, Boisd.); 77, _Limenitis_, Fabr.; 81, _Neptis_, Fabr.; 82,
_Athyma_, Westw.; and finally with those of genus 90, _Euthalia_,
Hübn.--which, according to Horsfield’s figures, possess only two rows
of spines, these being remarkably long and curved, and fringing both
sides. It may be safely assumed that the intermediate genera would
agree in possessing this important character of the Nymphalideous
larvæ, viz., spines.

After the genus 90 there are 22 more genera, and these are spineless,
at least in the case of the two chief genera, 93, _Apatura_, and
104, _Nymphalis_. Of the remainder I know neither figures nor
descriptions.[180] In the two genera named the larvæ are provided with
two or more spine-like tentacles on the head, and the last segment
ends in a fork-like process directed backwards. The body is otherwise
smooth, and differs also in form from that of the larvæ of the other
_Nymphalinæ_, being thickest in the middle, and tapering anteriorly and
posteriorly; neither is the form cylindrical, but somewhat flattened
and slug-shaped. If therefore we were to arrange these butterflies by
the larvæ instead of by the imagines, these two genera and their allies
would form a distinct family, and could not remain associated with the
90 other Nymphalideous genera.

We have here a case of _incongruence_; the imagines of the genera 1-90
and 91-112 are more closely allied than their larvæ.

From still another side there arises a similar disagreement. The larvæ
of the genera _Apatura_ and _Nymphalis_ agree very closely in their
bodily form and in their forked caudal appendage with the caterpillars
of another sub-family of butterflies, the _Satyrinæ_, whilst their
imagines differ chiefly from those of the latter sub-family in the
absence of an enlargement of certain veins of the fore-wings, an
essential character of the _Satyrinæ_.

This double disagreement has also been noticed by those systematists
who have taken the form of the caterpillar into consideration.
Thus, Morris[181] attempted to incorporate the genera _Apatura_ and
_Nymphalis_ into the family _Libytheidæ_, placing the latter as
transitional from the _Nymphalidæ_ to the _Satyridæ_. But although the
imagines of the genera _Apatura_, _Nymphalis_, and _Libythea_ may be
most closely related--as I believe they actually are--the larvæ are
widely different, being at least as different as are those of _Apatura_
and _Nymphalis_ from the remaining _Nymphalinæ_.

Now if we could safely raise _Apatura_ and _Nymphalis_ into a distinct
family--an arrangement which in the estimation of Staudinger[182] is
correct--and if this were interpolated between the _Satyridæ_ and
_Nymphalidæ_, such an arrangement could only be based on the larval
structure, and that of the imagines would thus remain unconsidered,
since no other common characters can be found for these two genera than
those which they possess in common with the other Nymphalideous genera.

The emperor-butterflies (_Apatura_), by the ocelli of their fore-wings
certainly put us somewhat in mind of the _Satyrinæ_, in which such
spots are always present; but this character does not occur in the
genus _Nymphalis_, and is likewise absent in most of the other genera
of this group. The genus _Apatura_ shows in addition a most striking
similarity in the markings of the wings to the purely Nymphalideous
genus _Limenitis_, and it is therefore placed, by those systematists
who leave this genus in the same family, in the closest proximity to
_Limenitis_. This resemblance cannot depend upon mimicry, since not
only one or another but _all_ the species of the two genera possess a
similar marking; and further, because similarity of marking alone does
not constitute mimicry, but a resemblance in colour must also be added.
The genus _Limenitis_ actually contains a case of imitation, but in
quite another direction; this will be treated of subsequently.

It cannot therefore be well denied that in this case the larvæ show
different relationships to the imagines.

If the “natural” system is the expression of the genetic relationship
of living forms, the question arises in this and in similar cases
as to whether the more credence is to be attached to the larvæ
or to the imagines--or, in more scientific phraseology, which
of the two inherited classes of characters have been the most
distinctly and completely preserved, and which of these, through its
form-relationship, admits of the most distinct recognition of the
blood-relationship, or, inversely, which has diverged the most widely
from the ancestral form? The decision in single instances cannot but be
difficult, and appears indeed at first sight impossible; nevertheless
this will be arrived at in most cases as soon as the ontogeny of the
larvæ, and therewith a portion of the phylogeny of this stage, can be
accurately ascertained.

As in the _Rhopalocera_ most of the families show a complete congruence
in the form-relationship of the caterpillars and perfect insects, so
a similar congruence is also found in the majority of the families
belonging to other groups. Thus, the two allied families of the group
_Sphingina_ can also be very well characterized by their larvæ;[183]
both the _Sphingidæ_ and the _Sesiidæ_ possess throughout a
characteristic form of larva.

Of the group _Bombycina_ the family of the _Saturniidæ_ possess
thick cylindrical caterpillars, of which the segments are beset with
a certain number of knob-like warts. It is true that two genera of
this family (_Endromis_ and _Aglia_) are without these characteristic
warts, but the imagines of these genera also show extensive and common
differences from those of the other genera. A distinct family has in
fact already been based on these genera (_Endromidæ_, Boisd.). Thus the
congruence is not thereby disturbed.

So also the families _Liparidæ_, _Euprepiidæ_, and _Lithosiidæ_ appear
sharply defined in both forms; and similar families occur likewise
under the _Noctuina_, although in this group the erection of families
presents great difficulties owing to the near relationship of the
genera, and is always to some extent arbitrary. It is important,
however, that it is precisely the transitional families which present
intermediate forms both as larvæ and as imagines.

Such an instance is offered by the _Acronyctidæ_, a family belonging
to the group _Noctuina_. The imagines here show in certain points an
approximation to the group _Bombycina_; and their larvæ, which are
thickly covered with hairs, likewise possess the characteristics of
many of the caterpillars of this group.[184]

A second illustration is furnished by the family _Ophiusidæ_, which is
still placed by all systematists under the _Noctuina_, its affinity to
the _Geometrina_, however, being represented by its being located at
the end of the _Noctuina_. The broad wings and narrow bodies of these
moths remind us in fact of the appearance of the “geometers;” and the
larvæ, like the imagines, show a striking resemblance to those of the
_Geometrina_ in the absence of the anterior abdominal legs. For this
reason Hübner in his work on caterpillars has termed the species of
this family “_Semi-Geometræ_.”

All these cases show a complete congruence in the two kinds of
form-relationship; but exceptions are not wanting. Thus, the family
_Bombycidæ_ would certainly never have been formed if the larval
structure only had been taken into consideration, since, whilst the
genera _Gastropacha_, _Clisiocampa_, _Lasiocampa_, _Odonestis_, and
their allies, are thickly covered with short silky hairs disposed in
a very characteristic manner, the caterpillars of the genus _Bombyx_,
to which the common silkworm, _B. Mori_, belongs, are quite naked and
similar to many Sphinx-caterpillars (_Chærocampa_). Are the imagines of
the genera united under this family, at any rate morphologically, as
unequally related as their larvæ? Whether it is correct to combine them
into one family is a question that does not belong here; we are now
only concerned with the fact that the two stages are related in form in
very different degrees.

An especially striking case of incongruence is offered by the family
_Notodontidæ_, under which Boisduval, depending only on imaginal
characters, united genera of which the larvæ differed to a very great
extent. In O. Wilde’s work on caterpillars this family is on this
account quite correctly characterized as follows:--“Larvæ of various
forms, naked or with thin hairs, sixteen or fourteen legs.”[185]
In fact in the whole order Lepidoptera there can scarcely be found
associated together such diverse larvæ as are here placed in one
imago-family; on one side the short cylindrical caterpillars of
the genus _Cnethocampa_, Steph. (_C. Processionea_, _Pithyocampa_,
&c.), which are covered with fine, brittle, hooked hairs, and are
very similar to the larvæ of _Gastropacha_ with which they were
formerly united; and on the other side there are the naked, humped,
and flat-headed larvæ of the genus _Harpyia_, Ochs., with their two
long forked appendages replacing the hindmost pair of legs, and the
grotesquely formed caterpillars of the genera _Stauropus_, Germ.,
_Hybocampa_, Linn., and _Notodonta_, Ochs.

The morphological congruence between larvæ and imagines declares itself
most sharply in genera, where it is the rule almost without exception.
In this case we can indeed be sure that a genus or sub-genus founded
on the imagines only will, in accordance with correct principles,
present a corresponding difference in the larvæ. Had the latter been
known first we should have been led to construct the same genera as
those which are now established on the structure of the imagines, and
these, through other circumstances, would have stood in the same degree
of morphological relationship as the genera founded on the imagines.
There is therefore a congruence in a double sense; in the first place
the differences between the larvæ and imagines of any two genera are
equally great, and, in the next place, the common characters possessed
by these two stages combined cause them to form precisely the same
groups defined with equal sharpness; the genera coincide completely.

So also the butterflies of the sub-family _Nymphalinæ_ can well be
separated into genera by the characters of the larvæ, and these, as
far as I am able to judge, would agree with the genera founded on the
imagines.

The genus _Melitæa_, for example, can be characterized by the
possession of 7-9 fleshy tubercles bearing hairy spines; the genus
_Argynnis_ may be distinguished by always having six hairy unbranched
spines on each segment, and the genus _Cethosia_ by two similar spines
on each segment; the genus _Vanessa_ shows sometimes as many as
seven branched spines; and the genus _Limenitis_ never more than two
branched blunt spines on each segment, and so forth. If we go further
into details it will be seen that the most closely related imagines,
as might indeed have been expected, likewise possess the most nearly
allied larvæ, whilst very small differences between the imagines are
also generally represented by corresponding differences in the larvæ.
Thus, for instance, the genus _Vanessa_ of Fabricius has been divided
into several genera by later authors. Of these sub-genera, _Grapta_,
Doubl. (containing the European _C.-album_, the American _Fabricii_,
_Interrogationis_, _Faunus_, _Comma_, &c.), is distinguished by the
fact that the larvæ not only possess branched spines on all the
segments with the exception of the prothorax, but these spines are
also present on the head; in the genus _Vanessa_ (_sensû strictiori_),
Doubl., the head and prothorax are spineless (_e.g._ _V. Urticæ_); in
the tropical genus _Junonia_, Hübn., which was also formerly (Godart,
1819[186]) united with _Vanessa_, the larvæ bear branched spines on all
the segments, the head and prothorax included.

It is possible to go still further and to separate two species of
_Vanessa_ as two new genera, although they have hitherto been preserved
from this fate even by the systematists most given to “splitting.”
This decision is certainly justifiable, simply because these species
at present stand quite alone, and the practical necessity of forming a
distinct genus does not make itself felt, and this practical necessity
moreover frequently comes into conflict with scientific claims: science
erects a new genus based on the amount of morphological difference, it
being quite immaterial whether one or many species make up this genus;
such an excessive subdivision is, however, a hindrance to practical
requirements, as the cumbrous array of names thereby becomes still
further augmented.

The two species which I might separate from _Vanessa_ on the ground of
their greater divergence, are the very common and widely distributed
_V. Io_ and _Antiopa_, the Peacock Butterfly and the Camberwell Beauty.
In the very remarkable pattern of their wings, both show most marked
characteristics; _Io_ possesses a large ocellus on each wing, and
_Antiopa_ has a broad light yellow border which is not found in any
other species of _Vanessa_. There can be no doubt but that each of
these would have been long ago raised into a genus if similarly marked
species of _Vanessa_ occurred in other parts of the world, as is the
case with the other species of the genus. Thus, it is well known that
there is a whole series of species resembling our _V. Cardui_, and
another series resembling our _V. C.-album_, the two series possessing
the same respective types of marking; indeed on these grounds the
sub-genera _Pyrameis_ and _Grapta_ have been erected.[187]

I should not have considered it worth while to have made these remarks
if it had not been for the fact that the caterpillars of _V. Io_ and
_V. Antiopa_ differ in small particulars from one another and from the
other species of the genus. These differences relate to the number and
position of the spines, as can be seen from the following table:--

SPECIES OF THE GENUS VANESSA, FABR.

  +-------------------+-----------------------------------------------+
  |                   |   Number of Spines on the head and segments   |
  |                   |                 of the larva.                 |
  |                   +-----+-----+-----+-----+-----+-----+-----+-----+
  |                   |Head.|Segm.|Segm.|Segm.|Segm.|Segm.|Segm.|Segm.|
  |                   |     |  I. | II. | III.| IV. |  V. |VI.- | XII.|
  |                   |     |     |     |     |     |     | XI. |     |
  |                   +-----+-----+-----+-----+-----+-----+-----+-----+
  |V. Io              |  0  |  0  |  2  |  2  |  4  |  6  |  6  |  4  |
  |V. Antiopa         |  0  |  0  |  4  |  4  |  6  |  6  |  7  |  4  |
  |V. Urticæ          |  0  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  |V. Polychloros     |  0  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  |V. Ichnusa         |  0  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  |V. Atalanta        |  0  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  |V. C.-album        |  2  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  |V. Interrogationis |  2  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  |V. Levana          |  2  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  +-------------------+-----+-----+-----+-----+-----+-----+-----+-----+

This character of the number of spines will not be considered as too
unimportant when we observe how perfectly constant it remains in the
nearly allied species. This is the case in the three consecutive
forms, _Urticæ_, _Polychloros_, and _Ichnusa_. Now when we see
that two species which differ in their imaginal characters present
correspondingly small differences in their larvæ, this exact systematic
congruence indicates a completely parallel phyletic development.

Exceptions are, however, to be met with here. Thus, Hübner has united
one group of the species of _Vanessa_ into the genus _Pyrameis_ just
mentioned, on account of certain characteristic distinctions of the
butterflies. I do not know, however, how this genus admits of being
grounded on the structure of the larvæ; the latter, as appears from the
above table, agree exactly in the number and position of the spines
with the caterpillars of _Vanessa_ (_sensû strictiori_), nor can any
common form of marking be detected which would enable them to be
separated from _Vanessa_.

Still more striking is the incongruence in the genus _Araschnia_,
Hübn. (_A. Prorsa-Levana_), which, like the genus _Pyrameis_, is
entirely based on imaginal characters. This is distinguished from all
the other sub-genera of the old genus _Vanessa_ by a small difference
in the venation of the wings (the discoidal cell of the hind-wings is
open instead of closed). Now it is well-known that in butterflies the
wing-venation, as most correctly shown by Herrich-Schäffer, is the
safest criterion of “relationship.” It thus happens that this genus,
typified by the common _Levana_, is in Kirby’s Catalogue separated
from _Vanessa_ by two genera, and according to Herrich-Schäffer[188]
by forty genera! Nevertheless, the larvæ agree so exactly in their
spinal formula with _Grapta_ that we should have no hesitation in
regarding them as a species of this sub-genus. It appears to me very
probable that in this case the form-relationship of the caterpillar
gives more correct information as to the blood-relationship of the
species than that of the imago--in any case the larvæ show a different
form-relationship to the imagines.

Just as in the case of butterflies there are many genera of _Sphingidæ_
which can be based on the structure of the larvæ, and which agree with
those founded on the imagines.

Thus, the genus _Macroglossa_ is characterized by a straight anal horn,
a spherical head, and by a marking composed of longitudinal stripes,
these characters not occurring elsewhere in this combination. The
nearly allied genus _Pterogon_, on the other hand, cannot be based
on the larvæ only, since not only is the marking of the adult larva
very distinct in the different species, but the anal horn is present
in two species, whilst in a third (_P. Œnotheræ_) it is replaced by
a knob-like eye-spot. The genus _Sphinx_ (_sensû strictiori_) is
distinguished by the simple, curved caudal horn, the smooth, egg-shaped
head and smooth skin, and by a marking mainly composed of seven oblique
stripes. The genus _Deilephila_ is distinguished from the preceding by
a dorsal plate, situated on the prothorax and interrupting the marking,
as well as by the pattern, which here consists of a subdorsal line
with ring-spots more or less numerous and developed; the skin also
is rough, “shagreened,” although it must be admitted that there are
exceptions (_Vespertilio_). The genus _Chærocampa_ admits also of being
based on the form-relationship of its caterpillars, although this is
certainly only possible by disregarding the marking and taking alone
into consideration the peculiar pig-like form of the larvæ. The genus
_Acherontia_, so nearly related to _Sphinx_, possesses in the doubly
curved caudal horn a character common to the genus (three species
known[189]). Finally may be mentioned the genus _Smerinthus_, of which
the larvæ, by their anteriorly tapering form, their shagreened skin and
almost triangular head with the apex upwards, their simply curved anal
horn, and by their seven oblique stripes on each side, constitute a
genus as sharply defined as that formed by the moths.

Although in all the systematic divisions hitherto treated of there are
cases where the form-relationship of the larva does not completely
coincide with that of the imago, such incongruences are of far more
frequent occurrence in the smallest systematic group, viz. species.

The larvæ of two species have very frequently a much nearer
form-relationship than their imagines. Thus, the caterpillars of
_Smerinthus Ocellatus_ and _S. Populi_ are closely allied in
structure, marking, and colouring, whilst the moths in these two last
characters and in the form of the wings are widely separated.[190]
Judging from the larvæ we should expect to obtain two very similar
moths, but in fact both _Populi_ and _Ocellatus_ have many near allies,
and these closely related species sometimes possess larvæ which differ
more widely than those of more distantly related species of imagines.

Thus, in Amur-land and North America there occur species of
_Smerinthus_ which closely resemble our _Ocellatus_ in colour, marking,
and form of wing, and which possess the characteristic large blue
ocellus on the hind-wings. _S. Excæcatus_ is quite correctly regarded
as the representative American form of our _Ocellatus_, but its
caterpillar, instead of being leaf-green, is of a chrome-yellow, and
possesses dark green instead of white oblique stripes, and has moreover
a number of red spots, and a red band on the head--in brief, in the
very characters (colour and certain of the markings) in which the
imagines completely agree it is widely different from _Ocellatus_. It
appears also to be covered with short bristles, judging from Abbot and
Smith’s figure.[191]

Just in the same way that the species having the nearest conceivable
form-relationship to _Ocellatus_ possesses a relatively strongly
diverging larva, so does the nearest form-relation of _Populi_ (imago)
offer a parallel case. This species, which is also North American,
lives on _Juglans Alba_. The imago of _Smerinthus Juglandis_ differs
considerably from _S. Populi_ in the form of the wings, but it
resembles the European species so closely in marking and colouring that
no doubt can exist as to the near relationship of the two forms. The
caterpillar of _S. Juglandis_,[192] however, differs to a great extent
from that of _Populi_ in colour--it is not possible to confound these
two larvæ; but those of _Populi_ and _Ocellatus_ are not only easily
mistaken for one another, but are distinguished with difficulty even by
experts.

In this same family of the _Sphingidæ_ cases are not wanting in which,
on the other hand, the moths are far more closely allied than the
larvæ. This is especially striking in the genus _Deilephila_, eight
species of which are allied in the imaginal state in a remarkable
degree, whilst the larvæ differ greatly from one another in colour,
and to as great an extent in marking. These eight species are _D.
Nicæa_, _Euphorbiæ_, _Dahlii_, _Galii_, _Livornica_, _Lineata_,
_Zygophylli_, and _Hippophaës_. Of these, _Nicæa_, _Euphorbiæ_,
_Dahlii_, _Zygophylli_, and _Hippophaës_ are so much alike in their
whole structure, in the form of the wings, and in marking, that
few entomologists can correctly identify them off-hand without
comparison. The larvæ of these four species, however, are of very
different appearances. Those of _Euphorbiæ_ and _Dahlii_ are most
alike, both being distinguished by the possession of a double row of
large ring-spots. _Zygophylli_ (see Fig. 50, Pl. VI.) possesses only
faint indications of ring-spots on a white subdorsal line; and in
_Hippophaës_ there is only an orange-red spot on the eleventh segment,
the entire marking consisting of a subdorsal line on which, in some
individuals, there are situated more or less developed ring-spots (see
Figs. 59 and 60, Pl. VII.). If we only compare the larvæ and imagines
of _D. Euphorbiæ_ and _Hippophaës_, we cannot but be struck with
astonishment at the great difference of form-relationship in the two
stages of development.

In the case of _D. Euphorbiæ_ and _Nicæa_ this difference is almost
greater. Whilst these larvæ show great differences in colour, marking,
and in the roughness or smoothness of the skin (compare Fig. 51, Pl.
VI. with Figs. 43 and 44, Pl. V.), the moths cannot be distinguished
with certainty. As has already been stated, the imago of the rare
_D. Nicæa_ is for this reason wanting in most collections; it cannot
be detected whether a specimen is genuine, _i.e._ whether it may not
perhaps be a somewhat large example of _D. Euphorbiæ_.

An especially striking instance of incongruence is offered by the
two species of _Chærocampa_ most common with us, viz., _Elpenor_ and
_Porcellus_, the large and small Elephant Hawk-moths. The larvæ are
so similar, even in the smallest details of marking, that they could
scarcely be identified with certainty were it not that one species
(_Elpenor_) is considerably larger and possesses a less curved caudal
horn than the other. The moths of these two species much resemble
one another in their dull green and red colours, but differ in the
arrangement of these colours, _i.e._ in marking, and also in the form
of their wings, to such an extent that _Porcellus_ has been referred to
the genus _Pergesa_[193] of Walker. If systemy, as is admitted on many
sides, has only to indicate the morphological relationship, this author
is not to blame--but in this case a special larval classification must
likewise be admitted, in a manner somewhat similar to that at present
adopted provisionally in text-books of zoology for the Hydroid Polypes
and inferior Medusæ. This case of _Porcellus_, however, shows that
those are correct who maintain that systemy claims to express, although
incompletely, the blood-relationship, and that systematists have always
unconsciously formed their groups as though they intended to express
the genetic connection of the forms. Only on this supposition can it
appear incorrect to us to thus separate two species of which the larvæ
agree so completely.

I cannot conclude this review of the various systematic groups without
taking a glance at the groups comprised within species, viz. varieties.
Whilst in species incongruence is of frequent occurrence, in varieties
this is the rule, for which reason it admits in this case of being more
sharply defined, since we are not concerned with a double difference
but only with the question whether in the one stage a difference or an
absolute similarity is observable. By far the majority of varieties are
either simply imaginal or merely larval varieties--only the one stage
diverges, the other is quite constant.

Thus, as has already been shown, in all the seasonally dimorphic
butterflies known to me the caterpillars of the two generations of
imagines, which are often so widely different, are exactly alike; and
the same obtains for the majority of purely climatic varieties of
butterflies. Unfortunately there are as yet no connected observations
on this point. The only certain instance that I can here mention is
that of the Alpine and Polar form of _Pteris Napi_. This variety,
_Bryoniæ_, the female of which differs so greatly in marking and
colouring, possesses larvæ which cannot be distinguished from those of
the ordinary form of _Napi_.(See part I. appendix I. p. 124.)

That caterpillars can also vary locally without thereby affecting the
imagines is shown by the frequently mentioned and closely investigated
cases of di- and polymorphism in the larvæ of a number of _Sphingidæ_
(_M. Stellatarum_, _A. Atropos_, _S. Convolvuli_, _C. Elpenor_, and
_Porcellus_, &c.). The same thing is still more clearly shown by those
instances in which there are not several but only one distinct larval
form occurring in each of two different localities.

To this class belongs the above-mentioned case of _Chærocampa Celerio_
(p. 197), supposing our information concerning this species to be
correct; likewise the recently-mentioned case of the Ligurian variety
of the caterpillar of _Saturnia Carpini_; and finally the case of
_Eriogaster Lanestris_, so well known to lepidopterists. This insect
inhabits the plains of Germany, and in the Alps extends to an elevation
of 7000 feet, where it possesses a larva differently marked and
coloured (_E. Arbusculæ_) to those of the lowlands whilst the moths are
smaller, but do not differ in other respects from those of the plains.

Among the Alpine species many other such cases may occur, but these
could only be discovered by making investigations having special
reference to this point. Of the Alpine butterflies, for example,
not a single species can have been reared from the caterpillar;
for this reason but few observations have on the whole been given
by entomologists respecting the Alpine larvæ, which are not known
sufficiently well to enable such a question to be decided.

The investigation of the form-relationships existing between larvæ on
the one hand and imagines on the other has thus led to the following
results:--

We learn on comparison that incongruences or inequalities of
form-relationship occur in all systematic groups from varieties to
families. These incongruences are of two kinds, in some cases being
disclosed by the fact that the larvæ of two systematic groups, _e.g._
two species, are more closely related in form than their imagines (or
inversely), whilst in other cases the larvæ form different systematic
groups to those formed by the imagines.

The results of the investigation into the occurrence of incongruences
among the various systematic groups may be thus briefly summarised:--

Incongruences appear to occur most frequently among varieties, since
it very frequently happens that it is only the larva or only the
imago which has diverged into a variety, the other stage remaining
monomorphic. The systematic division of varieties is thus very often
one-sided.

Among species also incongruences are of frequent occurrence. Sometimes
the imagines are much more nearly related in form than the larvæ, and
at others the reverse happens; whilst again the case appears also
to occur in which only the one stage (larva) diverges to the extent
of specific difference, the other stage remaining monomorphic (_D.
Euphorbiæ_ and _Nicæa_).

The agreement in form-relationship appears to be most complete in
genera. In the greater number of cases the larval and imaginal genera
coincide, not only in the sharpness of their limits, but also--as far
as one can judge--in the weight of their distinctive characters, and
therefore in the amount of their divergence. Of all the systematic
groups, genera show the greatest congruence.

In families there is again an increase of irregularity. Although larval
and imaginal families generally agree, there are so many exceptions
that the groups would be smaller if they were based exclusively
on the larval structure than if founded entirely on the imagines
(_Nymphalidæ_, _Bombycidæ_).

If we turn to the groups of families we find a considerably increased
incongruence; complete agreement is here again rather the exception,
and it further happens in these cases that it is always the larvæ
which, to a certain extent, remain at a lower grade, and which form
well defined families; but these can seldom be associated into groups
of a higher order having a common character, as in the case of the
imagines (_Rhopalocera_).

After having thus collected (so far as I am able) the facts, we have
now to attempt their interpretation, and from the observed congruence
and incongruence of form-relationship of the two stages to endeavour to
draw a conclusion as to the underlying causes of the transformations.

It is clear at starting that all cases of incongruence can only be the
expression or the consequence of a phyletic development which has not
been exactly parallel in the two stages of larva and imago--that one
stage must have changed either more rapidly or more slowly than the
other. An “unequal phyletic development” is thus the immediate cause of
incongruence.

Thus, the occurrence of different larvæ in species of which the
imagines have remained alike may be simply understood as cases in
which the imago only has experienced a change--has taken a forward
step in phyletic development, whilst the larvæ have remained behind.
If we conceive this one-sided development to be repeated several
times, there would arise two larval forms as widely different as those
of _Deilephila Nicæa_, and _Euphorbiæ_, whilst the imagines, as is
actually the case in these species, would remain the same.

The more commonly occurring case in which one stage has a greater
form-divergence than the other, is explicable by the one stage having
changed more frequently or more strongly than the other.

The explanation of the phenomena thus far lies on the surface, and it
is scarcely possible to advance any other; but why should one stage
become changed more frequently or to a greater extent than the other?
why should one portion be induced to change more frequently or more
strongly than another? whence come these inducements to change? These
questions bring us to the main point of inquiry:--Are the causes which
give rise to these changes internal or external? Are the latter the
result of a phyletic vital force, or are they only due to the action of
the external conditions of life?

Although an answer to this question will be found in the preceding
essay, I will not support myself on the results there obtained, but
will endeavour to give another solution of the problem on fresh
grounds. The answer will indeed be the same as before:--A phyletic
force must be discountenanced, since in the first place it does not
explain the phenomena, and in the second place the phenomena can be
well explained without its assumption.

The admission of a phyletic vital force does not explain the phenomena.
The assumption that there is a transforming power innate in the
organism indeed agrees quite well with the phenomenon of congruence,
but not with that of incongruence. Since a large number of cases of
the latter depend upon the fact that the larvæ are more frequently
influenced by causes of change than their imagines, or _vice versâ_,
how can this be reconciled with such an internal force? On this
assumption would not each stage of a species be compelled to change, if
not contemporaneously at least successively, with the same frequency
and intensity, by the action of an innate force? and how by means of
the latter can there ever result a greater form-divergence in the larvæ
than in the imagines?

It is delusive to believe that these unequal deviations can be
explained by assuming that the phyletic force acts periodically.
Granting that it does so, and that the internal power successively
compels the imago, pupa, and finally the larva to change, there would
then pass a kind of wave of transformation over the different stages
of the species, as was actually shown above to be the case in the
single larval stages. The only possible way of explaining the unequal
distances between larvæ and imagines would therefore be to assume that
two allied groups, _e.g._ species, were not contemporaneously affected
by the wave, so that at a certain period of time the imago alone of one
species had become changed, whilst in the other species the wave of
transformation had also reached the larva. In this case the imagines of
the two species would thus appear to be more nearly related than their
larvæ.

Now this strained explanation is eminently inapplicable to varieties,
still less to species, and least of all to higher systematic groups,
for the simple reason that every wave of transformation may be
assumed to be at the most of such strength as to produce a deviation
of form equal to that of a variety. Were the change resulting from
a single disturbance greater, we should not only find one-sided
varieties, _i.e._ those belonging to _one_ stage, but we should
also meet as frequently with one-sided species. If, however, a wave
of transformation can only produce a variety even in the case of
greatest form-divergence, the above hypothetical uncontemporaneous
action of such a wave in two species could only give rise to such
small differences in the two stages that we could but designate them
as varieties. An accumulation of the results of the action of several
successive waves passing over the same species could not happen,
because the distance from a neighbouring species would always become
the same in two stages as soon as _one_ wave had ended its course. In
this manner there could therefore only arise divergences of the value
of varieties, and incongruences in systematic groups of a higher rank
could not thus be explained.

All explanations of the second form of incongruence from the point of
view of a phyletic force can also be shown to be absurd. How can the
fact be explained that larval and imaginal families by no means always
coincide; or that the larvæ can only be formed into families whilst
the imagines partly form sharply defined groups of a higher order? How
can an internal directive force within the same organism urge in two
quite distinct directions? If the evolution of a definite system were
designed, and the admission of such a continually acting power rendered
necessary, why such an incomplete, uncertain, and confused performance?

I must leave others to answer these questions; to me a vital force
appears to be inadmissible, not only because we cannot understand the
phenomena by its aid, but above all because it is superfluous for their
explanation. In accordance with general principles the assumption of an
unknown force can, however, only be made when it is indispensable to
the comprehension of the phenomena.

I believe that the phenomena can be quite well understood without any
such assumption--both the phenomena of congruence and incongruence, in
their two forms of unequal divergence and unequal group-formation.

Let us in the first place admit that there is no directive force in
the organism inciting periodic change, but that every change is always
the consequence of external conditions, being ultimately nothing but
the reaction--the response of the organism to some of the influences
proceeding from the environment; every living form would in this case
remain constant so long as it was not compelled to change by inciting
causes. Such transforming factors can act directly or indirectly,
_i.e._ they can produce new changes immediately, or can bring about
a remodelling by the combination, accumulation, or suppression
of individual variations already present (adaptation by natural
selection). Both forms of this action of external influences have long
been shown to be in actual operation, so that no new assumption will be
made, but only an attempt to explain the phenomena in question by the
sole action of these known factors of species formation.

If, in the first instance, we fix our attention upon that form of
incongruence which manifests itself through unequal divergence of
form-relationship, it will appear prominently that this bears precise
relations to the different systematic groups. This form of incongruence
constitutes the rule in varieties of the order Lepidoptera, it is of
very frequent occurrence in species, but disappears almost completely
in genera, and entirely in the case of families and the higher groups.
On the whole, therefore, as we turn to more and more comprehensive
groups, the incongruence diminishes whilst the congruence increases,
until finally the latter becomes the rule.

Now if congruence presupposes an equal number of transforming
impulses, we perceive that the number of the impulses which have
affected larvæ and imagines agree with one another the more closely the
larger the systematic groups which are compared together. How can this
be otherwise? The larger the systematic group the longer the period of
time which must have been necessary for its formation, and the more
numerous the transforming impulses which must have acted upon it before
its formation was completed.

But if the supposition that the impulse to change always comes from
the environment in no way favours the idea that such impulses always
affect both stages contemporaneously, and are equal in number during
the same period of time, there is not, on the other hand, the least
ground for assuming that throughout long periods the larvæ or the
imagines only would have been affected by such transforming influences.
This could have been inferred from the fact that varieties frequently
depend only upon one stage, whilst specific differences in larvæ only
also occur occasionally, the imagines remaining alike; but no single
genus is known of which all the species possess similar larvæ. Within
the period of time during which genera can be formed the transforming
impulses therefore never actually affect the one stage only, but always
influence both.

But if this is the case--if within the period of time which is
sufficient for the production of species, the one stage only is but
seldom and quite exceptionally influenced by transforming impulses,
whilst both stages are as a rule affected, although not with the same
frequency, it must necessarily follow that on the whole, as the period
of time increases, the difference in the number of these impulses which
affect the larva and of those which affect the imago must continually
decrease, and with this difference the magnitude of the morphological
differences resulting from the transforming influences must at the same
time also diminish. With the number of the successively increasing
changes the difference in the magnitude of the change in the two stages
would always relatively diminish until it had quite vanished from our
perception; just in the same manner as we can distinguish a group of
three grains of corn from one composed of six, but not a heap of 103
grains from one containing 106 grains.

That the small systematic groups must have required a short period and
the large groups a long period of time for their formation requires no
special proof, but results immediately from the theory of descent.

All the foregoing considerations would, however, only hold good if
the transforming impulses were equal in strength, or, not to speak
figuratively, if the changes only occurred in equivalent portions of
the body, _i.e._ in such portions as those in which the changes are
of the same physiological and morphological importance to the whole
organism.

Now in the lower systematic groups this is always the case. Varieties,
species, and genera are always distinguished by only relatively small
differences; deep-seated distinctions do not here occur, as is implied
in the conception of these categories. The true cause of this is, I
believe, to be found in the circumstance that all changes take place
only by the smallest steps, so that greater differences can only arise
in the course of longer periods of time, within which a great number of
types (species) can, however, come into existence, and these would be
related by blood and in form in different degrees, and would therefore
form a systematic group of a higher rank.

The short periods necessary for the production of inferior groups, such
as genera, would not result in incongruences if only _un_typical parts
of the larvæ, such as marking or spines, underwent change, whilst in
the imagines typical parts--wings and legs--became transformed. The
changes which could have occurred in the wings, &c., during this period
of time would have been much too small to produce any considerable
influence on the other parts of the body by correlation; and two
species of which the larvæ and imagines, had changed with the same
frequency, would show a similar amount of divergence between the larvæ
and between the imagines, although on the one side only _un_typical
parts--_i.e._ those of no importance to the whole organization--and
on the other side typical parts, were affected. The _number_ of the
changes would here alone determine whether congruence or incongruence
occurred between the two stages.

The case would be quite different if, throughout a long period of time,
in the one stage only typical and in the other only untypical parts
were subjected to change. In the first case a complete transformation
of the whole structure would occur, since not only would the typical
parts, such as the wings, undergo a much further and increasing
transformation in the same direction, but these changes would also lead
to secondary alterations.

In this manner, I believe, must be explained the fact that in the
higher groups still greater form-divergences of the two stages occur;
and if this explanation is correct, the cause of this striking
phenomenon, viz., that incongruence diminishes from varieties to
genera, in which latter it occurs but exceptionally, whilst in families
and in the higher groups it again continually increases, is likewise
revealed. Up to genera the incongruence depends entirely upon the one
stage having become changed _more frequently_ than the other; but
in families and groups of families, and in the orders Diptera and
Hymenoptera, as will be shown subsequently, in sub-orders and tribes,
it depends upon the _importance of the part of the body_ affected by
the predominant change. In the latter case the number of changes is
of no importance, because these are so numerous that the difference
vanishes from our perception; but an equal number of changes, even
when very great, may now produce a much greater or a much smaller
transformation in the entire bodily structure according as they affect
typical or untypical portions, or according as they keep in the same
direction throughout a long period of time, or change their direction
frequently.

Those unequal form-divergences which occur in the higher systematic
groups a re always associated with a different formation of groups--the
larvæ form different systematic groups to the imagines, so that one
of these stages constitutes a higher or a lower group; or else the
groups are of equal importance in the two stages, but are of unequal
magnitude--they do not coincide, but the one overlaps the other.

Incongruences of this last kind appear in certain cases within families
(_Nymphalidæ_), but I will not now subject these to closer analysis,
because their causes will appear more clearly when subsequently
considering the orders Hymenoptera and Diptera. Incongruences of the
first kind, however, admit of a clear explanation in the case of
butterflies. They appear most distinctly in the groups composed of
families.

Nobody has as yet been able to establish the group _Rhopalocera_ by
means of any single character common to the larvæ; nevertheless, this
group in the imagines is the sharpest and best defined of the whole
order. If we inform the merest tyro that clubbed antennæ are the chief
character of the butterflies, he will never hesitate in assigning
one of these insects to its correct group. Such a typical character,
common to all families, is, however, absent in the larvæ; and it might
be correctly said that there were no Rhopalocerous larvæ, or rather
that there were only larvæ of _Equites_, _Nymphales_, and _Heliconii_.
The larvæ of the various families can be readily separated by means
of characteristic distinctions, and it would not be difficult for an
adept to distinguish to this extent in single cases a Rhopalocerous
caterpillar as such; but these larvæ possess only _family_ characters,
and not those of a higher order.

This incongruence partly depends upon the circumstance that the
form-divergence between a Rhopalocerous and a Heterocerous family is
much greater on the side of the imagines than on that of the larvæ.
Were there but a single family of butterflies in existence, such as
the _Equites_, we should be obliged to elevate this to the rank of
a sub-order on the side of the imagines, but not on that of the
larvæ. Such cases actually occur, and an instance of this kind will be
mentioned later in connection with the Diptera. But this alone does not
explain why, on the side of the imagines, a whole series of families
show the same amount of morphological divergence from the families
of other groups. There are two things, therefore, which must here be
explained:--First, why is the form-divergence between the imagines of
the Rhopalocera and Heterocera greater than that between their larvæ?
and, secondly, why can the imagines of the Rhopalocera be formed into
one large group by means of common characters whilst the larvæ cannot?

The answers to both these questions can easily be given from our
present standpoint. As far as the first question is concerned, this
finds its solution in the fact that the form-divergence always
corresponds exactly with the divergence of function, _i.e._ with the
divergence in the mode of life.

If we compare a butterfly with a moth there can be no doubt that the
difference in the conditions of life is far greater on the side of the
imagines than on that of the larvæ. The differences in the mode of life
of the larvæ are on the whole but very small. They are all vegetable
feeders, requiring large quantities of food, and can only cease
feeding during a short time, for which reason they never leave their
food-plants for long, and it is of more importance for them to remain
firmly attached than to be able to run rapidly. It is unnecessary for
them to seek long for their food, as they generally find themselves
amidst an abundance, and upon this depends the small development of
their eyes and other organs of sense. On the whole caterpillars live
under very uniform conditions, although these may vary in manifold
details.

The greatest difference in the mode of life which occurs amongst
Lepidopterous larvæ is shown by wood feeders. But even these, which
by their constant exclusion from light, the hardness of their food,
their confinement within narrow hard-walled galleries, and by the
peculiar kind of movement necessitated by these galleries, are so
differently situated in many particulars to those larvæ which live
openly on plants, have not experienced any general change in the
typical conformation of the body by adaptation to these conditions of
life. These larvæ, which, as has already been mentioned, belong to
the most diverse families, are more or less colourless and flattened,
and have very strong jaws and small feet; but in none of them do we
find a smaller number of segments, or any disappearance, or important
transformation of the typical limbs; they all without exception possess
sixteen legs, like the other larvæ excepting the _Geometræ_.

Now if even under the most widely diverging conditions of life
adaptation of form is produced by relatively small, and to a
certain extent superficial, changes, we should expect less typical
transformations in the great majority of caterpillars which live
on the exterior of plants or in their softer parts (most of the
Micro-lepidoptera). The great diversity in the forms of caterpillars
depends essentially upon a different formation of the skin and its
underlying portions. The skin is sometimes naked, and can then acquire
the most diverse colours, either protective or conspicuous, or it may
develop offensive or defensive markings; in other cases it may be
covered with hairs which sting, or with spines which prick; certain
of its glands may develop to an enormous size, and acquire brilliant
colours and the power of emitting stinking secretions (the tentacles of
the _Papilionidæ_ and Cuspidate larvæ); by the development of warts,
angles, humps, &c., any species of caterpillar may be invested with the
most grotesque shape, the significance of which with respect to the
life of the insect is as yet in most cases by no means clear: _typical
portions_ are not, however, essentially influenced by these manifold
variations. At most only the form of the individual segments of the
body, and with these the shape of the whole insect, become changed
(onisciform larvæ of _Lycænidæ_), but a segment is never suppressed,
and even any considerable lengthening of the legs occurs but very
seldom (_Stauropus Fagi_).[194]

We may therefore fairly assert that the structure of larvæ is on the
whole remarkably uniform, in consequence of the uniformity in the
conditions of life. Notwithstanding the great variety of external
aspects, the general structure of caterpillars does not become
changed--it is only their outward garb which varies, sometimes in one
direction, and sometimes in another, and which, starting from inherited
characters, becomes adapted to the various special conditions of life
in the best possible manner.

All this is quite different in the case of the imagines, where we
meet with very important differences in the conditions of life. The
butterflies, which live under the influence of direct sunlight and a
much higher temperature, and which are on the wing for a much longer
period during the day, must evidently be differently equipped to the
moths in their motor organs (wings), degree of hairiness, and in the
development of their eyes and other organs of sense. It is true that we
are not at present in a condition to furnish special proofs that the
individual organs of butterflies are exactly adapted to a diurnal life,
but we may safely draw this general conclusion from the circumstance
that no butterfly is of nocturnal habits.[195] It cannot be stated
in objection that there are many moths which fly by day. It certainly
appears that no great structural change is necessary to confer upon a
Lepidopteron organized for nocturnal life the power of also flying by
day; but this proves nothing against the view that the structure of
the butterflies depends upon adaptation to a diurnal life. Analogous
cases are known to occur in many other groups of animals. Thus, the
decapodous Crustacea are obviously organized for an aquatic life; but
there are some crabs which take long journeys by land. Fish appear
no less to be exclusively adapted to live in water; nevertheless the
“climbing-perch” (_Anabas_) can live for hours on land.

It is not the circumstance that some of the moths fly by day which is
extraordinary and demands a special explanation, but the reverse fact
just mentioned, that no known butterfly flies by night. We may conclude
from this that the organization of the latter is not adapted to a
nocturnal life.

If we assume[196] that the Lepidopterous family adapted to a diurnal
life gives rise in the course of time to a nocturnal family, there
can be no doubt but that the transformation of structure would be far
greater on the part of the imagines than on that of the larvæ. The
latter would not remain quite unchanged--not because their imagines
had taken to a nocturnal life which for the larva would be quite
immaterial, but because this change could only occur very gradually
in the course of a large number of generations, and during this long
period the conditions of life would necessarily often change with
respect to the larvæ. It has been shown above that within the period of
time necessary for the formation of a new species impulses to change
occur on both sides; how much more numerous therefore must these be
in the case of a group of much higher rank, for the establishment of
which a considerably longer period is required. In the case assumed,
therefore, the larvæ would also change, but they would suffer much
smaller transformations than the imagines. Whilst in the latter almost
all the typical portions of the body would undergo deep changes in
consequence of the entirely different conditions of life, the larvæ
would perhaps only change in marking, hairs, bristles, or other
external characters, the typical parts experiencing only unimportant
modifications.

In this manner it can easily be understood why the larvæ of a family
of _Noctuæ_ do not differ to a greater extent from those of a family of
butterflies than do the latter from some other Rhopalocerous family, or
why the imagines of a Rhopalocerous and a Heterocerous family present
much greater form-divergences than their larvæ. At the same time is
therefore explained the unequal value that must be attributed to any
single family of butterflies in its larvæ and in its imagines. The
unequal form-divergences coincide exactly with the inequalities in the
conditions of life.

When whole families of butterflies show the same structure in their
typical parts (antennæ, wings, &c.), and, what is of more importance,
can be separated as a systematic group of a higher order (_i.e._ as a
section or sub-order) from the other Lepidoptera whilst their larval
families do not appear to be connected by any common character, the
cause of this incongruence lies simply in the circumstance that the
imagines live under some peculiar conditions which are common to them
all, but which do not recur in other Lepidopterous groups. Their larvæ
live in precisely the same manner as those of all the other families of
Lepidoptera--they do not differ in their mode of life from those of the
Heterocerous families to a greater extent than they do from one another.

We therefore see here a community of form within the same compass as
that in which there is community in the conditions of life. In all
butterflies such community is found in their diurnal habits, and in
accordance with this we find that these only, and not their larvæ, can
be formed into a group having common characters.

In the larvæ also we only find agreement in the conditions of life
within a much wider compass, viz. within the whole order. Between
the limits of the order Lepidoptera the conditions of life in the
caterpillars are, as has just been shown, on the whole very uniform,
and the structure of the larvæ accordingly agrees almost exactly in all
Lepidopterous families in every essential, _i.e._ typical, part.

In this way is explained the hitherto incomprehensible phenomenon that
the sub-ordinal group _Rhopalocera_ cannot be based on the larvæ, but
that Lepidopterous caterpillars can as a whole be associated into a
higher group (order); they constitute altogether families and an order,
but not the intermediate group of a sub-order. By this means we at the
same time reply to an objection that may be raised, viz. that larval
forms cannot be formed into high systematic groups because of their
“low and undeveloped” organization.

To this form of incongruence, viz. to the formation of systematic
groups of unequal value and magnitude, I must attach the greatest
weight with respect to theoretical considerations. I maintain that
this, as I have already briefly indicated above, is wholly incompatible
with the admission of a phyletic force. How is it conceivable that such
a power could work in the same organism in two entirely different
directions--that it should in the same species lead to the constitution
of quite different systems for the larvæ and for the imagines, or that
it should lead only to the formation of families in the larvæ and to
sub-orders in the imagines? If an internal force existed which had
a tendency to call into existence certain groups of animal forms of
such a nature that these constituted one harmonious whole of which
the components bore to one another fixed morphological relationships,
it would certainly have been an easy matter for such a power to have
given to the larvæ of butterflies some small character which would
have distinguished them as such, and which would in some measure have
impressed them with the stamp of “_Rhopalocera_.” Of such a character
we find no trace however; on the contrary, everything goes to show that
the transformations of the organic world result entirely from external
influences.



III.

INCONGRUENCES IN OTHER ORDERS OF INSECTS.


Although the order Lepidoptera is for many reasons especially
favourable for an investigation such as that undertaken in the previous
section, it will nevertheless be advantageous to inquire into the
form-relationships of the two chief stages in some other orders of
metamorphic insects, and to investigate whether in these cases the
formation of systematic groups also coincides with common conditions of
life.


HYMENOPTERA.

In this order there cannot be the least doubt as to the
form-relationship of the imagines. The characteristic combination of
the pro- and meso-thorax, the number and venation of the wings, and the
mouth-organs formed for biting and licking, are found throughout the
whole order, and leave no doubt that the Hymenoptera are well based on
their imaginal characters.

But it is quite different with the larvæ. It may be boldly
asserted that the order would never have been founded if the larvæ
only had been known. Two distinct larval types here occur, the
one--caterpillar-like--possessing a distinct horny head provided with
the typical masticatory organs of insects, and a body having thirteen
segments, to which, in addition to a variable number of abdominal legs,
there are always attached three pairs of horny thoracic legs: the
other type is maggot-shaped, without the horny head, and is entirely
destitute of mouth-organs, or at least of the three pairs of typical
insect jaws, and is also without abdominal and thoracic legs. The
number of segments is extremely variable; the larvæ of the saw-flies
have thirteen besides the head, the maggot-shaped larvæ of bees possess
fourteen segments altogether, and the gall-flies and ichneumons only
twelve or ten. We should be much mistaken also if we expected to find
connecting characters in the internal organs. The intestine is quite
different in the two types of larvæ, the posterior opening being absent
in the maggot-like grubs; at most only the tracheal and nervous systems
show a certain agreement, but this is not complete.

The order Hymenoptera, precisely speaking and conceived only
morphologically, exists therefore but in the imagines; in the larvæ
there exist only the caterpillar- and maggot-formed groups. The former
shows a great resemblance to Lepidopterous larvæ, and in the absence
of all knowledge of the further development it might be attempted to
unite them with these into one group. The two certainly differ in
certain details of structure in the mouth-organs and in the number of
segments, abdominal legs, &c., to a sufficient extent to warrant their
being considered as two sub-orders of one larval order; but they would
in any case be regarded as much more nearly related in form than the
caterpillar- and maggot-like types of the Hymenopterous larvæ.

Is it not conceivable, however, that the imagines of the
Hymenoptera--that ichneumons and wasps may be only accidentally alike,
and that they have in fact arisen from quite distinct ancestral forms,
the one having proceeded with the Lepidopterous caterpillars from one
root, and the other with the grub-like Dipterous larvæ from another
root?

This is certainly not the case; the common characters are too
deep-seated to allow the supposition that the resemblance is here
only superficial. From the structure of the imagines alone the common
origin of all the Hymenoptera may be inferred with great probability.
This would be raised into a certainty if we could demonstrate the
phyletic development of the maggot-formed out of the caterpillar-formed
Hymenopterous larvæ by means of the ontogeny of the former. From the
beautiful investigations of Bütschli on the embryonic development of
bees[197] we know that the embryo of the grub possesses a complete
head, consisting of four segments and provided with the three typical
pairs of jaws. These head segments do not subsequently become formed
into a true horny head, but shrivel up; whilst the jaws disappear with
the exception of the first pair, which are retained in the form of soft
processes with small horny points. We know also that from the three
foremost segments of the embryo the three typical pairs of legs are
developed in the form of round buds, just as they first appear in all
insects.[198] These rudimentary limbs undergo complete degeneration
before the birth of the larva, as also do those of the whole[199] of
the remaining segments, which, even in this primitive condition, show a
small difference to the three foremost rudimentary legs.

The grub-like larvæ of the Hymenoptera have therefore descended from
forms which possessed a horny head with antennæ and three pairs
of gnathites and a 13-segmented body, of which the three foremost
segments were provided with legs differing somewhat from those of the
other segments; that is to say, they have descended from larvæ which
possessed a structure generally similar to that of the existing saw-fly
larvæ. The common derivation of all the Hymenoptera from one source is
thus established with certainty.[200]

But upon what does this great inequality in the form-relationship of
the larvæ and imagines depend? The existing maggot-like grubs are
without doubt much further removed from the active caterpillar-like
larvæ than are the saw-flies from the Aculeate Hymenoptera. Whilst
these two groups differ only through various modifications of the
typical parts (limbs, &c.), their larvæ are separable by much
deeper-seated distinctions; limbs of typical importance entirely
vanish in the one group, but in the other attain to complete
development.

In the Hymenoptera there exists therefore a very considerable
incongruence in the systems based morphologically, _i.e._ on the pure
form-relationships of the larvæ and of the imagines. The reason of this
is not difficult to find: _the conditions of life differ much less in
the case of the imagines than in that of the larvæ_. In the former the
conditions of life are similar in their broad features. Hymenoptera
live chiefly in the air and fly by day, and in their mode of obtaining
food do not present any considerable differences. Their larvæ, on the
other hand, live under almost diametrically opposite conditions. Those
of the saw-flies live after the manner of caterpillars upon or in
plants, in both cases their peculiar locomotion being adapted for the
acquisition and their masticatory organs for the reduction of food.
The larvæ of the other Hymenoptera, however, do not as a rule require
any means of locomotion for reaching nor any organs of mastication for
swallowing their food, since they are fed in cells, like the bees and
wasps, or grow up in plant galls of which they suck the juice, or are
parasitic on other insects by whose blood they are nourished. We can
readily comprehend that in the whole of this last group the legs should
disappear, that the jaws should likewise vanish or should become
diminished to one pair retained in a much reduced condition, that the
horny casing of the head, the surface of attachment of the muscles of
the jaws, should consequently be lost, and that even the segments of
the head itself should become more or less shrivelled up as the organs
of sense therein located became suppressed.

The incongruence manifests itself however in yet another manner than
by the relatively greater morphological divergence of the larvæ: a
different grouping is possible for the larvæ and for the imagines. If
we divide the Hymenoptera simply according to the form-relationships of
the imagines, the old division into the two sub-orders _Terebrantia_
or _Ditrocha_ and _Aculeata_ or _Monotrocha_ will be the most correct.
The distinguishing characters of a sting or ovipositor and a one- or
two-jointed trochanter are still of the greatest value. But these
two sub-orders do not by any means correspond with the two types of
larvæ since, in the _Terebrantia_, there occur families with both
caterpillar-formed and maggot-formed larvæ.

The cause is to be found in that a portion of these families possess
larvæ which are parasitic in other insects or in galls, their bodily
structure having by these means become transformed in a quite different
direction. The mode of life of the imagines is, on the other hand,
essentially the same.

We have here therefore another case like that which we met with among
the Rhopalocerous Lepidoptera, in which the imagines appear to be
capable of being formed into a higher group than the larvæ, because the
former live under conditions of life which are on the whole similar
whilst the latter live under very divergent conditions.

The old division of the Hymenoptera into two sub-orders has certainly
been abandoned in the later zoological text-books; they are now divided
into three:--saw-flies, parasitic, and aculeate Hymenoptera; but even
this arrangement has been adopted with reference to the different
structure of the larvæ. Whether this system is better than the older,
_i.e._ whether it better expresses the _genealogical_ relationship, I
will not now stop to investigate.[201]


DIPTERA.

The imagines of the Diptera (_genuina_), with the exception of the
_Aphaniptera_ and _Pupipara_, agree in all their chief characters,
such as the number and structure of the wings, the number and joints
of the legs, the peculiar formation of the thorax (fusion of the three
segments);[202] and even the structure of the mouth organs varies only
within narrow limits. This is in accordance with their mode of life,
which is very uniform in its main features: all the true Diptera live
in the light, moving chiefly by means of flight, but having also the
power of running; all those which take food in the imago condition
feed upon fluids. Their larvæ, on the other hand, are formed on two
essentially different types, the one--which I shall designate as the
gnat-type--possessing a horny head with eyes, three pairs of jaws,
and long or short antennæ, together with a 12- or 13-segmented body,
which is never provided with the three typical pairs of thoracic legs,
but frequently has the so-called abdominal legs on the first and last
segments. The other Dipterous larvæ are maggot-shaped and without a
horny head, or in fact without any head, since the first segment, the
homologue of the head, can in no case be distinguished through its
being larger than the others; it is on the contrary much smaller. The
typical insect mouth-parts are entirely absent, being replaced by a
variously formed and quite peculiar arrangement of hooks situated on
the mouth and capable of protrusion. Never more than eleven segments
are present besides the first, which is destitute of eyes; neither are
abdominal legs ever developed.

The mode of life differs very considerably in the two groups of larvæ.
Although the Dipterous maggots are not as a rule quite incapable of
locomotion like the grubs of the Hymenoptera (bees, ichneumons), the
majority are nevertheless possessed of but little power of movement
in the food-substance on which they were deposited as eggs. They do
not go in search of food, either because they are parasitic in other
insects in the same manner as the ichneumons (_Tachina_), or else they
live on decaying animal or vegetable substances or amidst large swarms
of their prey, like the larvæ of the _Syrphidæ_ amongst _Aphides_.
They generally undergo pupation in the same place as that which they
inhabit as larvæ and indeed in their larval skin which hardens into an
oval pupa-case. Some few leave their feeding place and pupate after
traversing a short distance (_Eristalis_).

As in the case of the Hymenoptera the structure of the larvæ can here
also be explained by peculiarities in their mode of life. Creatures
which live in a mass of food neither require special organs of
locomotion nor specially developed organs of sense (eyes). They have
no use for the three pairs of jaws since they only feed on liquid
substances, and the hooks within the mouth do not serve for the
reduction of food but only for fastening the whole body. With the jaws
and their muscular system there likewise disappears the necessity for a
hard surface of attachment, _i.e._ a corneous head.

The mode of life of the larvæ of the gnat-type is quite different
in most points. The majority, and indeed the most typically formed
of these, have to go in search of their food, whether they are
predaceous, such as the _Culicidæ_ and many of the other _Nemocera_
(_Corethra_, _Simulium_), or whether they feed on plants, which they
in some cases weave into a protective dwelling tube (certain species
of _Chironomus_). Many live in water and move with great rapidity;
others bury in the earth or in vegetable substances; and even those
species which live on fungi sometimes wander great distances, as in
the well-known case of the “army worm” where thousands of the larvæ of
_Sciara Thomæ_ thus migrate.

Now the two types of larvæ correspond generally with the two large
groups into which, as it appears to me correctly, the Diptera
(_genuina_) are as a rule divided. In this respect there is therefore
an equality of form-relationship--the grouping is the same, and
the incongruence depends only upon the form-divergence between the
two kinds of larvæ being greater than between the two kinds of
imagines.[203]

That the form-divergence is greater in the larvæ than in the imagines
cannot be doubted; that this distant form-relationship cannot,
however, be referred to a very remote common origin, _i.e._ to a very
remote blood-relationship, not only appears from the existence of
transition-forms between the two sub-orders, but can be demonstrated
here, as in the case of the Hymenoptera, by the embryonic development
of the maggot-like larvæ.

Seventeen years ago I showed[204] that the grub-formed larvæ of the
_Muscidæ_ in the embryonic state possessed a well-developed head with
antennæ and three pairs of jaws, but that later in the course of the
embryonic development a marked reduction and transformation of these
parts takes place, so that finally the four head segments appear as a
single small ring formed from the coalesced pairs of maxillæ, whilst
the so-called “fore-head” (the first head segment), together with the
mandibles, becomes transformed into a suctorial-head armed with hooks
and lying within the body. At the time of writing I drew no conclusion
from these facts with reference to the phyletic development of these
larval forms; nor did Bütschli, six years later, in the precisely
analogous case of the larvæ of the bees. The inference is, however, so
obvious that it is astonishing that it should not have been drawn till
the present time.[205]

There can be no doubt that the maggot-like larvæ of insects are not by
any means ancient forms, but are, on the contrary, quite recent, as
first pointed out by Fritz Müller,[206] and afterwards by Packard[207]
and Brauer,[208] and as is maintained in the latest work by Paul
Mayer[209] on the phylogeny of insects.

The Dipterous maggots have evidently descended from a larval form
which possessed a horny head with antennæ and three pairs of jaws, but
which had no appendages to the abdominal segments; they are therefore
ordinary Dipterous larvæ of the gnat-type which have become modified
in a quite peculiar manner and adapted to a new mode of life, just as
the grubs of the Hymenoptera are larvæ of the saw-fly type, which have
become similarly transformed, although by no means in the same manner.
The resemblance between the two types of larva is to a great extent
purely external, and depends upon the process designated “convergence”
by Oscar Schmidt, _i.e._ upon the adaptation of heterogeneous animal
forms to similar conditions of life. By adaptation to a life within
a mass of fluid nutriment, the caterpillar-formed larvæ of the
Hymenoptera and the _Tipula_-like larvæ of the Diptera have acquired
a similar external appearance, and many similarities in internal
structure, or, in brief, have attained to a considerable degree of
form-relationship, which would certainly have tended to conceal the
wide divergence in blood-relationship did not the embryological forms
on the one side and the imagines on the other provide us with an
explanation.

It is certainly of great interest that in another order of insects--the
Coleoptera--grub-formed larvæ occur quite irregularly, and their origin
can be here traced to precisely the same conditions of life as those
which have produced the grubs of bees. I refer to the honey-devouring
larvæ of the _Meloïdæ_ (_Meloë_, _Sitaris_, _Cantharis_). The case is
the more instructive, inasmuch that the six-legged larval form is not
yet relegated to the development within the egg, but is retained in the
first larval stage. In the _second larval stage_ the maggot-form is
first assumed, although this is certainly not so well pronounced as in
the Diptera or Hymenoptera, as neither the head nor the thoracic legs
are so completely suppressed as in these orders. Nevertheless, these
parts have made a great advance in the process of transformation.

The grub-like larvæ of the Hymenoptera and Diptera appear to me
especially instructive with reference to the main question of the
causes of transformation. The reply to the questions: what gives the
impetus to change? is this impetus internal or external? can scarcely
be given with greater clearness than here. If these larvæ have
abandoned their ancestral form and have acquired a widely divergent
structure, arising not only from suppression but partly also from an
essentially new differentiation (suctorial head of the _Muscidæ_), and
if these structural changes show a close adaptation to the existing
conditions of life, from these considerations alone it is difficult
to conceive how such transformations can depend upon the action of a
phyletic force. The latter must have foreseen that at precisely this
or that fixed period of time the ancestors of these larvæ would have
been placed under conditions of life which would make it desirable
for them to be modified into the maggot-type. But if at the same
time the imagines are removed in a less degree from those of the
caterpillar-like larvæ, this divergence being in exact relation with
the deviations in the conditions of life, I at least fail to see how we
can escape the consequence that it is the external conditions of life
which produce the transformations and induce the organism to change.
It is to me incomprehensible how one and the same vital force can in
the same individual induce one stage to become transformed feebly and
the other stage strongly, these transformations corresponding in extent
with the stronger or weaker deviations in the conditions of life to
which the organism is exposed in the two stages; to say nothing of the
fact that by such unequal divergences the idea of a perfect system
(creative thought) is completely upset.

Nor can the objection be raised that we are here only concerned with
insignificant changes--with nothing more than the arrested development
of single organs and so forth, in brief, only with those changes which
can be ascribed to the action of the environment.

We are here as little concerned with a mere suppression of organs
through arrested development as in the case of the Cirripedia;
the transformation and reconstruction of the whole body goes even
much further than in these Crustacea, although not so conspicuous
externally. Where do we elsewhere find insects having the head inside
a cavity of the body (sectorial head of the _Muscidæ_), and of
which the foremost segment--the physiological representative of the
head--consists entirely of the coalesced antennæ and pairs of maxillæ?

The incongruences in the form-relationships are, however, exceedingly
numerous in the case of the Diptera, and a special treatise would
be necessary to discuss them thoroughly. I may here mention only one
case, because the inequality shows itself in this instance in a quite
opposite sense.

Gerstäcker, who is certainly a competent entomologist, divides the
Diptera into three tribes, viz. the _Diptera genuina_, the _Pupipara_,
and the _Aphaniptera_. The latter, the fleas, possess in their divided
thoracic segments and in their jointed labial appendages characters so
widely divergent from those of the true Diptera and of the _Pupipara_
that Latreille and the English zoologists have separated them entirely
from the Diptera and have raised them into a separate order.[210] Those
who do not agree in this arrangement, but with Gerstäcker include the
fleas under the Diptera, will nevertheless admit that the morphological
divergence between the _Aphaniptera_ and the two other tribes is far
greater than that which exists between the latter. Now the larvæ of the
fleas are completely similar in structure to those of the gnat-type,
since they possess a corneous head with the typical mouth parts and
antennæ and a 13-segmented body devoid of legs. Were we only acquainted
with the larvæ of the fleas we should rank them with the true Diptera
under the sub-order _Nemocera_. On first finding such a larva we should
expect to see emerge from the pupa a small gnat.

While the imagines of the _Nemocera_ and _Aphaniptera_ thus show but a
very remote form-relationship their larvæ are very closely allied. Can
any one doubt that in this case it is not the larva but the imago which
has diverged to the greatest extent? Have not the fleas moreover become
adapted to conditions of life widely different from those of all other
Diptera, whilst their larvæ do not differ in this respect from many
other Dipterous larvæ?

We have here, therefore, another case of unequal phyletic development,
which manifests itself in the entirely different form-relationship
of the larvæ and the imagines. Thus in this case, as in that of the
Lepidoptera, it is sometimes the larval and at other times the imaginal
stage which has experienced the greatest transformation, and, as in the
order mentioned, the objection that a phyletic vital force produces
greater and more important differentiations in the higher imaginal
stage than in the lower or less developed larval stage, is equally
ineffectual.

If, however, it be asked whether the unequal phyletic development
depends in this case upon an unequal number of transforming impulses
which the two stages may have experienced during an equal period of
time, this must be decidedly answered in the negative. The unequal
development obviously depends in this case, as in the higher systematic
groups of the Lepidoptera, upon the unequal value of the parts affected
by the changes. These parts are on the one side of small importance,
and on the other side of great importance, to the whole structure of
the insect. This is shown in the last-mentioned case of the fleas,
where, of the typical parts of the body, only the wings have become
rudimentary, whilst the antennæ, mouth-parts, and legs, and even the
form and mode of segmentation (free thoracic segments), must have
suffered most important modifications; their larvæ, on the other hand,
can have experienced only unimportant changes, since they still agree
in all typical parts with those of the gnat-type.

Although therefore in this and in similar cases a greater number of
transforming impulses may well have occurred on the one side than on
the other--and it is indeed highly probable that this number has not
been absolutely the same--nevertheless the chief cause of the striking
incongruence is not to be found therein, but rather in the _strength_
of the transforming impulses, if I may be permitted to employ this
figure, or, more precisely expressed, in the importance of the parts
which become changed and at the same time in the amount of change.

In this conclusion there is implied as it appears to me an important
theoretical result which tells further against the efficacy of a
phyletic force.

If the so-called “typical parts” of an animal disappear completely
through the action of the environment only, and still further, if these
parts can become so entirely modified as to give rise to quite new and
again typical structures (suctorial head of the _Muscidæ_) without
the typical parts of the other stage of the same individual being
thereby modified and transformed into a new type of structure, how can
we maintain a distinction between typical and non-typical parts with
respect to their origin? But if a difference exists with respect only
to the physiological importance of such parts, _i.e._ their importance
for the equilibrium of the whole organization, while, with reference to
transformation and suppression, exactly the same influences appear to
be effective as those which bring about a change in or a disappearance
of the so-called adventitious parts, where is there left any scope
for the operation of the supposed phyletic force? What right have we
to assume that the typical structures arise by the action of a vital
force? Nevertheless this is the final refuge of those who are bound
to admit that a great number of parts or characters of an animal can
become changed, suppressed, or even produced by the action of the
environment.



IV.

SUMMARY AND CONCLUSION.


The question heading the second section of this essay must at the
conclusion of the investigation be answered in the negative. The
form-relationship of the larvæ does not always coincide with that
of the imagines, or, in other words, a system based entirely on the
morphology of the larvæ does not always coincide with that founded
entirely on the morphology of the imagines.

Two kinds of incongruence here present themselves. The first arises
from the different amount of divergence between two systematic groups
in the larvæ and in the imagines, these groups being of equal extent.
The second form of incongruence consists essentially in that the
two stages form systematic groups of different extents, either the
one stage constituting a group of a higher order than the other and
therefore forming a group of unequal value, or else the two stages form
groups of equal systematic value, these groups, however, not coinciding
in extent, but the one overlapping the other.

This second form of incongruence is very frequently connected with the
first kind, and is mostly the direct consequence of the latter.

The cause of the incongruences is to be found in unequal phyletic
development, either the one stage within the same period of time having
been influenced by a greater number of transforming impulses than the
other, or else these impulses have been different in strength, _i.e._
have affected parts of greater or less physiological value, or have
influenced parts of equal value with unequal strength.

In all these cases in which there are deep-rooted form-differences,
it can be shown that these correspond exactly with inequalities in
the conditions of life, this correspondence being in two directions,
viz. in strength and in extent: the former determines the _degree_ of
form-difference, the latter its _extent_ throughout a larger or smaller
group of species.

The different forms of incongruence are manifested in the following
manner:--

(1.) Different amount of form-divergence between the larvæ on the one
side and the imagines on the other. Among the Lepidoptera this is found
most frequently in varieties and species, and there is evidence to
show that in this case the one stage has been affected by transforming
influences, either alone (varieties), or at any rate to a greater
extent (species). In the last case it can be shown in many ways that
one stage (the larva) has actually remained at an older phyletic grade
(_Deilephila_ species). Incongruences of this kind depending entirely
upon the more frequent action of transforming impulses can only become
observable in the smaller systematic groups, in the larger they elude
comparative examination. In the higher groups unequal form-divergence
may be produced by the transforming impulses affecting parts of unequal
physiological and morphological value, or by their influencing parts
of equal value in different degrees. All effects of this kind can,
however, only become manifest after a long-continued accumulation of
single changes, _i.e._ only in those systematic groups which require
a long period of time for their formation. By this means we can
completely explain why the incongruences of form-divergence continually
diminish from varieties to genera, and then increase again from genera
upwards through families, tribes, and sub-orders: the first diminishing
incongruence depends upon an _unequal number_ of transforming impulses,
the latter increasing incongruence depends upon the _unequal power_ of
these impulses.

Cases of the second kind are found among the Lepidopterous families,
and especially in the higher groups (_Rhopalocera_ and _Heterocera_),
and appear still more striking in the higher groups of the Hymenoptera
and Diptera. Thus the caterpillar shaped and maggot-formed larvæ of
the Hymenoptera differ from one another to a much greater extent
than their imagines, since the latter have experienced a complete
transformation of typical parts; whilst in the caterpillar-formed larvæ
these parts vary only within moderate limits. Similarly in the case
of the Diptera, of which the gnat-like larvæ diverge more widely from
those of the grub type than do the gnats from the true flies. On the
other hand the divergence between the imagines of the fleas and gnats
is considerably greater than that between their larvæ--indeed the larvæ
of the fleas would have to be ranked as a family of the sub-order of
the gnat-like larvæ if we wished to carry out a larval classification.
By this it is also made evident that these unequal divergences, when
they occur in the higher systematic groups, always induce at the same
time the second form of incongruence--that of the formation of unequal
systematic groups.

In general whenever such unequal divergences occur in the higher groups
they run parallel with a strong deviation in the conditions of life.
If these differ more strongly on the side of the larvæ, we find that
the structure of the latter likewise diverges the more widely, and that
their form-relationship is in consequence made more remote (saw-flies
and ichneumons, gnats and flies); if, on the other hand, the difference
in the conditions of life is greater on the side of the imagines,
we find among the latter the greater morphological divergence
(butterflies and moths, gnats and fleas).

(2.) The second chief form of incongruence consists in the formation
of different systematic groups by the larvæ and the imagines, if the
latter are grouped simply according to their form-relationship without
reference to their genetic affinities. This incongruence again shows
itself in two forms--in the formation of groups of unequal value, and
the formation of groups equal in value but unequal in extent, _i.e._ of
overlapping instead of coinciding groups.

Of these two forms the first arises as the direct result of a different
amount of divergence. Thus the larvæ of the fleas, on account of their
small divergence from those of the gnats, could only lay claim to the
rank of a _family_, whilst their imagines are separated from the gnats
by such a wide form-divergence that they are correctly ranked as a
distinct _tribe_ or _sub-order_.

The inequalities in the lowest groups, varieties, can be regarded in a
precisely similar manner. If the larva of a species has become split up
into two local forms, but not the imago, each of the two larval forms
possesses only the rank of a _variety_, whilst the imaginal form has
the value of a _species_.

Less simple are the causes of the phenomenon that in the one stage the
lower groups can be combined into one of higher rank, whilst the other
stage does not attain to this high rank. Such a condition appears
especially complicated when the two stages can again be formed into
groups of a still higher rank.

This is the case in the tribe _Rhopalocera_, which is founded on the
imagines alone, the larvæ forming only families of butterflies. Both
stages can however be again combined into the highest systematic group
of the Lepidoptera.

In this case also the difference in the value of the systematic groups
formed by the two stages corresponds precisely with the difference in
the conditions of life. This appears very distinctly when there are
several sub-groups on each side, and not when, as in the fleas, only
one family is present as a tribe on the one side and on the other as
a family. Thus in the butterflies, on the one side there are numerous
families combined into the higher rank of a sub-order (imagines),
whilst on the other side (larvæ) a group of the same extent cannot
be formed. In this instance it can be distinctly shown that the
combination of the families into a group of a higher order, as is
possible on the side of the imagines, corresponds exactly with the
limits in which the conditions of life deviate from those of other
Lepidopterous families. The group of butterflies corresponds with an
equally large circle of uniform conditions of life, whilst a similar
uniformity is wanting on the side of the larvæ.

The second kind of unequal group formation arises from the circumstance
that groups of equal value can be formed from the two stages, but these
groups do not possess the same limits--they overlap, and only coincide
in part.

This is most clearly seen in the order Hymenoptera, in which both larvæ
and imagines form two well-defined morphological sub-orders, but in
such a manner that the one larval form not only prevails throughout the
whole of the one sub-order of the imagines, but also extends beyond and
spreads over a great portion of the other imaginal sub-order.

Here again the dependence of this phenomenon upon the influence
of the environment is very distinct, since it can be demonstrated
(by the embryology of bees) that the one form of larva--the
maggot-type--although the structure now diverges so widely, has
been developed from the other form, and that it must have arisen by
adaptation to certain widely divergent conditions of life.

This form of incongruence is always connected with unequal divergence
between the two stages of the one systematic group--in this
case the _Terebrantia_. The larvæ of this imaginal group partly
possess caterpillar-like (_Phytospheces_) and partly maggot-formed
(_Entomospheces_) larvæ, and differ from one another to a considerably
greater extent than the saw-flies from the ichneumons.[211] The final
cause of the incongruence lies therefore in this case also in the fact
that one stage has suffered stronger changes than the other, so that
a deeper division of the group has occurred in the former than in the
latter.

The analogous incongruences in single families of the Lepidoptera may
have arisen in a similar manner, as has already been more clearly shown
above; only in these cases we are as yet unable to prove in detail that
the larval structure has become more strongly changed through special
external conditions of life than that of the imagines.

In the smallest systematic group--varieties, it has been possible to
furnish some proof of this. The one-sided change here depends in part
upon the _direct action_ of external influences (seasonal dimorphism,
climatic variation), and it can be shown that these influences
(temperature) acted only on the one stage, and accordingly induced
change in this alone whilst the other stage remained unaltered.

It has now been shown--not indeed in every individual case, but for
each of the different kinds of incongruence of form-relationship--that
there is an exact parallelism corresponding throughout with the
incongruence in the conditions of life. Wherever the forms diverge
more widely in one stage than in the other we also find more widely
divergent conditions of life; wherever the morphological systemy of
one stage fails to coincide with that of the other--whether in the
extent or in the value of the groups--the conditions of life in that
stage also diverge, either more widely or at the same time within
other limits; whenever a morphological group can be constructed from
one stage but not from the other, we find that this stage alone is
submitted to certain common conditions of life which fail in the other
stage.

The law that the divergence in form always corresponds exactly with
the divergence in the conditions of life[212] has accordingly received
confirmation in all cases where we have been able to pronounce
judgment. Unequal form-divergences correspond precisely with unequal
divergence in the conditions of life, and community of form appears
within exactly the same limits as community in the conditions of life.

These investigations may thus be concluded with the following law:--In
types of similar origin, _i.e._ having the same blood-relationship,
the degree of morphological relationship corresponds exactly with the
degree of difference in the conditions of life in the two stages.

With respect to the question as to the final cause of transformation
this result is certainly of the greatest importance.

The interdependence of structure and function has often been insisted
upon, but so long as this has reference only to the agreement of each
particular form with some special mode of life, this harmony could
still be regarded as the result of a directive power; but when in
metamorphic forms we not only see a double agreement between structure
and function, but also that the transformation of the form occurs in
the two chief developmental stages in successive steps at unequal
rates and with unequal strength and rhythm, we must--at least so it
appears to me--abandon the idea of an inherent transforming force; and
this becomes the more necessary when, by means of the opposite and
extremely simple assumption that transformations result entirely from
the response of the organism to the actions of the environment, all the
phenomena--so far as our knowledge of facts at present extends--can be
satisfactorily explained. A power compelling transformation, _i.e._
a phyletic vital force, must be abandoned, on the double ground that
it is incapable of explaining the phenomena (incongruence and unequal
phyletic development), and further because it is superfluous.

Against the latter half of this argument there can at most be raised
but the one objection that the phenomena of transformation are not
completely represented by the cases here analysed. In so far as this
signifies that the whole organic world, animal and vegetable, has not
been comprised within the investigation this objection is quite valid.
The question may be raised as to the limit to which we may venture to
extend the results obtained from one small group of forms. I shall
return to this question in the last essay.

But if by this objection it is meant that the restricted field of
the investigation enables us to actually analyse only a portion of
the occurring transformations,[213] and indeed only those cases,
the dependence of which upon the external conditions of life would
be generally admitted, I will not let pass the opportunity of once
more pointing out at the conclusion of the present essay that the
incongruences shown to exist by no means depend only upon those more
superficial characters the remodelling of which in accordance with
the external conditions of life may be most easily discerned and
is most difficult to deny, but that in certain cases (maggot-like
Dipterous larvæ) it is precisely the “typical” parts which become
partly suppressed and partly converted into an entirely new structure.
From the ancient typical appendages there have here arisen new
structures, which again have every right to be considered as typical.
This transformation is not to be compared with that experienced
by the swimming appendages of the _Nauplius_-like ancestor of an
_Apus_ or _Branchipus_ which have become mandibulate, nor with the
transformation which the anterior limbs must have gone through in
the reptilian ancestors of birds. The changes in question (Dipterous
larvæ) go still further and are more profound. I lay great emphasis
upon this because we have here one of the few cases which show that
typical parts are quite as dependent upon the environment as untypical
structures, and that the former are not only able to become adapted to
external conditions by small modifications--as shown in a most striking
manner by the transformations of the appendages in the Crustacea and
Vertebrata--but that these parts can become modelled on an entirely
new type which, when perfected, gives no means of divining its mode
of origin. I may here repeat a former statement:--With reference to
the causes of their origination we have no grounds for drawing a
distinction between typical and untypical structures.

It may be mentioned in concluding that quite analogous although less
sharply defined results are arrived at if, instead of fixing our
attention upon the different stages of a systematic group in their
phyletic development, we only compare the different functional parts
(organs in the wide sense) of the organisms.

A complete parallel can be drawn between the two classes of
developmental phenomena. From the very different systematic values
attached by taxonomists to this or that organ in a group of animals,
it may be concluded that the individual parts of an organism are to
a certain extent independent, and that each can vary independently,
when affected either entirely alone or in a preponderating degree by
transforming impulses, without all the other parts of the organism
likewise suffering transformation, or at least without their becoming
modified in an equal degree. Did all the parts and organs in two groups
of animals diverge from each other to the same extent, the systematic
value of such parts would be perfectly equal; we should, for example,
be able to distinguish and characterize two genera of the family of
mice by their kidneys, their liver, their salivary glands, or by the
histological structure of their hair or muscles, or even by differences
in their myology, &c. equally as well as by their teeth, length of
toes, &c. It is true that such a diagnosis has yet to be attempted; but
it may safely be predicted that it would not succeed. Judging from all
the facts at present before us, the individual parts--and especially
those connected in their physiological action, _i.e._ the system of
organs--do not keep pace with reference to the modifications which the
species undergoes in the course of time; at one period one system and
at another period some other system of organs advances while the others
remain behind.

This corresponds exactly with the result already deduced from the
unparallel development of the independent ontogenetic stages. If the
inequality in the phyletic development is more sharply pronounced in
this than in the last class of cases, this can be explained by the
greater degree of correlation which exists between the individual
systems of organs in any single organism as compared with that existing
between the ontogenetic stages, which, although developed from one
another, are nevertheless almost completely independent. We should have
expected _à priori_ that a strong correlation would have here existed,
but as a matter of fact this is not the case, or is so only in a very
small degree.

Just as in the stages of metamorphosis the inequality of phyletic
development becomes the more obliterated the more distant and
comprehensive, or, in other words, the greater the period of existence
of the groups which we compare, so does the unequal divergence of the
systems of organs become obliterated as we bring into comparison larger
and larger systematic groups.

It is not inconceivable--although a clear proof of this is certainly
as yet wanting--that a variety of the ancestral species would differ
only in one single character, such as hairiness, colour, or marking,
and such instances would thus agree precisely with the foregoing cases
in which only the caterpillar or the butterfly formed a variety.
All the more profound modifications however--such for instance as
those which determine the difference between two species--are never
limited to one character, but always affect several, this being
explicable by correlation, which, as Darwin has shown in the case of
dogs, may cause modifications in the skull of those breeds having
hanging ears in consequence of this last character alone. It must be
admitted however that one organ only would be originally affected by
a modifying influence. Thus, I am acquainted with two species of a
genus of Daphniacea which are so closely allied that they can only be
distinguished from one another by a close comparison of individual
details. But whilst most of the external and internal organs are almost
identical in the two species the sperm-cells of the males differ in a
most striking manner, in one species resembling an Australian boomerang
in form and in the other being spherical! An analogous instance is
furnished by _Daphnia Pulex_ and _D. Magna_, two species which were
for a long time confounded. Nearly all the parts of the body are here
exactly alike, but the antennæ of the males differ to a remarkable
extent, as was first correctly shown by Leydig.

Similarly in the case of genera there may be observed an incongruence
of such a kind that individual parts of the body may deviate to
a greater or to a less extent than the corresponding parts in an
allied genus. If, for instance, we compare a species of the genus
of Daphniacea, _Sida_, with a species of the nearly allied genus
_Daphnella_, we find that all the external and internal organs are in
some measure dissimilar--nevertheless certain of these parts deviate to
an especially large extent, and have without question become far more
transformed than the others. This is the case, for example, with the
antennæ and the male sexual organs. The latter, in _Daphnella_, open
out at the sides of the posterior part of the body as long, boot-shaped
generative organs, and in _Sida_ as small papillæ on the ventral side
of this region of the body. If again we compare _Daphnella_ with the
nearly allied genus _Latona_, it will be found that no part in the one
is exactly similar to the corresponding part in the other genus, whilst
certain organs differ more widely than others. This is the case for
instance with the oar-like appendages which in _Latona_ are triramous,
but in _Daphnella_, as in almost all the other Daphniacea, only
biramous.

In families the estimation of the form-divergence of the systems of
organs and parts of the body becomes difficult and uncertain: still it
may safely be asserted that the two Cladocerous families _Polyphemidæ_
and _Daphniidæ_ differ much less from one another in the structure of
their oar-like appendages than in that of their other parts, such as
the head, shell, legs, or abdominal segments. In systematic groups of
a still higher order, _i.e._ in orders, and still more in classes, we
might be inclined to consider that all the organs had become modified
to an equally great extent. Nevertheless it cannot be conclusively
said that the kidneys of a bird differ from those of a mammal to the
same extent as do the feathers from mammalian hair, since we cannot
estimate the differences between quite heterogeneous things--it can
only be stated that both differ greatly. Here also the facts are not
such as would have been expected if transformation was the result of
an internal developmental force; no uniform modification of _all_
parts takes place, but first one part varies (variety) and then others
(species), and, on the whole, as the systematic divergence increases
all parts become more and more affected by the transformation and
all tend continually to appear changed to an equal extent. This is
precisely what would be expected if the transforming impulses came
from the environment. An equalization of the differences caused by
transformation must be produced in two ways; first by correlation,
since nearly every primary transformation must entail one or more
secondary changes, and secondly because, as the period of time
increases, more numerous parts of the body must become influenced by
primary transforming factors.

A tempting theme is here also offered by attempting to trace the
inequality of phyletic development to dissimilar external influences,
and by demonstrating that individual organs have as a rule become
modified in proportion to the divergence in the conditions of life by
which they have been influenced, this action, during a given period of
time, having been more frequent in the case of one organ than in that
of the others, or, in brief, by showing the connection between the
causes and effects of transformation.

It would be quite premature, however, to undertake such a labour at
present, since it will be long before physiology is able to account
for the fine distinctions shown by morphology, and further because we
have as yet no insight into those internal adjustments of the organism
which would enable us _à priori_ to deduce definite secondary changes
from a given primary transformation. But so long as this is impossible
we have no means of distinguishing correlative changes from the primary
modifications producing them, unless they happen to arise under our
observation.



APPENDIX I.[214]

ADDITIONAL NOTES ON THE ONTOGENY, PHYLOGENY, &C., OF CATERPILLARS.


_Ontogeny of the Noctua larvæ._--References have already been given
in a previous note (67, p. 166) to observations on the number of legs
and geometer-like habits of certain _Noctua_-larvæ when newly hatched.
This interesting fact in the development of these insects furnishes
a most instructive application of the principle of ontogeny to the
determination of the true affinities, _i.e._ the blood-relationship of
certain groups of Lepidoptera. While the foregoing portions of this
work have been in course of preparation for the press, some additional
observations on this subject have been published, and I may take the
present opportunity of pointing out their systematic bearing--not,
indeed, with a view to settling definitively the positions of the
groups in question, as our knowledge is still somewhat scanty--but with
the object of stimulating further investigation.

Mr. H. T. Stainton has lately recorded the fact that the young larva
of _Triphæna Pronuba_ is a semi-looper (Ent. Mo. Mag. vol. xvii. p.
135); and in a recently published life-history of _Euclidia Glyphica_
(_Ibid._ p. 210) Mr. G. T. Porritt states that this caterpillar is
a true looper when young, but becomes a semi-looper when adult. To
these facts Mr. R. F. Logan adds (_Ibid._ p. 237) that “nearly all
the larvæ of the _Trifidæ_ are semi-loopers when first hatched.” The
_Cymatophoræ_ appear to be an exception, but Mr. Logan points out
that this genus is altogether aberrant, and seems to be allied to the
_Tortricidæ_. Summing up the results of these and the observations
previously referred to, it will be seen that this developmental
character has now been established in the case of species belonging
to the following families of the section _Genuinæ_:--_Leucaniidæ_,
_Apameidæ_, _Caradrinidæ_, _Noctuidæ_, _Orthosiidæ_, _Hadenidæ_,
and _Xylinidæ_, as well as the other _Trifidæ_ (excepting
_Cymatophora_).[215] The larvæ of the _Minores_ and _Quadrifidæ_ are
as a rule semi-loopers when adult and may be true loopers when young,
although further observations on this point are wanted. These facts
point to the conclusion that the _Noctuæ_ as a whole are phyletically
younger than the _Geometræ_, whilst the _Genuinæ_ and _Bombyciformes_
have further advanced in phyletic development than the _Minores_ and
_Quadrifidæ_. The last two sections are therefore the most closely
related to the _Geometræ_, as correctly shown by the arrangement given
in Stainton’s “Manual;” whilst that adopted in Doubleday’s “Synonymic
List,” where the _Geometræ_ precede the _Noctuæ_, is most probably
erroneous.

_Additional descriptions of Sphinx-larvæ._--In the foregoing essay on
“The Origin of the Markings of Caterpillars,” Dr. Weismann has paid
special attention to the larvæ of the _Sphingidæ_ and has utilized
for this purpose, in addition to his own studies of the ontogeny of
many European species, the figures in the chief works dealing with
this family published down to the time of appearance of his essay
(1876).[216] In order to amplify this part of the subject I have added
references to more recent descriptions and figures of Sphinx-larvæ
published by Burmeister and A. G. Butler, and I have endeavoured in
these cases to refer the caterpillars as far as possible to their
correct position in the respective groups founded on the ontogeny
and phylogeny of their allies. It is, however, obvious that for the
purposes of this work figures or descriptions of adult larvæ are of but
little value, except for the comparative morphology of the markings;
and even this branch of the subject only becomes of true biological
importance when viewed in the light of ontogeny. As our knowledge of
the latter still remains most incomplete in the case of exotic species,
it would be at present premature to attempt to draw up any genealogy
of the whole family, and I will here only extend the subject by adding
some few descriptions of species which are interesting as having been
made from the observations of field-naturalists, and which contain
remarks on the natural history of the insects.

Mr. C. V. Riley in his “Second Annual Report on the Noxious,
Beneficial, and other Insects of the State of Missouri, 1870,” gives
figures and describes the early stages and adult forms of certain
grape-vine feeding larvæ of the sub-family _Chærocampinæ_. The
full-grown larva of _Philampelus Achemon_, Drury, “measures about 3½
inches when crawling, which operation is effected by a series of sudden
jerks. The third segment is the largest, the second but half its size,
and the first still smaller, and when at rest the two last-mentioned
segments are partly withdrawn into the third.... The young larva is
green, with a long slender reddish horn rising from the eleventh
segment and curving over the back.” Mr. Riley then states that full
grown specimens are sometimes found as green as the younger ones, but
“they more generally assume a pale straw or reddish-brown colour, and
the long recurved horn is invariably replaced by a highly polished
lenticular tubercle.” The specimen figured was the pale straw variety,
this colour deepening at the sides, and finally merging into a rich
brown. The markings appear to consist of an interrupted brown dorsal
line, a continuous subdorsal line of the same colour, and six oblique
scalloped white bars along the side. Whether the colour and marking
is adapted to the vine, as is the case with the two varieties of the
dimorphic _Chærocampa Capensis_ (_q.v._), is not stated. The larva of
_Philampelus Satellitia_, Linn., when newly hatched, and for some time
afterwards is “green with a tinge of pink along the sides, and with an
immensely long straight pink horn at the tail. This horn soon begins
to shorten, and finally curls round like a dog’s tail.” The colour
of the insect changes to a reddish-brown as it grows older, and the
caudal horn is entirely lost at the third moult. The chief markings
appear to be five oblique cream-yellow patches with a black annulation
on segments 6-10, and a pale subdorsal line. The caterpillar crawls
by a series of sudden jerks, and often flings its “head savagely from
side to side when alarmed.” “When at rest, it draws back the fore
part of the body and retracts the head and first two joints into the
third.” Two points in connection with these species are of interest
with respect to the present investigations. The green colour and the
possession of a long caudal horn when young shows that these larvæ,
like those of _Chærocampa Elpenor_ (p. 178), _C. Porcellus_ (p. 184),
and _Philampelus Labruscæ_ (p. 195, note), are descended from ancestors
which possessed these characters in the adult state.[217] The next
point of interest is the attitude of alarm assumed by these larvæ,
and effected by withdrawing the head and two front segments into the
third.[218] The importance of this in connection with the similar
habit of ocellated species will be seen on reading the remarks on page
367 bearing upon the initial stages of eye-spots. The other species
figured by Mr. Riley are _Chærocampa Pampinatrix_, Smith and Abbot, and
_Thyreus Abboti_, Swains. The latter has already been referred to (p.
256).

In a paper “On a Collection of Lepidoptera from Candahar” (Proc. Zoo.
Soc., May 4th, 1880), Mr. A. G. Butler has described and figured, from
materials furnished to him by Major Howland Roberts, the larvæ of
three species of _Sphingidæ_. _Chærocampa Cretica_, Boisd., feeds on
vine; out of 100 specimens examined, there was not one black variety,
while in another closely allied species, found at Jutogh and Kashmir,
the larva is stated to be as often black as green. The general colour
of the caterpillar harmonizes with that of the underside of the vine
leaves; it possesses a thread-like dorsal, and a pale yellow subdorsal
line; also “a subdorsal row of eye-spots, each consisting of a green
patch in a yellow oval, the first spot on the fifth segment being the
largest and most distinct, those on each following segment becoming
smaller, more flattened, and less distinct, till lost on the twelfth
segment, sometimes becoming indistinct after the seventh or eighth
segment; these spots are only distinct as eye-spots on the fifth and
sixth segments, that on the sixth being flatter than that on the fifth,
those on the remaining segments appearing like dashes while the larvæ
is green, but more like eyes on its changing colour when full fed.”
The change here alluded to is the dark-brown coloration so generally
assumed by green Sphinx-larvæ previous to pupation, and which, as I
have stated elsewhere (Proc. Zoo. Soc., 1873, p. 155), is probably an
adaptation advantageous to such larvæ when crawling over the ground
in search of a suitable place of concealment. Making the necessary
correction for the different mode of counting the segments, it will be
seen that the primary ocelli of this species are in the same position
as those of the other species of this genus as described in a previous
part of this essay, and that it belongs to the second phyletic group
treated of at p. 193. The interesting fact that this species does not
display dimorphism, whilst the closely allied form from Kashmir is
dimorphic, shows that in the present species the process of double
adaptation has not taken place; and this will probably be found to be
connected with the habits of life, _i.e._ the insect being well adapted
to the colour of its food-plant may not conceal itself on the ground
by day. The caterpillar of _Deilephila Robertsi_, Butl., is found at
Candahar on a species of _Euphorbia_ growing on the rocky hills, and
is so abundant that at the end of May every plant with any leaves left
on it had several larvæ feeding upon it. “The larvæ are very beautiful
and conspicuous, and are very different in colouring according to their
different stages of growth.” The general colour is black with white
dots and spots; a subdorsal row of large roundish spots, one on each
segment, either white, yellow, orange or red; dorsal stripe variable
in colour, and sometimes only partially present or altogether absent.
“At the end of May most of the larvæ found presented a different
appearance; the black disappears more or less, and with it many of
the small white spots. In some cases the black only remains as a ring
round the larger white spots; the ground-colour therefore becomes
yellowish-green or yellow, varying very considerably.” The larva does
not change colour previous to pupation. This species, according to
the outline figure given (_loc. cit._, Pl. XXXIX., Fig. 9), appears
to belong to the first of Dr. Weismann’s groups, comprising _D.
Euphorbiæ_, _D. Dahlii_ and _D. Nicæa_ (see p. 199), and is therefore
in the seventh phyletic stage of development (p. 224). From the
recorded habits it seems most probable that the colours and markings
of this caterpillar are signals of distastefulness. It is much to
be regretted that Major Roberts has not increased the value of his
description of this species by adding some observations or experiments
bearing on this point. _Eusmerinthus Kindermanni_, Lederer, feeds on
willow. “General colour green, covered with minute white dots and seven
long pale yellow oblique lateral bands. (The ground-colour is the same
as the willow-leaves on which the larva feeds, the yellow stripes
the same as the leaf-stalks, and the head and true legs like the
younger branches).” As no subdorsal line is mentioned or figured, this
species must be regarded as belonging to the third stage of phyletic
development (see p. 242).

I have recently had an opportunity of inspecting a large number of
drawings of Sphinx-larvæ in the possession of Mr. F. Moore, and of
those species not mentioned in the previous portions of this work the
following may be noticed:--_Chærocampa Theylia_, Linn., like _Ch.
Lewisii_ (note 82, p. 194), appears to be another form connecting the
second and third phyletic groups of this genus. _Ch. Clotho_, Drury,
belongs to the third group (figured by Semper; see note 3 to this
Appendix). The larva of _Ch. Lucasii_, Walk., offers another instance
of the retention of the subdorsal line by an ocellated species. The
larva of _Ch. Lycetus_, Cram., of which Mr. Moore was so good as to
show me descriptions made at the various stages of growth, presents
many points of interest. It belongs to the third phyletic group, and
all the ocelli appear at a very early stage. The dimorphism appears
also in the young larvæ, some being green, and others black, a fact
which may be explained by the law of “backward transference” (see
p. 274). A most suggestive feature is presented by the caudal horn,
which in the young caterpillar is stated to be _freely movable_.
It is possible that this horn, which was formerly possessed by the
ancestors of the _Sphingidæ_, and which is now retained in many genera,
is a remnant of a flagellate organ having a similar function to the
head-tentacles of the _Papilio-larvæ_, or to the caudal appendages of
_Dicranura_ (see p. 289).

_Lophostethus Dumolinii_, Angas.--The larva of this species differs so
remarkably from those of all other _Sphingidæ_, that I have thought it
of sufficient interest to publish the following description, kindly
furnished by Mr. Roland Trimen, who in answer to my application sent
the following notes:--“My knowledge of the very remarkable larva
of this large and curious Smerinthine Hawk-moth is derived from a
photograph by the late Dr. J. E. Seaman, and from drawings and notes
recently furnished by Mr. W. D. Gooch. The colour is greenish-white,
inclining to grey, and in the male there is a yellow, but in the female
a bluish, tinge in this. All the segments but the second and the head
bear strong black spines, having a lustre of steel blue, and springing
from a pale yellow tubercular base. The longest of these spines are in
two dorsal rows from the fourth to the eleventh segment, the pairs on
the fourth and fifth segments being longer than the rest, very erect,
and armed with short simple prickles for three-fourths of their upper
extremity. The anal horn, which is shorter than the spines, is of the
same character as the latter, being covered with prickles, and much
inclined backwards. Two lateral rows of similar shorter spines extend
from the fourth to the 12th segment, and on each of the segments
6-11 the space between the upper and lower spines is marked with a
conspicuous pale yellow spot. Two rows of smaller similar spines extend
on each side (below the two rows of larger ones) from the second to the
thirteenth segment, one spine of the lowermost row being on the fleshy
base of each pro-leg. All the pro-legs are white close to the base, and
russet-brown beyond. Head smooth, unarmed in adult, greenish-white with
two longitudinal russet-brown stripes on face.

“The young larvæ have proportionally much longer and more erect spines
with distinct long prickles on them. There is a short pair besides,
either on the back of the head or on the second segment. Moreover,
the dorsal spines of the third and fourth segments, and the anal horn
(which is quite erect, and the longest of all), are longer than the
rest, and distinctly _forked_ at their extremity.

“Mr. Gooch notes that these young larvæ might readily be mistaken for
those of the _Acrææ_, and suggests that this may protect them. He also
states that the yellow lateral spots are only noticed after the last
moult before pupation, and that the general resemblance of the larva as
regards colour is to the faded leaves of its food-plant, a species of
_Dombeia_.”

The forked caudal horn in the young larva of this species is of
interest in connection with the similar character of this appendage in
the young caterpillar of _Hyloicus Pinastri_, p. 265.

_Retention of the Subdorsal Line by Ocellated Larvæ._--It has already
been shown with reference to the eye-spots of the _Chærocampa_-larvæ,
that these markings have been developed from the subdorsal line, and
that, in accordance with their function as a means of causing terror,
this line has in most species been eliminated in the course of the
phylogeny from those segments bearing the eye-spots in order to give
full effect to the latter (see p. 379). In accordance with the law
that a character when it has become useless gradually disappears, the
subdorsal is more or less absent in all those species in which the
ocelli are most perfectly developed; and it can be readily imagined
that in cases where adaptation to the foliage exists the suppression
of this line would under certain conditions be accelerated by natural
selection. On the other hand, it is conceivable that the subdorsal
line may under other conditions be of use to a protectively coloured
ocellated species by imitating some special part of the food-plant,
under which circumstances its retention would be secured by natural
selection.

Such an instance is offered by _Chærocampa Capensis_, Linn.; and as
this case is particularly instructive as likewise throwing light upon
the retention of the subdorsal by certain species having oblique
stripes (see p. 377, and note 166, p. 378), I will here give some details
concerning this species which have been communicated to me by Mr.
Roland Trimen, the well-known curator of the South African Museum, Cape
Town. The caterpillar of _C. Capensis_, like so many other species
of the genus, is dimorphic, one form being a bright (rather pale)
green, and the other, which is much the rarer of the two, being dull
pinkish-red. Both these forms are adapted in colour to the vine on
which they feed, the red variety according to some extent with the
faded leaves of the cultivated vines, but to a greater extent with the
young shoots and underside of the leaves of the South African native
vine (_Cissus Capensis_), on which it also feeds. There are two
eye-spots in this species in the usual positions; they are described as
being blue-grey in a white ring, and raised so as to project a little.
The subdorsal is white, and is bordered beneath by a wide shade of
bluish-green irrorated with white dots, and crossed by an indistinct
white oblique ray on each segment. These last markings are probably
remnants of an oblique striping formerly possessed by the progenitor of
this and other species of the genus (see, for instance, Fig. 25, Pl.
IV., one of the young stages of _C. Porcellus_). It is possible that
these rudimentary oblique stripes are now of service in assisting the
adaptation of the larva to its food-plant, but this cannot be decided
without seeing the insect _in situ_.

The subdorsal line extends from immediately behind the second eye-spot
to the base of the very short and much curved violet anal horn.
With reference to the protective colouring Mr. Trimen writes:--“The
difficulty of seeing these large and beautifully-coloured larvæ on
the vines is quite surprising; six or more may be well within sight,
and yet quite unnoticed. The subdorsal stripe greatly aids in their
concealment, as it well represents in its artificial light and shade
the leaf-stalks of the vine.” When this larva withdraws its front
segments the eye-spots stand out very menacingly; but in spite of this
it is greedily eaten by fowls and shrikes (_Fiscus Collaris_), and
Mr. Trimen also found that a tame suricate (_Rhyzæna Suricata_) and a
large monitor lizard (_Regenia Albogularis_) did not refuse them. The
failure of the eye-spots in causing terror in these particular cases
cannot be regarded as disproving their utility in all instances. It
must always be borne in mind that no protective character can possibly
be of service against _all_ foes; natural selection only requires that
such characters should be advantageous with respect to the majority
of the enemies of any species, and further experiments with this
caterpillar may show that in the case of smaller foes the eye-spots
are effective as a means of causing alarm. The dimorphism of the larva
of _C. Capensis_ is of special interest, although we are not yet
sufficiently acquainted with the habits of this species to offer a
complete explanation. According to Dr. Weismann’s conclusions (p. 297),
the dimorphism of the _Chærocampa_-larvæ is due to a double adaptation,
the insects first having acquired the habit of concealing themselves
by day, and the dark form having then been produced by the action of
natural selection, in order to adapt such varieties to the colour of
the soil, whilst others retained the green colour which adapts them
to the foliage of their food-plants. In accordance with this, _C.
Capensis_ may have a similar habit of concealment, or (should this be
found not to be the case) it is possible that this insect at a former
period possessed this habit and fed upon some other plant, when it
would have become dimorphic in the manner explained, and the _existing_
dimorphism may be a survival of the more ancient dimorphism, the red
form (corresponding to the older dark form) having been subsequently
modified so as to become also adapted to the new food-plant. Much light
would be thrown upon this by studying the ontogeny of the species.

_Phytophagic Variability._--A number of observations bearing on the
phytophagic variability of the Sphinx-larvæ and other caterpillars
have been recorded in a previous note (p. 305), and reference has also
been made to the food-plants of _Acherontia Atropos_ in South Africa
(note 121, p. 263). I am now enabled to add some further observations
on this species, from notes furnished to me by Mr. Roland Trimen, who
states that for many years he has noticed that at the Cape this larva
varies greatly in the depth and shade of the green ground-colour, the
variability being in strict accordance with the colour of the leaves
of the particular plant on which the individual feeds. The phenomenon
was particularly noticeable in larvæ feeding on _Buxia Grandiflora_, a
shrub in common cultivation in gardens, and of which the foliage is of
a very dull pale greyish-green. Another striking instance was noticed
in some very fine caterpillars feeding on a large shrubby _Solanum_,
which, excepting the bright yellow bands bordering the dorsal violet
bars, were generally dull ochreous-yellow, like the leaves and stalks
of the _Solanum_. On plants with bright green or deep green leaves,
the colour of the larvæ is almost in exact agreement. Mr. Trimen
adds:--“These remarks apply principally to the underside and pro-legs
and lower lateral regions, the dorsal colours of violet and yellow
varying but little. The protection afforded is very considerable, as
the larvæ almost always cling to the lower side of the twigs of their
food-plants, so that their uniformly-coloured under-surface is upwards,
and turned towards the light, and their variegated upper surface turned
downwards.”

These observations are of the highest importance, not only as adding
another instance to the recorded cases of phytophagic variation, but
likewise as showing that with this variability a protective habit
has been acquired. It is to be hoped that such a promising field
for experimental investigation as is offered by this and analogous
cases will not long remain unexplored. In attacking the problem two
chief questions have in the first place to be settled: (1) Is the
variability truly phytophagic, _i.e._ are the colour variations
actually brought about by the chemico-physiological action of the
food-plant? and (2) Are the larvæ at any period of growth susceptible
to the action of phytophagic influences? The first question could be
decided by feeding larvæ from the same batch of eggs on different
food-plants from the period of their hatching. The second question
could be settled by changing the food-plants of a series of selected
specimens at various stages of growth, and observing whether any
change of colour was produced. In accordance with the principles
advocated in a previous note (p. 305), it is conceivable _à priori_
that phytophagic variability may occur by direct chemico-physiological
action, quite irrespective of any of the changes of colour being of
protective use. In the case of brightly-coloured distasteful species
phytophagic variability might thus have full play, but in the case of
protectively-coloured edible species, phytophagic variability would be
under the control of natural selection. These considerations raise a
question of the greatest theoretical interest in connection with this
phenomenon. If phytophagic variability can have full play uncontrolled
by natural selection in brightly-coloured caterpillars, ought not this
phenomenon to be of more common occurrence in such species than in
those protectively coloured? Although our knowledge of this subject is
still very imperfect, as a matter of fact brightly coloured larvæ, so
far as I have been able to ascertain, do not appear to be susceptible
of phytophagic influences. But this apparent contradiction, instead
of opposing actually confirms the foregoing views, as will appear on
further consideration. The colours of protected species are as a whole
much inferior in brilliancy to those of inedible species, so that any
phytophagic effect would be more perceptible in the former than in the
latter, in which the highest possible standard of brilliancy appears
in most cases to have been attained. Now phytophagic variations of
colour appear to be of but small amount, or, in other words, such
variations fluctuate within comparatively restricted limits, and as
the cases at present known are mostly _adaptive_ it is legitimate
to conclude that they have been produced and brought to their
present standard by natural selection, _i.e._ that they have arisen
from phytophagic influences as a cause of variability. The initial
stages of phytophagic variations must therefore have been still less
perceptible than the now perfected final results; and this leads to the
conclusion that minute variations of this character were of sufficient
importance to protectively-coloured species to be taken advantage of
by natural selection. But minute variations in a dull-coloured larva
would, as previously pointed out, produce a comparatively much greater
effect than such variations in a brilliantly-coloured species; and as
protection is required by the former, the initial phytophagic effects
would be accumulated, and the power of adaptability conferred by the
continued action of natural selection, whilst in vividly-coloured
species where no power of adaptability is required this cause of
variation would not only produce a result which, as compared with its
effects upon dull species, may be regarded as a “vanishing quantity,”
but this result would be too insignificant to be taken advantage of
by natural selection, which is in these cases dealing only with large
“quantities,” and striving to make the caterpillars as brilliant as
possible. The fact that vividly-coloured distasteful larvæ do not show
phytophagic variation is to my mind explained proximately by these
considerations; the ultimate cause of phytophagic variability regarded
as a chemico-physiological action requires further investigation.

_Sexual Variation in Larvæ._--Since most of the markings of
caterpillars can be explained by the two factors of adaptation and
inheritance, or, in other words, by their present and past relations
to the environment, and since sexual selection can have played no
direct part in producing these colours and markings, I feel bound to
record here some few observations on the sexual differences in larvæ in
addition to the cases of _Anapæa_ and _Orgyia_ already recorded (note
i., p. 308) and of _Lophostethus Dumolinii_ (p. 527).

Mr. C. V. Riley states[219] with reference to the larva of _Thyreus
Abboti_ that the ground-colour appears to depend upon the sex, Dr.
Morris having described the insect as “reddish-brown with numerous
patches of light green,” and having expressly stated that “the _female_
is of a uniform reddish-brown with an interrupted dark-brown dorsal
line and transverse striæ.” Mr. W. D. Gooch, who has reared the South
African butterflies _Nymphalis Cithæron_ and _N. Brutus_ from their
larvæ, states[220] that these “differed sexually in both instances.”
Of _Brutus_ only a few were bred, but of _Cithæron_ many. “The sexual
difference of the latter was that the females had a large dorsal
sub-cordate cream mark, which was wanting, or only shown by a dot, in
the males, and the colour was more vivid in the edgings to the frontal
horns.”

Although such cases appear to be at present inexplicable, they are of
interest as examples of those “residual phenomena” which, as is well
known, have in many branches of science so often served as important
starting-points for new discoveries and generalizations.[221]



APPENDIX II.


The following paper by Dr. Fritz Müller[222] forms the third of
a series of communications on Brazilian butterflies published in
“Kosmos,” and as it bears upon the investigations made known in the
third essay of the present work, I will here give a translation, by
permission of the publisher, Herr Karl Alberts.


“ACRÆA AND THE MARACUJÁ BUTTERFLIES AS LARVÆ, PUPÆ, AND IMAGINES.

“In a thoughtful essay on ‘Phyletic Parallelism in Metamorphic
Species,’ Weismann has shown that in the case of Lepidoptera the
developmental stages of larva, pupa, and imago vary independently,
and that a change occurring in one stage is without influence upon
the preceding and succeeding stages, so that the course which has
been followed by the individual stages in their developmental history
has not been in all cases identical. This want of agreement may
manifest itself both by unequal divergence of form-relationship, and
by unequal group formation. With respect to unequal form-divergence
the caterpillars are sometimes more closely related in form than their
imagines, and at other times the reverse is the case. With respect
to unequal group formation again, two cases are possible; the larvæ
and imagines may form groups of unequal value, the one stage forming
higher or lower groups than the other, or they may form groups of
unequal size, _i.e._, groups which do not coincide but which overlap.
Form-relationship and blood-relationship do not therefore always agree;
the resemblances among the caterpillars would lead to a quite different
arrangement to that resulting from the resemblances among the imagines,
and it is probable that neither of these arrangements would correspond
with the actual relationships.

“Starting from this fact, which he establishes by numerous examples,
Weismann proceeds to show most convincingly that an innate power of
development or of transformation, such as has been assumed under
various names by many adherents of the development theory, has no
existence, but that every modification and advancement in species has
been called forth by external influences.

“A most beautiful illustration of the want of ‘phyletic parallelism,’
as Weismann designates the different form-relationships of the
larvæ, pupæ, and imagines, is furnished by the five genera _Acræa_,
_Heliconius_, _Eueides_, _Colænis_, and _Dione_ (= _Agraulis_). This
instance seems to me to be of especial value, because it offers the
rare case of pupæ showing greater differences than the larvæ and
imagines.

“The species of which I observed the larvæ and pupæ are _Acræa Thalia_
and _Alalia_, _Heliconius Eucrate_, _Eueides Isabella_, _Colænis Dido_
and _Julia_, _Dione Vanillæ_ and _Juno_; besides these I noticed the
pupa of _Eueides Aliphera_.

“The following remarks apply only to these species, although we may
suppose with great probability that the whole of the congeneric
forms--excepting perhaps the widely ranging species of _Acræa_--would
display similar characters to their Brazilian representatives.

“The imagines of the five genera mentioned form two sharply defined
families, the _Acræidæ_ and the butterflies of the Maracujá group.[223]
The latter comprises the three genera _Heliconius_, _Eueides_, and
_Colænis_, which differ only in very unimportant characters; _Eueides_
is distinguished from _Heliconius_ by its shorter antennæ, and
_Colænis_ differs from _Eueides_ in having the discoidal cell of the
hind-wings open. The genus _Dione_ is further removed by the different
structure of the legs, and the silvery spots on the underside of
the wings. Certain species resemble those of other genera in a most
striking manner, and much more closely both in colour and marking, and
even in the form of their wings, than they do their own congeners. This
is the case with _Acræa Thalia_ and _Eueides Pavana_, with _Heliconius
Eucrate_ and _Eueides Isabella_, and with _Eueides Aliphera_ and
_Colænis Julia_, which are deceptively alike, and the last two are
connected with _Dione Juno_, at least by the upper side of the wings.
The difficulty of judging of the relationships of the single species is
thus much aggravated; it cannot be said how much of this resemblance
is to be attributed to blood-relationship, and how much to deceptive
imitation.

“As larvæ all the Brazilian species must be placed _in one genus_, as
they agree exactly in the number and arrangement of their spines (4
spines, not in a transverse row, on segments 2 and 3; 6 spines, in a
transverse row, on segments 4-11; 4 spines, not in a transverse row,
on the last (12th) segment). They differ from one another much less
in this respect than do the German species of _Vanessa_, such, for
instance, as _V. Io_ or _Antiopa_ from _V. Polychloros_, _Urticæ_,
and _Atalanta_.[224] The larvæ of _Acræa Thalia_ are certainly
without the two spines on the head which the others possess, and, on
the other hand, they have a well-developed pair of spines on the
first segment, which, in most of the other species, are completely
absent; but this does not justify their separation, since the head
spines of _Heliconius_, _Eueides_, and _Colænis Dido_, which are of
a considerable length, are shorter than those of the next segment in
_Colænis Julia_, and _Dione Vanillæ_, and in _Dione Juno_ they dwindle
down to two minute points, this last species also bearing a short pair
on the first segment. The larva of _Dione Juno_ is thus as closely
related to that of _Acræa Thalia_ as it is to that of its congener
_Dione Vanillæ_.

“If it were desired to form two distinct larval groups this could not
be effected on the basis of their differences in form, but could only
be based on their food-plants. The larvæ of _Heliconius_, _Eueides_,
_Colænis_, and _Dione_ live on species of Maracujá (Passiflora);
those of _Acræa Thalia_ and _Alalia_ on Compositæ (_Mikania_ and
_Veronia_). These larval groups would agree with those founded on the
form-relationships of the imagines, but unlike the imaginal groups,
which can be formed into families, they would scarcely possess a
generic value.

“If we arrange the single species of caterpillars according to their
resemblances, this arrangement does not agree with that based on the
resemblances of the imagines, even if we disregard the different values
of the groups. The result is somewhat as follows:--

[Illustration:

                                         IMAGINES.
                   (Nymphalideous butterflies with tufts on wing-veins.)
                   /---------------------------------------------------\
 (Families.)                MARACUJÁ-GROUP.                     ACRÆIDÆ.
                    /---------------------------\                  |
            /-------------------\    /-------------------\         |
 (Genera.)  Heliconius.  Eueides.   Colænis.         Dione.      Acræa.
              |                    /-------\       /-------\       |
 (Species.) Eucrate.  Isabella.  Dido.  Julia.  Vanillæ.  Juno.  Thalia.
              |          |         |      |        |        |      |
              \----------+---------/      \--------/        \------/
                         \--------------------------------------/
                                         LARVÆ.”
]

       *       *       *       *       *

[Here follow the remarks on the habits of the larvæ in connection with
their colours, &c., which have already been quoted in illustration of
the use of the spiny protection (note 133, p. 293). From these facts
the author draws the conclusion that the form-relationships of the
caterpillars depend rather upon their mode of life than upon their
blood-relationships, assuming the latter to be correctly expressed by
the arrangement of the imagines at present adopted.]

[Illustration: Figs. 1-4. Pupæ of _Acræa Thalia_; _Heliconius Eucrate_;
_Eueides Isabella_, and _Colænis Dido_; life size.]

“A glance at the above figures of the pupæ of _Heliconius Eucrate_
(Fig. 2), _Eueides Isabella_ (Fig. 3), and _Colænis Dido_ (Fig. 4),
will show how great are the differences between these pupæ as compared
with the close form-relationship of all the Maracujá butterflies, and
with the no less close resemblance of their larvæ. A family which
comprised three such dissimilar pupæ would also be capable of including
that of _Acræa Thalia_ (Fig. 1).

“The pupa of this last species has nothing peculiar in its general
appearance, but possesses the ordinary pupal form; it is tolerably
rounded, without any great elevations or depressions; a minute pointed
projection is situated on the head over each eye-cover, and a similar
process projects from the roots of the wings. Its distinguishing
characters are five pairs of spines on the back of the abdominal
segments. These spines are found also in _Acræa Alalia_, but appear
to be absent in other species, _e.g._ in the Indian _A. Violæ_.
Last summer, among some batches of _Thalia_ larvæ--each batch being
the progeny from one lot of eggs--I found certain individuals which
differed from the others in having much shorter spines, and these
changed into pupæ in which the five pairs of spines were proportionally
shorter than usual, thus being an exception to the rule that changes in
one stage of development are without influence on the other stages. I
may remark, by the way, that this law, enunciated by Weismann, can only
be applied to imagines and pupæ with certain restrictions. The skin of
the pupa forms a sheath or cover for the eyes, antennæ, trunk, legs,
and wings of the imago, and if these parts undergo any considerable
modification in the latter, corresponding changes must appear in the
pupa. This is shown, for instance, by many ‘Skippers’ (_Hesperidæ_),
in which the extraordinarily long trunk necessitates a sheath of a
corresponding length. The colour of the pupa of _Acræa Thalia_ is
whitish, the wing-veins with some other markings and the spines are
black; metallic spots are absent.

“In the pupa of _Heliconius Eucrate_ the laterally compressed region of
the wings is raised into a large projection, the antennal sheaths lying
on the edges of the wings are serrated and beset with short pointed
spines; instead of the minute projections of _Acræa Thalia_, the head
bears two large humped processes; the body is raised on each side into
a foliaceous border carrying five spines of different lengths, the
foremost pair, directed towards the head, being the longest. The pupa
is brown, and ornamented with four pairs of brilliant metallic spots,
one pair close behind the antennæ, and three pairs, almost coalescent,
on the back before the longest pair of spines. A short spine projects
from the middle of each of the latter somewhat arched metallic patches.

“In the pupa of _Colænis Dido_ (which resembles that of _Colænis
Julia_, and to which may be added those of _Dione Vanillæ_ and _Juno_)
the spines are absent, the wing region is but moderately arched, and
the antennæ marked only by small elevations; instead of the leaf-like
border, there are on each side of the back five knotty or humped
processes. The metallic spots are similar in number and position to
those of _Heliconius Eucrate_; those on the back have a wart-like
process in the middle, instead of a spine.

“The pupæ of _Heliconius_ and _Colænis_ when moving their posterior
segments rapidly, as they do whenever they are disturbed, produce a
very perceptible hissing noise by the friction of these segments,
this sound, which is especially noticeable in the case of _Heliconius
Eucrate_, perhaps serving to terrify small foes. (So loud is the sound
produced in this manner by the pupæ of _Epicalia Numilia_, that my
children have named them ‘_Schreipuppen_.’)

“The pupæ of _Heliconius_ and _Colænis_ thus differ to a much greater
extent than the imagines or larvæ, and the same holds good for
_Eueides_ in a much higher degree as compared with its above-mentioned
allies. The larvæ of _Eueides_ have no distinctive characters, and
even the generic rank of the imagines is doubtful; as pupæ, on the
other hand, they are far removed (even by their mode of suspension)
not only from the remainder of the Maracujá group and from the whole
of the great Nymphalideous group (_Danainæ_, _Satyrinæ_, _Elymniinæ_,
_Brassolinæ_, _Morphinæ_, _Acræinæ_ and _Nymphalinæ_), but from almost
all other butterflies. The larva pupates on the underside of a leaf;
the pupa is fastened by the tail, but does not hang down like the pupæ
of the other _Nymphalidæ_,--its last segments are so curved that the
breast of the chrysalis is in contact with the underside of the leaf.
I am not acquainted with any other pupa among those not suspended by
a girdle which assumes such a position. Something similar occurs,
however, in the pupa of _Stalachtis_, which is without a girdle, and
according to Bates, is ‘kept in an inclined position by the fastening
of the tail.’ By this peculiarity Bates distinguishes the _Stalachtinæ_
from the _Libytheæ_ with pupæ ‘freely suspended by the tail.’

“Besides through this peculiar position of the body, the pupa of
_Eueides Isabella_ is distinguished by short hooked and long narrow
sabre-like pairs of processes on the back and head. Its colour is
whitish, yellowish, or sordid yellowish-grey; in the last variety both
the four long dorsal processes and the surrounding portions, as well as
the points of the other processes, remain white or yellowish. The pupa
_Eueides Aliphera_ is very similar, only all the processes are somewhat
shorter, the four longest (dorsal) and some other markings being black.

“Now if, as Weismann has attempted to show for larvæ and imagines,
the form-divergence always ‘corresponds exactly with the divergence
in the mode of life,’ the question arises as to what difference
in the conditions of life has brought about such a considerable
form-divergence between the pupæ of such closely allied species as the
Maracujá butterflies. In pupæ which do not eat or drink, and which
have neither to seek in courtship nor to care for progeny, it is only
protection from foes that can concern us. But in the pupæ of nearly
allied species of which the larvæ feed on kindred plants in the same
districts at the same periods of the year, can the enemies be so
different as to produce such a considerable divergence in form? One
might answer this question in the negative with some confidence, and
affirm that in this case the difference in the pupæ does not result
from the ‘divergence in the mode of life,’ or from the difference
in the external conditions, but is accidental, _i.e._ a consequence
of some fortunate variation induced by some external cause, which
variation afforded protection against common foes--to one species in
one way, and to the other species in some other way; this course,
once entered upon, having been urged on by natural selection, until
at length the wide divergence now shown is attained. How in the case
of any of the species the peculiarity in colour or form can actually
serve as a protection, I must confess myself at fault in answering.
Only in the case of the pupa of _Eueides Isabella_ will I venture to
offer a supposition. That it is not green like other pupæ which suspend
themselves among foliage (_Siderone_, _Epicalia,_ _Callidryas_, &c.),
but contrasts more or less brightly with the dark green of the leaves,
precludes the idea of concealment; on the other hand its colour is
too dull to serve as a conspicuous sign of distastefulness. In either
case the meaning of the wonderful processes of the pupa would remain
unexplained.

“We are thus compelled to seek another possibility in mimicry, by which
foes would be deceived by deceptive resemblance. But what is the object
imitated? Dead insects overgrown by fungi are often found on leaves,
the whitish or yellowish fungi growing from their bodies in various
fantastic forms. Such insects of course no longer serve as tempting
morsels. The processes of the pupa of _Eueides_ suggest such fungoid
growths, although I certainly cannot assert that to our eyes in broad
daylight the resemblance is very striking. But the pupæ hang among the
shadows of the leaves, and a less perfect imitation may deceive foes
that are not so sharp-sighted; protective resemblance must commence
moreover with an imperfect degree of imitation.”



EXPLANATION OF THE PLATES.


PLATE III.

Figs. 1-12 represent larvæ of _Macroglossa Stellatarum_, all bred from
one batch of eggs. Most of the figures are enlarged, but sometimes to a
very small extent only; the lines show the natural length.

Fig. 1. Stage I.; a caterpillar immediately after hatching. Natural
length, 0.2 centim.

Fig. 2. Stage II.; shortly after the first moult. Natural length, 0.7
centim.

Figs. 3-12. Stage V.; the chief colour-varieties.

Fig. 3. The only lilac-coloured specimen in the whole brood. Natural
length, 3.8 centim.

Fig. 4. Light-green form (rare) with subdorsal shading off beneath.

Fig. 5. Green form (rare) with strongly-pronounced dark markings
(dorsal and subdorsal lines). Natural length, 4.9 centim.

Fig. 6. Dark-brown form (common). Natural length, 4 centim. In this
figure the fine shagreening of the skin is indicated by white dots;
in the other figures these are partially or entirely omitted, being
represented only in Figs. 8 and 10.

Fig. 7. Light-green form (common). Natural length, 4 centim.

Fig. 8. Light-brown form (common). Natural length, 3.5 centim.

[Illustration:

  Plate III.

  Aug. Weismann pinx.      Lith. J.A. Hofmann, Würzburg.
]

[Illustration:

  Plate IV.

  Aug. Weismann pinx.      Lith. J.A. Hofmann, Würzburg.
]

[Illustration:

  Plate V.

  Aug. Weismann pinx.      Lith. J.A. Hofmann, Würzburg.
]

[Illustration:

  Plate VI.

  Aug. Weismann pinx.      Lith. J.A. Hofmann, Würzburg.
]

Fig. 9. Parti-coloured specimen, the only one out of the whole brood.
Natural length, 5.5 centim.

Fig. 10. Grey-brown form (rare).

Fig. 11. One of the forms intermediate between the dark-brown and green
varieties, dorsal aspect.

Fig. 12. Light-green form with very feeble dorsal line (shown too
strongly in the figure), dorsal aspect.

Figs. 13-15. _Deilephila Vespertilio._

Fig. 13. Stage III.(?); the subdorsal bearing yellow spots. Natural
length, 1.5 centim.

Fig. 14. Stage IV.; the subdorsal interrupted throughout by complete
ring-spots, the white “mirrors” of which are bordered with black, and
contain in their centres a reddish nucleus. Natural length, 3 centim.

Fig. 15. Stage V.; shortly after the fourth moult. Subdorsal line
completely vanished; ring-spots somewhat irregular, with broad black
borders; natural length, 3.5 centim.

Fig. 16. _Sphinx Convolvuli_, Stage V., brown form. Subdorsal line
retained on segments 1-3, on the other segments present only in small
remnants; at the points where the (imaginary) subdorsal crosses the
oblique stripes there are large bright spots; natural length, 7.8
centim.


PLATE IV.

Figs. 17-22. Development of the markings in _Chærocampa Elpenor_.

Fig. 17. Stage I.; larva one day after hatching. Natural length, 7.5
millim.

Fig. 18. Stage II.; larva after first moult. Length, 9 millim.

Fig. 19. Stage II.; immediately before the second moult (Fig. 30
belongs here). Length, 13 millim.

Fig. 20. Stage III.; after second moult. Length, 20 millim.

Fig. 21. Stage IV.; after third moult (Figs. 32 and 33 belong here).
Length, 4 centim.

Fig. 22. Stage V.; after fourth moult. A feeble indication of an
eye-spot can be seen on the third segment besides those on the fourth
and fifth. Ocelli absent on segments 6-10.

Fig. 23. Stage VI.; after fifth moult. The subdorsal line is feebly
present on segments 6-10, and very distinctly on segments 11 and 1-3.
Ocelli repeated as irregular black spots above and below the subdorsal
line on segments 6-11; a small light spot near the posterior border of
segments 5-10 (dorsal spots) and higher than the subdorsal line. Larva
adult.

Figs. 24-28. Development of the markings of _Chærocampa Porcellus_.

Fig. 24. Stage I.; immediately after emergence from the egg. Length,
3.5 millim.

Fig. 25. Stage II.; after first moult. Length, 10 millim.

Fig. 26. Stage III.; after second moult. Length, 2.6 centim.

Fig. 27. Eye-spots at this last stage; subdorsal much faded, especially
on segment 4. Position the same as in last Fig.; magnified.

Fig. 28. Stage IV.; after third moult; corresponds exactly with Stage
VI. of _C. Elpenor_. Dorsal view, with front segments partly retracted
(attitude of alarm). Ocelli on segment 5 less developed than in
_Elpenor_; repetitions of ocelli as diffused black spots on all the
following segments to the 11th; two light spots on each segment from
the 5th to the 11th, exactly as in _Elpenor_; subdorsal line visible
only on segments 1-3. Length, 4.3 centim.

Fig. 29. _Chærocampa Syriaca_. From a blown specimen in Lederer’s
collection, now in the possession of Dr. Staudinger. Length, 5.3
centim.

Fig. 30. First rudiments of the eye-spots of _Chærocampa Elpenor_,
Stage II. (corresponding also with Fig. 19 in position, the head of
the caterpillar being to the left). Subdorsal line slightly curved on
segments 4 and 5.

Fig. 31. Eye-spots at Stage III. of the larva Fig. 20 somewhat further
developed (larva immediately before third moult). Position as in Fig.
20.

Fig. 32. Eye-spots at Stage IV. corresponding to Fig. 21, A being the
eye-spot of the fourth and B that of the fifth segment.

Fig. 33. Eye-spot at Stage V. of the larva of _C. Elpenor_; fourth
segment.

Figs. 30-33 are free-hand drawings from magnified specimens.

Fig. 34. _Darapsa Chærilus_ from N. America. Adult larva with front
segments retracted. Copied from Abbot and Smith.

Fig. 35. _Chærocampa Tersa_, from N. America. Adult larva copied from
Abbot and Smith.


PLATE V.

Fig. 36. Sixth segment of adult _Papilio_-larvæ; A, _P. Hospiton_,
Corsica; B, _P. Alexanor_, South France; C, _P. Machaon_, Germany; D,
_P. Zolicaon_, California.

Figs. 37-44. Development of the markings of _Deilephila Euphorbiæ_.

Fig. 37. Stage I.; young caterpillar shortly after emergence. Natural
length, 5 millim.

Fig. 38. Similar to the last, more strongly magnified. Natural length,
4 millim.

Fig. 39. Stage II.; larva immediately after first moult. The row of
spots distinctly connected by a light stripe (residue of the subdorsal
line). Natural length, 17 millim.

Fig. 40. Stage III.; after second moult; magnified drawing of the last
five segments. Only one row of large white spots on a black ground
(ring-spots); subdorsal completely vanished; the shagreen-dots formerly
absent now appear in vertical rows interrupted only by the ring-spots.
Below the latter are some enlarged shagreen-dots which subsequently
become the second ring-spots. Natural length of the entire caterpillar,
21 millim.

Fig. 41. Stage IV.; the same larva after the third moult.
Transformation of the ground-colour from green to black, owing to the
spread of the black patches proceeding from the ring-spots in Fig. 40
in such a manner as to leave between them only a narrow green triangle.
The shagreen dots below the ring-spots have increased in size, but have
not yet coalesced.

Fig. 42. Stage III.; larva, same age as Fig. 40, but with _two rows_ of
ring-spots. Natural length of the whole caterpillar, 32 millim.

Fig. 43. Stage V.; larva from Kaiserstuhl. Variety with only one row of
ring-spots, and with red nuclei in the mirror-spots. Natural length, 5
centim.

Fig. 44. Stage V.; larva from Kaiserstuhl (like the three preceding).
The green triangles on the posterior edges of the segments in Fig. 42
have become changed into red. Natural length, 7.5 centim.

Fig. 45. _Deilephila Galii_; Stage IV. Subdorsal with open ring-spots.
Natural length, 3.4 centim.

Fig. 46. _D. Galii_; adult larva; Stage V. Brown variety with feeble
shagreening; subdorsal completely vanished. Natural length, 6 centim.


PLATE VI.

Fig. 47. The same species at the same stage. Black variety strongly
shagreened; similar to _Deil. Euphorbiæ_.

Fig. 48. Similar to the last. Yellow var. without any trace of
shagreening.

Fig. 49. _Deilephila Vespertilio._ Three stages in the life of the
species, representing three phyletic stages of the genus. A, life-stage
III.=phyletic stage 3 (subdorsal with open ring-spots); B, life-stage
IV.=phyletic stage 4 (subdorsal with closed ring-spots); C, life-stage
V.=phyletic stage 5 (subdorsal vanished, only _one_ row of ring-spots).

Fig. 50. _Deilephila Zygophylli_, from S. Russia; stage V. From a
blown specimen in Staudinger’s collection. In this specimen the
ring-spots are difficult to distinguish on account of the extremely
dark ground-colour; they are nevertheless present, and would probably
be more distinct in the living insect. A, open ring-spot from another
specimen of this species in the same collection.

Fig. 51. _Deilephila Nicæa_, from South France; Stage V. Copied from
Duponchel.

Fig. 52. _Sphinx Convolvuli_; Stage V., segments 10-8. Brown variety,
with distinct white spots at the points of intercrossing of the
vanished subdorsal with the oblique stripes.

Fig. 53. _Anceyrx Pinastri_; A and B, larvæ immediately after hatching.
Natural length, 6 millim.

Fig. 54. Same species; Stage II. Subdorsal, supra- and infra-spiracular
lines developed. Natural length, 15 millim.

Fig. 55. _Smerinthus Populi_; Stage I. Immediately after hatching; free
from all marking. Length, 6 millim.

Fig. 56. Same species at the end of first stage; lateral aspect.
Length, 1.3 centim.

Fig. 57. Same species; Stage II. Subdorsal indistinct; the first and
last oblique stripes more pronounced than the others. Length, 1.4
centim.

Fig. 58. _Deilephila Hippophaës_; Stage III. Subdorsal with open
ring-spot on the 11th segment. A, segment 11 somewhat enlarged. Length,
3 centim.


PLATE VII.

Fig. 59. _Deilephila Hippophaës_; Stage V. Secondary ring-spots on six
segments (10-5).

Fig. 60. Same species; Stage V. One or two red shagreen dots on
segments 10-4 in the position of the ring-spots of Fig. 59. Length, 6.5
centim.

Fig. 61. Same species; Stage V. Segments 9-6 of another specimen, more
strongly magnified. A ring-spot on segments 9 and 8 showing its origin
from two shagreen-dots; two red shagreen-dots on segment 7, on segment
6 only one.

Fig. 62. _Deilephila Livornica_ (Europe) in the last stage. Green form.
Copied from Boisduval.

Fig. 63. _Pterogon Œnotheræ_; Stage IV. Length, 3.7 centim.

Fig. 64. The same species at the same stage; dorsal view of the last
segment.

Fig. 65. The same segment in Stage V. Eye-spot completely developed.

Fig. 66. _Saturnia Carpini_, larva from Freiburg; Stage III. Natural
length, 15 millim.

Fig. 67. Same species; larva from Genoa; Stage IV. Length, 20 millim.

Fig. 68. Same species; larva from Freiburg; Stage III. Segments 8 and 9
in dorsal aspect. Length, 15 millim.

Fig. 69. The same caterpillar; lateral view of segment 8.

Fig. 70. _Smerinthus Ocellatus_; adult larva with distinct subdorsal
on the six foremost segments. The shagreening is only shown in the
contour, elsewhere omitted. Length, 7 centim.

[Illustration:

  Plate VII.

  Aug. Weismann pinx.      Lith. J. A. Hofmann, Würzburg.
]

[Illustration:

  Plate VIII.

  Aug. Weismann pinx.      Lith. J. A. Hofmann, Würzburg.
]


PLATE VIII.

Figs. 71-75 represent segments 8 and 9 of the larva of _Saturnia
Carpini_ (German form) in dorsal aspect, all at the fourth stage.
The head of the caterpillar is supposed to be above, so that the top
segment is the eighth.

Fig. 71. _Saturnia Carpini._ Darkest variety.

Fig. 72. Lighter variety.

Fig. 73. Still lighter variety.

Fig. 74. One of the lightest varieties; the black extends further on
segments 9 and 10 than on the 8th.

Fig. 75. Lightest variety.

Figs. 76-80 are only represented on a smaller scale than the remaining
Figs. in order to save space; were they enlarged to the same scale they
would be larger than the other figures.

Fig. 76. _Saturnia Carpini_ (Ligurian form); Segment 8; Stage V.

Fig. 77. Same form; same segment in stage VI.

Figs. 78, 79, and 80. _Saturnia Carpini_ (German form); dorsal aspect
of 8th segment in Stage V. (the last of this form).

Fig. 78. Darkest variety.

Fig. 79. Lighter variety.

Fig. 80. Lightest variety.

Figs. 81-86. _Saturnia Carpini_ (German form); Stage IV. Side view of
the 8th segment in six different varieties. Fig. 81 shows only two
small green spots at the bases of the upper warts besides the green
spiracular stripes. Fig. 82 shows the spots enlarged and increased by a
third behind the warts; the pro-legs have also become green.

Fig. 83. Two of the three green spots, which have become still more
enlarged, are coalescent.

Fig. 84. All three spots coalescent; but here, as also in

Fig. 85, various residues of the original black colour are left as
boundary-marks.

Fig. 86. Lightest variety.


END OF PART II.



STUDIES IN THE THEORY OF DESCENT.

=Part III.=

ON THE FINAL CAUSES OF TRANSFORMATION.



=III.=

THE TRANSFORMATION OF THE MEXICAN AXOLOTL INTO AMBLYSTOMA.


INTRODUCTION.

Since the time when Duméril made known the transformation of a number
of Axolotls into the so-called Amblystoma form, this Mexican Amphibian
has been bred in many European aquaria, chiefly with the view to
establish the conditions under which this transformation occurred, so
as to be enabled to draw further conclusions as to the true causes of
this exceptional and enigmatical metamorphosis.

Although the Amphibians propagated freely, the cases in which
transformation occurred remained extremely rare, and it was not once
possible to reply to the main question, viz. whether this metamorphosis
was determined by external conditions or by purely internal causes;
to say nothing of the possibility of there perhaps being discoverable
certain definite external influences by means of which the
metamorphosis could have been induced with certainty. But while these
points are undecided all attempted theoretical interpretations of the
phenomenon must be devoid of a solid basis.

It appeared to me from the first that the history of this
transformation of the Axolotl was of special theoretical value; indeed
I believed that it might possibly furnish a special case for deciding
the truth of those ground-principles, according to which the origin of
this species is represented by the two conflicting schools as a case of
transformation or as one of heterogenesis. I therefore determined to
make some experiments with the Axolotl myself, in the hopes of being
fortunate enough to be able to throw some light upon the subject.

In the year 1872 Prof. v. Kölliker was so good as to leave with me
five specimens of his Axolotls, bred in Würzburg, and these furnished
a numerous progeny in the following year. With these I carried out the
idea, the theoretical bearing of which will be shown subsequently,
whether it would not be possible to force all the larvæ, or at any
rate, the greater majority, to undergo transformation by exposing them
to conditions of life which made the use of gills difficult, and that
of lungs more easy; in other words, by compelling them to live partly
on land at a certain stage of life.

During that year indeed I obtained no results, most of the larvæ
perishing before the time for such an experiment had arrived, and the
few survivors did not undergo transformation, but lived on to the
following spring and then also died one after the other. Through long
absence from Freiburg, necessitated by other labours, I had evidently
left them without sufficient care and attention. I was thus led to
the conviction, which was more fully confirmed subsequently, that no
results can be obtained without the greatest care and attention in
rearing, towards which single object all one’s interest should be
concentrated, and it must not be considered irksome to have to devote
daily for many months a large amount of time to this experiment. As it
was evident that I could not afford this time without calling in other
aid, I hailed with pleasure an opportunity of witnessing the experiment
performed by other hands.

A lady living here (Freiburg), Fräulein v. Chauvin, undertook to rear
a number of my larvæ of the following year which had just hatched,
and in accordance with my idea to make the experiment of forcibly
compelling them to adopt the Amblystoma form. How completely this was
accomplished will be seen from the following notes by the lady herself,
and it will no less appear that these results were only obtained by
that care in treatment and delicacy of observation which she devoted to
the experiments.


EXPERIMENTS.

“I began the experiments on June 12th, 1874, with five larvæ about
eight days old, these being the only survivors out of twelve. Owing
to the extraordinary delicacy of these creatures, the quality and
temperature of the water, and the nature and quantity of their food
exerts the greatest influence, especially in early life, and one cannot
be too cautious in their treatment.

“The specimens were kept in a glass globe of about thirty centimeters
in diameter, the temperature of the water being regulated; as food at
first Daphnids, and afterwards larger aquatic animals were introduced
in large numbers. By this means all the five larvæ throve excellently.
At the end of June the rudiments of the front legs appeared in the most
vigorous specimens, and on the 9th of July the hind legs also became
visible. At the end of November I noticed that one Axolotl remained
constantly at the surface of the water, and this led me to suppose that
the right period had now arrived for effecting the transformation
into Amblystoma. For brevity I shall designate this as No. I., and the
succeeding specimens by corresponding Roman numerals.

“In order to bring about this metamorphosis, on December 1st, 1874, No.
I. was placed in a large-sized glass vessel containing earth arranged
in such a manner that, when the vessel was filled with water, only one
portion of the surface of the earth was entirely covered by the liquid,
and the creature in the course of its frequent perigrinations was thus
more or less exposed to the air. The water was gradually diminished on
the following days, during which period the first changes made their
appearance in the Amphibian--_the gills commenced to shrivel up_, and
at the same time the creature showed a tendency to seek the shallowest
spots. On December 4th, it took entirely to the land, and concealed
itself among some damp moss which I had placed on a heap of sand on the
highest portion of the earth in the glass vessel. At this period the
first ecdysis occurred. Within the four days from the 1st to the 4th of
December, a striking change took place in the external appearance of
No. I., the gill-tufts shrivelled up almost entirely, the dorsal crest
completely disappeared, and the tail, which had hitherto been broad,
became rounded and similarly formed to that of a land salamander. The
grey-brown colour of the body changed gradually into a blackish hue;
isolated spots, at first of a dull white, made their appearance and
these in time increased in intensity.

“When the Axolotl left the water on December 4th the gill-clefts were
still open, but these closed gradually, and after about eight days were
overgrown with skin and no longer to be seen.

“Of the other larvæ three appeared at the end of November (_i.e._ at
the same time when No. I. came to the surface of the water) to have
kept pace in development with No. I., an indication that for these
also the right period had arrived for accelerating the developmental
processes. They were therefore submitted to the same treatment as No.
I. No. II. became transformed at the same time and exactly in the same
manner as the latter; its gill-tufts were complete when it was first
placed in the shallow water, but after four days these had almost
entirely disappeared; in the course of about ten days after it took to
the land, the overgrowth of skin on the gill-clefts and the complete
assumption of the salamander form occurred. During this last period the
creature took food, but only when urged to do so.

“In Nos. III. and IV. the development proceeded more slowly. Neither of
these so frequently sought the shallow spots, nor did they as a rule
remain so long exposed to the air, so that the greater part of January
had expired before they took entirely to the land. Nevertheless the
dessication of the gill-tufts did not take a longer time than in Nos.
I. and II. as the first ecdysis occurred as soon as they took to the
land.

“No. V. showed still more striking deviations in its transformation
than Nos. III. and IV., but as this specimen appeared much weaker
than the others from the beginning and was retarded in growth to a
most notable extent, this is by no means surprising. It took fourteen
instead of four days before the transformation had advanced far enough
to enable it to leave the water. It was especially interesting to
observe the behaviour of this specimen during this period. Its weak
and delicate constitution evidently made it much more susceptible to
all external influences than the others. If exposed to the air for too
long a time it acquired a light colour, and when annoyed or alarmed it
emitted a peculiar odour, similar to that of a salamander. As soon as
these phenomena were observed it was at once placed in deeper water,
into which it immediately plunged and gradually recovered itself,
the gills always becoming again expanded. The same experiment was
repeated several times and always led to the same result, from which
we may venture to conclude that by accelerating the transformation
too energetically, the process may come to a standstill, and even by
continued compulsion may end in death.

“It yet remains to be mentioned with respect to Axolotl No. V. that
this specimen, unlike all the others, did not emerge from the water at
the first ecdysis, but at the time of the fourth.

“All the Axolotls are now (July, 1875) living, and are healthy and
vigorous, so that with respect to their state of nourishment there is
nothing to prevent their propagating. Of the first four the largest is
fifteen centim. long; Axolotl No. V. measures twelve centim.

“The preceding statements appear to demonstrate the correctness of the
views advanced in the Introduction:--Axolotl larvæ generally but not
always complete their metamorphosis if, in the first place, they emerge
sound from the egg and are properly fed; and if, in the next place,
they are submitted to the necessary treatment for changing aquatic
into aërial respiration. It is obvious that this treatment must only
be applied very gradually, and in such a manner as not to overtax the
vital energy of the Amphibian.”

       *       *       *       *       *

To the foregoing remarks of Fräulein v. Chauvin I may add that in all
five cases the transformation was complete, and not to be confounded
with that change which occurs more or less in all Axolotls in the
course of time when confined in small glass vessels. In this last case
there frequently appear changes in the direction of the Amblystoma form
without the latter being actually reached. In the five adult Axolotls
which I possessed for a short time, and of which two were at least four
years old, the gills were much shrivelled, but the aquatic tail and
dorsal crest were unchanged. The crest may, however, also disappear,
and the tail become shortened without these changes being due to a
transformation into Amblystoma, as will be shown further on.

With respect to the duration of the transformation, this amounted
in Axolotls Nos. I. to IV. altogether to twelve or fourteen days.
Of these, four days were taken by the first changes which occurred
while the creature was still in the water; the remaining time, to the
completion of the metamorphosis, was passed on land. Duméril gives the
duration of the metamorphosis as sixteen days.

The following results of the experiments just described appear to me
to be especially noteworthy:--The five Axolotl larvæ which can alone
be taken into consideration, the others having soon perished, all
experienced metamorphosis, and without an exception became Amblystomas.
Only one of them, No. I., by persistently swimming at the surface, as
was observed at the end of six months, showed a decided tendency to
undergo metamorphosis and to adopt aërial respiration. With respect to
this specimen it may therefore be confidently assumed that it would
have taken to the land, and that metamorphosis would have occurred
without artificial aid, just as was the case in the thirty specimens
which Duméril altogether observed.

Respecting Nos. II., III., and IV., on the other hand, such a
supposition is but little probable. These three larvæ endeavoured
to keep in deep water and avoided as long as possible the shallow
places which would have enforced them to take entirely to lung
breathing. Metamorphosis thus occurred more than a month later in these
individuals.

Finally, there can scarcely be any doubt that No. V. would not have
become transformed without forcible adaptation to an aërial life.

From these results we may venture to conclude that most Axolotl larvæ
change into the Amblystoma form when, at the age of six to nine
months, they are placed in such shallow water that they are compelled
to respire chiefly by their lungs. The experiments before us are
certainly at present but very few in number, but such a conclusion
cannot be termed premature if we consider that out of several hundred
Axolotls (the exact number is not given) Duméril obtained only about
thirty Amblystomas, while v. Kölliker bred only one Amblystoma out of a
hundred Axolotls.

It now only remains questionable whether _each larva_ could have been
forced to undergo metamorphosis, but this could only be decided by
new experiments. It was originally my intention to have delayed the
publication of the experiments till Fräulein v. Chauvin had repeated
them in larger numbers, but as my Axolotls have not bred this year
(1875) I must abandon my scheme, and this can be done the more readily
because, for the theoretical consideration of the facts, it is
immaterial whether _all_ or only _nearly all_ the Axolotls could have
been compelled to undergo transformation. I must not, however, omit to
mention that Herr Gehrig, the curator of our Zoological Museum, bred
a considerable number of larvæ from the same brood as that with which
Fräulein v. Chauvin experimented, and that of these larvæ six lived
over the winter _without undergoing metamorphosis_. They were always
kept in deep water and thus furnished the converse experiment to those
recorded above; they further prove that this whole brood did not have a
previous tendency to undergo metamorphosis.

If these new facts are to be made use of to explain the nature of this
extraordinary process of transformation in accordance with our present
conception, the data already known must in the first place be called to
our aid.

It has first to be established that _Siredon Mexicanus_ never, as
far as we know, undergoes metamorphosis in its native country. This
Amphibian is there only known in the _Siredon_ form, a statement which
I have taken from De Saussure,[225] who has himself observed the
Axolotl in the Mexican lakes. This naturalist never found a single
Amblystoma in the neighbourhood of the lakes, “nevertheless the larva
(Axolotl) is so common there that it is brought into the market by
thousands.” De Saussure believes that in Mexico the Axolotl does not
undergo transformation.[226] The same statement is distinctly made by
Cope,[227] whose specimens of _Siredon Mexicanus_ bred in America,
even in captivity showed “no tendency to become metamorphosed.” On the
other hand Tegetmeier observed[228] that one out of five specimens
obtained from the Lake of Mexico underwent metamorphosis, and this
accordingly establishes the second fact, viz. that the true Axolotl
becomes transformed under certain conditions into an Amblystoma when in
captivity.

This last remark would be superfluous if, as was for a long time
believed, the Paris Axolotls, of which the metamorphosis was first
observed and which at the time made such a sensation, were actually
_Siredon Mexicanus_, _i.e._ the _Siredon_ which alone in its native
country bears the name of Axolotl. In his first communication Duméril
was himself of this opinion; he then termed the animal “_Siredon
Mexicanus vel Humboldtii_,”[229] but subsequently, in his amplified
work[230] on the transformation of the Axolotl observed in the Jardin
des Plantes, he retracted this view, and after a critical comparison of
the five described species of _Siredon_, he came to the conclusion that
the species in the possession of the Paris Museum was probably _Siredon
Lichenoides_ (Baird). All the transformations of Axolotls observed
in Europe must consequently be referred to this species, since they
were--at least as far as I know--all derived from the Paris colony. My
own experimental specimens were also indirectly descended from these.

Now it must be admitted that this does not coincide with the fact that
the Amblystoma form which Duméril first obtained from his Axolotls
agreed with Cope’s species, _A. Tigrinum_, while on the other hand we
learn from Marsh[231] that _Siredon Lichenoides_ (Baird), when it does
undergo metamorphosis, becomes transformed into _Amblystoma Mavortium_
(Baird).

Marsh found _Siredon Lichenoides_ in mountain lakes (7000 feet above
the sea) in the southwest of the United States (Wyoming Territory),
and obtained from them, by breeding in aquaria, _Amblystoma Mavortium_
(Baird). He considers it indeed doubtful whether the Amphibian
undergoes this transformation in its native habitat, although he
certainly states this opinion without rigorous proof on purely
theoretical considerations, because, according to his view, “the low
temperature is there less favourable.”[232]

If I throw doubt upon this last statement it is simply because
_Amblystoma Mavortium_ is found native in many parts of the United
States, viz:--in California, New Mexico, Texas, Kansas, Nebraska, and
Minnesota. It is indeed by no means inconceivable that in the mountain
lakes where Marsh obtained this species, it may behave differently
with respect to metamorphosis than in other habitats, and this appears
probable from certain observations upon _Triton_ which will be
subsequently referred to.

Meanwhile, in the absence of further observations, we must admit that
the Paris Axolotls were not _Siredon Lichenoides_, but some nearly
allied and probably new species. But little information is furnished
by observing the course of the transformation, although it is at least
established that this Axolotl in its native habitat does not undergo
metamorphosis or does so as exceptionally as in Europe. Unfortunately
in his papers Duméril gives no precise statement respecting the
locality of this species imported from “Mexico”--it is probable that
he was himself unacquainted with it, so that I can only state on the
authority of Cope that Amblystoma has never been brought from south of
the provinces of Tamaulipas and Chilhuahua, _i.e._ south of the Tropic
of Cancer.[233]

This last statement, however, gives no certainty to the matter. Of much
more importance is the above-mentioned fact, that the true Axolotl
of the lakes about the city of Mexico does not, as a rule, become
transformed into an Amblystoma in that locality, although this species
in certain cases undergoes metamorphosis when in confinement. From
this circumstance and from the fact that the Paris Axolotl likewise
experienced but a very small percentage of metamorphosis in captivity,
we may venture to conclude that this species also, in its native
habitat, either does not become transformed at all or does so only
exceptionally.

But there is yet another body of facts which come prominently into
view on considering the history of the transformations. I refer to the
existence of numerous species of _Amblystoma_ in a natural state. In
the “Revision of the genera of _Salamandridæ_,” published some years
ago by Strauch,[234] this author, following Cope,[235] gives twenty
species of _Amblystoma_ as inhabiting North America. Although some
of these species are based on single examples, and consequently, as
Strauch justly remarks, “may well have to be reduced in the course
of time,” there must nevertheless always remain a large number of
species which live and propagate as true Amblystomas, and of which the
habitat extends from the latitude of New York to that of New Mexico.
There are therefore true species of _Siredon_ which regularly assume
the Amblystoma form under their natural conditions of life, and which
propagate in this form, while, on the other hand, there are at least
two species which, under their existing natural conditions of life,
always propagate as _Siredon_. It is but another mode of expression
for the same facts if we say that the Mexican Axolotl and the Paris
_Siredon_--whether this is _Lichenoides_ or some other species--stand
at a lower grade of phyletic development than those species of
_Amblystoma_ which propagate in the salamander form. No one can raise
any objection to this statement, while the alternative view maintained
by all authors contains a theory either expressed or implied which is,
as I believe, incorrect, viz. that the Mexican Axolotl _has remained_
at an inferior stage of phyletic development.

All zoologists[236] who have expressed an opinion upon the
transformation of the Axolotl, and who are not, like the first observer
of this fact, embarrassed by Cuvier’s views as to the immutability of
species, regard the phenomenon as though a species, which owing to some
special conditions had hitherto remained at a low stage of development,
had now through some other influences been compelled to advance to a
higher stage.

I believed for a long time that the phenomenon could not otherwise be
comprehended, so little was I then in a position to bring all the facts
into harmony with this view. Thus in the year 1872 I expressed myself
as follows[237]:--“Why should not a sudden change in all the conditions
of life (transference from Mexico to Paris) have a direct action on
the organization of the Axolotl, causing it suddenly to reach a higher
stage of development, such as many of its allies have already attained,
and which obviously lies in the nature of its organization--a stage
which it would perhaps itself have reached, although later, in its
native country? Or is it inconceivable that the sudden removal from
8000 feet above the sea (Mexican plateau) to the altitude of Paris, may
have given the respiratory organs an impetus in the direction of the
transformation imminent? In all probability we have here to do with the
direct action of changed conditions of life.”

That the substance of this last statement must still hold good is
obvious from the experiments previously described, which show that
by the application of definite external influences, we have it to
a certain extent in our power to produce the transformation. It is
precisely in this last point that there lies the new feature furnished
by these experiments.

But are we also compelled to interpret the phenomenon in the above
manner? _i.e._ as a sudden advance in the phyletic development of the
species occurring, so to speak, at one stroke? I believe not.

What first made this view appear to me erroneous, was the appearance
of the living Amblystomas bred from my Axolotl larvæ. These creatures
by no means differed from the Axolotls merely in single characters,
but they were distinct from the latter in their entire aspect; they
differed in some measure in all their parts, in some but slightly and
in other parts strongly--in brief, they had become quite different
animals. In accordance with this, their mode of life had become
completely modified; they no longer lived in the water, but remained
concealed by day among the damp moss of the vivarium, coming forth only
by night in search of food in dry places.

I had been able to perceive the great difference between the two
stages of development from the anatomical data with which I had long
been familiar, and which Duméril had made known with respect to the
structure of his Amblystomas. But the collecting of numerous details
gives no very vivid picture of the metamorphosis; it was the appearance
of the living animal that first made me conscious how deep-seated was
the transformation which we have here before us, and that this change
not merely affected those parts which would be directly influenced
by the change in the conditions of life, such as the gills, but that
most if not all the parts of the animal underwent a transformation,
which could in part be well explained as morphological adaptation to
new conditions of life, and partly as a consequence of this adaptation
(correlative changes), but could not possibly be regarded as the sudden
action of these changed conditions.

Such at least is my view of the case, according to which a _per saltum_
development of the species of such a kind as must here have taken
place, is quite inconceivable.

I may venture to assume that most observers of the metamorphosis of
Axolotl have, like myself, not been hitherto aware of the extent of
the transformation, and it may thus be explained why the theoretical
bearings of the case have on all sides been incorrectly conceived. We
have here obviously a quite extraordinary case of the first order of
importance. I believe that it can easily be shown that the explanation
of the history of the metamorphosis of the Paris Axolotl which has
hitherto been pretty generally accepted, necessarily comprises a very
far-reaching principle. If this interpretation is correct, then in my
opinion must also hold good the ideas of those who, like Kölliker,
Askenasy, Nägeli, and, among the philosophers, Hartmann and Hübner,
would refer the transformation of species in the first instance to a
power innate in the organism, to an active, _i.e._ a self-urging “law
of development”--a phyletic vital force.

Thus, if the Axolotls transformed into Amblystomas are regarded as
individuals which, impelled by external influences, have anticipated
the phyletic development of the others, then this advance can only
be ascribed to a phyletic vital force, since the transformation is
sudden, and leaves no time for gradual adaptation in the course of
generations. The _indirect_ influence of the external conditions of
life, _i.e._ natural selection, is thus excluded from the beginning.
But the _direct_ action of the changed conditions of life by no means
furnishes an explanation of the complete transformation of the whole
structure, such as I have already alluded to, and which I will now
enter into more closely.

The differences between the Paris Axolotl and its Amblystoma according
to Duméril, Kölliker, and my own observations are as follow:--

1. The gills disappear; the gill-clefts close up, and of the branchial
arches only the foremost remain, the posterior ones disappearing. At
the same time the os _hyoideum_ becomes changed (Duméril).

2. The dorsal crest completely disappears (Duméril).

3. The aquatic tail becomes transformed into one like that of the
salamanders (Duméril), which, however, is not as in the salamander
cylindrical, but somewhat compressed laterally (Weismann).

4. The skin becomes yellowish white, irregularly spotted on the sides
and back (Duméril), whilst at the same time its former grey-black
ground-colour changes into a shining greenish black (Weismann); it
loses, moreover, the slimy secretion of the skin, and the cutaneous
glands become insignificant (Kölliker).

5. The eyes become prominent and the pupils narrow (Kölliker), and
eyelids capable of completely covering the eyes are formed; in Axolotl
only a narrow annular fold surrounds the eyes, so that these cannot be
closed (Weismann).

6. The toes become narrowed and lose their skin-like appendages
(Kölliker), or more precisely, the half webs which connect the proximal
extremities of the toes on all the feet (Weismann).

7. The teeth are disposed in this species, as in all other
_Amblystomæ_, in transverse series; whilst in Axolotl, as in _Triton_
larvæ, they are arranged at the sides of the jaw in the form of a
curved arch-like band beset with several rows of teeth.[238] (Duméril.
See his fig., _loc. cit._ p. 279).

8. In Axolotl the lower jaw, in addition to the teeth on the upper edge
of the bone, also bears “de très petites dents disposées sur plusieurs
rangs;” these last disappear after metamorphosis (Duméril). I will add
that the persistent teeth belong to the _os dentale_ of the lower jaw,
and those that are shed to the _os operculare_.[239]

9. The surface of the posterior moveable part of the body is slightly
concave both before and after transformation; the anterior part is,
however, less concave in _Amblystoma_ than in _Siredon_ (Duméril).

I have not yet been able to verify Duméril’s 7th and 9th statements,
as I did not want to kill any of my living Amblystomas,[240] simply
in order to confirm the observations of a naturalist in whom one may
certainly place complete confidence. Neither have I as yet observed the
transformation of the branchial arches, but all the other statements of
Kölliker and Duméril I can entirely corroborate.

The structural differences between Axolotl and Amblystoma are
considerably greater and of more importance than those between allied
genera, or indeed than between the families of the Urodela. The genus
_Siredon_ undoubtedly belongs to a different sub-order to the genus
_Amblystoma_ into which it occasionally becomes transformed. Strauch,
the most recent systematic worker at this group, distinguishes
the sub-order _Salamandrida_ from that of the _Ichthyodea_ by the
possession of eyelids, and by the situation of the palatine teeth in
single rows on the posterior edge of the palatal bone: in _Ichthyodea_
the eyelids are wanting and the palatine teeth are either “situated on
the anterior edge of the palatal bone,” or “cover the whole surface of
the palatal plates in brush-like tufts.”

How is it possible to regard such widely divergent anatomical
characters as changes suddenly produced by the action (but once
exerted) of deviating conditions of life? Hand in hand with the
shedding of the old and the appearance of new palatine teeth, there
occurs a change in the anatomical structure of the vertebral column,
and also--as we may fairly conclude from Kölliker’s correct observation
of the cessation of the slimy secretion--in the histological structure
of the skin. Who would undertake to explain all these profound
modifications as the direct and sudden action of certain external
influences? And if any one were inclined to explain such changes as a
consequence of the disappearance of the gills, _i.e._ as correlative
changes, what else is such a correlation than the phyletic vital force
under another name?

If from one change arising from the direct action of external
agencies, the whole body can in two days become transformed in all
its parts, in the precise manner which appears best adapted for the
new conditions of life under which it is henceforward to exist, then
the word “correlation” is only a phrase which explains nothing, but
which prevents any attempt at a better explanation, and it would be
preferable to profess simply the belief in a phyletic vital force.

Moreover, it is hardly permissible to seek such an explanation, since
Urodela are known which have no gills in the adult state, and which
nevertheless possess all the other characters of the _Ichthyodea_,
viz. want of eyelids, characteristic palatine teeth, and the tongue
bone. This is the case with the genera _Amphiuma_ (Linn.), _Menopoma_
(Harl.), and _Cryptobranchus_ (v. d. Hoev.). The two first genera, as
is known, still possess gill-clefts, but _Cryptobranchus_ has even
lost these clefts, which, as in _Amblystoma_, are overgrown by skin;
nevertheless _Cryptobranchus_ is, according to the concurrent testimony
of all systematists, a true salamander in habits, tongue bone, palatine
teeth,[241] &c. It must further be added that the Axolotl itself can
lose the gills without thereby becoming transformed into an Amblystoma.
I have previously mentioned that in Axolotls which were kept in shallow
water the gills frequently became diminutive, and it also sometimes
happens that they completely shrivel up. I possess an Axolotl preserved
in alcohol in which the gills have shrivelled up into small irregular
bunches, and the dorsal crest is also so completely absent that its
place is occupied by a long furrow, and even on the tail the crest
has entirely disappeared from the lower edge and about half from the
upper edge. Notwithstanding this, the creature is widely removed from
Amblystoma in structure; it possesses the arched branchial apparatus,
the palatine teeth, the skin, &c., of the Axolotl.

These facts prove, therefore, that the shedding of the gills by no
means always entails all the other modifications which we observe in
the metamorphosis of Axolotl, so that these modifications are thus
not by any means the necessary and immediate consequence of such gill
shedding.

Whether these modifications will occur after a long series of
generations--whether the successors of _Cryptobranchus_ will also one
day acquire the salamandriform structure is another question, and one
which I could not exactly answer in the negative. But this question
does not here come into consideration, as we are now only concerned
with the _immediate result_ of the shedding of the gills.

The problem appears therefore to be as follows:--Either the hitherto
received interpretation of the transformational history of the Axolotl
as a further development of the species is incorrect, or else the case
of Axolotl incontestably proves the existence of a phyletic vital force.

We have now to ask whether the facts of this transformational history
are not capable of another explanation.

I believe that this is certainly possible, and that another
interpretation can be shown to be correct with some degree of
probability.

I am of opinion that those Amblystomas which have been developed
in captivity in certain instances from _Siredon Mexicanus_ (_S.
Pisciformis_), as well as from the Paris Axolotls, are not progressive,
but reversion forms; I believe that the Axolotls which now inhabit
the Mexican lakes were Amblystomas at a former geological (or better,
zoological) epoch, but that owing to changes in their conditions of
life, they have reverted to the earlier perennibranchiate stage.

I was undoubtedly first led to this conception by the results which
arose from my studies on the seasonal dimorphism of butterflies.[242]
In this case we were also concerned with the two different forms under
which one and the same species appears, and of which it was shown to
be probable that the one is phyletically older than the other. The
younger summer form, according to my view, has arisen, through the
gradual amelioration of the climate, from the winter form, which at an
earlier zoological epoch was the only one in existence; but the latter,
the primary form, has not for this reason ceased to exist, but now
alternates in each year as a winter form with the secondary summer form.

Now with seasonally dimorphic butterflies, it was easily possible to
induce the summer brood to assume the winter form by exposing their
pupæ for a long time to a low temperature; and it was shown to be
highly probable that this abrupt and often very extensive change or
transformation, only apparently takes place suddenly, and is but the
apparent result of the action of cold upon this generation, whilst in
fact it depends upon reversion to the primary form of the species, so
that the low temperature, which is only once applied, gives but the
impetus to reversion, and is not the true cause of the transformation.
This cause must rather be sought in the long continued action of the
cold to which the ancestors of our existing butterflies were subjected
for thousands of generations, and of which the final result is the
winter form.

If we assume for an instant that my interpretation of the
transformation of Axolotl as just offered is correct, we should have
conditions in many respects analagous to those of seasonal dimorphism.
It is true that in this case the two forms no longer alternate
regularly with each other, but the primary form may occasionally
appear instead of the secondary form, owing to the action of external
conditions.

Just as in the case of seasonal dimorphism it is possible to compel
the summer generation to abandon the summer form, and to assume the
winter guise by the action of cold; so in the present case we are able
to induce the Axolotl to adopt the Amblystoma form by making aërial
respiration compulsory at a certain stage of life; and further, just as
in seasonal dimorphism it can be shown that this artificially produced
change is only apparently an abrupt transformation, and is actually a
reversion to the much older winter form; so here we have not an actual,
but only an apparent remodelling of the species--a reversion to the
phyletically older form.

This certainly appears a paradox, inasmuch as a form here arises by
reversion which must yet undoubtedly rank as the more highly developed.
I believe, however, that much which seems paradoxical in this statement
will disappear on further examination.

It must in the first place be taken into consideration that the
phyletic development of species need not by any means always take place
by advancement. We have indeed many cases of retrogressive development,
although in a somewhat different sense, as with parasites and those
forms which have degenerated from free locomotion to a sedentary mode
of life.[243] I do not confuse this kind of retrogressive development,
arising from the arrest of certain organs and systems of organs, with
true reversion. The latter is a return to a form which has already
been once in existence; but in the former case, in spite of all
simplification of the organization, some entirely new feature always
comes into existence. But I am not able to see any absurdity in the
assumption that even true reversion, whether of a whole species or of
the individuals of a certain district, may be regarded as possible,
and I require no further concession. Why, for example, should it be
inconceivable that at a very remote period the Axolotl was adapted to
a life on land; that through the direct and indirect action of changed
conditions of life it gradually acquired the salamander form, but that
subsequently, through new and unfavourable changes in the conditions of
life, it again relapsed to the older form, or at least to one nearly
related thereto?

At any rate such an assumption contains nothing opposed to known facts,
but can be supported in many ways, and finally it commends itself, at
least in my opinion, as offering the only admissible explanation of the
facts before us.

The existence of a whole series of species of _Amblystoma_, as already
mentioned, at once shows that species of _Siredon_ can become elevated
into the salamander form, and can propagate regularly in this state,
and further, that this phyletic advance has already actually taken
place in many species.

That degeneration may also occur from this high stage to a lower stage
of development, is shown by many observations on our water-salamanders.
It is known that under certain circumstances Tritons, as it is
generally expressed, become “sexually mature in the larval condition.”

In the year 1864 De Filippi[244] found fifty Tritons in a pool at
Andermatten, in the neighbourhood of Puneigen, and of these only two
showed the structure of the adult water-salamander; all the others
still possessed gills, but notwithstanding this, they agreed in both
sexes, in size and in the development of the sexual organs, with mature
animals. De Filippi established that these “sexually mature larvæ” not
only resembled larvæ externally through the possession of gills, but
that they also possessed all the other anatomical characters of the
larvæ, _i.e._ the characteristic bunches of palatine teeth situated
on both sides in the position of the subsequent single rows, and a
vertebral column represented throughout its whole length by the _chorda
dorsalis_.

According to my view this would be a case of the reversion of the
Triton to the immediately anterior phyletic stage, _i.e._ to the
perennibranchiate stage, and in the present instance the majority
of zoologists who take their stand by the theory of descent, would
certainly concur in this view. I should at least consider it to be a
useless play upon words did we here speak of larval reproduction, and
thereby believe that we had explained something. The animal certainly
becomes sexually mature in the same condition as that in which it
first appears as a larva, but we first get an insight into the nature
of this process by considering that this so-called “sexually mature
larva” has the precise structure which must have been possessed by
the preceding phyletic stage of the species, and that an individual
reversion to the older phyletic stage of the species is consequently
before us. I maintain that Duméril is in error in regarding this case
of the Triton as parallel with the true larval reproduction of Wagner’s
_Cecidomyia_ larva. In this last case it is certainly not reversion
to an older phyletic stage that confers the power of reproduction
upon the larvæ, since the latter do not represent an older phyletic
stage of the species, but must have arisen contemporaneously with this
last stage. The enormous structural difference between the larvæ and
the imagines is not explained by the latter having arisen from the
former supplementarily as a finished production, but by both having
been contemporaneously adapted to continually diverging conditions
of life.[245] Considered phyletically, these larvæ are by no means
necessarily transitional to the origination of the flies. They could
have been quite different without the form of the imagines having
been thereby modified, since the stages of insect metamorphosis vary
independently of each other in accordance with the conditions of life
to which they are subjected, and exert scarcely any, or only a very
small form-determining influence upon each other, as has been amply
proved in the preceding essay. In any case the power of these larvæ
(the _Cecidomyiæ_) to propagate themselves asexually was first acquired
as a secondary character, as appears from the fact that there exist
numerous species of the same genus which do not “nurse.” In the form
which they now possess they could never have played the part of the
final stage of the ontogeny, nor could they formerly have possessed
the power of sexual reproduction.[246] In brief, we are here concerned
with true larval reproduction, whilst in Triton we have reversion to an
older phyletic stage.[247]

I cannot agree with my friend Professor Haeckel when he occasionally
designates the reversion of the Tritons as an “adaptation” to a purely
aqueous existence.[248] We could here only speak of “adaptation” if
we took the word in a quite different sense to that in which it was
first introduced into science by Darwin and Wallace. These naturalists
thereby designate a gradual bodily transformation appearing in the
course of generations in correspondence with the new requirements
of altered conditions of life or, in other words, the action of
natural selection, and not the result of a suddenly and direct acting
transforming cause exerted but once on a generation.

Just because the word “adaptation” can be used in ordinary language in
many senses, it is desirable that it should have only _one_ precise
signification, and above all that we should not speak of adaptation
where scarcely any morphological change occurs, but only a kind of
functional change in the sense used by Dohrn.[249] This is the case
for example, when Forel[250] shows that fresh water Pulmonifera,
the organization of which is attributed to the direct respiration
of air, can nevertheless become settled in the greatest depths of
mountain lakes through their lungs being again employed as gills.
That not the least change in the lungs hereby takes place is shown
by the observations of Von Siebold,[251] who saw the shallow water
Pulmonifera using their lungs alternately for direct aërial and
aquatic respiration, according to the amount of air contained in the
water. If with Von Siebold we merely apply the word “adaptation” to
such cases, this expression would lose the special sense which it
originally conveyed, and the word would have to be abandoned as a
_terminus technicus_; still, such cases may perhaps be spoken of as
_physiological_ adaptation.

In any case the reproductive “larvæ” of the Tritons as little present
a case of true adaptation as the Axolotl, which occasionally becomes
transformed into an Amblystoma. In both cases the transformation
referred to is by no means indispensable to the life of the individual.
Mature Tritons (devoid of gills) can exist, as I have myself seen,
for many months, and probably also for a year in deep water, although
adapted for purely pulmonary respiration; whilst Axolotls, as I have
already mentioned, can live well for a year in shallow water poor in
air. If their gills by this means become shrivelled up or completely
disappear, even this is not adaptation in the Darwinian sense, but
the effect of directly acting external influences, and chiefly of
diminished use.

A case entirely analagous to that of Filippi’s was observed by
Jullien in 1869. Four female larvæ of _Lissotriton Punctatus_
(Bell)--(synonymous with _Triton Tæniatus_, Schnd.), taken from a
pool, proved to be sexually mature. They contained mature eggs in
their ovaria ready for laying, and two of them actually deposited
eggs. Four male larvæ found in the same pool, appeared to be equally
developed with respect to size, but their testicles contained no free
spermatozoa, but only sperm-cells.[252]

I have met with a third case of a similar kind mentioned by Leydig in
his memoir, rich in interesting details, “on the tailed Amphibians
of the Wurtemburg fauna.”[253] Schreibers, the former director of
the Vienna Museum, also found “larvæ” of Tritons with well-developed
gills, but of the size of the “adult male individuals,” and, as shown
by anatomical investigation, with well “developed sexual organs,” the
ovaria especially being distended with eggs.

It is thus established that species which long ago reached the
salamander stage in phyletic development, may occasionally degenerate
to the perennibranchiate stage. This fact obviously makes my
conception of the Axolotl as a reversion form appear much less
paradoxical--indeed, the cases of reversion in Triton are precisely
analagous to the process which I suppose to have taken place in the
Axolotl. We have only to substitute Amblystomas for Tritons, to
imagine the pool in which De Filippi found his “sexually mature Triton
larvæ” enlarged to the size of the Lake of Mexico, and to conceive the
unknown, and perhaps here transitory, causes of the reversion to be
permanent, and we have all that is necessary, so far as we at present
know, for the restoration of the Axolotl; we obtain a perennibranchiate
population of the lake.

It has not yet been determined whether the perennibranchiate form of
the Triton actually prevailed permanently in De Filippi’s pool, since,
so far as I know, this has not since been examined.

Let us, however, assume for an instant that this is really the
case, and that there exists at that spot a colony of sexually
reproductive perennibranchiate Tritons: should we wonder if a true
Triton occasionally appeared among their progeny, or if we were able
to induce the majority of the individuals of this brood to become
metamorphosed into Tritons by keeping them in shallow water? According
to my view this is precisely the case of the Mexican Axolotl.

I need not, however, restrict myself to this in order to support my
hypothesis, but must also directly combat the view hitherto received,
since the latter is in contradiction with facts.

Did there really exist in the Axolotl a tendency to sudden phyletic
advancement, then one fact would remain quite incomprehensible, viz.
the sterility of the Amblystomas.

Out of about thirty Amblystomas obtained by Duméril down to the
year 1870, there was not one in a state of sexual maturity; neither
copulation nor deposition of eggs took place, and the anatomical
investigation of single specimens showed that the eggs were immature,
and that the spermatozoa, although present, were without the undulating
membrane characteristic of the salamanders, but were not devoid of
all power of movement, only, as established by Quatrefages, were
“incompletely motile.”[254]

So also the five Amblystomas about which I have been writing, show up
to the present time no appearance of reproduction.

The objection raised by Sacc,[255] that the sterility of the
Amblystomas bred from Axolotls is attributable to “bad nourishment,” is
obviously of but little avail. How is it that the Axolotls, which are
fed in a precisely similar manner, propagate so readily? Moreover, I am
able to expressly assert that my Amblystomas were very well fed. It is
true that they have as yet scarcely reached the age of two years, but
the Axolotl propagates freely in the second year, and some of Duméril’s
Amblystomas were five years old in 1870.

This fact of the sterility is strongly opposed to the idea that these
Amblystomas are the regular precursors of the phyletically advancing
genus _Siredon_.[256] I will by no means assert that my theory of
reversion actually explains the sterility, but it is at least not
directly opposed to it. Mere reversion forms may die off without
propagating themselves; but a _new form_ called forth by the action
of a phyletic vital force should not be sterile, because this is the
precise “aim” which the vital force had in view. The conception of a
vital force comprises that of teleology.

The sterility of Amblystoma moreover, although not completely
explicable from our standpoint, can be shown to be a phenomenon
not entirely isolated. In the above mentioned case of _Lissotriton
Punctatus_, the female “larvæ” were certainly sexually mature and laid
eggs, but the males of the same period contained in their testicles no
fully developed spermatozoa.

Other cases of this kind are unknown to me; at the time when I made the
experiments with butterflies already recorded (see the first essay),
this point of view was remote, and I therefore neglected to examine
the artificially bred reversion forms with respect to their organs of
reproduction. But general considerations lead to the supposition that
atavistic forms may easily remain sterile.

Darwin[257] finds the proximate causes of sterility in the first place
in the action of widely diverging conditions of life, and in the next
place in the crossing of individuals widely different in constitution.
Now it is certainly deviating conditions of life which lead to the
metamorphosis of the Axolotl, and from this point of view it cannot be
surprising if we find those individuals sterile which show themselves
so especially affected by these changed conditions as to revert to the
salamander form.

By this it is not in any way meant to be asserted that reversion
is invariably accompanied by sterility, and one cannot raise as an
objection to my interpretation of the metamorphosis of the Axolotl,
that a reproductive colony of Axolotls could never have arisen by
reversion. On the contrary, Jullien’s egg-depositing female Triton
larvæ show that also with reversion the power of reproduction may be
completely preserved.[258] From the above-mentioned general causes
of sterility, it may even be inferred that fertility can be lost in
different degrees, and it can be further understood to a certain
extent why this fertility is more completely lost by reversion
to the Amblystoma, than by the reversion of the Triton to the
perennibranchiate form.

If in these cases the reversion is brought about by a change in the
conditions of life, we may perhaps suppose that the magnitude of this
change would determine the degree of fertility, and the preservation
of the reversion form. Still more, however, would the fertility be
influenced by the extent of the morphological difference resulting
from the reversion. We know that the blending of very different
constitutions (_e.g._ the crossing of different species) produces
sterility. Something similar results from the sudden reversion to a
stage of development widely different in its whole structure. Here
also we have in a certain sense the union of two very different
constitutions in one individual--a kind of crossing.

From this point of view it can in some measure be comprehended why
sterility may be a result of reversion; on the other hand, we thereby
obtain no explanation why, with the same amount of morphological
difference, in one case complete sterility, and in another relative
fertility occurs. The morphological difference between Axolotl and
Amblystoma is exactly the same as between Triton and its “sexually
mature larva;” the difference between the two cases of reversion
depends entirely upon the direction of the leap, that taken in the
former case being precisely opposite in direction to that taken in the
latter.

Herein might be sought the explanation of the different strength with
which the reproductive power is affected; not indeed in the direction
of the leap itself, but in the differences in the ontogeny which
are determined by the differences in the direction of the leap. The
reversion of the Triton to an older phyletic stage coincides with
the arrest at a younger ontogenetic stage; or, in other words, the
older stage of the phylogeny to which reversion takes place is still
entirely comprised in the ontogeny of each individual. Each Triton is
perennibranchiate throughout a long period of its life; the reverting
individual simply reverts to the older phyletic stage by remaining at
the larval stage of its individual development.

But it is quite different with the reversion of the Axolotl to the
formerly acquired, but long since abandoned Amblystoma form. This is
not retained in the ontogeny of Axolotl, but has been completely lost;
for a long series of generations--so must we suppose--the ontogeny
has always only attained to the perennibranchiate form. Now if at the
present time certain individuals were compelled to revert to the
Amblystoma form, certainly no greater leap would have been made from
a morphological point of view, than in the reversion of Triton to the
perennibranchiate form, but at the same time the leap would be in
another direction, viz. over a long series of generations back to a
form which the species had not produced for a long period, and which
had to a certain extent become foreign to it. We should thus have here
also the grafting of a widely different constitution upon that of the
Axolotl, or, if one prefers it, the commingling of two widely different
constitutions.

Of course I am far from wishing to pretend that this “explanation” is
exact; it is nothing more than an attempt to point out the direction
in which the causes affecting the reproductive powers in different
degrees are to be looked for. A deeper penetration into and special
demonstration of the manner in which these causes bring about such
results, must be reserved for a future period. For the present it
must suffice to have indicated that there is an essential distinction
between the two kinds of reversion, and to have made it to some
extent comprehensible that this distinction may be the determining
impulse with respect to the question of sterility. Perhaps the law
here concealed from us may one day be thus formulated:--Atavistic
individuals lose the power of reproduction the more completely, the
greater the number of generations of their ancestors whose ontogeny no
longer comprises the phyletically older stage to which the reversion
takes place.

The hypothesis which interprets the transformation of the Axolotl as
a case of reversion, thus holds out the possibility of our being able
to comprehend the sterility of the Amblystomas arising in this manner,
whilst, on the other hand, for the adherents of a phyletic vital
force, not only is this observed sterility as Duméril expresses it “un
véritable énigme scientifique,” but an absolute paradox. We should
expect such a directive and inciting principle to call into existence
new forms having vitality and not destined to perish, the more so when
it is concerned with a combination of structural characters which, when
originating in another manner (viz. from other species of _Siredon_),
have long since shown themselves to have vitality and reproductive
power. We are indeed acquainted with species of _Amblystoma_ which
propagate as such, and each of which arises from an Axolotl-like larva.
Thus we cannot regard the sterile Amblystomas produced by the Paris
Axolotls as abortive attempts of a vital force--an interpretation which
is certainly in itself already sufficiently rash.

Now if it be asked what change in the conditions of life could have led
to the reversion in the Lake of Mexico[259] of the Amblystoma to the
Siredon form, I must admit that I can only offer a conjectural reply,
having but a conditional value so long as it is not supported by a
precise knowledge of the conditions there obtaining, and of the habits
both of the Axolotl and of the Amblystoma.

It may be supposed generally that reversion is brought about by
the same external conditions as those which formerly produced the
perennibranchiate stage. This supposition is in the first place
supported by the experiments here recorded, since it is evidently the
inducement to aërial respiration which causes the young Axolotl to
revert to the Amblystoma form, _i.e._ the inciting cause under whose
domineering influence the Amblystoma form must have arisen.

Here again the case is quite similar to that of seasonally dimorphic
butterflies. Reversion of the summer brood to the winter form is there
most easily caused by the action of cold, _i.e._ by the same influence
as that under whose sway the winter form was developed.

We know indeed that reversion may also arise by the crossing of
races and species, and I have attempted to show that reversion in
butterflies may also be brought about by other influences than cold;
but still the most probable supposition obviously is, that reversion
would be caused by the persistent action of the same influences as
those which in a certain sense created the perennibranchiate form.
That the latter was produced under the influence of an aquatic life
there can be no doubt, and thus, in accordance with my supposition, the
hypothetical _Amblystoma Mexicanum_, the supposed ancestral form of
the Axolotl of the Mexican Lake, might have been caused to revert to
the perennibranchiate form by a reduction in the possibilities of its
living upon land, and by its being compelled to frequent the water.

I will not here return to the consideration of every other opinion
_ab initio_. It is very advisable to distinguish between the mere
impulses which are able to produce sudden reversion, and between
actual transforming causes which result directly or indirectly in the
remodelling of a species. Thus, it is conceivable _à priori_ that
reversion may occur by the action of an inciting cause having nothing
to do with the origin of the phyletically older form. Temperature
can certainly have played no part, or only a very small part, in the
formation of the perennibranchiate form; nevertheless cold may well
have been one of the inciting causes which induced the Amblystoma at
one time to revert to the _Siredon_ form, and we cannot at present
consider De Saussure to be incorrect when he maintains that the low
temperature of the Mexican winter might prevent that transformation
(of the Axolotl into the Amblystoma) which would occur “in the warm
reptile-house” of the Jardin des Plantes. He supports this view by
stating that “Tschudi has found the Amblystoma” (of course another
species) “in the hottest parts of the United States.” “On the Mexican
plateau, however, it snows every winter, and if the lake does not
actually freeze, its temperature must fall very considerably in the
shallowest parts.”

But although this view is not opposed by any theoretical
considerations, I still hold it to be incorrect. I doubt whether it
is temperature that has brought about the reverse transformation of
the Amblystoma into the Axolotl, or which, according to De Saussure’s
conception, at the present time prevents the transformation of the
Axolotl in the Lake of Mexico. I doubt this because Amblystomas are now
known from all parts of the United States as far north as New York, a
proof that a winter cold considerably greater than that of the Mexican
plateau is no hindrance to the metamorphosis of the Axolotl, and that
the genus does not show itself to be in this respect more sensitive
than our native genera of _Salamandridæ_.

The following observations of De Saussure, in which he calls attention
to the nature of the Mexican Lake, appear to me to be more worthy of
consideration:--“The bottom of this lake is shallow, and one passes
imperceptibly from the lake into extensive marshy regions before
reaching solid ground; perhaps this circumstance makes the Axolotl
incapable of reaching dry land, and prevents the transformation.”

In any case the Lake of Mexico offers very peculiar conditions for
Amphibian life. My esteemed friend Dr. v. Frantzius has called my
attention to the fact that this lake--as well as many other Mexican
lakes--is slightly saline. At the time of the conquest of Mexico by
Ferdinand Cortez, this circumstance led to the final surrender of the
city, as the Spaniards cut off the supply of water to the besieged, and
the water of the lake is undrinkable. The ancient Mexicans had laid
down water-conduits from the distant mountains, and the city is still
supplied with water brought through conduits.

Now this saltness cannot in itself be the cause of the degeneration
to the perennibranchiate form, but it may well be so in combination
with other pecularities of the lake. The narrowest part of the lake
is the eastern, and it is only in this part that the Axolotl lives.
Now in winter, violent easterly gales rush down from the mountains and
blow continuously, driving the water before them to such an extent
that it becomes heaped up in the western portion of the lake, where it
frequently causes floods, whilst 2000 feet of the shallow eastern shore
are often laid completely dry.[260]

Now if we consider these two peculiarities, viz. salineness and
periodical drying up of a part of the bottom of the lake through
continuous gales, we certainly have for the Axolotl, conditions of
life which are only to be found in few species. One might certainly
attempt to apply these facts in a quite opposite sense, and to regard
them as unfavourable to my theory, since the retreat of the water
from a great portion of the bottom of the lake would--so one might
think--rather facilitate transition to a life upon land, and indeed
compel the adoption of such a mode of existence. But we should thus
forget that the exposed bottom of the lake is a sterile surface without
food or place of concealment, and, above all, without vegetation;
and further, that owing to the considerable salineness of the water
(specific gravity = 1.0215),[261] the whole of the exposed surface must
be incrusted with salt, a circumstance which would render it quite
impossible for the creatures to feed upon land. Sodic chloride and
carbonate are dissolved in the water in such considerable quantities,
that they are regularly deposited upon the shores of the lake as a
crust, which is collected during the dry season of the year and sent
into the market under the name of “tequisquite” (Mühlenpfordt).[262]

Thus the supposition is not wanting in support, that peculiar
conditions make it more difficult for the creature to obtain its food
upon land than in the water, and this alone may have been sufficient
to have induced it to acquire the habits of a purely aquatic existence,
and thus to revert to the perennibranchiate or Ichthyodeous form.

But enough of supposition. We must not complain that we are unable
from afar to discover with precision the causes which compelled the
Axolotl to abandon the Amblystoma stage, as long as we are not able to
explain the much nearer cases of reversion in Filippi’s and Jullien’s
Tritons; nevertheless, in these cases also, the causes affecting the
whole colony of Tritons must be general, since--at least in the case
noticed by Filippi--the greater majority of the individuals remained
in the larval condition. Experiments with Triton larvæ could throw
greater light upon this subject; it would have in the first place to be
established whether reversion could be artificially induced, and if so,
by what influences.

From the previously mentioned experiments with butterflies, as well
as from the results obtained with Axolotls, we should expect that
in Tritons, reversion to the Ichthyodeous form would take place if
we allowed the inciting cause, viz. the bathing of the gills and of
the whole body with water, to act persistently, and at the same time
withheld that influence under whose action the salamander form became
developed, viz. the bathing of the gills, the skin, and the surfaces of
the lungs with air.

Old experiments of this kind are to be met with, but they were never
carried on for a sufficient time to entirely allay the suspicion, that
the specimens concerned would perhaps have undergone the ordinary
metamorphosis if their existence had been prolonged.

Thus, Schreibers[263] relates that “by confining tadpoles of the
salamander found at large in their last stage of growth, under water by
means of an arrangement (net?), and feeding them with finely chopped
earthworms, he was able to keep them for several months--and indeed
throughout the winter--in this condition, and in this way to forcibly
defer their final change, and their transition from the tadpole stage
to that of the perfected creature during this period.” It is not stated
whether the animals finally underwent transformation, so that it cannot
be decided whether we have here a case of reversion or simply one of
retarded development. That metamorphosis may occur after a long period
of time, is shown by experiments upon the tadpole of _Pelobates_
conducted by Professor Langer in Vienna.[264] The creatures were kept
in deep water in such a manner that they were not able to land, and
by this means three out of a large number of individuals had their
metamorphosis delayed till the second summer; notwithstanding this,
transformation then occurred.

It cannot be objected to my reversion hypothesis, that it opposes on
the one side what on the other it postulates, viz. a _per saltum_
change of structure. Reversion is characterized by the sudden
acquisition of an older, _i.e._ a formerly existing phyletic stage.
That reversion occurs is a fact, whilst nobody has hitherto been able
to prove, or even to make probable, that a stage of the future (_sit
venia verbo_) has been attained at once (_per saltum_).

Now if it is possible to find influences in the present conditions
of life of the Axolotl which make it difficult or quite impossible
for it to live upon land, and which therefore appear as incentives
to the reversion to the Ichthyodeous form, the other portion of my
hypothesis--the assumption that the Axolotl had become an Amblystoma at
a former period--can also be supported by facts.

We know from Humboldt[265] that the level of the Lake of Mexico at a
comparatively recent period was considerably higher than at present. We
know further that the Mexican plateau was covered with forest, which
has now been destroyed wherever there are human, and especially Spanish
settlements. Now if we suppose that at some post-glacial period the
mountain forests extended to the borders of the lake, at that time
deep, with precipitous sides and much less saline, not only should we
thus have presented different conditions of life to those at present
existing, but also such as would be most favourable for the development
of a species of salamander.

On the whole, I believe that my attempt to explain the exceptional
metamorphosis of the Axolotl of the Mexican lake cannot be objected
to as being a too airy phantasy. In any case it is the only possible
explanation which can be opposed to that which supposes that the
occasional transformation of the Axolotl is not reversion, but an
attempt at advancement. This last assumption must, in my judgment, be
rejected on purely theoretical grounds by those who hold that a sudden
transformation of a species, when connected with adaptation to new
conditions of life, is inconceivable--by those who regard adaptation,
not as the sudden work of a magic power, but as the end result of a
long series of natural, although minute and imperceptible causes.

If my interpretation of the facts be correct, there arises certain
consequences which I may here briefly mention in conclusion.

First, with regard to more obvious results. If _Siredon Mexicanus_,
Shaw, only by occasional reversion assumes the Amblystoma form,
and never, or only exceptionally, propagates as such, but only
as _Siredon_, the more recent systematists are not justified in
striking out the genus _Siredon_ and in placing _S. Mexicanus_ as an
undeveloped form in the genus _Amblystoma_. So long as there exists
not one only, but several species of _Siredon_ which as such regularly
propagate themselves, the genus exists; and although we would not
deprive systematists of all hope of these species of _Siredon_ being
one day re-elevated to _Amblystomæ_, it nevertheless better accords
with the actually existing state of affairs if we allow the genus
_Siredon_ to remain as before among the genera of _Salamandrina_, and
to include therein all those species which, like the Paris Axolotl,
_S. Mexicanus_, Shaw, and probably also _S. Lichenoides_, Baird, only
exceptionally, or through artificial influences, assume the Amblystoma
form, but without propagating regularly in this condition. On the other
hand, we should correctly comprise under the genus _Amblystoma_ all
those species which propagate in this state regularly, and in which the
perennibranchiate stage occurs only as a larval condition.

To arrive at a decision in single cases would chiefly concern the
American naturalists, whose ever increasing activity may lead us
to hope soon for a closer investigation of the reproduction of the
numerous species of _Amblystoma_ of their native country. I should
rejoice if the facts and arguments which I have here offered should
give an impetus to such researches.

The second consequence to which I may refer, is of a purely
theoretical nature, and concerns a corollary to the “fundamental
biogenetic law” first enunciated by Fritz Müller and Haeckel. This, as
is well known, consists of the following law:--The ontogeny comprises
the phylogeny, more or less compressed and more or less modified.

Now according to this law, each step in phyletic development when
replaced by a later one, must remain preserved in the ontogeny, and
must therefore appear at the present time as an ontogenetic stage
in the development of each individual. But my interpretation of the
transformation of the Axolotl appears to stand in contradiction to
this, since the Axolotl, which at a former period was an Amblystoma,
retains nothing of the latter in its ontogeny. The contradiction is,
however, only apparent. As long as we are concerned with an actual
advance in development, and therefore with the attainment of a new step
never formerly reached, the older stages will be found in the ontogeny.
But this is not the case when the new stage is not an actual novelty,
but formerly represented the final stage of the individual development;
or, in other words, when we are concerned with the reversion, not
of single individuals, but of the species as such, to the preceding
phyletic stage, _i.e._ with a phyletic degeneration of the species.
In this case the former end-stage of the ontogeny would be simply
eliminated, and we should then only be able to recognize its former
existence by its occasional appearance in a reversion form. Thus, under
certain conditions the Triton sinks back to the perennibranchiate
stage; not in such a manner that the individual first becomes a Triton
and then undergoes perennibranchiate re-modification, but simply, as I
have already shown above, by its remaining at the Ichthyodeous stage
and no longer attaining to the Salamander form. So also, according
to my hypothesis, the salamandrine _Amblystoma Mexicanum_, formerly
inhabiting the shores of the Lake of Mexico, has degenerated to the
perennibranchiate stage, and the only trace that remains to us of its
former developmental status is the tendency, more or less retained
in each individual, to again ascend to the salamander stage under
favourable conditions.

The third and last consequence which my interpretation of the facts
entails, is the change in the part played by reversion in organic
nature. Whilst atavistic forms have hitherto been known only as
isolated and exceptional cases, interesting indeed in the highest
degree, but devoid of significance in the course of the development
of organic nature, a real importance in this last respect must now be
attached to them.

I may assume that reversion can in two ways be effectual for the
preservation or re-establishment of a living form. In the first
place, where, as in Axolotl, the new and organically higher form
becomes untenable through external influences, instead of simply
perishing--since advancement in another direction does not appear to
be possible--a reversion of the species to the older and more lowly
organized stage occurs. In the second place, the older phyletic form
may not be abandoned while a newer form is being developed therefrom,
but the former may alternate with the latter, as we see in the case
of seasonally dimorphic butterflies. It can hardly be objected if I
regard the alternation of the summer and winter form in this case as a
periodic reversion to the phyletically older (winter) form.

Although the reversion of an entire species, such as I suppose to have
been the case with the Axolotl, may be of rare occurrence, this is
certainly not the case with periodic or cyclical reversion; the latter
plays a very important part in the development of the various forms of
alternating or cyclical propagation.[266]


POSTSCRIPT.

In the previous portion of this essay it was pointed out that the
causes to which I attributed the reversion of the hypothetical
_Amblystoma Mexicanum_ to the existing Axolotl, did not appear to me
to amount to a complete explanation of the phenomenon. In the first
place these seemed to me too local, since they could only be applied
with any certainty to the Axolotl of the lake of the Mexican capital,
whilst the Paris Axolotls obtained from other parts of Mexico still
required an explanation. On the other hand, these causes did not appear
to me sufficiently cogent. Should we even learn subsequently that the
Paris Axolotl is also derived from a salt lake which is exposed to
similar winds to the Lake of Mexico, we still have in this peculiarity
of the lakes only a cause tending to make it difficult for the larva
to undergo metamorphosis, and to reach a suitable new habitat on the
land. The impossibility of doing this, or the complete absence of such
habitat, does not however follow as a necessary consequence.

It would obviously be a much more solid support for my hypothesis if it
were possible to point to some physical conditions of the land which
there precluded the possibility of the existence of Amblystomas.

For a long time I was indeed unable to discover such causes, and I
therefore concluded the previous portion of this essay and went to
press. Afterwards, when residing in one of the highest valleys of our
Alps in the Upper Engadine, an idea accidentally occurred to me, which
I do not now hesitate to regard as correct after having tested it by
known facts.

It happens that in the Upper Engadine there live only such Amphibia
as persistently, or at least frequently resort to the water. I found
frogs up to nearly 7000 feet above the sea, and Tritons at 6000 feet
(Pontresina and Upper Samaden). On the other hand, the land-living
mountain salamander, _S. Atra_,[267] was absent, although suitable
stations for this species were everywhere present, and it would have
wanted for food as little as do its allies the water-newts. Neither
would the great elevation above the sea offer any obstacle to its
occurrence, since it occasionally ascends to a height of 3000 metres
(Fatiot).[268]

Now it is well known that the atmosphere of the Upper Engadine,[269]
like that of other elevated Alpine valleys enclosed by extensive
glaciers, is often extraordinarily dry for a long period, a condition
which appears to me to explain why the black land-salamander is there
absent,[270] whilst its near water-living ally occurs in large numbers.
The skin of the naked Amphibia generally requires moisture, or else
it dries up, and the creature is deprived of a necessary breathing
apparatus, and often dies as rapidly as though some important internal
organ had been removed. Decapitated frogs hop about for a long time,
but a frog which escapes from a conservatory and wanders about for one
night in the dry air of a room, is found the following day with dry and
dusty skin half dead in some nook, and perhaps perishes in the course
of another day if left without moisture.

All that we know of the biology of the Amphibia is in accordance with
this. Thus, all the land-salamanders of southern Italy avoid the hot
and dry air of summer by burying in the ground, where they undergo
a summer sleep. This is the case with the interesting _Salamandrina
Perspicillata_,[271] and with the land-living Sardinian Triton, the
remarkable _Euproctus Rusconii_, Gené,[272] (_Triton Platycephalus_,
Schreiber). With respect to _Geotriton Fuscus_ I learn from Dr.
Wiedersheim, who has studied the life conditions of this, the lowest
European Urodelan, in its own habitat, that in Sardinia it sleeps
uninterruptedly from June till the winter; whilst on the coast of
Spezia and at Carrara, where it also occurs, it avoids the summer sleep
in a very peculiar manner. It makes use of the numerous holes in the
calcareous formation of that region, and for some months in the year
becomes a cave-dweller. As soon as the great heat occurs, often in May,
it withdraws into the holes, and again emerges in November during the
wet weather. In these lurking holes it does not fall into a sleep, but
is found quite active, and its stomach, filled chiefly with scorpions,
shows that it goes successfully in search of food; the moist air of the
holes makes it unnecessary for it to bury in the earth.

In the same sense it appears to me must be conceived the fact
that the solitary species of frog of the Upper Engadine, _Rana
Temporaria_,[273] the brown grass frog, is there much more a frequenter
of the water than in the plains. It is true that I can find no remark
to this effect in the excellent work of Fatiot, already referred to
above, and I am therefore obliged to resort to my own observations,
which, although often repeated, have always been carried on for only a
short time. I was much struck with the circumstance that the Engadine
frogs were to be found in numbers in the water long after the pairing
season, which, according to Fatiot, lasts at most to the end of June.
In the numerous pools around Samaden I found them in July and August,
whilst in the plains they only take to the water at the time of
reproduction, and seek winter quarters in the mud on the first arrival
of this season. (Fatiot, p. 321.) In the Engadine they have therefore
in some measure adopted the mode of life of the aquatic frogs, but this
of course does not prevent them from returning in damp weather to their
old habits and roving through meadows and woods.

After these considerations had made it appear to me very probable that
the dry air of the Upper Engadine accounted for the absence of the
black land-salamander, the question at once arose whether the absence
of Amblystomas from the Mexican plateau might not perhaps be due to
the same cause, _i.e._ whether such a dryness of the atmosphere might
not perhaps prevail also in that region, so that Amphibia, or at least
salamander-like Amphibia, could not long exist on the land. The height
above the sea is still greater (7000 to 8000 feet), and the tropical
sun would more rapidly dessicate everything in a country poor in water.

As I was at the time without any books that might have enlightened
me on the meteorological conditions of Mexico, I wrote to Dr. v.
Frantzius, who, by many years residence in Central America was familiar
with the climate of this region, and solicited his opinion. I received
the reply that on the high plains of Mexico an extraordinary dryness
of the atmosphere certainly prevails. “The main cause of the dryness
of the high plains is to be found in the geographical position,
the configuration of the land, and the physical structure. The
north-eastern trade-wind drives the clouds against the mountains, on
the summits of which they deposit their moisture, so that no vapour is
carried over; as long as the north-east trade-wind blows, the streams
feeding the rivers flowing into the Atlantic Ocean are abundantly fed
with water, whilst on the western slopes, and especially on the high
plains, the clouds give no precipitation. In the second half of the
year also, during our summer, the so-called rainy season brings but
little rain[274]--little in comparison with the more southern regions,
where the heavy tropical thunderstorms daily deluge the earth with
water. Mexico lies much too northerly, and does not reach the zone of
calms, within which region these tropical rains are met with.”

Thus, in the high degree of dryness of the air lasting throughout the
year, I do not doubt that we have the chief cause why no Amblystomas
occur on these elevated plains; they simply cannot exist, and would
become dried up if taken there, supposing them not to be able to change
their mode of life and to take to the water. If therefore in former
times Amblystomas inhabited Mexico, the coming on of the existing
climatic conditions left them only the alternative of becoming extinct,
or of again taking to the aquatic life of their Ichthyodeous ancestors.
That this was not _directly_ possible--that the Amblystoma form was not
able to become aquatic without a change of structure, is shown by the
fact that even in the Lake of Mexico no Amblystoma occurs. A retreat to
an aqueous existence could, as it appears, only be effected by complete
reversion to the Ichthyodeous form, which then also took place.

But my hypothesis of the transformation of the Axolotl not only
requires the proof that Amblystomas cannot exist under present
conditions in Mexico, but also the further demonstration that at a
former period other conditions prevailed there, and these of such a
nature as to make the existence of land-salamanders possible.

With respect to my question, whether we might not perhaps assume that
at some post-glacial period the conditions of atmospheric moisture on
the high plains of Mexico were essentially different from those at
present prevailing, I recollected Dr. v. Frantzius and the above-quoted
observation of Humboldt’s,[275] who discovered in the neighbourhood
of the Lake of Tezenco (Mexico) distinct evidence of a much higher
former level of the water. “All such elevated plains were certainly at
a former period so many extensive water-basins, which gradually became
filled, and are still filling up with detritus. The evaporation from
such large surfaces of water must at that time have caused a very moist
atmosphere, favourable to vegetation and adapted for the life of naked
Amphibia.”

From this side also my hypothesis thus receives support, and we may
assume with some certainty that at the beginning of the diluvial
period[276] the woods surrounding the Mexican lakes were inhabited
by Amblystomas, which, as the lakes subsequently became more and more
dried up and the air continually lost moisture, found it more difficult
to exist on the land. They would at length have completely died out,
had they not again become aquatic by reversion to the Ichthyodeous
form. It may perhaps be supposed that the above-mentioned physical
conditions--desolate, salt-incrusted shores--co-operated in the
production of the reversion, by making it difficult for the larvæ to
quit the water; but we can only judge with certainty upon this point
when, by means of experiment, we have discovered the causes which
produce reversion in the Amphibia.


ADDENDUM.

I have lately met with another interesting notice on the reproduction
of the native North American Amblystomas. Professor Spence F. Baird, of
Washington, has often observed the development from the egg of various
species, and especially of _Amblystoma Punctatum_ and _A. Fasciatum_.
His observations do not appear to be as yet published, so that I was
unable to discover any account of the development of _Amblystoma_ in
existing literature.[277] I am authorized to extract the following
brief data from a letter addressed to Dr. v. Frantzius.

In order to deposit their eggs the Amblystomas go into the water,
where the eggs are laid enclosed in a jelly-like mass, but never more
than fifteen to twenty together. The spherical eggs are very large,
perhaps a quarter of an inch in diameter. They soon develop into a
_Siredon_-like larva, which remains several months in this condition.
The gills then shrivel up, the creature begins to crawl, and gradually
passes through the different transformations to the complete Amblystoma
form.

It appears from this communication that the Amblystomas lay much larger
and much fewer eggs than the Axolotl, and that their development
throughout resembles that of our salamanders.

In concluding I may mention an anatomical fact which most strongly
supports my view that the Mexican Axolotl is a reverted Amblystoma.
I learn from Dr. Wiedersheim that the Axolotl possesses the
“intermaxillary gland” which occurs in all the land Amphibia. This
organ, lying in the intermaxillary cavity, appears, whenever it occurs,
to produce a kind of birdlime, _i.e._ a very glutinous secretion,
which serves to attach the prey to the rapidly protrusible tongue.
Although this secretion may perhaps also have another function, from
the absence of the intermaxillary gland in all exclusively aquatic
Amphibia, it follows that it must be devoid of importance for, and
inapplicable to feeding in the water. The intermaxillary gland is
absent in all _Perennibranchiata_ and _Derotremata_ which Wiedersheim
has hitherto investigated, viz. in _Menobranchus_, _Proteus_,
_Siren_, _Cryptobranchus_, _Amphiuma_, and _Menopoma_, all of which
are indeed without the cavity in which the gland is situated in the
_Salamandrina_, _i.e._ the _cavum intermaxillare_.

Now in the _Salamandrina_ the gland appears at an early stage. It is
possessed in a well-developed state by the larvæ both of species of
_Triton_ and of _Amblystoma_, where indeed the glandular structure
completely fills the _cavum intermaxillare_.

Were the Axolotl a species retarded in phyletic development,
the presence of a gland which does not occur in any other
_Perennibranchiata_, and which is only of use for life upon land, would
be quite inexplicable.

The matter becomes still more enigmatical through the fact that
the gland, although present, is quite rudimentary. Whilst in the
_Salamandrina_ the capacious intermaxillary cavity is entirely filled
by the tubes of the gland in question, in Axolotl this cavity is almost
completely filled with a closely woven connective tissue, in which
there can only be found a small number of gland-tubes--in the extreme
front, and at the base immediately over the intermaxillary teeth--these
tubes agreeing in the details of their histological structure with the
elements of the same gland in the _Salamandridæ_.

I give these anatomical details from Dr. Wiedersheim’s verbal
communication. An amplified account will subsequently appear in another
place.[278]

An explanation of this rudimentary intermaxillary gland in the Axolotl
only appears to me possible on the supposition that the latter is an
atavistic form. From this point of view it is evident that the gland
already present in all _Amblystoma_-larvæ must have been taken over
by the perennibranchiate form of the existing Axolotl, through the
reversion of the hypothetical _Amblystoma Mexicanum_ of the “diluvial
period.”[279] It can also be easily understood that this organ would
become more and more rudimentary in the course of time, since it has
no further use in the water, and the gap thus arising in the formerly
present _cavum intermaxillare_ would become filled with connective
tissue.

While the German edition of this work was going through the press
I obtained, through the kindness of my friend Dr. Emil Bessels of
Washington, the Mexican memoir upon the new Axolotl,[280] which even in
Mexico regularly, or at least in many cases, becomes developed into the
Amblystoma form.

The facts are briefly as follows:--The small Lake of Santa Isabel
is some hours’ journey from the Mexican capital. In this lake there
lives a species of Axolotl which had hitherto remained unknown, and
was described by Señor Velasco as _Siredon Tigrinus_. This species
propagates itself indeed in the Axolotl state, but in many cases it
becomes transformed into Amblystoma and takes to the land. Although
propagation in the Amblystoma condition was not observed, it can hardly
be doubted that it also propagates in this form.

At first sight these facts appear to refute my hypothesis, that
the extreme dryness of the air of the Mexican plateau precludes
the existence of land Amphibia. Nevertheless I do not abandon this
hypothesis for the former one, since a closer study of the data
furnished by Velasco confirms rather than refutes my supposition.

Velasco expressly corroborates the statement that the Axolotl hitherto
known from the great Mexican lake which never dries up (Lake of
Xochimilco and Chalco), is only met with in its native habitat in the
_Siredon_ form, _i.e._ as _Siredon Humboldtii_. According to Velasco
the cause of the frequent assumption of the Amblystoma form by the new
_Siredon Tigrinus_, is to be found in the local conditions of life of
this species. The Lake Santa Isabel is shallow, its greatest depth
amounting to three meters, and it is liable to a periodical drying up,
which is so complete that one can pass dry-shod through it in several
places. The species must therefore have long since died out had it
not been able to adapt itself periodically to a land life. Now it
could have become transformed into a land Amphibian--as Señor Velasco
observed--at various stages of growth; and indeed this author believes
that “the Creator has implanted an instinct in this creature,” which
enables it to always undergo metamorphosis at the right time.

This last assumption may or may not be taken as correct, but this much
is established, viz. that numerous individuals of this species take to
the land, and remain there during a period of many months.

But does this contain the proof that salamander-like animals are
actually able to lead a land life in Mexico--that the dry air is
advantageous, or at least supportable to them? It does not appear so
to me, but rather that all which has been reported of this Amblystoma
by Señor Velasco goes to show that the animal does not, properly
speaking, live upon land like the North American Amblystomas, or like
our land-salamanders, but that _it only experiences a summer sleep
lasting over the period of drought_. These Amblystomas were observed
as they left the dried-up lake at night in order to seek some moist
lurking-place in the neighbourhood, where they might remain concealed.
They are only known in the villages situated near the lake, and were
only seen there at large just when they were wandering from the lake
to their place of concealment. At other times they were mostly found
in the earth, buried under walls, the pavement of the market-place,
&c. When laying down a line of railway, a workman found in the earth
a whole nest of twelve Amblystomas lying close together. All these
are not mere lurking-holes which could be abandoned at any moment;
it would rather appear that we have here places of refuge for the
entire duration of the period of drought, and that these would only
be forsaken when the water of the rainy season penetrated the soil.
I am not myself in a favourable position for investigating these
suppositions more closely, but this could be done by Señor Velasco,
who lives in Mexico, and science would be much indebted to him if he
would examine as precisely as possible into the habits and conditions
of life of this, and of the other species of Mexican Axolotls.
Unfortunately this gentleman can, it would appear, have seen only the
French publications upon the transformation of the Axolotl, and could
not therefore have asked himself questions arising from my conception
of the facts; otherwise many of his observations would have led to more
definite results. The above conclusion can however be still further
supported by Señor Velasco’s data.

One might indeed insist that with us also the land-salamanders conceal
themselves in moist places during dry weather, and often lie hidden,
as in Mexico, in a hole, in a cluster of as many as ten together;
but with us they leave their lurking-place from time to time and go
in search of food. Señor Velasco mentions nothing with respect to
this. What especially struck me was the statement that the Mexican
Amblystomas _were also to be found in the water_.[281] When Lake Santa
Isabel is drained, the fishermen stretch large nets across the exit
channels, and in these they not only find ordinary Axolotls, but also
some “_sin aretes_,” which they also designate “_mochos_,” _i.e._
hornless Axolotls, because they have no gills, but have already reached
the Amblystoma stage. Our land-salamanders live in the water only
as larvæ, but they also love and require moisture. Only the female
enters the water when she wants to deposit her young (eggs with mature
larvæ), and then only at the margin of shallow pools or small brooks.
The Mexican Amblystoma thus much more resembles in its habits our
water-salamanders (Tritons), which remain in the water at least during
the whole period of reproduction. These also leave the water later,
and, like the land-salamander, seek concealment in the earth. They have
this habit also in those districts which possess a very dry atmosphere;
and especially in the Engadine, where I first conceived the idea of
taking into account the dryness of the air, I found in the pools at the
end of August and the beginning of September only larvæ of Tritons. The
older Amphibians must therefore have been on the land, presumably in
their places of winter concealment.

From what we have hitherto learnt from Señor Velasco, the mode of
life of _Amblystoma Tigrinum_ must resemble that of our Tritons,
although its structure is that of a land-salamander. I would thus
offer the following explanation of the facts at present known:--Owing
to the periodic drying up of the lake of Santa Isabel, the _Siredon
Tigrinus_ would be again compelled to undergo metamorphosis. Whether
this was formerly entirely abandoned, or whether it always occurred
in solitary individuals, is almost immaterial; in any case the habit
of metamorphosis must have been very rapidly acquired through natural
selection, and must have again become general, if the faculty was only
present in the species, although latent. Through the dryness of the
air, the Amblystomas that had taken to the land would be compelled to
bury themselves at once, and to remain asleep till the recurrence of
the rainy season, when they would hasten back into the water and would
there live as a species of Triton.

Now one might feel inclined to ask why the species of the great Mexican
lake has not also taken to this mode of life. To this it may be simply
replied that the water of this lake never dries up, and that the
Axolotls have thus never been reduced to the alternative of undergoing
metamorphosis or of perishing. If therefore the conditions of existence
in water were more favourable than on land, the tendency to abandon
metamorphosis would increase from generation to generation, and the
deportment at present observed would finally result, _i.e._ propagation
would take place exclusively in the Axolotl state. As has already been
mentioned above, the latest observations of Velasco furnish further
confirmation that the Axolotl of the great lake is never met with in
the Amblystoma condition, “although it (the Axolotl) is brought daily
from Mexico into the market throughout the whole year.” I should not
however regard it as a refutation of my view if prolonged investigation
should show that this species also (_Siredon Humboldtii_) occasionally
developed into an Amblystoma; on the contrary, it would not at all
surprise me if such cases of reversion occurred in Mexico as well as in
Europe. The fact that an immense majority of the Amphibians propagate
in the Axolotl state would not be thereby affected, and would still
require an explanation: this I am still inclined to see in the dryness
of the air of the high plains, which is so unfavourably adapted for a
life passed entirely on land.



=IV.=

ON THE MECHANICAL CONCEPTION OF NATURE.



INTRODUCTION.


In the first of the three preceding essays it was attempted to solve
the question whether the transformations of a given complex of
characters in a certain systematic group could be completely explained
by the sole aid of Darwinian principles. It was attempted to trace
the origin of the marking and colouring of the Sphinx-caterpillars to
individual variability, to the influences of the environment, and to
the laws of correlation acting within the organism. These principles
as applied to the origin of a certain well-defined, although narrowly
restricted range of forms, were tested in order to see whether they
were alone sufficient to explain the transformation of the forms.

It appeared that this was certainly the case. In all instances, or
at least where the facts necessary to obtain a complete insight were
available, the transformations could be traced to these known factors;
there remained no inexplicable residual phenomena, and we therefore had
no reason for inferring the existence of some still unknown modifying
cause lying concealed in the organism. In this region of the marking
and colouring of caterpillars, the assumption of a phyletic vital force
had to be abandoned, as being superfluous for the explanation of the
facts.

In the second essay the attempt was next made with reference to double
form-relationship, as presented for observation in metamorphic insects,
to draw conclusions as to the causes of the transformations. It
appeared here that form- and blood-relationship do not always coincide,
since the larvæ of a species, genus, or family, &c., may show quite
different form-relationships to their imagines. These facts alone told
very decisively against the existence of an internal developmental
power, so that the latter had likewise to be set aside by the method
of elimination, since the observed incongruences as well as the
congruences of form-relationship, found sufficient explanation in the
action of the environment on the organism.

This investigation thus also led to the denial of a phyletic vital
force.

In the third essay I finally sought to prove that the only case
of transformation of one species into another at present actually
observed[282], could not without further evidence be interpreted as
the result of the action of a phyletic vital force, but that more
probably we had here only an apparent case of new formation, which was
in reality but a reversion to a stage formerly in existence.

If this last investigation removes the only certain observation which
could have been adduced in favour of the hypothesis of a phyletic vital
force, so also do the two former essays show that this hypothesis, at
least in the case of insects, must be abandoned as inadequate.

The question now arises whether this conclusion, based on such a
limited range of inquiry, can also be applied to the other groups of
the organic world without further evidence.

The supporters of a principle of organic development will deny this in
each individual case, and will demand special proof for each group of
organisms; I believe this position, however, to be incorrect. Here,
if anywhere, it appears to me justifiable to apply the conclusions
inductively from special cases to general ones, since I cannot at all
see why a power of such pre-eminent and fundamental importance as a
phyletic vital force should have its activity limited to solitary
groups in the organic world. If such a power exists it must be the
inciting cause of organic development in general, and must be equally
necessary in every part of creation, as no advancement could take place
without it. In this case, however, the force would be recognizable and
demonstrable at every point; the phenomena should nowhere stand in
opposition to its admission, and should in no case be explicable or
comprehensible without it. The same laws and forces which caused the
development of one group of forms must underlie the development of the
whole organic world.

I therefore believe that we are correct in applying to the whole living
world the results furnished by the investigation of insects, and in
thus denying the existence of an innate metaphysical developmental
force.

There is, however, a quite distinct method which leads to the same
results, and to the preliminary, if not to the complete and definitive
rejection of such a principle; _the admission of this power is directly
opposed to the laws of natural science_, which forbid the assumption of
_unknown_ forces as long as it is not demonstrated that _known_ forces
are insufficient for the explanation of the phenomena. Now nobody will
assert that this has in any case been proved; the test of applying the
known factors of transformation has only just commenced, and wherever
it has been made they have proved sufficient as causal forces. Thus,
even without the foregoing special investigations we should deny a
phyletic vital force; the more so as its admission is fraught with
the greatest consequences, since it involves a renunciation of the
possibility of comprehending the organic world. We should, on this
assumption, at once cut ourselves off from all possible mechanical
explanation of organic nature, _i.e._ from all explanation conformable
to law. But this signifies no less than the renunciation of all further
inquiry; for what is investigation in natural science but the attempt
to indicate the mechanism through which the phenomena of the world
are brought about? Where this mechanism ceases science is no longer
possible, and transcendental philosophy alone has a voice.

This conception represents very precisely the well-known decision of
Kant:--“Since we cannot in any case know _à priori_ to what extent the
mechanism of Nature serves as a means to every final purpose in the
latter, or how far the mechanical explanation possible to us reaches,”
natural science must everywhere press the attempt at mechanical
explanation as far as possible. This obligation of natural science will
be conceded even by those who lay great stress upon the necessity for
assuming a designing principle. Thus, Karl Ernst von Baer states that
we have no right “to assert of the individual processes of Nature, even
when these evidently lead to a definite result, that some Mind has
originated them designedly. The naturalist must always commence with
details, and may then afterwards ask whether the totality of details
leads him to a general and final basis of intentional design.”[283]

But even if we are precluded on these grounds only from assuming
the existence of a directive power, _i.e._ a phyletic vital
force, for explaining detailed phenomena, and are at the same
time debarred from the possibility of arriving at a physical or
mechanical explanation--which amounts to no less than the abandoning
of the scientific position--it certainly cannot be asserted that
the development of the organic world is already conceived of as
a mechanical process. We rather acquiesce in the belief that the
processes both of organic and of inorganic nature depend most probably
upon purely causal powers, and that the attempt to refer these to
mechanical principles should not therefore be abandoned. There is no
ground for renouncing the possibility of a mechanical explanation, and
the naturalist _must not_ therefore resign this possibility; for this
reason he cannot be permitted to assume a phyletic power so long as it
is not demonstrated that the phenomena can never be understood without
such an assumption.

It cannot be raised as an objection that even for the explanation of
individual life a vital power was long ago admitted, as there was not
then sufficient material at hand to enable the phenomena of life to be
traced to physical forces. It is now no longer questionable that this
assumption was a useless error--a false method--at the time when made
certainly very excusable, since the aspect of the question was then,
owing to the imperfect basis of facts, very different to the present
analogous question as to the causes of derivative development. Thus,
although it is now easy to prove this assumption to be erroneous, it
was in the former sense correct, as it was in accordance with the
existing state of knowledge. At that time there was hardly one of the
numerous bridges which now connect inorganic with organic nature,
so that the supposition that life depended upon forces which had no
existence outside living beings was sufficiently near.

In any case the philosophers of that period cannot be blamed for
filling up the gaps in the existing knowledge by unknown powers,
and in this manner seeking to establish a finished system. The task
of philosophy is different to that of natural science; the former
strives at every period to set up a completely finished representation
of the universe in accordance with the existing state of knowledge.
Natural science on the other hand is only concerned in collecting
this knowledge; she need not therefore always finish off, and indeed
can never close her account, since she will never be in a position to
solve all problems.[284] But science must not for this reason pronounce
any question to be insoluble simply because it has not yet been
completely solved; this she does, however, as soon as she renounces
the possibility of a mechanical explanation by invoking the aid of a
metaphysical principle.

That this is the correct mode of scientific investigation is seen
by the abandoning of the (ontogenetic) vital force. The latter is
no longer admitted by anybody, now that we have turned from mere
speculation to the investigation of Nature’s processes; nevertheless
its non-existence has not been demonstrated, nor are we yet in a
position to prove that all the phenomena of life must be traced to
purely physico-chemical processes, to say nothing of our being actually
able to thus trace them. Von Baer also states “that the abolishment of
the vital force is an important advance; it is the reduction of the
phenomena of life to physico-chemical processes, although these indeed
still contain many gaps.” He points out how very far we are still
removed from being able to reduce to physical causes, the processes
through which the fertilized yelk of an egg becomes developed into a
chicken.

How comes it therefore that we all have a conviction that such a
complete reduction will in time become possible, or if not this, that
the development of the individual depends entirely upon the same forces
which are in operation without the organism? For what reason have we
rejected the “vital force”?

Simply because we see no reason for assuming that known forces are
insufficient for explaining the phenomena, and because we are not
justified in admitting directive forces as long as we have any hope of
one day furnishing a mechanical explanation.

But if it is not only permissible, but even necessary, to explain the
_ontogenetic_ vital power by known forces, and to commence to indicate
the mechanism which produces the individual life, why should it not
be equally necessary to abandon that assumption of a _phyletic_ vital
force which stifles any deeper inquiry, and to attempt to point out
that here also the co-operation of mechanical forces has brought about
the multitudinous and wonderful phenomena of the organic world?

The renunciation of the old vital force was certainly an immediate
consequence of the acquisition of new facts--of the knowledge that the
same compounds which compose organic bodies can be produced without the
latter. This discovery, due to Wöhler and his followers, showed that
organic products could be prepared artificially.[285] In brief, the
decline of the vital force followed from the knowledge that at least
one portion of the processes of life was governed by known forces.

But in the domain of the development of the organic world have we not
quite analogous proofs of the efficacy of known forces? Is not the
_variability_ of all types of forms a fact? and must not this under the
action of natural selection and heredity lead to permanent changes? Has
not the problem of explaining the subserviency of all organic form to
law as a _result_ without invoking its aid as a _principle_ been thus
successfully solved? It is true that we have not directly observed the
process of natural selection from beginning to end; neither has anybody
directly observed the mode in which the heat of the animal body is
generated by the processes of combustion going on in the blood and
in the tissues; nevertheless, this is believed as a certainty, and a
“vital force” is not invoked.

Now the above-mentioned Darwinian principles of transmutation are
certainly not simple forces of nature like those underlying the
development of the individual, _i.e._ chemico-physical forces, and it
cannot be said _à priori_ whether in one of these principles--perhaps
in variability or in correlation--there may not lie concealed a
metaphysical principle in addition to the physical forces. In fact it
has lately been asserted by Edward von Hartmann[286] that the theory
of selection is not a mechanical explanation, since it combines forces
which are only partly mechanical and in part directive.

It must therefore be next investigated whether this assertion is
tenable.



I.

ARE THE PRINCIPLES OF THE SELECTION THEORY MECHANICAL?


Edward von Hartmann may justly claim that his views should be
considered and tested by naturalists.[287] He would be correctly
classed with those philosophers who have approached this question with
a many-sided scientific preparation. It can nevertheless be perceived
in his case how difficult, and indeed how impossible, it is to estimate
the true value of the facts furnished by the investigation of nature,
when we attempt to take up only the results themselves, without being
practised in the methods by which these are reached, _i.e._ without
being completely at home in one of the scientific subjects concerned
through one’s own investigations. It appears to me that the denial of
the purely mechanical value of the Darwinian factors of transformation
arises in most part from an erroneous classification of the scientific
facts with which we have to deal. There can certainly be no mistake
that the entire philosophical conception of the universe, as laid down
by Von Hartmann in his “Philosophy of the Unconscious,” is unfavourable
to an unprejudiced estimate of scientific facts and to their mechanical
valuation.

Variability, heredity, and above all correlation, would not be regarded
by Von Hartmann as purely mechanical principles, but he would therein
assume a metaphysical directive principle.

In the first place, as regards variability, Von Hartmann endeavours
to show that it is only a quite unlimited variability which suffices
for the explanation of necessary and useful adaptations by means
of selection and the struggle for existence. But this does not
exist--variation rather takes place in a fixed direction only (in
Askenasy’s sense), and this can be nothing else than the expression of
an innate law of development, _i.e._ a phyletic vital force.

This deduction appears to me in two ways erroneous. In the first place
it is incorrect that a quite unlimited variability is a postulate
of the theory of selection, and in the next place the admission of
variability, which is in a certain sense “fixed in direction,” does not
necessitate the assumption of a phyletic vital force.

A mere unsettled variability, uniform in all possible directions, is,
according to Von Hartmann, necessary for the theory of selection,
because only then does the variability offer a certain guarantee “that
under given conditions of life the variations necessary for complete
adaptation will not be wanting.” But it is hereby overlooked that the
new life conditions to which the adaptation must take place are as
little fixed and unchangeable as the organism itself. In such a case of
transformation we have not to deal with a type of organization which
was before fixed and immutable, and which has to be squeezed into new
life-conditions as into a mould. The adaptation is not one-sided, but
mutual; a species in some measure selects its new conditions of life,
corresponding with those possible to its organization, _i.e._ with the
variations actually occurring. I will choose an instance which will
even be conceded by Von Hartmann as being only explicable by natural
selection, viz., a case of mimicry.

Supposing that among the South American _Heliconiidæ_ there occurred
a species of _Pieris_ which had no resemblance to these protected
butterflies, either in form, marking, or colouring; who can deny
that it would be most useful to this species to acquire the form and
colouring of a Heliconide, and thus, by taking to new conditions of
life, to avoid the persecutions of its foes? But if the physical
nature of the Pieride concerned precluded the occurrence of Heliconoid
variations, would this incapability of insinuating itself into these
new conditions necessitate the decline of the species? Could not its
existence be secured in some other manner? could not the destruction of
numerous individuals by foes be compensated for by increased fertility?
to say nothing of the numerous other means through which the number
of surviving individuals might become increased, and the existence
of the species secured. This case is not arbitrarily chosen; in the
districts where the _Heliconiidæ_ occur there are actually a large
number of Whites which do not possess the protective colours of the
former nauseous family. In the adoption of these new life conditions
we have not to deal therefore with survival or extermination, but only
with amelioration. It is not every species of “White” that can become
adapted to these conditions, because every species does not give rise
to the necessary colour variations; those that do, become in this way
modified, because they are thus better protected than before. And so
it is throughout; wherever we find protected insects enjoying immunity
from foes we see also mimickers, sometimes only single, sometimes
several, and generally from very diverse groups of insects, according
to the general resemblance which existed before the commencement of
the process of adaptation, and to the variations made possible by the
physical nature of the species concerned.

In the first essay of the second part of this work it was shown that in
certain Lepidopterous larvæ a process of adaptation is at the present
time still in progress, this depending upon the fact that while the
young caterpillar is very well protected by the leaf-green colour of
its body, this colour becomes insufficient to conceal the insect as
soon as it exceeds the leaf in size. All such caterpillars--and there
is a whole series of species--as they increase in size acquire the
habit of concealing themselves on the earth by day, and of feeding
only at night. New conditions of life are thus imposed, and these
are even compulsory, _i.e._ they could not be abandoned without
risking the existence of the species. Now in accordance with these
new conditions, some individuals in these species have lost the green
colouring of the young stages, and have acquired the brown coloration
of the dark surroundings of the insects which conceal themselves
by day. In one species this change has now occurred in almost all
individuals, in others in only a larger or smaller proportion of
them. Now supposing that among these species there occurred one, the
physical nature of which did not admit of the production of brown
shades of colour, would the species for this reason succumb? Is it not
conceivable that the want of colour adaptation might be compensated
for by better concealment, _i.e._ by burrowing into the earth, or by
a greater fertility of the species, or by the development of warning
signals--supposing the species to be unpalatable--or finally, by the
acquisition of a terrifying marking? In other words, could not the
caterpillar itself modify the new condition of life--that of being
concealed by day--in accordance with variations made possible by its
physical nature?

As a matter of fact in one of these species the green colour remains
unchanged in spite of the altered mode of life, and this species,
wherever it occurs, notwithstanding the persecution of entomologists,
is always common (_Deilephila Hippophaës_); it conceals itself better
and deeper however than those other species which, like _Sphinx
Convolvuli_, are difficult to detect on account of their brown colour.
In another species the striking yellowish green colouring is likewise
retained in the majority of individuals, but this species buries
itself by day in the loose soil (_Acherontia Atropos_).

To this it may be objected that there are also compulsory changes in
the conditions of life from which the species cannot withdraw itself,
but in which adaptation must necessarily follow, or extermination would
take place.

Such compulsory conditions of life do most assuredly occur, and there
is indeed no doubt that many living forms have perished through not
becoming transformed. I believe, however, that such conditions occur
much more rarely than one is inclined to admit at first sight. As
a rule the alternative of immediate change or of extermination is
offered only by such changes in the conditions of life as occur very
rapidly. The sudden appearance of a new and dominant enemy, such as
man, has already caused the extinction of the Dodo (_Didus ineptus_),
and of Steller’s Sea Cow (_Rhytina Stelleri_), and of other vertebrate
animals, and constantly leads to the extermination of many other
species of different classes. When in America hundreds of thousands of
acres of primeval forest are annually destroyed, the conditions of life
of a numerous fauna and flora must be thereby suddenly changed, leaving
no choice but extermination.

Such abrupt changes in the conditions of life occur, however, but
seldom in nature unless caused by man, and must therefore have very
rarely happened in former epochs of the earth’s history. Even climatic
changes, which we might at first regard as of this character, and
which produce a modification in one fixed direction, occur always so
gradually that the species has time either to adapt itself to the
conditions in this or that direction, according to the variations
possible to its physical nature, or else to emigrate.

It thus appears to me erroneous to suppose that variability must be
“merely undetermined” in order to complete its part in Darwin’s theory
of selection, and its “illimitedness” seems to me also as little
necessary for this purpose. Von Hartmann imagines that it is only
unlimited variability that furnishes a guarantee that any type, to
whatever extent diverging from its point of departure, will be reached
by the Darwinian method of gradual transmutation by means of selection
and the struggle for existence.

But who has ever asserted that _any_ type can be reached from any
point? Or if anybody has said such nonsense, who can prove that its
admission is necessary for the theory of selection? Nowhere in systemy
do we see any point of support for such an assumption. But when Von
Hartmann imagines that the “unlimited” variability which he postulates
for Darwin “is in itself unlimited, the limits of its divergence in
a given direction being found, not in itself, but only in external
obstacles,” he conceives variability to be something independent of,
and in some way added to, the animal body, and not a mere expression
for the fluctuations in the type of the organism. If, however, we
conceive variability in this latter, the true scientific sense, it
is in no way “quantitatively unlimited,” nor are its limits even
determined by _external_ influences, but essentially by _internal_
influences, _i.e._ by the underlying physical nature of the organism.
Darwin has indeed already shown this in a most beautiful manner in his
investigations upon the correlations of organs and systems of organs
of the body. To make use of a metaphor, the forces acting within the
body are in equilibrium; if one organ becomes changed this causes a
disturbance in the forces, and the equilibrium must be restored by
changes in other parts, and these again entail other modifications,
and so forth. Herein lies the reason why the primary change cannot
exceed a certain amount if the restoration of the equilibrium is
not to be quite impossible. This is but a metaphor, and I do not
wish to assert that we are at present in a position to formulate and
demonstrate mathematically for any particular case, how much an organ
can become changed in any one species before an interruption of the
internal harmony of the body takes place. But such impossibility of
demonstration does not appear to me to furnish a sufficient reason
for regarding variability as the expression of a directive power--as
an “innate tendency to variation conformable to law.”[288] On the
contrary, it is to me easily conceivable that we only learn to analyse
the processes of nature in detail very slowly, because of their
necessary complexity. It thus appears to me quite useless when in this
sense Wigand makes use of the objection, that “the gooseberry has not
undergone any enlargement since 1852, although it is inconceivable
why it should not attain the size of a pumpkin if variability was
not internally limited.” It may well be that this is for the present
“inconceivable;” nevertheless, this does not justify us in setting
up a hypothetical “force of variation” which will not admit of the
gooseberry surpassing the pumpkin in size. We are bound to maintain
that it is the action and reaction of known forces which sets a limit
to the enlargement of this fruit.

In more simple instances the causes of such limitations to growth can
be well perceived. Several decades have passed since Leuckart proved
in how exact a relation the proportion of volume and surface stood to
the degree of organization of an animal. In animals of a spherical
form the surface is quite sufficient for respiration, so long as they
are of microscopic size. But such an organism cannot become enlarged
at pleasure, because the ratio of the surface to the volume would
become quite different. The surface increases as the square, whilst
the volume increases as the cube, so that very soon the surface of
the more rapidly increasing bodily mass can no longer suffice for
respiration.[289] This sort of limitation is in no way equivalent to
that purely external kind which, for instance, manifests itself in such
a manner as to prevent the indefinite lengthening of the tail feathers
of the Bird of Paradise. In this case feathers that were too long would
hinder flight, and such individuals would accordingly be eliminated by
natural selection. The cause is in the former case purely internal,
depending upon the equilibrium of the forces governing the organism.

Von Hartmann is entirely in the right when he asserts that variability
is neither qualitatively nor quantitatively unlimited. In both
senses it is limited (in direction as well as in amount) by the
physico-chemical forces acting in some contrary way in each specific
organism--by the physical nature of each living form. He errs,
however, both in making absolute illimitability a necessary postulate
of the theory of selection, as also in inferring the existence of
a directive principle from that limitation of variability which is
certainly present. “Tendencies to variation” do however exist, not in
the sense of a directive power, but as expressions of the different
physical constitutions of species, which necessarily cause unequal
reactions to the same external actions, as will be more clearly proved
below.[290]

This is, of course, a modification of Darwin’s original assumption of
an unbounded variability not limited in direction; but Darwin himself
has later coincided in the view that the quality of the variations is
essentially determined by the nature of the organism.[291]

I now turn to the consideration of the second factor of the theory of
selection--heredity. This also, according to Von Hartmann is not a
mechanical principle. Darwin himself has now become convinced how great
is the probability against the hereditary retention of modifications
which, whether feebly or strongly pronounced, appear _only in single
individuals_, _i.e._ of those so-called “fortuitous” variations which
are not the expression of a directive developmental principle. “But as
among the numberless possible directions of an indefinite variability,
useful modifications can only occur in single cases, Darwin has by this
supplementary admission himself retracted an inadmissible assumption of
his theory of selection,” and so forth. A “regular, designed tendency
to variation, acting from within and contemporaneously affecting a
large number of individuals,” must therefore be assumed “in order to
insure the by itself improbable inheritance.”

But even from the unbounded variability laid down by the author, it
by no means follows that useful variations can only occur in single
individuals. In the whole category of _quantitative_ variations the
reverse is always the case. Is it the lengthening of some part that
is concerned; so would a large number of individuals always possess
the useful variation, since we are not dealing with an _absolute_
enlargement, but only with the fact that the part concerned is longer
than in other individuals.[292]

But if _qualitative_ variations come into consideration, it may be
asked whether Darwin’s “supplementary admission” does not go too
far. Such calculations as those quoted by Darwin from the article in
the _North British Review_ of March 1867 are extremely deceptive,
since we have no means of measuring the amount of protection afforded
by a useful variation, and we can therefore hardly compute with
any certainty, in how great a percentage of individuals a change
must contemporaneously occur in order to have a chance of becoming
transferred to the following generation. If our blue rock-pigeon could
exist in a polar climate, and if we had the power of introducing it
gradually, but not suddenly, into these regions in a wild state, who
can doubt that it would assume the white colour of all polar animals?
Nevertheless, among wild rock-pigeons white varieties do not occur
more frequently than among swallows, crows, or magpies. Or must the
white colour of polar animals, the yellow colour of desert species, and
the green colour of leaf-frequenting forms, be always referred to a
“regular, designed, fixed tendency to variation acting from within,”
and causing a “large number of individuals” to vary in a similar manner?

There is, however, a grain of truth in the foregoing; variations which
occur singly have but little chance of becoming predominant characters,
and this is obviously what Darwin concedes. But this is by no means
equivalent to the assumption that only those variations which from the
first occur in numerous individuals have a chance of being perpetuated.
Let us keep to the facts. We have not the slightest reason either
for regarding the white colour of polar animals as the direct action
of cold, or for considering that the green colour of foliage-living
caterpillars depends upon direct action arising from the habit of
resting upon the leaves;[293] both these characters are explicable only
by natural selection, and there is nothing to favour the assumption
(which Von Hartmann postulates as necessary for success) that many
individuals varied into white at the same time. We know no single
extra-polar species of a dark colour which frequently, _i.e._ in many
individuals of every generation, varies into white, but we know many
species which from time to time produce single white individuals. Now
when, on the other hand, we find that all polar animals to which the
white coloration is advantageous, and indeed none but species of which
the nearest allies vary only individually into white, possess this
colour, must we not conclude from this alone that _single_ variations
can, under favourable conditions, become predominant characters?

It appears to me that in this question one weighty factor has been too
little regarded, even by the supporters of the selection theory, viz.,
the slowness of most, and especially of climatic changes, which I have
already insisted upon. If the transformation of a temperate into an
arctic climate occurred so rapidly that the species exposed to it had
the alternative either of becoming white in ten or twenty generations
or of being unable to exist, then the hasty intervention of a directive
power could alone save them from extermination by causing hundreds of
thousands of individuals to become similarly coloured with all speed.
But it is quite different if the change of climate takes place only in
the course of several thousand generations; and this, according to the
geological evidence, must have been the true state of the case.

Let us take a definite example--the well-known one of the hare. With
us this animal remains brown in the winter and but seldom produces
white varieties, whilst its ally the Alpine hare is white during
seven months of the year, the Norwegian hare during nine months, and
the Greenland hare throughout the whole year. If our climate became
transformed into an arctic one, after a given time there would arrive
a period when the older coloration no longer possessed any advantage
over the occasional and singly-appearing white variations; the winter
days during which the ground was covered with snow would have become so
numerous, that the protection afforded to the white animals would be
equal to the protection enjoyed by the brown individuals on the equally
numerous days free from snow. From this time forth the hares that
were white in winter would not be subjected to a greater decimation
by foxes, &c., than the brown individuals. This period must however
be represented as consisting of one or more centuries, and it would
be strange if from the individual white hares, which now had an equal
chance of existing, some white families did not become established.
But the state of affairs would gradually become reversed--the brown
hares would experience greater decimation, and wherever there were
white families these would possess an advantage in the struggle for
existence. It does not follow that the dark individuals would be
forthwith extirpated; on the contrary, the advantage in favour of the
white would be but small throughout a long period of time, and these
individuals would only gradually increase to a higher percentage of
the total population; nevertheless their numbers would constantly but
very slowly augment. In the course of time this increase would become
more rapid for two reasons--first, because even a very small advantage
in favour of the increasing number of individuals would always leave a
greater number of these victorious; and secondly, because on the whole
as the climate became more arctic, the advantage of being white would
continually become more decisive in determining which should live and
which should succumb.

Thus I see no reason why individual variations which do not appear only
once, but which frequently recur in the course of generations, should
not acquire predominance under favourable conditions. All facts are
in accord with this. Even the common hare shows us that it would be
quite capable of becoming coloured in a similar manner. In the museum
of Stuttgart there are three specimens of _Lepus timidus_, killed in
Wurtemburg, which are completely white, and several others which are
silver-grey or spotted with white. In eastern Russia the common hare
possesses a light grey, almost white, winter coat, and Seidlitz[294]
makes known the interesting observation that such light specimens occur
_singly_ in Livonia, where “the common hare has become naturalized
since the commencement of the century.”

As I have already insisted upon above, from the point of view
of the conditions of life there is no reason for assuming rapid
transformations; the change of conditions is almost always extremely
slow; and indeed in numerous instances no objective change occurs,
but simply a subjective one, if we may thus designate those cases in
which the alteration in the conditions of life depends upon a change
in the animal form which is undergoing transformation, and not in that
of the environment. This is the case in the above-mentioned instances
of mimicry, where the whole change in the conditions of life arises
from one species becoming similar to another. The process of natural
selection has here as long a period of time as it requires to perfect
its results. It is quite similar in all cases of special protective
adaptations of form and colour. In all these it is always _improvement_
that is concerned, and not the question “to be or not to be” with which
we have to deal.

It is just cases of this last kind, however, which are best fitted for
exposing the improbability and insufficiency of the assumption of a
variational tendency as a distinct directive power. We have only to
fix our attention upon some particular case of sympathetic colouring,
or, still better, of mimicry. A “tendency to variation” implies that a
large number of individuals produce varieties resembling the model to
be imitated, and this--at least according to Von Hartmann--must take
place in each of the successive generations, so that by this means,
combined with heredity, the useful variation becomes increased. But how
comes it that this “tendency to variation” coincides with the existence
of the model both in time and place? Can this be due to accident if
the two have not a common cause? The upholders of a directive power
will certainly not admit this; so that there remains only Leibnitz’s
assumption of a pre-established harmony contained in the first organic
germ, which, after innumerable transformations of the organic form
and after millions of years, gave rise in the midst of the Amazonian
region to an inedible Heliconide with certain yellow, black, and white
markings on the wings, and at precisely the same time developed the
tendency in a Pieride at the same spot on the globe to imitate this
Heliconide as a model!

In addition to this assumption, which is certainly but little worthy
of consideration, there is perhaps one other remaining, viz., that all
or many Pierides and other species of butterflies possessed the same
tendency to a Heliconoid variation and were always everywhere striving
to develop this type, but succeeded only where they accidentally
coincided in time and place with the model, the “tendency” being thus
furthered by natural selection. But the facts negative this assumption,
since such imitative variations have never been observed to a
perceptible extent in other species.[295]

All variations which are demonstrably useful can be similarly dealt
with if their origin is explained by variational tendencies.

We perceive that the objection which Von Hartmann brings against
heredity is only valid on the ground that this process affords no
security for the preservation of variations which occur singly. That
heredity itself is a mechanical process is not directly disputed; it is
simply assumed that new characters can be transferred by inheritance
only when they are produced by the metaphysical “developmental
principle,” and not when they arise “accidentally.” This critic does
not therefore direct his attack against heredity, but rather against
the mechanical origin of variability.

Von Hartmann might have said here that a reference of the phenomenon
of heredity to purely mechanical causes, _i.e._ a mechanical theory
of heredity, is up to the present time wanting. That he has not done
so proves on the one hand that he despised the dialectical art, but,
on the other hand, that he himself has not overlooked the subserviency
of the total phenomenon to law, and that he grants the possibility of
finding a mechanical explanation therefor. If, in fact, the power of
inheritance does not depend upon mechanical principles, I know not
what organic processes we are entitled to regard as mechanical, since
they are all dependent in essence upon heredity, with which process
they are at one, and from which they cannot be thought of as isolated.
Haeckel correctly designates reproduction as surplus individual
growth, and accordingly refers the phenomena of heredity to those of
growth. Conversely, growth may also be designated reproduction, since
it depends upon a continuous process of multiplication of the cells
composing the organism, from the germ-cell to the innumerable congeries
of variously differentiated cells of the highly developed animal body.
Who can fail to see that these two processes, the reproduction of the
germ-cell and its offspring in the economy of the individual, and the
reproduction of individuals and species in the economy of the organic
world, show an exact and by no means simply superficial analogy?[296]
But whoso grants this must also conceive both processes to depend upon
the same cause--he cannot assume for the one a causal power and for
the other a directive principle. If nutrition and cell-multiplication
are purely mechanical processes, so also is heredity. Although it
has not yet been possible to demonstrate the mechanism of this
phenomenon, it can nevertheless be seen broadly that by means of a
minimum of living organic matter (_e.g._ the protoplasm of the sperm
and germ-cell) certain motions are transferred, and these can be
regarded as directions of development, as I have already briefly laid
down in a former work.[297] The power of organisms to transmit their
properties to their offspring appears to me to be only conceivable in
such a manner “that the germ of the organism by its chemico-physical
composition together with its molecular structure, has communicated to
it a fixed direction of development--the same direction of development
as that originally possessed by the parental organism....” (_loc.
cit._ p. 24). This is confessedly nothing more than a hint, and we do
not learn therefrom the means by which developmental direction can be
possibly transferred to another organism.

Recently Haeckel, that indefatigable pioneer to whom we are indebted
for such a rich store of new ideas, has attempted to bridge over this
gap in his essay on “The Perigenesis of the Plastidule,” Berlin, 1876.
The basic idea, that heredity depends upon the transference of motion,
and variability upon a change of this motion, completely corresponds
with the conviction gained in the province of physical science, that
“all laws must finally be merged in laws of motion” (Helmholtz[298]).
I hold this view to be the more completely justifiable--although
certainly not in the remotest degree as proved--because I formerly
designated the acquired individual variations as the “diversion of the
inherited direction of development.” Haeckel’s hypothesis in so far
accomplishes more than Darwin’s pangenesis, in which a transference
of matter, and not of a species of motion peculiar to this matter, is
assumed. But although the germ of a mechanical theory of heredity may
be contained in Haeckel’s hypothesis, this nevertheless appears to me
to be somewhat remote from completely solving the problem. It brings
well into prominence one portion of the process of inheritance; under
the image of a molecular motion of the plastidule, which motion is
modifiable by external influences, we can well understand the fact of
a change gradually taking place in the course of generations. On the
other hand, the assumption of consciousness in the plastidule,--however
admissible philosophically--although only as a formula, scarcely
furnishes any deeper knowledge. In the light of a theory, detailed
instances which were formerly obscure should become comprehensible.
I fail to see, however, how the various forms of atavism, _e.g._ the
reversions which so commonly occur by crossing different races, become
more comprehensible by assuming consciousness in the plastidule. If
in both parents the plastidule long ago acquired different molecular
motions, why, in its rencounters in the germ, does it recollect past
times and reassume the older and long abandoned motion? That it does
acquire the latter is indeed a fact if we once refer the directional
development of the individual to molecular motion of the plastidule;
the wherefore does not appear to me, however, to become clearer by
assuming consciousness in the plastidule. A mechanical theory of
heredity must rather be able to show that the plastidule movements of
the male and female germ-cells, in their rencounter in the case of the
crossing of widely divergent forms, become mutually modified in such
a manner that the motion of the common ancestral form must occur as
the resultant. To such demonstration there is however as yet a long
step. Haeckel himself moreover points out that his hypothesis is by
no means a “mechanical theory of heredity,” but only an introduction
to this theory, which he hopes “will be capable of being elevated to
the rank of a genetic molecular theory” (_loc. cit._ p. 17). But
although we must also confess with the critic of the “Philosophy of the
Unconscious,” that “the facts of heredity have hitherto defied every
scientific explanation,”[299] this furnishes us with no excuse for
flying to a metaphysical explanation, “which is here certainly least
able to satisfy the inability to understand the connection arising from
natural laws.”

It is not to be wondered at that Von Hartmann, on the ground of the
“Unconscious” on which he takes his stand, speaks of the law of
correlation as an unconscious acknowledgment of a “non-mechanical
universal principle on the side of Darwinism.” By “correlation” he
understands something quite different to the idea which we attach to
this expression. He supposes that “Darwinism sees itself compelled
to acknowledge through empirical facts the uniform correlation of
characters pertaining to the specific type; but it thereby contradicts
its mechanical principles of explanation, all of which amount to the
same thing as conceiving the type as a mosaic, chequered, superficial,
and accidental aggregate of characters, which have been singly
acquired, contemporaneously or successively, by selection or habit.”
I do not believe, however, that any such conception has ever been
admitted either by Darwin or any one else. The admission that not all,
but only every deep-seated _physiological_ detailed modification, is
or may be bound up with a system of correlated changes, indeed implies
that we on our side also acknowledge an internal harmony of parts--an
equilibrium, as I have above expressed it.

But does this include the admission of a teleological principle, or
exclude a mechanical explanation? Do we thereby acknowledge a “specific
type” in the sense of an inseparably connected complex of characters,
none of which can be taken away without all the others becoming
modified? Does such a view agree generally with the empirical facts?

Neither of these views appears to me to represent the case.

I will first answer the second question. On all possible sides the
earlier view of the absolute nature of species is contradicted; there
is no boundary between species and varieties. But when Von Hartmann
assumes that by the transformation of one species “into another” the
“whole uniformly connected complex must become changed,” he falls
back into the old doctrine of the absolute nature of species, which
is sharply contradicted by multitudes of facts. We not unfrequently
observe varieties which differ from the parent-form by only a single
character, whilst others show numerous differences, and again others
may be seen in which the differences predominate. This last deviation
would then be designated by many systematists as a new species, but not
so by others.

The “specific type” is thus indeed a kind of mosaic-work, but it is
a structure to which all the single characters--the stones of the
mosaic--belong and build up one harmonious whole, and not a meaningless
confusion. Some of the stones or groups of stones can be taken away
and replaced by others differently coloured without the structure
being thereby necessarily distorted, _i.e._ destroyed as a structure;
but the larger the stones which are exchanged the more necessary will
corrections in the other parts of the structure become, in order that
the harmony of the whole may be preserved.

Still more weighty than those insensible transitions which in various
groups of animals so frequently connect species with species, appear
to me, however, the facts made known in the second essay of the
second part of this volume, which prove that the two forms in which
one species appears can change entirely independently of one another.
The caterpillar changes and becomes a new variety or even species
(according to the form-value of the change), whilst the butterfly
remains unaltered. How could this occur if some other law than that
of physiological equilibrium linked together the parts or characters
and permitted them to become severed? Must not the two stages become
changed with and through one another, like the parts of one body, since
they first together constitute the specific type? Is not the fact of
this not happening a proof that the whole “uniformly connected complex”
of the specific type is not bound and held together by a metaphysical
principle, but simply by natural laws?

Now when Von Hartmann comprises the relations of different species
to one another under the idea of correlation, such for instance as
the relation of dependence in which orchidaceous flowers stand with
respect to the insects which visit them, he completely abandons the
scientific conception which should be associated with this expression,
and compares together two heterogeneous things which have nothing in
common excepting that they are both considered by him as a result
of the “Unconscious.” The consequence which is then deduced from
this correlation of his own construction, viz., that an organic law
of correlation is only another expression for a “law of organic
development” in the sense of a metaphysical power, obviously cannot be
admitted.

By correlation we understand nothing more than the dependence of one
part of the organism upon the others and the mutual inter-relations
of these parts, which depend entirely upon a “physiological relation
of dependence,” as Von Hartmann himself has correctly designated it.
Herein is evidently comprised the total morphology of the organism--the
structure as a whole, the length, thickness and weight of the single
parts, as well as the histological structure of the tissues, since upon
all these depends the performance of the single parts. But when, under
correlation, Von Hartmann comprises “also a morphological, systematic,
inter-action of all the elements of the organism with reference both
to the typical ground-plan of the organization as well as to the
microscopic anatomical structure of the tissues,” he drags into the
idea something foreign to it, not on the ground of facts, but actually
in opposition to them, and supported only by a supposed “innate
developmental principle” which “is not of a mechanical nature.”

The living organism has already been often compared with a crystal, and
the comparison is, _mutatis mutandis_, justifiable. As in the growing
crystal the single molecules cannot become joined together at pleasure,
but only in a fixed manner, so are the parts of an organism governed
in their respective distribution. In the crystal where nothing but
homogeneous parts become grouped together their resulting combination
is likewise homogeneous, and it is obvious that they offer but very
little possibility of modification, so that the governing laws thus
appear restricted and immutable. In the organism, whether regarded
microscopically or macroscopically, various parts become combined, and
these therefore offer numerous possibilities of modification, so that
the governing laws are more complex, and appear less restricted and
unchangeable. In neither instance do we know the final causes which
always lead to a given state of equilibrium; in the case of a crystal
it has not occurred to anybody to ascribe the harmonious disposition
of the parts to a teleological power; why then should we assume such
a force in the organism, and thus discontinue the attempt, which has
already been commenced, to refer to its natural causes that harmony of
parts which is here certainly present and equally conformable to law?

On these grounds the assertion that the theory of selection is not an
attempt at a “mechanical” explanation of organic development appears to
me to be incorrect. Variability and heredity, as well as correlation,
admit of being conceived as purely mechanical, and must be thus
regarded so long as no more cogent reasons can be adduced for believing
that some force other than physico-chemical lies concealed therein.

But we certainly cannot remain at the purely empirical conception as
laid down by Darwin in his admirable work on the “Origin of Species.”
If the theory of selection is to furnish a method of mechanical
explanation, it is essential that its factors should be formulated in a
precise mechanical sense. But as soon as we attempt to do this it is
seen that, in the first enthusiasm over the newly discovered principle
of selection, the one factor of transformation contained in this
principle itself has been unduly pushed into the background, to make
way for the other more apparent and better known factors.

I have for many years insisted that the first, and perhaps most
important, or in any case the most indispensable, factor in every
transformation, is _the physical nature of the organism itself_.[300]

It would be an error to believe that it is entirely the external
conditions which determine what changes shall appear in a given
species; the nature of these changes depends essentially upon the
physical constitution of the species itself, and a modification
actually arising can obviously be only regarded as the resultant of
this constitution and of the external influences acting thereon.

But if an essential or perhaps even a preponderating share in
determining new characters is to be undoubtedly ascribed to the
organism itself, for a mechanical representation of organic
developmental processes everything depends upon our being able
to conceive this most important factor in a definite theoretical
manner, and to comprise under one common point of view its apparently
contradictory manifestations of constancy and variability.

Now every change of considerable extent is certainly considered by
Darwin to be the direct or indirect consequence of external actions;
but indirect action always presupposes a certain small variability
(individual variability), without which larger modifications cannot
be brought about. Empirically this small amount of variability is
doubtless present, but the question arises, upon what does it depend?
Can it be conceived as arising mechanically, or is it perhaps just at
this point that the metaphysical principle steps in and offers those
minute variations which make possible that course of development which,
according to this view, is immutably pre-determined? It is certainly
the absence of a theoretical definition of variability which always
leaves open a door for smuggling in a teleological power. A mechanical
explanation of variability must form the basis of this side of the
theory of selection.

This explanation is not difficult to find. All dissimilarities of
organisms must depend upon the individuals having been affected by
dissimilar external influences during the course of the development of
organic nature. If we ascribe to the organism the power of giving rise
by multiplication only to exact copies of itself, or, more correctly,
the power of transmitting unaltered to its successors the motion of its
own course of development, each “individual variation” must depend upon
the power of the organism to react upon external influences, _i.e._ to
respond by changes of form and of function, and consequently to modify
its original (inherited) developmental direction.

It has sometimes been insisted upon, that the “individuals of the same
species” or the offspring of one mother cannot be absolutely equal,
because, from the commencement of their existence, they have been
subjected to dissimilar actions of the environment. But this implies
that by perfectly equal influences they would become equal, _i.e._
it supposes that variability is not inseparably bound up with the
essence of the organism, but is only the consequence of developmental
tendencies which are in themselves equal being unequally influenced. As
a matter of fact the first germs of an individual certainly cannot be
supposed to be perfectly equal, because the individual differences of
the ancestors must be contained therein in different degrees according
to their constitution, and we should have to go back to the primordial
organism of the earth in order to find a perfectly homogeneous root,
a _tabula rasa_ from which the descendants would commence their
development. Whether such a homogeneous root ever existed is however
doubtful; it is much more probable that _numerous_ organisms first
arose spontaneously,[301] and these cannot be presumed to have been
absolutely equal, since the conditions under which they came into
life cannot have been perfectly identical. Let us, however, for the
sake of simplicity assume a single primordial organism; the first
generation which took its rise from this by reproduction could only
have possessed such individual differences as were produced by the
action of dissimilar external influences. But the third generation,
together with self-acquired, would also have shown _inherited_,
dissimilarities, and in each succeeding generation the number of
tendencies to individual difference imparted to the germ by heredity
must have increased to a certain degree, so that it may be said that
all germs, from their first origination, bear in themselves a tendency
to show individual peculiarities, and would develop these even if
they should not be again affected by dissimilar influences. This is
obviously the case, since the youngest egg-cells in the ovary of an
animal are, as can be demonstrated, always exposed to unequal external
conditions with respect to nutrition and pressure.[302] Hence, if it
were possible that two germs were exactly equal with respect to the
direction of development imparted to them by heredity, they would
nevertheless furnish two incongruent individuals; and if, conversely,
it were possible that two individuals could be exposed to absolutely
the same external influences from the formation of the embryo, these
also could not be identical, because the individual differences of the
ancestors would entail small differences, even in asexual reproduction,
in the direction of development transmitted to the egg. The differences
between individuals of similar origin thus finally depend entirely upon
the dissimilarity of external influences--on the one side upon those
which divert the development of the progenitors, and on the other side
upon those which divert the individual itself from its course, _i.e._
from the developmental direction transmitted hereditarily. Although
I thus essentially agree with Darwin and Haeckel in so far as these
authors refer the “universal individual dissimilarity” to dissimilar
external actions, I differ from Darwin in this, that I do not see an
essential distinction between the direct and indirect production of
individual differences, if by the latter is meant only the unequal
influencing of the germ in the parental organism. Haeckel is certainly
correct in referring the “primitive differences of the germs produced
by the parents” to the inequalities of nutrition to which the single
germs must inevitably have been exposed in the parent organism;
but another dissimilarity of the germs must evidently be added--a
dissimilarity which has nothing to do with unequal nutrition, but
which depends upon unequal inheritance of the individual differences
of the ancestors, a source of dissimilarity which must arise to a
greater extent in sexual than in asexual reproduction. Just as in
sexual propagation there occurs a blending of the characters (or
more precisely, developmental directions) of two _contemporaneous
individuals_ in _one_ germ, so in every mode of reproduction there meet
together in the same germ the characters of a whole _succession of
individuals_ (the ancestral series), of which the most remote certainly
make themselves but seldom felt in a marked degree.

The fact of individual variability can in this way be well understood;
the living organism contains in itself no principle of variability--it
is the _statical element_ in the developmental processes of the
organic world, and would always reproduce exact copies of itself if the
inequality of the external influences did not affect the developmental
course of each new individual; these influences are therefore the
_dynamical elements_ of the process.

From this conception of variability two important empirically
established facts can be theoretically deduced, viz. the limitability
of variation with respect to quality, which has already been previously
mentioned, and the origination of transformations by the direct action
of external conditions of life.

If the differences in individuals of the same origin depend upon
the action of unequal influences, variation itself is nothing else
than the reaction of the organism to a definite external inciting
cause, the quality of the variation being determined by the quality
of the inciting cause and by that of the organism. In the cases of
individual variation hitherto considered, the quality of the organism
is equal but that of the inciting cause is unequal, and in this way
there arise minute differences in organisms of an equal physical
constitution--variations of a different quality.

The same result, viz., different qualities of variation, may also arise
in a reverse manner by organisms of a different physical nature being
affected by equal external influences. The response of the organism to
the cause inciting change would be different according to its nature,
or, in other words, organisms of different natures react differently
when affected by equal modifying influences. The physical nature of
the organism plays the chief part with respect to the quality of the
variations; each specific organism can thus give rise to extremely
numerous, but not to all conceivable, variations; that is, only to
such variations as are made possible by its physical composition.
From this it follows further that the possibilities of variation in
two species are more widely different, the wider they diverge in
physical constitution (including bodily morphology)--that a cycle of
variation is peculiar to every species. In this manner we are led to
the knowledge that there must certainly exist a “fixed direction of
variation,” but not in the sense of Askenasy and Von Hartmann, as the
result of an unknown internal principle of development, but as the
necessary, _i.e._ mechanical, consequence of the unequal physical
nature of the species, which must respond even to the same inciting
cause by unequal variations.

The facts, as far as we know them, agree very well with this
conclusion. Allied species vary in a similar manner, whilst species
which are more distantly related vary in a different manner, even
when acted upon by the same external influences. Thus, in the first
part of these “Studies” I have remarked that many butterflies under
the influence of a warm climate acquire an almost black coloration
(_Polyommatus Phlæas_), whilst on the other hand others become lighter
(_Papilio Podalirius_).

We can thus understand why always certain courses of development are
followed, a fact which cannot be completely explained by the nature of
the conditions of life which induce the variations. But as soon as we
clearly perceive that the quality of the changes essentially depends
upon the physical nature of the organism itself, we arrive at the
conclusion that species of widely diverging constitutions must give
rise to different variations, whilst those of allied constitutions
would produce similar variations. But definite courses of development
are thus traced out, and we perceive that from any point of the organic
developmental series, it is impossible that any other point can be
attained at pleasure. Variation in a definite direction thus by no
means necessitates the acknowledgment of a metaphysical developmental
principle, but can be well conceived as the mechanical result of the
physical constitution of the organism.

The manner in which the dissimilar physical constitution of organisms
must arise can also be easily shown, although the first commencement of
the whole developmental series, _i.e._ the oldest living forms must be
assumed to have been almost homogeneous in their physical constitution.
The quality of the variation is, as said before, not merely the
product of the physical constitution, but the resultant of this and of
the quality of the changing external conditions. Thus from the first
“species” there proceeded, through the dissimilar influence of external
conditions of life, several new “species,” and as this took place the
former physical nature of the organism at the same time became changed,
necessitating also a new mode of reacting upon external influences,
_i.e._ another direction of variation. The difference from the primary
“species” must certainly be conceived as having been very minute, but
it must have increased with each new transformation, and must have
proceeded exactly parallel with the degree of physical change connected
with each transformation. Thus, hand in hand with the modifications,
the power of modification, or mode of reaction of the organism to
changing influences, must have continually become re-modified, and we
finally obtain an endless number of differently constituted living
forms, of which the variational tendencies are different in exact
proportion to their physical divergence, so that nearly allied forms
respond similarly, and widely divergent forms very differently, to the
same inciting causes.

Individual variation arises, as I have attempted to show, by each
individual having been continually affected by different, and indeed
by constantly changing, influences. Let us, however, imagine on the
contrary, that a large group of individuals is affected by the same
influences--in fact by such influences as the remaining individuals of
the species are not exposed to: this group of individuals would then
vary in a nearly similar manner, since both factors of variation, viz.
the external influence and the physical constitution, are equal or
nearly so. Such local variations would first become prominent when the
same external influence had acted upon a series of generations, and
the minima of variation produced in the individual by the once-exerted
action of the cause inciting change had become augmented by heredity.
Transformations of some importance (up to the form-value of species)
can thus arise simply by the direct action of the environment, in the
same way as that in which individual differences are produced--only
the latter fluctuate from generation to generation, since the inciting
influences continually change; whilst, in the former, the constant
external cause inciting modification always reproduces the same
variation, so that an accumulation of the latter can take place.
Climatic varieties can be thus explained.

A more efficacious augmentation of the variations arising in the single
individual is certainly brought about by the _indirect_ action of the
environment upon the organism. It is not here my intention to explain
once more the processes of natural selection; I mention this only in
order to point out that in these cases transformation depends upon a
_double action_ of the environment, since the latter first induces
small deviations in the organism by direct action, and then accumulates
by selection the variations thus produced.

By regarding variability in this manner--by considering each variation
as the reaction of the organism to an external action, as a diversion
of the inherited developmental direction, it follows that without a
change in the environment no advance in the development of organic
forms can take place. If we imagine that from any period in the earth’s
history the conditions of life remain completely unchanged, the species
present on the earth at this period would not, according to our view,
undergo any further modification. Herein is clearly expressed the
difference of this view from that other one according to which the
inciting principle of modification is not in the environment, but lies
in the organism itself in the form of a phyletic vital force.

I cannot here refrain from once more returning to the old (ontogenetic)
vital force of the natural philosophers, since the parallel between
this and its younger sister, the “phyletic vital force” which
appears in so many disguises, is indeed striking. Were the inciting
principle of the development of the individual actually an independent
vital force acting within the organism, the birth and growth of
the individual would be able to take place without the continuous
encroachment of the environment, such as occurs in nutrition and
respiration. Now this is known to be impossible, so that those who
support the existence of such a force, if any still exist, would be
driven to the obscure idea of a co-operation between the designing
power and the influences of the environment, just in the same manner
as such a co-operation is at present postulated by the defenders of
the phyletic vital force. I shall further on take the opportunity of
pointing out that this last idea is quite untenable; with respect to
the (ontogenetic) vital force any clearer proof cannot well be adduced,
but it will be admitted that the confused notion of the co-operation
and inter-action of teleological and causal powers is, from our
point of view, opposed to those very simple and clear ideas which
are in harmony with the views on phyletic development. As in racial
development each change of the organic type is entirely dependent upon
the action of the environment upon the organism, so in the development
of the individual, the totality of the phenomena of the personal life
must depend upon similar actions. Physiology, as is known, herein
entirely supports our view, since this shows that without the continual
alternating action of the environment and of the organism there can be
no life, and that vital phenomena are nothing but the reactions of the
organism to the influences of the environment.

It will be immediately perceived how exactly the processes of phyletic
and of ontogenetic development coincide, not merely in their external
phenomena but in their nature, if we trace the consequences of the
existing knowledge of the structure of the animal body. Although we
may not entirely agree with Haeckel’s doctrine of individuality in
its details, its correctness must on the whole be conceded, since it
cannot be disputed that the notion of individuality is a relative one,
and that several categories of morphological individuals exist, which
appear not only _singly_ as _physiological individuals_, _i.e._ as
independent living beings of lowest grade, but which can also _combine_
to form beings of a higher order.

But if we admit this, we should see with Haeckel nothing but
reproduction in the origination of a high organism from a single cell,
the egg; this reproduction being at the same time combined with various
differentiations of the offspring, _i.e._ with adaptations of the
latter to various conditions of life. Not even in the fact that the
tissues and organs of a single physiological individual stand in great
dependence upon one another through physical causes,[303] is there any
striking difference between this view and the phyletic composition of
the animal (and vegetable) kingdom out of physiological individuals
(Haeckel’s “_Bionten_”), since contemporaneous animals (individuals and
species) are known to influence one another in the most active manner.

Now if we further consider that the same units (cells) which, by
their reproduction and division of labour, at present compose the
body of the highest organism, must at one time have constituted as
independent beings the beginning of the whole of organic creation, and
that consequently the same processes (division of cells) which now
lead to the formation of a mammal, at that time led only to a long
series of different independent beings, it will be admitted that both
developmental series must depend upon the same inciting powers, and
that with reference to the causes of the phenomena it is not possible
that any great gap can exist between ontogeny and phylogeny, _i.e._
between the life-phenomena of the individual and those of the type.
According to our view both depend upon that co-operation of the same
material physical forces which admits of being briefly summarized
as the reaction of organized living matter to influences of the
environment.

Our opponents either cannot boast of such harmony in their conception
of nature, or else they must, together with the phyletic vital force,
re-admit into their theory the old ontogenetic vital force. I know not
indeed why they should not do so. Whoever inclines to the view that
organic nature is governed not merely by causal, but at the same time
by teleological, forces, may admit that the latter are as effective as
inciting causes of individual, as they are of phyletic, development.
According to my idea they are even bound to admit this, since it cannot
be perceived why the adaptations of the ontogeny should not depend upon
the same metaphysical principle assumed for each individual, as the
adaptations of the phylogeny; the latter are indeed only brought about
by the former. I believe therefore that the vital force (ontogenetic)
of the ancients stands or falls with the modern (phyletic) vital force.
We must admit both or neither, since they both rest on the same basis,
and are supported or opposed by the same arguments. Whoever feels
justified in setting up a metaphysical principle where complete proof
that _known forces_ are sufficient for the explanation of the phenomena
has not yet been adduced, must do the same with respect to individual,
as he does to phyletic, development, since this proof is in both cases
very far from being complete, and still contains large and numerous
gaps.[304]

The theoretical conception of variation as the reaction of the organism
to external influences has also not yet been experimentally shown to
be correct. Our experiments are still too coarse as compared with the
fine distinctions which separate one individual from another; and
the difficulty of obtaining clear results is greatly increased by
the circumstance that a portion of the individual deviations always
depends upon heredity, so that it is frequently not only difficult,
but absolutely impossible, to separate those which are inherited from
those which are acquired. Still further are we removed from being
able to refer variation to its final mechanical causes, _i.e._ from a
mechanical theory of reproduction, which would bring within the range
of mathematical calculation both the phenomena of stability (heredity)
and of change (variability).

But although sufficient proofs of the correctness of the views
here advocated cannot at present be adduced, these views are not
contradicted by any known facts--they are, on the contrary, supported
by many facts which they in turn make comprehensible (local forms,
different cycles of variation in heterogeneous species). These views
are finally completely justified by their furnishing the only possible
theoretical formulation of variability on which a mechanical conception
of organic development can be based. That such a conception is not only
admissible, but is unavoidable, at least to the naturalist, I have
already attempted to prove.



II.

MECHANISM AND TELEOLOGY.


In the third volume of his smaller works Karl Ernst von Baer submits
the theory of selection to a most searching examination. Without
actually calling in question its scientific admissibility, he believes
that this theory is dependent upon its satisfying one condition, viz.
that it should connect the teleological with the mechanical principle.

“The Darwinian hypothesis, as stated by its supporters, always ends in
denying to the processes of nature any relation to a future, _i.e._ any
relation of aim or design. Since such relations appear to me _quite
evident_,” &c. And further:--“If the scientific correctness of the
Darwinian hypothesis is to be admitted, it must accommodate itself to
this universal striving after a purpose. If it cannot do this we should
have to deny its value.”

These words appear almost equivalent to passing a sentence of doom
upon the theory of selection and the mechanical conception of nature,
for how can one and the same process be effected simultaneously by
necessity and by designing powers? The one excludes the other, and we
must--so it appears--take our stand either on one side or the other.

Nevertheless we cannot set aside Von Baer’s proposition without
further examination simply because it is apparently incapable of being
fulfilled, since it contains a truth which should not be overlooked,
even by those who uphold the mechanical theory of nature. It is the
same truth which is also made use of by the philosophical opponents of
this theory, viz. that the universe as a whole cannot be conceived as
having arisen from blind necessity--that the endless harmony revealed
in every nook and corner by all the phenomena of organic and of
inorganic nature cannot possibly be regarded as the work of chance,
but rather as the result of a “vast designed process of development.”
It is also quite correct when, in reply to the supposed objection
that the mechanical theory of nature is not concerned with chances
but with necessities, Von Baer answers that the operations of a
series of necessities which “are not connected together” can only be
termed accidents in their opposing relations. He illustrates this by
instancing a target. If I hit the latter by a well-aimed shot, nobody
would explain this as the result of an accident, but if “a horseman is
riding along a gravelly road past this target, and one of the pebbles
thrown up by the hoof of the galloping horse hits the mark, this would
be termed an accident of extremely rare occurrence. My target was not
the mark for the pebble, therefore the hit was purely accidental,
although the projection of the stone in this precise direction with
the velocity which it had acquired, was sufficiently explained by
the kick given by the horse. But the hit was accidental because the
kick of the galloping horse, although it necessarily projected the
pebble, had no relation at all to my target. For the same reason we
must regard the universe as an immense accident if the forces which
move it are not designedly regulated--the more immense because it is
not a single motion of projection that acts here, but a large number
of heterogeneous powers, _i.e._ a large number of variously acting
necessities which are, as a whole, devoid of purpose, but which
nevertheless accomplish this purpose, not only at any single moment,
but constantly. A truly admirable series of desirable accidents!”[305]

The same idea is expressed, although in a very different manner, by
Von Hartmann, in the concluding chapter of his work already quoted.
He thinks that “design is a necessary and certain consequence of the
mechanical laws of nature.” “Were the mechanism of natural laws not
teleological there would be no mechanically regulated laws, but a weak
chaos of obstinate and capricious powers. Not until the causality of
the laws of inorganic nature had superseded the expression “dead”
nature, and had shown itself as the mainspring of life and of a
conformability to design visible on all sides, did it deserve the
name of mechanical lawfulness; just as a complication of wheels and
machinery made by man, which move in some definite manner with respect
to one another, only acquires the name of a mechanism or of a machine
when the immanent teleology of the combination and of the various
movements of the parts is revealed.”[306]

Against the correctness of the idea underlying these statements
scarcely anything can in my opinion be said. The harmony of the
universe and of that portion of it which we designate organic nature,
cannot be explained by chance, _i.e._ without a common ground for
co-operating necessities; by the side of mere mechanism it is
impossible not to acknowledge a teleological principle--the only
question is, in what manner can we conceive this as acting without
abandoning the purely mechanical conception of nature?

This is obviously effected if, with Von Baer and Von Hartmann, we
permit the metaphysical principle to interrupt the course of the
mechanism of nature, and if we consider both the former and the latter
to work together with equal power. Von Hartmann expressly makes such an
admission under the name of an “internal principle of development,” to
which he attributes such an important share that one cannot understand
why it should have any need for the employment of causal powers, and
why it does not simply do everything itself. Von Baer expresses himself
much less decisively, and even in many places insists upon the purely
mechanical connection of organic natural phenomena; but that with him
also the idea of interruption by a metaphysical principle is present,
is principally shown by his assuming, at least partly, the _per saltum_
development of species. This necessarily involves an actively internal
power of development.

Although I have already brought forward many arguments against the
existence of such a power, and although in refuting it every form of
development by directive powers is at the same time overthrown, it
nevertheless appears to me not to be superfluous in such a deeply
important question to show that a _per saltum_ development, and
especially the so-called heterogeneous generation, is inconceivable,
not only on the ground of the arguments formerly employed against the
phyletic vital force in general, but quite independently of these.

In the first place it must be said that the positive basis of this
hypothesis is insecure. Cases of sudden transformation of the whole
organism with subsequent inheritance are as yet quite unknown. It has
been shown that the occasional transformation of the Axolotl must most
probably be regarded in a different light. Another case, taken for
heterogeneous generation, viz. the budding of twelve-rayed Medusæ in
the gastric cavity of an eight-rayed species, has lately been shown by
Franz Eilhard Schulze[307] to be a kind of parasitism or commensalism.
The buds of the _Cuninæ_ do not spring, as was supposed, from the
_Geryonia_, but are developed from a _Cunina_ egg. But even if we
recall here the cases of alternation of generation and heterogenesis,
this would not be of any value by way of proof; it would only be thus
indicated how one might picture to oneself a sudden transformation.
That in alternation of generation, or generally, in every mode of
cyclical reproduction, we have not to deal with the abandonment of
one type of organization and the transition to some other, is proved
by the continual return to the type of departure--by the cyclical
character of the entire transformation. That two quite heterogeneous
types can belong to one cycle of development is, however, capable
of a far better and more correct explanation than would be given
by the supporters of _per saltum_ development. If we trace cyclical
reproduction to the adaptation of different developmental stages
or generations to deviating conditions of life, we thus not only
explain the exact and often striking agreement between form and mode
of life--we not only bridge over the gap between metamorphosis and
alternation of generation, but we can also understand how, within one
and the same family of Hydrozoa, species can occur with or without
alternation of generation, and further how other species can exist in
which the alternation of generation (the production of free Medusæ) is
limited to the one sex; we can understand in general how one continuous
series of forms may lead from the simple sexual organ of the Polypes
to the independent and free swimming sexual form of the Medusæ, and
how hand in hand with this the simple reproduction becomes gradually
cyclical. It is just these intermediate steps between the two kinds of
reproduction that make quite untenable the idea that the heterogeneous
forms in cyclical propagation arise through so-called “heterogeneous
generation,” _i.e._ through sudden _per saltum_ transformation. It
is excusable if philosophers to whom these facts are strange, or who
have to take the trouble of working them up, should adduce alternation
of generation as an instance of “heterogeneous generation,” but by
naturalists this should be once and for ever abandoned.

All other facts which have hitherto been referred to “heterogeneous
generation” are still less explicable as such, inasmuch as they always
relate to changes in single parts of an organism, such as the sudden
change of fruit or flower in cultivated plants. The notion of _per
saltum_ development, however, demands a total transformation--it
comprises (as Von Hartmann quite correctly and logically admits) the
idea of a _fixed specific type_ which can only be re-modelled _as a
whole_, and cannot become modified piecemeal. It must further be added,
that the observed variations which have arisen abruptly in single parts
are not as a rule inherited:[308] fruit-trees are only propagated by
grafting, _i.e._ by perpetuating the individual, and not by ordinary
reproduction by seeds. Now, if we nowhere see sudden variations of
large amount perpetuated by heredity, whilst we everywhere observe
small variations which can all be inherited, must it not be concluded
that _per saltum_ modification is not the means which Nature
employs in transforming species, but that an accumulation of small
variations takes place, these leading in time to large differences?
Is it logical to reject the latter conclusion because our period of
observation is too brief to enable us to directly follow long series
of accumulations, whilst _per saltum_ variation is admitted, although
unsupported by a single observation? As long as there remains any
prospect of tracing large deviations to the continually observed
phenomenon of small variations, I believe we have no right to resort to
the purely hypothetical explanation afforded by _per saltum_ variations.

But the hypothesis of “heterogeneous generation” is not only without a
basis of facts--it can also be directly shown to be untenable. Since
the operation of an internal power of transformation does not explain
adaptation to the conditions of life, the claims of natural selection
to explain these transformations must be admitted; but the co-operation
of a phyletic vital force and natural selection is inconceivable if we
imagine the modifications to occur _per saltum_.

The supposed “heterogeneous generation” is always illustrated by the
example of alternation of generation. The origination of a new animal
form is thus conceived to take place in the same manner as we now see,
in the cyclical reproduction of the Medusæ, free swimming, bell-shaped
Medusoids, produced from fixed polypites, or _Cercariæ_ from Trematode
worms by internal budding; in brief, it is imagined that one animal
form suddenly gives rise to another widely deviating form by purely
internal causes. Now on this theory it would be an unavoidable
postulate, that by such a process of _per saltum_ development there
arises not merely a new type of some species, but at the same time
individuals capable of living and of persisting under, and fitted to,
given conditions of life. But every naturalist who has attempted to
completely explain the relation between structure and mode of life
knows that even the small differences which separate one species from
another, always comprise a number of minute structural deviations which
are related to well defined conditions of life--he knows that in every
species of animal the whole structure is adapted in the most exact
manner _in every detail_ to special conditions of life. It is not an
exaggeration when I say in every detail, since the so-called “purely
morphological parts” could not be other than they are without causing
changes in other parts which exercise a definite function. I will not
indeed assert that in the most closely related species all the parts
of the body must in some manner differ from one another, if only to a
small extent; it seems to me not improbable, however, that an exact
comparison would very frequently give this result. That animals which
are so widely removed in their morphological relations as Medusæ and
Polypes, or Trematoda and their “nurses,” are differently constructed
in each of their parts can, however, be stated with certainty.

Now if this wide deviation in every part were in itself no obstacle to
the assumption of a designing and re-modelling power, it would become
so by the circumstance that all the parts of the organism must stand in
the most precise relation to the external conditions of life, if the
organism is to be capable of existing--all the parts must be exactly
adapted to certain conditions of life. How can this be brought about
by a transforming force acting spasmodically? Von Hartmann--who, in
spite of his clear perception and widely extended scientific knowledge,
cannot possibly possess a strong conviction of that harmony between
structure and life-conditions prevailing throughout the whole system
of the organism, and which personal research and contemplation are
alone able to give--simply bridges over the difficulty by permitting
natural selection to come to his aid as an “auxiliary principle” of
the re-modelling power. It would not be supposed that naturalists
would resort to the same device--nevertheless those who support the
phyletic force and _per saltum_ development generally invoke natural
selection as the principle which governs adaptation. But when does
this agency come into operation? When by germinal metamorphosis a
new form has arisen, this, from the first moment of its existence,
must be adapted to the new conditions of life or it must perish. No
time is allowed for it to continue in an unadapted state throughout
a series of generations until adaptation is luckily reached through
natural selection. Let us have either natural selection or a phyletic
force--both together are inconceivable. If there exists a phyletic
force, then it must itself bring about adaptation.

It might perhaps be here suggested that the same objection applies to
that process of modification which is effected by small steps, but that
it does so only when the change occurs suddenly. This, however, as I
have already attempted to show, but very rarely takes place; in many
cases (mimicry) the conditions even change in the first place through
the change in form and therefore, as is evident, as gradually as the
latter. It must be the same in all other cases where transformation of
the existing form and not merely extinction of the species concerned
takes place. The transmutation must always keep pace with the change
in the conditions of life, since if the latter change more rapidly the
species could not compete with rival species--it would become extinct.

The abrupt transformation of species implies sudden change in the
conditions of life, since a Medusa does not live like a Polype,
nor a Trematode like its “nurse.” For this reason it is impossible
that natural selection can be an aiding principle of “heterogeneous
generation.” If such abrupt transformation takes place it must produce
the new form instantly equipped for the struggle for existence,
and adapted in all its organs and systems of organs to the special
conditions of its new life. But would not this be “pure magic”? It
is not thereby even taken into consideration that here--as in the
cases of mimicry--time and place must agree. The requirements of a
pre-established harmony (“_prästabilirte Harmonie_”) further demand
that an animal fitted for special conditions of life should only make
its appearance at that precise period of the earth’s history when these
special conditions are all fulfilled, and so forth.

But he who has learnt to perceive the numerous and fine relations
which, in every species of animal, bring the details of structure into
harmony with function, and who keeps in view the impelling power of
these conditions, cannot possibly hold to the idea of a _per saltum_
development of animal forms. If development has taken place, it must
have occurred gradually and by minute steps--in such a manner indeed
that each modification had time to become equilibrated to the other
parts, and in this way a succession of modifications gradually brought
about the total transformation of the organism, and at the same time
secured complete adaptation to new conditions of life.

Not only abrupt modification however, but every transformation is
to be rejected when based upon the interference of a metaphysical
principle of development. Those to whom the arguments already advanced
against such a principle appear insufficient may once more be asked,
how and where should this principle properly interfere? I am of
opinion that one effect can have but one sufficient cause; if this
suffices to produce it, no second cause is required. The hand of a
watch necessarily turns once round in a circle in a given time as soon
as the spring which sets the mechanism in movement is wound up; in an
unwound watch a skilful finger can perhaps give the same movement to
the hand, but it is impossible that the latter can receive both from
the operator and from the spring at _the same time, the same motion_ as
that which it would receive through either of these two powers _alone_.
In the same manner it appears to me that the variations which lead to
transformation cannot be at the same time determined by physical and by
metaphysical causes, but must depend upon either one or the other.

On no side will it be disputed that at least one portion of the
processes of organic life depends upon the mechanical co-operation of
physical forces. How is it conceivable that sudden pauses should occur
in the course of these causal forces, and that a directive power should
be substituted therefor, the latter subsequently making way again for
the physical forces? To me this is as inconceivable as the idea that
lightning is the electric discharge of a thunder-cloud, of which the
formation and electrical tension depends upon causal forces, and of
which the time and place are purely determined by such forces, but that
Jupiter has it nevertheless in his power to direct the lightning flash
according to his will on to the head of the guilty.

Now although I deny the possibility or conceivability of the
contemporaneous co-operation of teleological and of causal forces
in producing any effect, and although I maintain that a purely
mechanical conception of the processes of nature is alone justifiable,
I nevertheless believe that there is no occasion for this reason to
renounce the existence of, or to disown, a directive power; only we
must not imagine this to interfere directly in the mechanism of the
universe, but to be rather behind the latter as the final cause of this
mechanism.

Von Baer himself points this out to us, although he does not follow
up the complete consequences of his arguments. He especially insists
in his book, which abounds in beautiful and grand ideas, that the
notions of necessity (causality) and of purpose by no means necessarily
exclude one another, but rather that they can be connected together in
a certain manner. Thus, the watchmaker attains his end, the watch, by
combining the elastic force of a spring with wheel-work, _i.e._ by
utilizing physical necessities; the farmer accomplishes his purpose,
that of obtaining a crop of corn, by sowing the seed in suitable land,
but the seed must germinate as an absolute necessity when exposed to
the influences of warmth, soil, moisture, &c. Thus, in these instances
a chain of necessities is undoubtedly connected with a teleological
force, the human will; and it directly follows from such cases that
wherever we see an aim or result attained through necessities, the
directive force does not interrupt the course of the series of
necessities which have already commenced, but is active before the
first commencement of these necessities, since it combines and sets the
latter in movement. From the moment when the mechanism of the watch
is combined harmoniously and the spring wound up, it goes without the
further interference of the watchmaker, just as the corn-seed when once
placed in the earth develops into a plant without assistance from the
farmer.

If we apply this argument to the development of the organic world,
those who defend mechanical development will not be compelled to deny a
teleological power, only they would have with Kant[309] to think of the
latter in the only way in which it can be conceived, viz. _as a Final
Cause_.

In the region of inorganic nature nobody any longer doubts the purely
mechanical connection of the phenomena. Sunshine and rain do not now
appear to us to be whims of a deity, but divine natural laws. As the
knowledge of the processes of nature advances, the point where the
divine power designedly interrupts these processes must be removed
further back; or, as the author of the criticism of the philosophy
of the Unconscious[310] expresses it, all advance in the knowledge
of natural processes depends “upon the continual elimination of the
idea of the miraculous.” We now believe that organic nature must be
conceived as mechanical. But does it thereby follow that we must
totally deny a final Universal Cause? Certainly not; it would be a
great delusion if any one were to believe that he had arrived at a
comprehension of the universe by tracing the phenomena of nature to
mechanical principles. He would thereby forget that the assumption
of eternal matter with its eternal laws by no means satisfies our
intellectual need for causality. We require before everything an
explanation of the fact that relationships everywhere exist between the
parts of the universe--that atoms everywhere act upon one another.[311]
He who can content himself with the assumption of matter may do so,
but he will not be able to show that the assumption of a Universal
Cause underlying the laws of nature is erroneous.

It will not be said that there is no advantage in assuming such a
Final Cause, because we cannot conceive it, and indeed cannot so much
as demonstrate it with certainty. It certainly lies beyond our power
of conception, in the obscure region of metaphysics, and all attempts
to approach it have never led to anything but an image or a formula.
Nevertheless there is an advance in knowledge in the assumption of this
Cause which well admits of comparison with those advances which have
been led to by certain results of the new physiology of the senses. We
now know that the images which give us our sense of the external world
are not “actual representations having any degree of resemblance,”[312]
but are only signs for certain qualities of the outer world, which
do not exist as such in the latter, but belong entirely to our
consciousness. Thus we know for certain that the world is not as we
perceive it--that we cannot perceive “things in their essence”--and
that the reality will always remain transcendental to us. But who will
deny that in this knowledge there is a considerable advance, in spite
of its being for the most part of a negative character? But just as we
must assume behind the phenomenal world of our senses an actual world
of the true nature of which we receive only an incomplete knowledge
(_i.e._ a knowledge corresponding only in reality with the relations of
time and space), so behind the co-operating forces of nature which “aim
at a purpose” must we admit a Cause, which is no less inconceivable
in its nature, and of which we can only say one thing with certainty,
viz., that it must be teleological. Just as the former first leads
us to perceive the true value of our sensual impressions, so does
the latter knowledge lead us to foresee the true significance of the
mechanism of the universe.

It is true that in neither case do we learn more than that there is
something present which we do not perceive, but in both instances this
knowledge is of the greatest value. The consciousness that behind
that mechanism of the universe which is alone comprehensible to us
there still lies an incomprehensible teleological Universal Cause,
necessitates quite a different conception of the universe--a conception
absolutely opposed to that of the materialist. Most correctly and
beautifully does Von Baer say that “a purpose cannot be otherwise
conceived by us than as proceeding from a will and consciousness; in
this would the ‘aiming at a purpose,’ which appears to us as reasonable
as it is necessary, have its deepest root.” If we conceive in this
world a divine Universal Power exercising volition as the ultimate
basis of matter and of the natural laws resident therein, we thus
reconcile the apparent contradiction between the mechanical conception
and teleology. In the same way that Von Hartmann, somewhere speaks of
the immanent teleology of a machine, we might speak of the immanent
teleology of the universe, because the single forces of matter are
so exactly adjusted that they must give rise to the projected world,
just as the wheels and levers of a machine bring forth a required
manufactured article. I admit that these are grossly anthropomorphic
ideas. But as mortals can we have any other ideas? Is not the notion
of purpose in itself an equally anthropomorphic one? and is there
any certainty that the idea of causality is less so? Do we know that
causality is unlimited, or that it is universally valid? In the
absence of this knowledge, should it not be permissible to satisfy
as far as we can the craving of the human mind for a spiritual First
Cause of the universe, by speaking of it in terms conceivable to
human understanding? We can take up such a final position and still
be conscious that we thereby form no certain conception, and indeed
come no nearer to the reality. The materialist still makes use of the
notion of “eternity,” and frequently handles it as though it were a
perfectly known quantity. We nevertheless do not seriously believe that
by the expression “eternal matter,” any true idea resulting from human
experience is gained.

If it is asked, however, how that which in ourselves and in the
remainder of the animal world is _intellectual_ and _perceptive_, which
_thinks_ and _wills_, is ascribable to a mechanical process of organic
development--whether the development of the mind can be conceived as
resulting from purely mechanical laws? I answer unhesitatingly in the
affirmative with the pure materialist, although I do not agree with
him as to the manner in which he derives these phenomena from matter,
since thinking and extension are heterogeneous things, and one cannot
be considered as a product of the other. But why should not the ancient
notion of “conscious matter” given out by Maupertuis and Robinet, not
be again entertained, as pointed out in recent times by Fechner?[313]
Would there not thus be found a useful formula for explaining phenomena
hitherto quite incomprehensible?

Von Hartmann in criticizing himself, designates the sensibility of
atoms as an “almost inevitable hypothesis” (p. 62), “inevitable
because if sensibility were not a general and original property of the
constituent elements of matter, it would be absolutely incomprehensible
how through its potentiality and integration that sensibility known
to us as being possessed by the organism could have arisen.” “It is
impossible that from purely external elements devoid of all internality
(_Innerlichkeit_) there should suddenly appear, by a certain mode
of combination, an internality which becomes more and more richly
developed. The more certainly science becomes convinced that in
the sphere of externality (_Äusserlichkeit_) the higher (organic)
phenomena are only results of combination, or are the aggregate
phenomena of the elementary atomic forces, the more surely, when she
once seriously concerns herself with this other question, will she not
fail to be convinced that the sensibility possessed by higher stages
of consciousness can be only combination-results, or the aggregate
phenomena of the elementary sensations of atoms, although these
atomic sensations as such always remain below the level of the higher
combinations of consciousness.” In confusing this double-sided nature
of the objective phenomenon “lies the main error of all materialism
and of all subjective idealism. Just as the attempt of the latter
(subjective idealism) to construct the external phenomena of existence
in space out of functions of internality and their combinations is
impossible, so is the endeavour of the former (materialism) to build up
internal sensation out of any combinations of force acting externally
in space equally impossible.”

I have no intention of going any deeper into these questions. I mention
them only in order to point out that even from this side there appears
to me no obstacle in the way of a purely mechanical conception of the
processes of the universe. The naturalist may be excused if he attempts
to penetrate into the region of philosophy; it arises from the wish to
be able to contribute a little towards the reconciliation of the latest
knowledge of the naturalist with the religious wants of the human
mind--towards the aim striven for by both sides, viz. a satisfactory
and harmonious view of the universe, according with the state of
knowledge of our time.

I believe that I have shown that the theory of selection by no means
leads--as is always assumed--to the denial of a teleological Universal
Cause and to materialism, and I thereby hope that I have cleared the
way for this doctrine, the importance of which it is scarcely possible
to over-estimate. Many, and not the most ill-informed, do not get so
far as to make an unbiassed examination into the facts, because they
are at the outset alarmed by the to them inevitable consequence of the
materialistic conception of the universe. Mechanism and teleology do
not exclude one another, they are rather in mutual agreement. Without
teleology there would be no mechanism, but only a confusion of crude
forces; and without mechanism there would be no teleology, for how
could the latter otherwise effect its purpose?[314]

Von Hartmann correctly says:--“The most complete mechanism conceivable
is likewise the most completely conceivable teleology.” We may thus
represent the phenomenal universe as such a completely conceivable
mechanism. With this conception vanish all apprehensions that the new
views would cause man to lose the best that he possesses--morality and
purely human spiritual culture. He who, with Von Baer, considers the
laws of nature as the “permanent expressions of the will of a creative
principle,” will clearly perceive that a further advance in the
knowledge of these laws need not divert man from the path of increasing
improvement, but must further him in this course--that the knowledge
of truth, whatever may be its purport, cannot possibly be considered
a backward step. Let us take our stand boldly on the ground of new
knowledge, and accept the direct consequences thereof, and we shall
not be obliged to give up either morality or the comforting conviction
of being part of an harmonious world, as a necessary member capable of
development and perfection.

Any other mode of interference by a directive teleological power in
the processes of the universe than by the appointment of the forces
producing them, is however, at least to the naturalist, inadmissible.
We are still far removed from completely understanding the mechanism by
means of which the organic world is evoked--we still find ourselves at
the very beginning of knowledge. We are, however, already convinced
that both the organic and the inorganic worlds are dependent only upon
mechanical forces, for to this conclusion we are led, not only by the
results of investigators who have restricted themselves to limited
provinces, but also by the most general considerations. But although
the force of these arguments may not be acknowledged, and although
one might maintain that the inductional proofs against the existence
of a “phyletic vital force” have been directed only against points of
detail, or have never been completely demonstrated, _i.e._ for all
points, it must nevertheless be conceded, that for the naturalist
the mechanical conception of Nature is the only one possible--that
he is not at all justified in abandoning this view so long as the
interference of teleological forces _in the course_ of the processes
of organic development has not been demonstrated to him. Thus, it will
not be immaterial whether a conception of Nature which to many seems
inevitable is consistent with the idea of universal design, or a final
directive universal principle, since the value which we may attach to
our own lives and aims, essentially depends thereon. The final and main
result of this essay will thus be found in the attempted demonstration
that the mechanical conception of Nature very well admits of being
united with a teleological conception of the Universe.


THE END.



FOOTNOTES


[171] [The slight variability in the colour of this pupa, opens up
the interesting question of the photographic sensitiveness of this
and other species, which is stated to cause them to assimilate in
colour to the surface on which the larva undergoes its final ecdysis.
Some experiments upon this subject have been recorded by Mr. T. W.
Wood, Proc. Ent. Soc. 1867, p. xcix, but the field is still almost
unexplored. R.M.]

[172] “Über den Einfluss der Isolirung auf die Artbildung.” Leipzig,
1872, p. 20.

[173] In some instances _Deilephila Lineata_ has also been seen by day
hovering over flowers.

[174] It is true that I only reared one brood, but from this fifty
specimens were obtained. It would be interesting to know whether this
variety of the caterpillar is distributed over the whole of Southern
Europe.

[175] In this sense Lubbock says:--“It is evident that creatures which,
like the majority of insects, live during the successive periods
of their existence in very different circumstances, may undergo
considerable changes in their larval organization in consequence of
forces acting on them while in that condition; not, indeed, without
affecting, _but certainly without affecting to any corresponding
extent_, their ultimate form.”--“Origin and Metamorphoses of Insects,”
London, 1874, p. 39.

[176] “Grundzüge der Zoologie,” 1875.

[177] [Lepidopterists are of course aware that even these distinctions
are not absolute, as no single character can be named which does not
also appear in certain moths. The definition in this case, as in that
of most other groups of animals and plants, is only a general one.
See, for instance, Westwood’s “Introduction to the Classification of
Insects,” vol. ii. pp. 330-332. Also some remarks by C. V. Riley in his
“Eighth Annual Report” on the insects of Missouri, 1876, p. 170. With
reference to the antennæ as a distinguishing character, see Mr. A. G.
Butler’s article in “Science for All,” 1880, part xxvii. p. 65. R.M.]

[178] The genus of _Morphinæ, Discophora_, possesses hairs very similar
to those of the genus _Cnethocampa_ belonging to the _Bombycidæ_.

[179] [The larvæ of genera 14, _Phyciodes_, and 35, _Crenis_, are
likewise spiny. See Edwards’ “Butt. of N. Amer.” vol. ii. for figures
of the caterpillar of _Phyc. Tharos_: for notes on the larvæ of
_Crenis Natalensis_ and _C. Boisduvali_ see a paper by W. D. Gooch,
“Entomologist,” vol. xiv. p. 36. The larvæ of genus 55, _Ageronia_, are
also spiny. (See Burmeister’s figure of _A. Arethusa_, “Lép. Rép. Arg.”
Pl. V. Fig. 4). The larvæ of genus 98, _Aganisthos_, also appear to be
somewhat spiny (see Burmeister’s figure of _A. Orion_, _loc. cit._ Pl.
V. Fig. 6), and this raises the question as to whether the genus is
correctly located in its present position. The larvæ of the following
genera figured in Moore’s “Lepidoptera of Ceylon,” parts i. and ii.,
are all spiny:--6, _Cirrochroa_ (Pl. XXXII.); 7, _Cynthia_ (Pl. XXVI.);
27, _Kallima_ (Pl. XIX.); and 74, _Parthenos_ (Pl. XXIV.). Many species
of caterpillars which are spiny when adult appear to be spineless,
or only slightly hairy when young. See Edwards’ figures of _Melitæa
Phaeton_, _Argynnis Diana_, and _Phyc. Tharos_ (_loc. cit._) and his
description of the larva of _Arg. Cybele_, “Canad. Entom.” vol. xii. p.
141. The spiny covering thus appears to be a character acquired at a
comparatively recent period in the phyletic development. R.M.]

[180] [The larvæ of the 110th genus, _Paphia_, Fabr. (_Anæa_, Hübn.)
are also smoothed-skinned. See Edwards’ figure (_loc. cit._ vol. i. Pl.
XLVI.) of _P. Glycerium_. Also C. V. Riley’s “Second Annual Report” on
the insects of Missouri, 1870, p. 125. Burmeister figures the larva of
a species of _Prepona_ (genus 99) which is smooth (_P. Demophon_, _loc.
cit._ Pl. V. Fig. 1). The horns on the head of _Apatura_, &c., may
possibly be a survival from a former spiny condition. R.M.]

[181] “Synopsis of the described Lepidoptera of North America.”
Washington, 1862.

[182] “Catalog der Lepidopteren des Europäischen Faunengebietes.”
Dresden, 1871.

[183] This group of moths (“Schwärmer”) is regarded as of very
different extents by systematists; when I here comprise under it only
the _Sphingidæ_ proper and the _Sesiidæ_, I by no means ignore the
grounds which favour a greater extension of the group; the latter is
not rigidly limited. [The affinities of the _Sesiidæ_ (_Ægeriidæ_) are
by no means clearly made out: it appears probable that they are not
related to the _Sphingidæ_. See note 160, p. 370. R.M.]

[184] [For Mr. A. G. Butler’s observations on the genus _Acronycta_,
see “Trans. Ent. Soc.” 1879, p. 313; and note 68, p. 169, of the present
volume. R.M.]

[185] [The following characters are given in Stainton’s “Manual of
British Butterflies and Moths,” vol. i. p. 114:--“Larva of very
variable form: at one extreme we find the singular _Cerura_ larvæ,
with only fourteen legs, and two long projecting tails from the last
segment; at the other extreme we have larvæ with sixteen legs and no
peculiarity of form, such as _Chaonia_ and _Bucephala_; most have,
however, the peculiarity of holding the hind segment of the body erect
when in repose; generally quite naked, though downy in _Bucephala_ and
rather hairy in _Curtulu_; very frequently there are projections on the
back of the twelfth segment.” R.M.]

[186] Encyl. Meth. ix. p. 310.

[187] [The genus _Vanessa_ (in the wide sense) appears to be in a
remarkable condition of what may be called phyletic preservation.
Thus, the group of species allied to _V. C.-album_ passes by almost
insensible steps into the group of butterflies typified by our
“Tortoiseshells.” The following is a list of some of the intermediate
species in their transitional order:--_I.-album_, _V.-album_, _Faunus_,
_Comma_, _California_, _Dryas_, _Polychloros_, _Xanthomelas_,
_Cashmirensis_, _Urticæ_, _Milberti_, &c. Similarly, our _Atalanta_
and _Cardui_ are connected by a number of intermediate forms, showing
a complete transition from the one to the other. The following is the
order of the species so far as I am acquainted with them:--_Atalanta_,
_Dejeanii_, _Callirhoë_, _Tammeamea_, _Myrinna_, _Huntera_,
_Terpsichore_, _Carye_, _Kershawii_, and _Cardui_. R.M.]

[188] “Prodromus Systematis Lepidopterorum.” Regensburg, 1864.

[189] [The larva of _Acherontia Morta_, figured by Butler (see note 121,
p. 262), possesses the characteristically recurved horn; that of _Ach.
Medusa_ figured by the same author, does not appear to possess this
character in any marked degree. R.M.]

[190] [See note 97, p. 233. R.M.]

[191] _Loc. cit._ Pl. XXV. [This species is referred by Butler to the
genus _Paonias_, Hübn. R.M.]

[192] Abbot and Smith, Pl. XXIX. [Placed by Butler in the genus
_Cressonia_, Grote and Robinson. Abbot and Smith state that this
larva is sometimes green. According to Mr. Herman Strecker (Lepidop.
Rhopal. and Hetero, Reading, Pa. 1874, p. 54) it feeds upon black
walnut (_Juglans Nigra_), hickory (_Carya Alba_), and ironwood
(_Ostrya Virginica_). Of the North American species of _Smerinthus_,
the following, in addition to _Excæcatus_, closely resemble our
_Ocellatus_:--_S. (Calasymbolus) Geminatus_, Say; _(C.) Cerisii_,
Kirby; and _Ophthalmicus_, Boisd. In addition to _S. (Cressonia)
Juglandis_, _S. (Triptogon) Modesta_ much resembles our _Populi_. The
larva of _Geminatus_, according to Strecker, is “pale green, lightest
above, with yellow lateral granulated stripes; caudal horn violet;
stigmata red. It feeds on the willow.” R.M.]

[193] Cat. Brit. Mus.

[194] [This lengthening of the true legs is mimetic according to
Hermann Müller, and causes the anterior portion of the caterpillar to
resemble a spider. See note 129, p. 290. R.M.]

[195] [Certain butterflies appear to be crepuscular, if not nocturnal
in their habits. Thus in his “Notes on the Lepidoptera of Natal,” Mr.
W. D. Gooch states that he never saw _Melanitis_, _Leda_, or _Gnophodes
Parmeno_ on the wing by day, but generally during the hour after
sunset. He adds:--“My sugar always attracted them freely, even up to
10 or 11 p.m.” Many species of _Hesperidæ_ are also stated to be of
crepuscular habits by this same observer. See “Entomologist,” vol xvi.
pp. 38 and 40. R.M.]

[196] I only make this assumption for the sake of simplicity, and not
because I am convinced that the existing _Rhopalocera_ are actually the
oldest Lepidopterous group.

[197] Zeitschrift für wissenschaftl. Zoologie, vol. xx. p. 519.

[198] [See for instance Lubbock’s “Origin and Metamorphoses of
Insects,” chap. iii.; and F. M. Balfour’s “Comparative Embryology,”
vol. i., 1880, pp. 327--356. This last work contains an admirable
_résumé_ of our knowledge of the embryonic development of insects up to
the date of publication. R.M.]

[199] Are not the 4th, 11th, and 12th segments destitute of the
rudiments of legs as in the larvæ of all existing saw-flies? I might
almost infer this from Bütschli’s figures (see for instance Pl. XXV.,
Fig. 17A).

[200] [The grub-formed Hymenopterous larvæ, like the larvæ of all
other holometabolous insects, thus represent an acquired degenerative
stage in the development, _i.e._ an adaptation to the conditions of
life at that stage. Bearing in mind the above-quoted observations of
Bütschli and the caterpillar-like form of the Terebrantiate group of
Hymenopterous larvæ, the following remarks of Balfour’s (_loc. cit._ p.
353), appear highly suggestive:--“While in a general way it is clear
that the larval forms of insects cannot be expected to throw much light
on the nature of insect ancestors, it does nevertheless appear to me
probable that such forms as the caterpillars of the Lepidoptera are not
without a meaning in this respect. It is easy to conceive that even a
secondary larval form may have been produced by the prolongation of one
of the embryonic stages; and the general similarity of a caterpillar to
_Peripatus_, and the retention by it of post-thoracic appendages, are
facts which appear to favour this view of the origin of the caterpillar
form.” See also Sir John Lubbock, _loc. cit._, pp. 93 and 95. R.M.]

[201] [In the most recent works dealing with this order six
groups, based on the character of the imagines are recognized,
viz.:--_Tubulifera_, _Terebrantia_, _Pupivora_, _Heterogyna Fossores_,
and _Mellifera_. (See, for instance, F. P. Pascoe’s “Zoological
Classification,” 2nd ed. p. 147.) Of these groups the larvæ of the
_Terebrantia_ as thus restricted are all of the caterpillar type
(_Tenthredinidæ_ and _Siricidæ_), whilst those of the other groups are
maggot-shaped. For a description of the development of the remarkable
aberrant larva of _Platygaster_, see Ganin in Zeit. f. wissenschaftl.
Zool., vol. xix. 1869. R.M.]

[202] [For recent investigations on the structure of the thorax in
Diptera, see a paper by Mr. A. Hammond, in Journ. Linn. Soc., Zoology,
vol xv. p. 9. R.M.]

[203] I am familiar with the fact that the two sub-orders of true
Diptera, the short-horned (_Brachycera_), and the long-horned
(_Nemocera_), are not sharply limited; and I am likewise well
acquainted with the circumstance that there are forms which connect the
two larval types. The connecting forms of the imagines do not, however,
always coincide with the intermediate larval forms, so that there
here arises a second and very striking incongruence of morphological
relationship which depends only upon the circumstance that the one
stage has diverged in form more widely than the other through a greater
divergence in the conditions of life. The difficulty is in these cases
aggravated because an apparent is added to the true form-relationship
through convergence, so that without going into exact details the form
and genealogical relationships of the Diptera cannot be distinguished.
It would be of great interest for other reasons to make this
investigation, and I hope to be able to find leisure for this purpose
at some future period.

[204] “Entwicklung der Dipteren.” Leipzig, 1864.

[205] Lubbock concludes from the presence of thoracic legs in the
embryonic larva of bees that these have been derived from a larva of
the _Campodea_ type, but he overlooks the fact that the rudiments of
the abdominal legs are also present; _loc. cit._, p. 28.

[206] “Für Darwin,” Leipzig, 1864, p. 8.

[207] Mem. Peabody Acad. of Science, vol. i. No. 3.

[208] Verhandl. Wien. Zoolog. Botan. Gesellsch. 1869, p. 310.

[209] Über Ontogenie und Phylogenie der Insekten. Eine akademische
Preisschrift. Jen. Zeitschrift. Bd. x. Neue Folge, iii. Heft 2. 1876.
[Some remarks by F. M. Balfour on the origin of certain larval forms
have already been quoted in a previous note (p. 485). This author
further states:--“The fact that in a majority of instances it is
possible to trace an intimate connection between the surroundings
of a larva and its organization proves in the clearest way _that
the characters of the majority of existing larval forms of insects
have owed their origin to secondary adaptations_. A few instances
will illustrate this point:--In the simplest types of metamorphosis,
_e.g._ those of the Orthoptera genuina, the larva has precisely the
same habits as the adult. We find that a caterpillar form is assumed
by phytophagous larvæ amongst the Lepidoptera, Hymenoptera, and
Coleoptera. Where the larva has not to go in search of its nutriment
the grub-like apodous form is assumed. The existence of such an apodous
larva is especially striking in the Hymenoptera, in that rudiments
of thoracic and abdominal appendages are present in the embryo and
disappear again in the larva.... It follows from the above that the
development of such forms as the Orthoptera genuina is more primitive
than that of the holometabolous forms, &c.” Comparative Embryology,
vol. 1, p. 352. R.M.]

[210] [The _Aphaniptera_ are now recognized in this country as
a sub-order of Diptera. See, for instance, Huxley’s “Anatomy
of Invertebrated Animals,” p. 425, and Pascoe’s “Zoological
Classification,” 2nd ed. p. 122. R.M.]

[211] [This illustration of course only applies to the old arrangement
of the Hymenoptera into _Terebrantia_ and _Aculeata_. See also note 201,
p. 488. R.M.]

[212] [Eng. ed. This law is perhaps a little too restricted, inasmuch
as it is theoretically conceivable that the organism may be able
to adapt itself to similar conditions of life in different ways;
differences of form could thus depend sometimes upon differences of
adaptation and not upon differences in the conditions of life, or, as
I have formerly expressed it, it is not necessary to allow always only
_one_ best mode of adaptation.]

[213] [It must be understood that the word rendered here and elsewhere
throughout this work as “transformation” is not to be taken in the
narrow sense of metamorphosis, but as having the much broader meaning
of a change of any kind incurred by an organism. Metamorphosis is in
fact but one phase of transformation. R.M.]

[214] By the Editor.

[215] Mr. C. V. Riley in his excellent “Annual Reports” already
quoted in previous notes, states that the larvæ of _Agrotis Inermis_,
_Leucania Unipuncta_ (Army-worm), and _L. Albilinea_ are all loopers
when newly hatched. (See First Report, p. 73; Eighth Report, p. 184;
and Ninth Report, p. 53.)

[216] The following species not referred to in the previous part of
this work are figured by Semper (Beit. zur Entwicklungsgeschichte
einiger ostasiat. Schmet.; Verhandl. d. k.k. zoo. bot. Gesell. in Wien,
1867):--_Panacra Scapularis_, Walk.; _Chærocampa Clotho_, Drury; and
_Diludia (Macrosila) Discistriga_, Walk. The following are figured by
Boisduval and Guenée. (Spéc. Gén. 1874):--_Smerinthus Ophthalmicus_,
Boisd.; _Sphinx Jasminearum_, Boisd.; _S. (Hyloicus) Plebeia_, Fabr.;
_S. (Hyloicus) Cupressi_, Boisd.; _S. (Pseudosphinx) Catalpæ_, Boisd.;
_Philampelus Jussiuæ_, Hübn. (= _Sphinx Vitis_, Linn.?); and _Ceratomia
Amyntor_, Hübn. As the works of Abbot and Smith, and Horsfield and
Moore have been exhausted by Dr. Weismann, it is quite unnecessary
to extend this note by giving a list of the species figured by these
authors.

[217] The same inference has already been drawn with respect to
_Pterogon (Proserpinus) Œnotheræ_, see pp. 257, 258.

[218] This would of course be the _fourth_ segment if the head be
considered the first, as on the Continent.

[219] “Second Annual Report,” 1870, p. 78.

[220] “Entomologist,” vol. xiv. p. 7.

[221] With reference to the habits of _C. Capensis_ (p. 531), I have
since been informed by Mr. Trimen that this species does not conceal
itself by day, so that the dimorphism may be regarded as a character
retained from an earlier period and adapted to the present life
conditions.

[222] “Kosmos,” Dec. 1877, p. 218. The paper is here introduced chiefly
with a view to illustrate an important case of incongruence among
Lepidopterous pupæ.

[223] [Maracujá, the local name for the Passiflora. R.M.]

[224] See p. 448.

[225] Verhandl. Schweiz. Naturforsch. Gesellschaft. Einsiedeln, 1868.

[226] [Eng. ed. In 1878 Señor José M. Velasco published a paper
entitled “Description, metamorfosis. y costumbres de una especie nueva
del genero _Siredon_.” Memor. Sociedad Mexicana de Historia Natural,
December 26th. See Addendum to this essay.]

[227] Dana and Silliman’s Amer. Journ., 3rd series, i. p. 89. Annals
Nat. Hist. vii. p. 246.

[228] Proc. Zoo. Soc. 1870, p. 160.

[229] Compt. Rend., vol. lx. p. 765 (1865).

[230] Nouvelles Archives du Muséum d’Histoire Nat. Paris, 1866, vol.
ii. p. 268.

[231] Proc. Boston Soc., vol. xii. p. 97; Silliman’s Amer. Journ., vol.
xlvi. p. 364; reference given in “Troschel’s Jahresbericht” for 1868,
p. 37.

[232] Proc. Boston Soc., vol. xii. p. 97; Silliman’s Amer. Journ., vol.
xlvi. p. 364. I have not been able to get a copy of this paper, and
quote from a reference in “Troschel’s Jahresbericht.” See preceding
note.

[233] Dana and Silliman’s Amer. Journ. See note 3.

[234] Proc. Acad. Philadelph. xix. 1867, pp. 166-209.

[235] Mém. Acad. Petersb. vol. xvi.

[236] [Eng. ed. Seidlitz is an exception, since in his work on
Parthenogenesis (Leipzig, 1872, p. 13) he states that “In the Axolotl,
Pædogenesis, which is not in this case ... monogamous, but sexual, and
indeed gynækogenetic, has already become so far constant that it has
perhaps entirely superseded the orthogenetic reproduction.”]

[237] Über den Einfluss der Isolirung auf die Artbildung. Leipzig,
1872, p. 33.

[238] Duméril represents the teeth of the _vomer_ as separated from
those of the _os palatinum_ by a gap. This is probably accidental,
since Gegenbaur (Friedrich u. Gegenbaur, the skull of Axolotl,
Würzburg, 1849) figures the rows of teeth as passing over from the one
bone to the other without interruption. This was the case with the
Axolotls which I have been able to examine on this point; but this
small discrepancy is, however, quite immaterial to the question here
under consideration.

[239] See O. Hertwig “Über das Zahnsystem der Amphibien und seine
Bedeutung für die Genese des Skelets der Mundhöhle.” Archiv. für
microsc. Anat., vol. xi. Supplement, 1874.

[240] [Eng. ed. These Amblystomas have since died and have been
minutely described by Dr. Wiedersheim. See his memoir, “Zur Anatomie
des _Amblystoma Weismanni_,” in Zeit. für wiss. Zool., vol. xxxii. p.
216.]

[241] See Strauch, _loc. cit._ p. 10.

[242] See Part I. of this volume.

[243] [This is the principle of “Degeneration” recognized by Darwin
(see “Origin of Species,” 6th ed. p. 389, and “Descent of Man,” vol.
i. p. 206), and given fuller expression to by Dr. Anton Dohrn (see
his work entitled “Der Ursprung der Wirbelthiere und das Princip des
Functionswechsels.” Leipzig, 1875). A large number of cases have been
brought together by Prof. E. R. Lankester, in his recent interesting
work on “Degeneration, a Chapter in Darwinism.” Nature series, 1880.
R.M.]

[244] “Sulla Larva del _Triton Alpestris_.” Archivio per la Zoologia.
Genova e Torino, 1861, vol. i. pp. 206-211.

[245] See also Lubbock “On the Origin and Metamorphoses of Insects,”
London, 1874.

[246] See the first essay “On the Seasonal Dimorphism of Butterflies,”
p. 82.

[247] [Eng. ed. It has frequently been objected to me that the existing
Axolotl is not a form resulting from atavism, but a case of “arrested
growth.” The expression “atavism” is certainly to be here taken in
a somewhat different sense than, for example, in the case of the
reversion of the existing Axolotl to the Amblystoma form. Further on, I
have myself insisted that in the first case the phyletic stage in which
the reversion occurred is still completely preserved in the ontogeny
of each individual, whilst the Amblystoma stage has become lost in the
ontogeny of the Axolotl. If, therefore, we apply the term “atavism”
only to such characters or stages (_i.e._ complexes of characters) as
are no longer preserved in the ontogeny, we cannot thus designate the
present arrest of the Axolotl at the perennibranchiate stage. Such a
restriction of the word, however, appears to me but little desirable,
since the process is identical in both cases, _i.e._ it depends upon
the same law of heredity, in accordance with which a condition formerly
occurring as a phyletic stage suddenly reappears through purely
internal processes. It is true that the reversion is not _complete_,
_i.e._ the present sexually mature Axolotl does not correspond in all
details with its perennibranchiate ancestors. Since Wiedersheim has
shown that the existing Axolotl possesses an intermaxillary gland, this
can be safely asserted. This gland occurs only in _land_ Amphibians,
and therefore originated with the Amblystoma form, afterwards becoming
transferred secondarily to the larval stage. Nevertheless, the present
Axolotl must resemble its perennibranchiate ancestors in most other
characters, and we should be the more entitled to speak of a reversion
to the perennibranchiate stage as we speak also of the reversion of
single characters. To this must be added that the Axolotl does not
correspond exactly with an Amblystoma larva, since Wiedersheim has
shown that the space for the intermaxillary gland is present, but that
the gland itself is confined to a few tubes which do not by any means
fill up this space. (“Das Kopfskelet der Urodelen.” Morph. Jahrbuch,
vol. iii. p. 149). By the expression “arrested growth” not much is
said, if at the same time the cause of the arrest is left unstated. But
what can be the cause why the whole organization remains stationary at
the perennibranchiate stage, the sexual organs only undergoing further
development? Surely only that law or force of heredity known by its
effects, but obscure with respect to its causes, through which old
phyletic stages sometimes suddenly reappear, or in other words, that
power through which reversion takes place. It must not be forgotten
that all these cases of “larval reproduction” in Amphibians appear
suddenly. The present sexually mature form of the Axolotl has not
arisen by the sexual maturity gradually receding in the ontogeny from
generation to generation, but by the occurrence of single individuals
which were sexually mature in the perennibranchiate stage, these having
the advantage over the _Amblystomæ_ in the struggle for existence under
changed climatic conditions.

By admitting a reversion, we perfectly well explain why arrest at the
perennibranchiate stage can be associated with complete development of
the sexual organs; the assumption of an “arrested growth” leaves this
combination of characters completely unexplained. Moreover, I am of
opinion that the expressions “arrested growth” or “reversion” are of
but little importance so long as the matter itself is clear.]

[248] See Haeckel’s “Anthropogenie,” p. 449.

[249] “Der Ursprung der Wirbelthiere und das Princip des
Functionswechsels,” Leipzig, 1875.

[250] Bull. Soc. Neuchâtel. vol. viii. p. 192. Reference given in
“Troschel’s Jahresbericht” for 1869.

[251] Sitzungsberichte d. math. phys. Klasse der Akad. d. Wiss. zu
München, 1875. Heft i.

[252] Compt. Rend. vol. lxviii. pp. 938 and 939.

[253] Archiv f. Naturgeschichte, 1867.

[254] Compt. Rend. vol. v. 1870, p. 70.

[255] Bull. Soc. Neuchâtel. vol. viii. p. 192. Reference given in
“Troschel’s Jahresbericht” for 1869.

[256] [Eng. ed. It was mentioned in the German edition of this work
that in the spring of 1876 a female Amblystoma of the Jardin des
Plantes in Paris had laid eggs (see Blanchard in the Compt. Rend. 1876,
No. 13, p. 716). Whether these eggs were fertile, or whether they
developed was not then made known. Thus much was however at the time
clear, that even if this had been the case, the reproduction of this
Amblystoma would have been only an _exceptional_ occurrence. At that
time there were in the Jardin des Plantes Amblystomas which had been
kept for more than ten years, and only on one occasion was there a
deposition of eggs, and this by only one specimen. That I was correct
in speaking of the “sterility” of these Amblystomas in spite of this
one exception, is proved by the latest communication from the Jardin
des Plantes. We learn from this (Compt. Rend. No. 14, July, 1879, p.
108) that in the years 1877 and 1878 none of the Amblystomas laid any
more eggs, although all means were exerted to bring about propagation.
In April, 1879, eggs were again laid by one female, and by a second in
May. These eggs certainly developed, as did those of 1876, and produced
tadpoles. These Amblystomas are therefore not absolutely, but indeed
relatively sterile. Whilst the Axolotl propagates regularly and freely
every year, this occurs with the Amblystoma but rarely and sparsely.
The degree of their sterility can only be approximately established
when we know the number of Amblystomas that have since been kept in the
Jardin des Plantes. Unfortunately nothing has been said with respect to
this.]

[257] Origin of Species, 6th ed. p. 252.

[258] In plants also reversion forms show sterility in different
degrees. Mr. Darwin has called my attention to the fact that the
peloric (symmetrical) flowers which occasionally appear as atavistic
forms in _Corydalis solida_ are partly sterile and partly fertile.
That in other causes of sterility, and above all by bastardizing, the
reproductive power is lost in the most varying degrees, has been known
since the celebrated observations of Kölreuter and Gärtner. [Eng.
ed. An Orchid (_Catasetum tridentatum_) has the sexes separate, and
the male flowers (_Myanthus barbatus_) differ considerably from the
female (_Monachanthus viridis_); besides these, there occurs a form
with bisexual flowers which must be considered as a reversion (_Cat.
tridentatum_) and _this is always sterile_. Darwin, “Fertilization of
Orchids,” 2nd ed. p. 199.]

[259] As we do not know the origin of the “Paris Axolotl” I must
restrict myself in the following remarks to _Siredon Mexicanus_ (Shaw).

[260] Mühlenpfordt, “Versuch einer getreuen Schilderung der Republik
Mejico,” Hanover, 1844, vol. ii. p. 252.

[261] [The specific gravity of sea water (Atlantic), according to the
determinations of Mr. Buchanan on board the “Challenger,” at 15.56°
C. varies from 1.0278 to 1.0240. That of the water of the Dead Sea is
1.17205.--Watts’ “Dict. of Chemistry,” vol. v., table, p. 1017. R.M.]

[262] _Loc. cit._ p. 252.

[263] “Über die specifische Verschiedenheit des gefleckten und des
schwarzen Erdsalamanders oder Molchs, und der höchst merkwürdigen, ganz
eigenthümlichen Fortpflanzungsweise des Letzteren.” Isis, Jahrg. 1833,
p. 527.

[264] The experiments referred to have not been made known; I am
indebted for them to a written communication kindly furnished by an
esteemed colleague.

[265] See Mühlenpfordt’s work already quoted, vol. i.

[266] In the province of botany such a case has already been made
known by Fritz Müller (Botan. Zeitung, 1869, p. 226; 1870, p. 149). I
may be here permitted to quote a passage from the letter in which Dr.
Müller calls attention to this interesting discovery. “As a proof of
the possibility that a reversion form can again become a persistent
character in a species or in the allied form of a particular district,
I may refer you to an _Epidendrum_ of the island of Santa Catharina.
In all Orchids (with the exception of _Cypripedium_) only one anther
is developed; in very rare cases well-formed anthers appear as
reversions among the aborted lateral anthers of the inner whorl. In the
_Epidendrum_ mentioned, these are however _always present_.”

[267] [This species is interesting as being ovoviviparous, the young
passing through the branchiate stage within the body of the mother.
Some experiments, which were partially successful, were made by
Fräulein v. Chauvin with a view to solve the question whether the
branchiate stage could be prolonged by taking the larvæ directly from
the mother before birth and keeping them in water. See “Zeit. für
wissen. Zoo.” vol. xxix., p. 324. R.M.]

[268] See Fatiot, “Les Reptiles et les Batraciens de la haute
Engadine.” Geneva, 1873.

[269] I can remember at Upper Engadine a peculiar kind of preserved
beef, prepared by simply drying in the air; also the mummification of
entire human bodies by drying in the open air, as is practised at Great
St. Bernard.

[270] “Faune des Vertébrés de la Suisse,” vol. iii. “Histoire Naturelle
des Reptiles et des Batraciens.” Geneva, 1873.

[271] See Wiedersheim, “Versuch einer gleichenden Anatomie der
Salamandrinen.” Würzburg, 1875.

[272] See Gené, “Memorie della Reale Acad. di Torino,” vol. i.

[273] _Rana esculenta_ never reaches Alpine regions, this species not
having been found higher than 1100 meters. (Fatiot, _loc. cit._, p.
318.)

[274] See also the excellent work upon Mexico by Mühlenpfordt already
quoted, vol. i., pp. 69-76.

[275] “Essai politique sur le Royaume de la Nouvelle Espagne,” 1805, p.
291.

[276] [The expression made use of by the author, viz. “Diluvialzeit,”
would perhaps be more in harmony with the views of English geologists
if rendered as the “pluvial period,” thereby indicating the period of
excessive rainfall which, according to Mr. Alfred Tylor, succeeded
to and was a consequence of the thawing of the great glaciers which
accumulated during the last glacial epoch. There is abundant evidence
to show that during the latter period glacial action extended in North
America at least as far south as Nicaragua. See Belt on “The Glacial
Period in North America,” Trans. Nova Scotian Inst. of Nat. Sci. 1866,
p. 93, and “The Naturalist in Nicaragua,” pp. 259-265. R.M.]

[277] [Eng. ed. A memoir by Samuel Clarke has since been published upon
the embryonic development of _Amblystoma punctatum_, Baird. Baltimore,
1879.]

[278] [Eng. ed. See this author’s work, “Das Kopfskelet der Urodelen.”
Leipzig, 1877, p. 149.]

[279] [See preceding note 52. R.M.]

[280] See note 226, p. 566.

[281] [Prof. Semper also remarks (“Animal Life,” note 47, p. 430) with
reference to the Axolotl of Lake Como in the Rocky Mountains, which he
states always becomes transformed into _Amblystoma Mavortium_, that
this metamorphosis “takes place in the water, and the Amblystomas, so
long as they are little, actually live exclusively in the water, as I
know by my own experience. A young Amblystoma which I kept alive for a
long time, never went out of the water of its own free will, while one
nearly twice as large lives entirely on land and only takes a bath now
and then. It always goes into the water when the temperature of the
air in the cellar, in which my aquaria stand, falls below that of the
water--down to about 6° or 8° C.” This statement appears to suggest
that the effect of temperature may be a factor in some way concerned
in these interesting cases of transformation, and would in any case
be well worthy of experimental investigation. Some further details
concerning the _Siredon Lichenoides_ of Lake Como have been recently
published by Mr. W. E. Carlin (Proc. U.S. National Museum, June, 1881).
The lake, which is shallow, is fed by a constant stream of fresh water,
but the water of the lake is intensely saline. The Siredon never enter
the fresh water stream, but congregate in large numbers in the alkaline
waters of the lake. “When about one hundred and fifty were placed
in fresh water they seemed to suffer no inconvenience, but it had a
remarkable effect in hastening their metamorphosis into the Amblystoma
form. Of an equal number kept in fresh water and in the lake water,
quite a change occurred with the former after twenty-four hours, while
the latter showed no change after several days of captivity. Those that
were kept well fed in jars usually began to show a slight change in
from two to three weeks, and all of them completed the change into the
Amblystoma inside of six weeks, while in some kept, but not specially
fed, there were but three changes in three months.” (Nature, Aug. 25th,
1881, p. 388.) R.M.]

[282] [Some experiments on the transformation of the Crustacean
_Artemia Salina_ into _A. Milhausenii_ by gradually increasing the
saltness of the water, and conversely, the transformation of _A.
Milhausenii_ into _A. Salina_ by diminishing the saltness of the water,
have been made by Schmankewitsch (Zeitschrift f. wiss. Zool. xxv.
Suppl. 103 and xxix. 429), but the changes which occur here are much
less considerable than in the case of the Axolotl. R.M.]

[283] “Reden und kleinere Aufsätze, Th. II.: Studien aus dem Gebiete
der Naturwissenschaften.” St. Petersburg, 1876, p. 81.

[284] This obviously does not imply that the naturalist should not
investigate Nature’s processes, and not only correlate these, but also
work them up into a universal conception; this is indeed both desirable
and necessary if natural knowledge is to be regarded in its true value.
The naturalist by this means becomes a philosopher, and the vitality
of the so-called “natural philosopher” has been inspired, not by the
necessity for investigation, but by philosophy proper.

[285] [The discovery here referred to is the synthesis of urea by
Wöhler in 1828 (Pogg. Ann. xii., 253; xv. 619), by the molecular
transformation of ammonium cyanate. Since that period large numbers
of organic syntheses have been effected by chemists, and many of the
compounds formerly supposed to be essential products of life have been
built up in the laboratory from their inorganic elements. The division
of chemistry into “organic” and “inorganic” is thus purely artificial,
and is merely retained as a matter of convenience, the former division
of the science being defined as the chemistry of the carbon compounds.
R.M.]

[286] “Wahreit und Irrthum im Darwinismus.” Berlin, 1875.

[287] [Eng. ed. I have been reproached by competent authorities for
having clothed my ideas upon the theory of selection in the form of
a reply to Von Hartmann. I willingly admit that this author cannot
be considered as the leader of existing philosophical views upon the
theory of descent in Germany; Frederick Albert Lange has certainly a
much greater claim to this position. Lange does not however combat
this theory; he accepts and develops it most beautifully and lucidly
on a sound philosophical basis in such a manner as has never been
done before from this point of view (“Geschichte des Materialismus,”
3rd. ed., 1877, vol. ii. pp. 253-277). On most points I can but agree
with Lange. Von Hartmann, however, whose objections appeared to me to
be supported by a wide scientific knowledge, afforded me a suitable
opportunity of developing my own ideas upon some essential points
in the theory of selection. In this sense only have I attempted to
interfere with this author, the refutation of his views, as such,
having been with me a secondary consideration.] [The chief exponent
of the doctrine of organic evolution in this country is Mr. Herbert
Spencer, in whose “Principles of Biology,” vol. i. chap. xii., will be
found a masterly treatment of the theory of descent from a “mechanical”
point of view. R.M.]

[288] [The above views on the nature of variability, which were also
broadly expressed in the first essay “On the Seasonal Dimorphism of
Butterflies” (pp. 114, 115), are fully confirmed by Herbert Spencer
(_loc. cit._ chaps. ix. and x.), and more recently by A. R. Wallace in
an article on “The Origin of Species and Genera” (_Nineteenth Century_,
vol. vii., 1880, p. 93). See also some remarks by Oscar Schmidt in his
“Doctrine of Descent and Darwinism,” Internat. Scien. Ser. 3rd. ed.
1876, p. 173. R.M.]

[289] [This law has been beautifully applied by Herbert Spencer in
order to explain why, with an unlimited supply of food, an organism
does not indefinitely increase in size. “Principles of Biology,” vol.
i. p. 121-126. R.M.]

[290] [Eng. ed. This idea, formerly expressed by me, occurs also
in Lange (“Geschichte des Materialismus,” ii. 265), and is there
exemplified in a very beautiful manner by illustrations from modern
chemistry. Lange compares what I have termed above the “physical
constitution” of the organism to the chemical constitution of one of
those organic acids which by substitution of single elements may become
transformed into more complicated acids, but which, as it were, always
undergo “further development” in only one determined and narrowly
restricted course. Here, as with the organism, the number of possible
variations is very great, but is nevertheless limited, since “what
can or cannot arise is determined beforehand by certain hypothetical
properties of the molecule.”]

[291] “Origin of Species.” 4th German ed., p. 19; 5th English ed., p. 6.

[292] [Mr. A. R. Wallace, in his article last referred to, quotes some
most valuable measurements of mammals and birds, showing the amount of
variation of the different parts. These observations were published
by J. A. Allen, in a memoir “On the Mammals and Winter Birds of East
Florida,” &c. (Bulletin of the Museum of Comparative Zoology at Harvard
College, Cambridge, Mass., vol. ii. No. 3.) R.M.]

[293] [See note 142, p. 310. R.M.]

[294] “Die Darwin’sche Theorie,” Dorpat, 1875.

[295] [A certain number of instances of mimicry are known to occur
between species both of which are apparently nauseous. A most able
discussion of this difficult problem is given by Fritz Müller, in the
case of the two butterflies _Ituna Ilione_ and _Thyridia Megisto_, in
a paper published in _Kosmos_, May, 1879 (p. 100). The author shows by
mathematical reasoning that such resemblances between protected species
can be accounted for by natural selection if we suppose that young
birds and other insect persecutors have to learn by experience which
species are distasteful and which can be safely devoured. See also
Proc. Ent. Soc. 1879, pp. xx-xxix. R.M.]

[296] See Haeckel’s “Generelle Morphologie,” ii. 107.

[297] “Über die Berechtigung der Darwin’schen Theorie,” Leipzig, 1868.

[298] “Populäre wissenschaftl. Vorträge,” vol. ii., Brunswick, 1871, p.
208.

[299] “Das Unbewusste vom Standpunkte der Physiologie u.
Descendenztheorie,” Berlin, 1872, p. 89. The second edition appeared in
1877, in Von Hartmann’s own name.

[300] “Über die Berechtigung,” &c., Leipzig, 1868. In this work will be
found briefly laid down the theoretical conception of variability here
propounded somewhat more broadly. [In the last edition of the “Origin
of Species” Darwin states, with respect to the direct action of the
conditions of life as producing variability, that in every case there
are two factors, “the nature of the organism and the nature of the
conditions.” 6th ed. p. 6. R.M.]

[301] [Although hardly necessary to the evolutionist, it may perhaps
be well to remind the general reader, that all experiments upon
spontaneous generation, or abiogenesis, have hitherto yielded negative
results; no life is produced when the proper precautions are taken for
excluding atmospheric germs. But although we have so far failed to
reproduce in our laboratories the peculiar combination of conditions
necessary to endow colloidal organic matter with the property of
“vitality,” the consistent evolutionist is bound to believe, from
the analogy of the whole of the processes of nature, that at some
period of the earth’s history the necessary physical and chemical
conditions obtained, and that some simple form or forms of life arose
“spontaneously,” _i.e._ by the operation of natural causes. R.M.]

[302] See Haeckel’s “Generelle Morphologie,” vol. ii. p. 203, and
Seidlitz, “Die Darwin’sche Theorie,” 1875, p. 92 _et seq._

[303] [In a recently published work by Dr. Wilhelm Roux this author has
attempted to work out the idea of an analogy between the struggle for
existence and survival of the fittest in individuals and species, and
the struggle for existence and survival of the parts in the individual
organism. See “Der Kampf der Theile im Organismus: ein Beitrag zur
Vervollständigung der mechanischen Zweckmässigkeitslehre,” Leipzig,
1881. R.M.]

[304] [Eng. ed. Meanwhile it has been shown by Oscar Schmidt that
Von Hartmann, under the name of “the Unconscious,” re-invests
the old vital force with some portion of its former power. “Die
naturwissenschaftlichen Grundlagen der Philosophie des Unbewussten,”
Leipzig, 1877, p. 41.]

[305] _Loc. cit._ p. 175.

[306] _Loc. cit._ p. 156.

[307] “Über die Cuninen-Knospenähren im Magen von Geryonien.” Reprint
from “Mittheil. des naturwiss. Vereines,” Graz, 1875.

[308] [See Darwin’s “Origin of Species,” 6th ed. pp. 33, 34, and
201-204. R.M.]

[309] [Eng. ed. See Kant’s “Allgemeine Naturgeschichte und Theorie des
Himmels.”]

[310] “Das Unbewusste vom Standpunke der Physiologie und
Descendenz-Theorie,” Berlin, 1872, p. 16.

[311] [Eng. ed. See Lotze’s “Mikrokosmos,” 1st ed., vol. iii. pp.
477-483.]

[312] See Helmholtz’s “Populäre wissenschaftl. Vorträge,” vol. ii.,
Brunswick, 1872.

[313] See also Fr. Vischer’s “Studien über den Traum. Beilage zur
Augsburger Allgem. Zeitung,” April 14th, 1876. Haeckel also includes
this idea in his recent essay already quoted, “Die Perigenesis der
Plastidule,” Berlin, 1876, p. 38 _et seq._

[314] See Von Hartmann, _loc. cit._ p. 158.



INDEX.


  A.

  Abbot and Smith, descriptions of larvæ, 192, 216, 261, 263, 268, 452,
            453.

  _Acherontia Atropos_, South African form of larva, 263, 324, 531;
    larva in Spain, 324;
    larva phytophagically variable, 531.

  _Acosmeryx anceus_, larva, 192.

  _Acræa_ and Maracujà butterflies as larvæ, pupæ, and imagines, 536.

  _Acronycta_, affinities of genus, 169.

  Adaptation, 281, 589;
    analogous, 376;
    physiological, 590;
    mutual, 647.

  Allen, J. A., variation in birds and mammals, 658.

  Alpine form of _P. Napi_, 40.

  Alpine hare, seasonal change of colour, 7, 660.

  Alternation of generations, 80, 699, 702.

  _Amblystoma._ _A. mavortium_ from _Siredon lichenoides_, 567;
    _A. tigrinum_ bred by Duméril, 567;
    distribution of species of genus, 569;
    habits of species, 573;
    a reversion form, 531, 592;
    sterility of, 593;
    oviposition by, 594, 623;
    definition of genus, 610;
    degeneration of, 612;
    causes of reversion of, 600, 608, 613, 620, 631;
    _A. punctatum_ and _A. fasciatum_, development of, 623;
    summer sleep of, 628;
    _A. mavortium_, metamorphosis of, 629.

  _Ambulyx gannascus_ and _A. liturata_, larvæ, 245.

  Amixia, the principle of, 46, 110.

  Ammonites, 275.

  _Ampelophaga rubiginosa_, larva, 192.

  Amphibia of Upper Engadine, 615.

  _Amphiuma_, characters of genus, 579.

  _Anceryx_, larva, 264;
    stages 3, 4, and 5, 266.

  _Aphaniptera_, characters of, 498.

  Aphides, organic reproduction of, 97.

  _Araschnia._ _A. levana_ and _A. prorsa_ seasonally dimorphic, 2;
    dimorphism of larvæ, 6;
    _A. levana_, first experiments with, 10;
    _A. var. porima_, artificial production of, 10, 17;
    _A. levana_, Dorfmeister’s experiments with, 11;
    explanation of seasonal dimorphism of, 19;
    northern, extension of, 20;
    _A. prorsa_ gradual origination of, 22;
    _A. levana_, reversion of, caused by high temperature, 37;
    the _levana_ form the most constant, 43, and sexually dimorphic, 43;
    summer and winter forms compared, 56;
    experiments with _A. levana_, 117;
    _A. levana_, variable in all three stages, 405;
    incongruence in genus, 449.

  Argent, W. J., phytophagic variability of larva of _Sphinx Ligustri_,
            306.

  _Argynnis paphia_, dimorphism of, 250.

  _Artemia salina_ and _A. Milhausenii_, transformations of, 635.

  _Ascaris nigrovenosa_, propagation of, 90.

  Askenasy, on limits of variability, 114.

  Atavism and arrested growth, 587.

  _Aterica meleagris_, colour variations of, 8.

  Atoms, actions of, 710;
    sensibility of, 714.

  Axolotl, transformation into Amblystoma, 555;
    experiments with, 558;
    duration of transformation, 563;
    conditions of metamorphosis of, 564;
    distinguishing characters, 575.


  B.

  Baer, Karl Ernst von, on the mechanism of nature, 639;
    on the vital force, 641;
    on the theory of selection, 694;
    on causality and purpose, 708.

  Baird, Prof., on the development of species of _Amblystoma_, 623.

  Balbiani, reproduction of Aphides, 97.

  Balfour, F. M., on insect phylogeny, 485, 494.

  Bates, H. W., local variation in Amazonian Lepidoptera, 60.

  Bees, embryological development of, 483.

  Belt, Thomas, on coloured frogs, 294;
    on the glaciation of North America, 622.

  Biogenetic law, corollary of, 611.

  Birchall, E., local variation in British Lepidoptera, 60.

  Blanchard, on oviposition of _Amblystoma_, 594.

  Boisduval, figures of larvæ referred to, 216, 218, 245;
    on snake-like appearance of the larva of _D. nicæa_, 342;
    on the relationship of _Euchloe belia_ and _E. ausonia_, 48.

  _Bombycidæ_, early stage of larvæ, 166.

  _Bombyx._ _B. cynthia_, colour changes in larva, 7;
    _B. neustria_, caterpillar refused by lizards, 336;
    _B. Rubi_, caterpillar eaten by lizards, 338.

  Boscher, E., dimorphism of larva of _S. ocellatus_, 241, 306.

  Brauer, insect phylogeny, 493.

  Burmeister, figures of larvæ referred to, 195, 196, 224, 232, 255,
            261, 263, 436, 437;
    early stages of larvæ of Argentine butterflies, 166.

  Butler, A. G., affinities of genus _Acronycta_, 169;
    genera of _Chærocampinæ_, 190, 192, 194, 196;
    genera of _Smerinthinæ_, 233, 243, 244;
    genera of _Macroglossinæ_, 253, 254;
    species of _Pterogon_, 256;
    genera of _Sphinginæ_ and _Acherontiinæ_, 262;
    on coloured larvæ being rejected, 336;
    affinities of _Sesiidæ_, 370;
    descriptions and figures of new Sphinx-larvæ, 524, 526.

  Bütschli, embryology of bees, 483.

  Butterflies, action of warmth in producing changes in colour and
            marking, 58;
    summer and winter larvæ and pupæ alike in seasonally dimorphic
            species, 64;
    seasonally dimorphic species hibernate as pupæ, 73;
    odoriferous organs, 102;
    early stages of larvæ, 165;
    di- and polymorphism of, 250;
    crepuscular habits of certain species, 475.


  C.

  _Callosune eupompe_, polymorphism of female, 251.

  Cameron, P., on larva of _S. ocellatus_, 241, 282;
    protective habits of saw-fly larvæ, 290, 294.

  Carlin, W. E., on _Siredon lichenoides_, 630.

  Caterpillars, origin of markings, 161;
    means of defence, 289;
    defensive habits, 290;
    food of brightly coloured species, 294;
    habit of concealment by day, 291, 296;
    phytophagic variability, 305, 531;
    seasonal variability, 305;
    experiments with ocellated species,
  330;
    experiments with annulated species, 336;
    sexual dimorphism of, 308, 527, 534;
    independent variability of stages, 416.

  _Cecidomyia_, metagenesis of, 82, 586.

  _Chærocampa._ _C. elpenor_ and _C. porcellus_, relationship of, 171;
    markings of larvæ of the genus, 177;
    _C. elpenor_, 1st and 2nd larval stages, 178;
      3rd and 4th stages, 180;
      5th stage, 181;
      6th stage, 183;
    _C. porcellus_, 1st larval stage, 184;
      2nd and 3rd stages, 185;
      4th stage, 186;
    _C. elpenor_ and _porcellus_, larval markings compared with other
            species of _Chærocampa_, 188;
    _C. elpenor_, experiments with larva, 331, 332;
    _C. Japonica_ and _C. Lewisii_, larvæ, 194;
    _C. cretica_, larva, 524;
    _C. lycetus_, larva, 527;
    _C. capensis_, larva, 529;
    eaten by birds, &c., 530.

  Characters, new, backward transference of, 279.

  Chauvin, Fräulein von, experiments with Axolotl, 558;
    experiments with _Salamandra atra_, 615.

  Chavannes, figure of larva referred to, 255.

  Clarke, development of _Amblystoma punctatum_, 623.

  Classification, objects of, 168.

  Claus, on the origination of heterogenesis, 89.

  Clemens, larva of _A. ello_, 268.

  Climatic variation, 45;
    in _P. Napi_, 47;
    in _E. Belia_, 47;
    nature of causes producing, 52, 105;
    by sex, 61, 62;
    climatic varieties very distinct from local, 45;
    climatic influences operate slowly, 58;
    climatic changes gradual, 652, 660.

  Cole, B. G., seasonal dimorphism of _E. punctaria_, 4.

  Coleoptera, grub-like larvæ of, 495.

  _Colias edusa_, dimorphism of female, 250.

  Colour, biological value of, 289.

  Conditions of life, compulsory changes in, 651.

  Congruence in Lepidopterous families, 435;
    in Lepidopterous genera, 444.

  _Coniferæ_, food-plants of _Anceryx_, 268.

  Convergence of characters in _Ophiuroidea_, 396;
    in Dipterous larvæ, 494.

  Cope, Prof., habits of _Siredon Mexicanus_, 566;
    on distribution of _Amblystoma_, 569.

  Correlation of growth, 278, 363, 388, 415;
    action of, 415, 516;
    Von Hartmann’s views of, 670;
    nature of, 673.

  _Cryptobranchus_, characters of, 579.

  Cyclical propagation, 82, 699;
    and reversion, 613.


  D.

  Daphniacea, differences between allied species and genera, 516.

  Darwin, C., influence of external conditions of life, 59;
    inheritance at corresponding periods, 63, 275, 431;
    markings of butterflies, 101;
    definition of eye-spots, 326;
    adaptation in larvæ, 392;
    correlation of growth, 516;
    degeneration, 583;
    adaptation, 589;
    causes of sterility, 596;
    limited variability, 656;
    factors of variability, 676;
    abrupt variations, 701.

  Degeneration, 583;
    phyletic, 611.

  _Deilephila_, division of larvæ into groups, 199;
    _D. Euphorbiæ_, 1st larval stage, 202;
      2nd and 3rd stages, 203;
      4th and 5th stages, 205;
    _D. nicæa_, larva, 207;
    _D. dahlii_, larva, 208;
    _D. vespertilio_, larva, 209;
    _D. Galii_, larva, 211;
      5th stage, 213;
    _D. livornica_, larva, 215;
    _D. Zygophylli_, larva, 217;
    _D. Hippophaës_, larva, 218, 650;
    division of larvæ according to phyletic stages, 223;
    genealogy of, 358;
    _D. Galii_, larva rejected, 340;
    _D. Euphorbia_, larva eaten, 340;
    habits of larvæ and imagines, 412;
    _D. Robertsi_, larva, 525.

  Development;
    of marking of Sphinx-larvæ, laws of, 274;
    phyletic, of markings of _Sphingidæ_, 370;
    unequal phyletic, 460.

  Dimorphism and polymorphism;
    of butterflies, 250;
    of Sphinx-larvæ, 300;
    of pupæ, 412.

  Diptera, incongruences among, 488;
    imaginal characters of, 488;
    divisions of, 498.

  Dodo, extinction of, 651.

  Dohrn, A.;
    on degeneration, 583;
    on functional change, 590.

  Dorfmeister, G.;
    experiments with _A. levana_, 11.

  Duponchel;
    figures of larvæ referred to, 207, 208, 218, 253.

  Duméril;
    experiments with Axolotl, 555, 564, 566, 593;
    characters of Axolotl, 375, 576.


  E.

  Edwards, W. H., seasonal dimorphism of _P. pseudargiolus_ and
            _P. violecea_, 4;
    of _P. Tharos_ and _P. Marcia_, 4;
    of _P. Ajax_, 30;
    of _L. Artemis_ and _L. Proserpina_, 32;
    experiments with _P. Ajax_ and _P. Tharos_, 33, 126, 140;
    pupal period of winter forms of _P. Ajax_, 36;
    experiments with _G. interrogationis_, 149;
    early stages of larvæ of N. American butterflies, 165;
    larva of _P. pseudargiolus_ and attendant ants, 290;
    figures of larvæ referred to, 165, 436, 437.

  Eimer, colour variation in _Lacerta muralis_, 361.

  _Emmelesia unifasciata_, larva phytophagically variable, 307.

  Environment, direct and indirect action of, 686.

  _Ephyra punctaria_ and _E. omicronaria_, seasonal dimorphism of, 4.

  _Eriogaster lanestris_, larva eaten by lizards, 336.

  _Eriotus Pteridis_, resemblance of larva, 320.

  _Euchelia Jacobææ_;
    larva and imago rejected by lizards, 336;
    constant in all three stages, 405.

  _Euchloe_:
    _E. belia_ and _E. ausonia_, seasonal dimorphism of, 3;
    _E. belia_, climatic variation of, 47;
    var. _Simplonia_ in Alps, 48;
    _E. belemia_, seasonal dimorphism and migration of, 78.

  _Euproctus Rusconii_, habits of, 617.

  _Eusmerinthus Kindermanni_, larva, 526.

  Evershed, J., larva of _C. porcellus_, 188.

  Eye-spots;
    repetition of, 277;
    development of, 325, 327;
    terrifying action, 330;
    resemble flower-buds, 334;
    useful when rudimentary, 366.


  F.

  Fatiot, Amphibia of Upper Engadine, 616.

  Fechner, on conscious matter, 714.

  Filippi, De, _Triton alpestris_, 585.

  Forel, Prof., fresh water _Pulmonifera_, 590.

  Form-relationship and blood-relationship, not always coincident, 395.

  Frantzius, Von, on climate of Mexico, 619.

  Frogs, 615, 616, 618;
    inedible species, 294.

  Function, change of, 365.


  G.

  Gegenbaur, Prof., the skull of Axolotl, 576.

  Gehrig, breeding of Axolotl, 565.

  Gené, habits of _Euproctus Rusconii_, 617.

  Genera, Lepidopterous, show greatest congruence, 459.

  Generation, heterogeneous, 679, 698, 702.

  Gentry, T. G., phytophagic variability of caterpillars, 306.

  Gerstäcker, division of the Diptera, 498.

  _Geotriton fuscus_, habits, 617.

  Glacial epoch; butterflies monogoneutic during, 19, 72;
    in N. America, 622.

  Gooch, W. D., description of larvæ referred to, 436;
    crepuscular butterflies, 476;
    sexual dimorphism of caterpillars, 535.

  Gooseberry, not increased in size, 654.

  Goss, H., larva of _C. porcellus_, 188.

  Gosse, P. H., embryonic larva of _P. Homerus_, 167.

  _Grapta interrogationis_, experiments with, 149.

  Green colour of caterpillars, 293, 310.

  Growth, law of, 655.

  Gynandromorphism in butterflies, 249.

  _Gynanisa Isis_, young larva, 166.


  H.

  Haeckel, Prof., on metagenesis, 80, 83;
    on ontogeny, 270;
    on adaptation, 281, 589;
    on the biogenetic law, 611;
    on reproduction and growth, 664;
    the plastidule theory, 667.

  Hare, white variation of, 660.

  Hartmann, E. von; his views examined, 645;
    on variability, 646, 652, 655, 664;
    on heredity, 657;
    his “Philosophy of the Unconscious,” 670;
    on correlation, 670, 673;
    on the nature of species, 671;
    on design in nature, 696;
    on the sensibility of atoms, 714.

  Helmholtz, Prof., on natural laws, 668;
    on the senses, 711.

  _Hemaris hylas_, larva, 255.

  Heredity, alternating and continuous, 24;
    cyclical, 66;
    homochronic, 63, 431;
    not mechanical, 657;
    nature of, 667.

  Herrich-Schäffer, wing neuration of butterflies, 449.

  Hertwig, on teeth of Amphibia, 376.

  Heterogenesis, definition of, 82, 96;
    parallelism between and metamorphosis, 86;
    origination of, 89.

  Heyden, Von, reproduction of _Aphis_, 97.

  Hilgendorf, on fossil shells, 109.

  _Hipparchia semele_, colour variations, 8.

  Hoffmann, Ernst, on immigration of European butterflies, 77.

  Horsfield and Moore, figures of caterpillars referred to, 193, 195,
            196, 243, 253, 261.

  Hübner, figures of caterpillars referred to, 193, 208, 211, 218, 231,
            267.

  Humboldt, Baron, on the Lake of Mexico, 608, 621.

  Huxley, Prof., “Anatomy of Invertebrated Animals” referred to, 498.

  _Hydromedusæ_, alternation of generations in, 83;
    want of parallelism in different generations of, 395.

  Hydrozoa, asexual reproduction and polymorphosis, 83.

  Hymenoptera, incongruence among, 481;
    imaginal characters, 481;
    causes of incongruence, 486.


  I.

  Ichthyodea, characters of, 577, 579.

  Incongruence; in Lepidopterous families, 437;
    genera, 446, 449;
    species, 451;
    varieties, 456;
    two kinds of, 458, 502;
    in Hymenoptera, 481;
    in Diptera, 488;
    different forms of, 503;
    dependent upon action of environment, 505, 507, 508.

  Inheritance; homochronic, 63, 431;
    definition of cyclical, 66;
    limited by sex, 68.

  Insects, ancestry of, 493.

  Intermaxillary gland of Axolotl, 623.


  J.

  Jullien, on _Lissotriton punctatus_, 591.


  K.

  Kant, referred to, 638, 709.

  Kirby, W. F., “Synonymic Catalogue” referred to, 2, 49, 435, 449.

  Kleemann, larva of _S. Ligustri_, 259.

  Knaggs, Dr. H. G., seasonal dimorphism of _S. illustraria_, 4.

  Kölliker, Prof., experiments with Axolotl, 564;
    characters of Axolotl, 575.


  L.

  _Lacerta muralis_, colour variations, 361.

  Lake of Mexico, 603, 608, 621.

  Lange, F. A., “History of Materialism” referred to, 645, 656.

  Langer, Prof., experiments with _Pelobates_ tadpoles, 607.

  Lankester, Prof. E. R., on degeneration, 583.

  _Lasiocampa Pini_, larva eaten by lizards, 336;
    variable in two stages, 405.

  Lepidoptera; foes, 8;
    larva and imago independently variable, 401;
    species constant in three stages, 405;
    species variable in three stages, 405;
    variable in two stages, 405;
    variable in one stage, 406;
    causes of variability of pupæ, 411;
    case of incongruence among pupæ, 540.

  _Leptodora hyalina_, development, 93.

  _Lepus timidus_, white variations of, 662.

  Leuckart, reproduction of _Ascaris nigrovenosa_, 90;
    on growth, 654.

  Leydig, on sexual larvæ of _Triton_, 591.

  _Limenitis Artemis_ and _L. Proserpina_, dimorphic in both sexes, 32;
    _L. Camilla_, constant in three stages, 405.

  Lines, dorsal, subdorsal, supra- and infra-spiracular, 175.

  _Lissotriton punctatus_, sexual larvæ, 591, 595.

  Lizards, colour variation of, 361;
    rejection of caterpillars by, 336.

  Local forms, distinct from climatic varieties, 45;
    produced by isolation, 109.

  Lockyer, B., young _Noctua_ larvæ, 166.

  _Lophostethus Dumolinii_, larva, 527.

  _Lophura hyas_, larva, 254.

  Lubbock, Sir John, insect metamorphosis, 65, 431;
    derivation of metagenesis, 83;
    colours of caterpillars, 294;
    ancestry of insects, 493.

  _Lycæna pseudargiolus_, larva and ants, 290.


  M.

  _Macroglossa_, larvæ of, 245;
    _M. stellatarum_, oviposition of, 245;
    1st, 2nd, and 3rd larval stages, 246;
    4th and 5th stages, 247;
    di- and polymorphism of larva, 247;
    pupa of, 250;
    _M. belis_, and _M. pyrrhosticta_, larvæ, 255.

  Markings, general biological value of, 285;
    special biological value of, 308;
    subordinate, 347;
    of _Sphingidæ_, evolution of, 380.

  Marsh, Prof., on _Siredon lichenoides_, 567.

  Matter, assumption of, 711;
    conscious, 714.

  Mayer, Paul, insect phylogeny, 493.

  McLachlan, R., phytophagic variability, 305.

  Mechanism and teleology, 694.

  _Medusæ_, internal parasitism, 699.

  _Melanopsis recurrens_, from _Paludina_ bed, 275.

  _Menopoma_, characters of, 579.

  Mérian, Madame, figures of caterpillars referred to, 195, 232, 263,
            268.

  Metagenesis, defined, 81, 96;
    derivation, 83.

  Metamorphosis, 430.

  Mexico, climate of, 619.

  Millière, figure of larva referred to, 254.

  Mimicry, 648, 663.

  Möller, L., phytophagic variability, 305.

  Mono- and digoneutic species, 16.

  Moore, F., figures of caterpillars referred to, 436, 526.

  Morris, affinities of _Apatura_ and _Nymphalis_, 438.

  Moths, seasonal dimorphism in, 4;
    resembling splinters, 292.

  Mühlenpfordt, on Lake of Mexico, 604.

  Müller, Fritz, odoriferous organs of butterflies, 102;
    sexual selection in butterflies, 103;
    habit and protective resemblance, 290;
    spiny caterpillars, 293;
    larva of _Papilio Evander_, 299;
    reversion in Orchids, 614;
    maggot-like larvæ of insects, 493;
    the biogenetic law, 611;
    mimicry, &c., 665.

  Müller, Hermann, larva of _Stauropus Fagi_, 290.

  Müller, P. E., _Leptodora hyalina_, 94.

  Murray, Andrew, seasonal change of colour, in animals, 7;
    green caterpillars, 293.

  _Muscidæ_, embryonic development, 492.


  N.

  Nägeli, reversion of cultivated plants, 107.

  Natural selection, 112, 361, 704, 705.

  Nature, mechanical conception of, 634;
    harmony of, 697.

  Neumayr and Paul, on _Melanopsis_, 275.

  _Noctuæ_, change of colour in larvæ, 166;
    number of legs in young larvæ, 166, 520;
    ontogeny of larvæ, 520.

  Noll, Dr., larva of _Ach. Atropos_, 324.

  North America, glaciation of, 622.

  Nurse-breeding in Hymenoptera and Diptera, 402.

  _Nymphalidæ_, larval classification, 435.


  O.

  Ontogeny, Fritz Müller and Haeckel on, 270;
    predominance of new characters in last stage of, 280, 283.

  Ontogeny and Phylogeny of Sphinx-markings, 177.

  Ophiuroidea, want of parallelism in, 396.

  Orchids, reversion in, 614.

  Organic compounds, synthesis of, 643.

  Organism, parts independently variable, 514.


  P.

  _Pachylia Ficus_, early stages of larva, 232.

  Packard, insect phylogeny, 493.

  Pangenesis, theory of, 668.

  _Papilio._ _P. Ajax_, seasonal forms, 30;
    _P. Turnus_, a dimorphic form, 32;
    _P. Machaon_ and _Podalirius_, seasonally dimorphic in Spain and
            Italy, 74;
    _P. Ajax_, experiments with, 126;
    _P. Merope_, young larva, 166;
    _P. Homerus_, young larva, 167;
    _P. Machaon_, larva rejected by lizards, 339.

  Parallelism, phyletic, in metamorphic species, 390;
    phyletic, law of, 510.

  _Pararga Ægeria_, sexually dimorphic, 68;
    climatic variation of, 68.

  Pascoe, F. P., “Zoological Classification” referred to, 488, 498.

  _Pelobates_, experiments with tadpoles, 607.

  _Pergesa Mongoliana_, larva, 194.

  _Philampelus achemon_ and _P. satellitia_, larvæ, 523;
    _P. labruscæ_, larva, 195;
    _P. vitis_, and _P. anchemolus_, larvæ, 232.

  Philosophy, function of, 640.

  _Phyciodes Tharos_ and _P. Marcia_, seasonal dimorphism of, 4;
    experiments with, 140.

  Phylogeny, conclusions from, 270;
    and ontogeny of Sphinx-markings, 177;
    of insects, 493.

  _Pierinæ_, seasonal dimorphism of, 13;
    analogous seasonal dimorphism in, 60, 108;
    experiments with, 122.

  _Pieris Napi_, experiments with, 13;
    summer form the younger, 29;
    reversion of, caused by mechanical vibration, 38;
    var. _Bryoniæ_, the potential winter form, 39;
    experiments with this var., 40;
    this var. the parent form of _Napi_, 41;
    a climatic var., 41;
    very variable, 43;
    variability due to crossing, 43;
    var. _æstiva_ more variable than var. _vernalis_, 43;
    _P. Napi_, summer and winter forms compared, 56;
    incongruence in this species, 457;
    _P. Krueperi_, seasonal dimorphism of, 78;
    _P. Brassicæ_, larva rejected by lizards, 337;
    constant in three stages, 405;
    photographic sensitiveness of pupa, 405;
    _P. Napi_, variable in two stages, 405.

  Plants, cultivated, 107;
    sterility of reverted, 596.

  Plastidule theory, 667.

  _Plebeius amyntas_ and _P. polysperchon_, seasonal dimorphism, 2;
    _P. Alexis_, do., 3;
    _P. pseudargiolus_ and _P. violacea_, do., 4;
    _P. agestis_, do., 50;
    polymorphism of females in this genus, 251.

  Pluvial period, 621.

  Polar animals, colour of, 658, 660.

  _Polyommatus phlæas_, distribution and climatic variation, 49;
    seasonal dimorphism, 50;
    Italian summer and winter forms compared, 55.

  _Polyptychus dentatus_, larva, 244.

  Ptarmigan, seasonal colours of, 6.

  _Pterogon_, larvæ of, 255;
    _P. ænotheræ_, larva, 256.

  _Pulmonifera_, functional change in lungs, 590.

  _Pygæra bucephala_, larva rejected by lizards, 337.


  Q.

  Quatrefages, De, spermatozoa of _Amblystoma_, 593.


  R.

  _Rana temporaria_ of Upper Engadine, 618.

  Ratzeburg, larva of _Anc. Pinastri_, 265.

  Reproduction and growth, 664.

  Reversion, of species, 611;
    two modes of, 612;
    periodic, 613.

  _Rhopalocera_, characters of, 433, 471.

  _Rhytina Stelleri_, extinction of, 651.

  Riley, C. V., descriptions of caterpillars referred to, 437, 521, 522;
    on sexual dimorphism in caterpillar, 535.

  Ring-spots, 325;
    development, 327;
    signs of distastefulness, 341;
    resembling berries, 344.

  Rösel, breeding of _Aras. Levana_ by, 6;
    on larva of _Deil. Euphorbiæ_, 206;
    of _D. Galii_, 213;
    of _Smer. Tiliæ_, 236;
    of _S. ocellatus_, 240;
    of _Anc. Pinastri_, 265, 268.

  Roux, Dr. W., struggle of parts in the organism, 689.

  Rutherford, D. G., colour variations of _Ater. meleagris_, 8.


  S.

  Sacc, on sterility of _Amblystoma_, 593.

  Salamanders, habits of Italian land species, 617;
    oviposition of, 630.

  _Salamandra atra_, experiments with, 615.

  _Salamandrina_, characters of, 577;
    _S. perspicillata_, habits of, 617.

  Sars, development of _Leptodora hyalina_, 94.

  _Saturnia._
    _S. Yamamai_, variable in first stage only, 406;
    _S. carpini_, development of larva of local forms, 419.

  _Satyrinæ_, hibernation of larvæ, 73.

  Saussure, De, on _Siredon Mexicanus_, 565, 602;
    physical character of Mexican Lake, 603.

  Saw-flies, protective habits of larvæ, 290.

  Schmankewitsch, transformation of _Artemia_, 635.

  Schmidt, Oscar, on convergence of character, 396, 459;
    on variability, 654.

  Schreibers, sexual _Triton_ larvæ, 591;
    arrested tadpoles, 607.

  Schulze, F. E., parasitism in _Medusæ_, 699.

  Science, function of, 640.

  Scudder, S. H., embryonic larvæ of butterflies, 166.

  Seasonal dimorphism, origin and significance, 1;
    seasonal change of colour in animals, 7;
    and climatic variation, 45;
    seasonal adaptation in caterpillars, 305.

  Seidlitz, reproduction of Axolotl, 571;
    white var. of hare, 662.

  _Selenia tetralunaria_, _S. illunaria_, _S. lunaria_, and
            _S. illustraria_, seasonal dimorphism, 4.

  Semper, C., origin of alternation of generations, 84;
    transformation of _Amblystoma mavortium_, 629.

  Semper, G., young Sphinx-larvæ, 166;
    figures of caterpillars referred to, 522.

  Senses, physiology of, 711.

  Sepp, figure of caterpillar referred to, 262.

  _Sesiidæ_, affinities of, 370.

  Sexual dimorphism, of butterflies, 32, 250;
    of caterpillars, 308, 527, 534;
    secondary sexual characters, 62;
    sexual selection, 69, 102.

  Siebold, Von, on _Pulmonifera_, 590.

  _Siredon._ _S. Mexicanus_, habits, 565;
    _S. lichenoides_, transformation, 567, 630;
    phyletic advance of genus, 584;
    retention of genus, 610;
    _S. tigrinus_ from L. St. Isabel, 626.

  Slater, J. W., food of gaily coloured caterpillars, 294.

  _Smerinthus_, larvæ of, 232;
    _S. Tiliæ_, 233;
    4th stage, 235;
    _S. Populi_, 236;
    and stage, 237;
    3rd and 4th stages, 238;
    5th stage, 239;
    _S. ocellatus_, 240;
    dimorphism of larva, 241;
    _S. Tiliæ_, variable in two stages, 405;
    North American species of, 453;
    _S. tatarinovii_, and _S. planus_, larvæ, 244.

  Species, causes of transformation of, 116;
    nature of, 671.

  Specific characters, 91;
    may originate through direct action, 100.
    Specific constitution, 112.

  Spencer, Herbert, theory of descent, 645;
    variability, 654;
    law of growth, 655.

  Spermatozoa, of _Amblystoma_, 593;
    of _Lissotriton punctatus_, 595.

  Speyer, Dr., seasonal dimorphism of moths, 4;
    of _P. phlæas_ in Germany, 50.

  _Sphingidæ_, oviposition, 164;
    congruence and incongruence in genera, 450.

  _Sphinx_-markings, ontogeny and morphology, 177.
    Larvæ of genus, 259;
    _S. Convolvuli_, 650.
    Origination of markings, 273;
    law of development, 274;
    forms of markings, 309;
    _S. Ligustri_, constant in three stages, 405.

  Stainton, H. T., larval characters of _Notodontidæ_, 443;
    young larvæ of _Triph. pronuba_, 520.

  Staudinger, Dr., seasonal dimorphism of _Euch. Belia_ and
            _E. Ausonia_, 3, 48;
    butterflies of Finnmark, 20;
    _P. Napi_ var. _Bryoniæ_ from Lapland, 44, 49;
    larvæ of _Ch. alecto_, 193;
    affinities of _Nymphalis_ and _Apatura_, 438.

  _Stauropus Fagi_, protective habits of larva, 290.

  Sterility, causes of, 596;
    a result of reversion, 597;
    law of, in reversion forms, 599.

  Stoat, seasonal change of colour, 7.

  Strauch, “Revision of _Salamandridæ_” referred to, 570, 577.

  Strecker, H., N. American species of _Smerinthus_, 453.

  Stripes, longitudinal, 312, 372;
    oblique, 317, 373.


  T.

  Tadpoles, arrested development, 607.

  Tegetmeier, W. B., metamorphosis of _Siredon_, 566.

  Teleology and Mechanism, 694.

  _Terebrantia_, larvæ, 508.

  Transformation, causes of, 460, 496;
    factors of, 676.

  _Trematoda_, alternation of generations in, 83.

  Trimen, Roland;
    larva of _Lopho. Dumolinii_, 527;
    of _Ch. Capensis_, 529;
    phytophagic variability of larva of _Ach. Atropos_, 531.

  _Triptogon roseipennis_, larva, 244.

  _Triton_;
    _T. alpestris_, sexual larva, 585;
    reproductive larvæ, 590.
    Degeneration in genus, 612.
    Species of Upper Engadine, 631.

  Typical parts, modified by environment, 501, 513.


  U.

  Upper Engadine;
    Amphibia, 615;
    climate, 616;
    frog from, 618;
    Tritons of, 631.

  Urea, synthesis of, 643.


  V.

  _Vanessa._
    _V. Urticæ_, climatic variation, 60;
    _V. Atalanta_, _Urticæ_, and _polychloros_, variable in two stages,
            405;
    _V. Io_, variable in one stage, 407.
    Congruence and incongruence in genus, 446;
    phyletic completeness of, 447;
    arrangement of spines on larvæ, 448.

  Variability, origin, 107;
    limited, 114, 362, 653, 655;
    independent in different stages, 402;
    definition of, 404;
    a relative term, 408;
    causes of, in pupæ, 411;
    primary and secondary, 416;
    independent in different larval stages, 416;
    phytophagic, 305, 531;
    Von Hartmann’s views, 646;
    not unlimited, 647, 652, 655, 683;
    factors of, 676, 684;
    mechanical theory of, 677;
    individual, 681, 685.

  Variation; climatic, 45, 686;
    causes of, 105;
    analogous, 683;
    abrupt not inherited, 701.

  Varieties, climatic, distinct from local, 45.

  Velasco, Señor, Axolotl of L. St. Isabel, 626.

  Vital force, phyletic, elimination of, 287;
    objections to, 352, 461, 511, 518;
    ontogenetic, abandonment of, 641, 687.


  W.

  Wallace, A. R.;
    introduction of the expression “seasonal dimorphism,” 1;
    on the dull colours of female butterflies, 8;
    sexual dimorphism of butterflies, 32;
    local variation in colour, 60;
    comparison of seasonal dimorphism with alternation of generations,
            80;
    the bright colours of caterpillars, 293;
    on adaptation, 589;
    on variability, 654, 658.

  Wallace, Dr., colour changes in larva of _B. Cynthia_, 7.

  Walsh, B. D., phytophagic variation, 305.

  Weale, J. P. M., on the young larvæ of _Pap. merope_ and _Gyn. Isis_,
            166;
    the larva of _Ach. Atropos_ in S. Africa, 263;
    habits of S. African Sphinx-caterpillars, 290.

  Weir, J. Jenner, the colour variations of _Hip. semele_, 8;
    experiments with brightly-coloured, hairy, and spiny caterpillars,
            336.

  Westwood, Prof. J. O., seasonal dimorphism of _Eph. punctaria_, 4;
    figure of caterpillar referred to, 243;
    gynandromorphic butterfly, 249.

  White colour, protection afforded by, 6, 659, 661.

  White, Dr. F. B., local variation in British Lepidoptera, 60;
    young _Notua_ larvæ, 166.

  Wiedersheim, on the anatomy of _Amblystoma_, 577;
    habits of _Geotriton fuscus_, 617;
    anatomy of Axolotl, 623.

  Wigand, on the non-increase in size of the gooseberry, 654.

  Wilde, O., on larva of _Dei. Galii_, 213;
    of _Dei. Hippophaës_, 218;
    larval characters of the _Notodontidæ_, 443.

  Wilson, Owen, figure of larva of _Ch. porcellus_ referred to, 188.

  Wöhler, Prof., the synthesis of urea, 643.

  Wood, T. W., photographic sensitiveness of Lepidopterous pupæ, 405.

  Würtemberger, on Ammonites, 275.


  Y.

  Yenisei, Swedish Expedition referred to, 20.


  Z.

  Zeller, P. C., seasonal dimorphism of _Pl. Polysperchon_ and
            _Pl. Amyntas_, 2;
    of Italian butterflies, 75.



ERRATA.


  Page 81, line 8 from top, and throughout essay, for “_Daphnidæ_” read
      “_Daphniidæ_.”

  Page 95, line 3 from bottom, for “Daphnoidea” read “Daphniacea.”

  Page 166, line 7 from bottom (note), for “p. 438” read “p. 433.”

  Page 245, line 17 from bottom (note), for “Ställ” read “Stoll.”

  Page 263, after the word “insects” (bottom line of note), add, “but
      the whole marking is suggestive of distastefulness.”

  Page 296, line 3 from bottom, for “Stähelina--collector” read
      “Stähelin--a collector.”

  Page 305, line 5 from bottom (note), for “In 1869” read “In 1865.”

  Page 434, bottom line of note, for “Geometræ” read “Bombycidæ.”

  Page 494, line 2 from top, for “from which a larval form” read “from
      a larval form which.”

  Page 542, line 12 from top, for “_Dione Vanilla_” read “_Dione
      Vanillæ_.”

  Page 544, line 15 from bottom, for “_Siderome_” read “_Siderone_.”



Transcriber’s Notes


Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in this book; otherwise they were not
changed.

Simple typographical errors were corrected; occasional unbalanced
quotation marks were corrected.

Ambiguous hyphens at the ends of lines were retained.

“Errata” at the end of this Volume have been applied to the relevant
text of this eBook.

This is Volume II of a two-volume set. The Table of Contents for both
volumes is in Volume I.

The Index for both volumes is in this one. References to pages 1-400
will be found in Volume I.

Footnotes and references to them have been renumbered into one
continuous sequence and moved to the end of the text of this eBook,
immediately preceding the Index. The sequence begins at “171” because
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Page 414: “Hippophae” was printed that way, rather than as “Hippophæ”.

Page 423: “Stage VII. Pupation.” was printed in the second column.

Page 436: “Phaeton” was printed that way, rather than as “Phæton”.

Page 642: “yelk” was printed that way.

Page 539: The names in the “Species” line were italicized, but the
underscores used here to represent italics have been omitted from that
line to make the diagram narrower.

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