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Title: Darwin, and After Darwin (Vol 3 of 3) - Post-Darwinian Questions: Isolation and Physiological Selection
Author: Romanes, George John, 1848-1894
Language: English
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DARWIN, AND AFTER DARWIN. An Exposition of the Darwinian Theory and a
Discussion of Post-Darwinian Questions.

    1. THE DARWINIAN THEORY. With portrait of Darwin. 460 pages. 125
        illustrations. Cloth, $2.00.

    2. POST-DARWINIAN QUESTIONS. Edited by Prof. C. Lloyd Morgan. With
        portrait of G. J. Romanes. 338 pages. Cloth, $1.50.

        Edited by Prof. C. Lloyd Morgan. With portrait of Mr. J. T.
        Gulick. 181 pages. Cloth, $1.00.

    All three volumes together, $4.00 net.

AN EXAMINATION OF WEISMANNISM. With portrait of Weismann. 236 pages.
Cloth, $1.00.

THOUGHTS ON RELIGION. Edited by Charles Gore, M.A., Canon of
Westminster. Third Edition. 184 pages. Cloth, gilt top, $1.25.


[Illustration: Frontispiece--John J. Gulick]





_Honorary Fellow of Gonville and Caius College, Cambridge_






The Lakeside Press


Of the six chapters which constitute this concluding volume of G. J.
Romanes' _Darwin, and after Darwin_, three, the first two and the last,
were in type at the time of his death. I have not considered myself at
liberty to make any alterations of moment in these chapters. For the
selection and arrangement of all that is contained in the other three
chapters I am wholly responsible.

Two long controversial Appendices have been omitted. Those marked A and
B remain in accordance with the author's expressed injunctions. In a
third, marked C, a few passages from the author's note-books or MSS.
have been printed.

The portrait of the Rev. J. Gulick, which forms the frontispiece, was
prepared for this volume before the author's death. Mr. Gulick's chief
contributions to the theory of physiological selection are to be found
in the Linnean Society's _Journal_ (_Zoology_, vols. xx and xxiii), and
in four letters to _Nature_ (vol. xli. p. 536; vol. xlii. pp. 28 and
369; and vol. xliv. p. 29).

I have to thank Mr. Francis Galton, D.C.L., F.R.S. and Mr. F. Howard
Collins for valuable assistance generously rendered for the sake of one
whom all who knew him held dear. For he was, if I may echo the words of
Huxley, "a friend endeared to me, as to so many others, by his kindly
nature, and justly valued by all his colleagues for his powers of
investigation and his zeal for the advancement of science."


BRISTOL, _May 1897_.


ISOLATION                                                  1

ISOLATION (_continued_)                                   28

PHYSIOLOGICAL SELECTION                                   41



ORGANIC EVOLUTION                                        101

GENERAL CONCLUSIONS                                      144

VIEWS ON PHYSIOLOGICAL SELECTION                         151

EVER ACTING ALONE                                        157

NOTE-BOOKS                                               169




This treatise will now draw to a close by considering what, in my
opinion, is one of the most important principles that are concerned in
the process of organic evolution--namely, Isolation. I say in _my_
opinion such is the case, because, although the importance of isolation
is more or less recognized by every naturalist, I know of only one other
who has perceived all that the principle involves. This naturalist is
the Rev. J. Gulick, and to his essays on the subject I attribute a
higher value than to any other work in the field of Darwinian thought
since the date of Darwin's death[1]. For it is now my matured conviction
that a new point of departure has here been taken in the philosophy of
Darwinism, and one which opens up new territories for scientific
exploration of an endlessly wide and varied character. Indeed I believe,
with Mr. Gulick, that in the principle of Isolation we have a principle
so fundamental and so universal, that even the great principle of
Natural Selection lies less deep, and pervades a region of smaller
extent. Equalled only in its importance by the two basal principles of
Heredity and Variation, this principle of Isolation constitutes the
third pillar of a tripod on which is reared the whole superstructure of
organic evolution.

    [1] It will be remembered that I regard Weismann's theory of
    heredity, with all its deductive consequences, as still _sub

By isolation I mean simply the prevention of intercrossing between a
separated section of a species or kind and the rest of that species or
kind. Whether such a separation be due to geographical barriers, to
migration, or to any other state of matters leading to exclusive
breeding within the separated group, I shall indifferently employ the
term isolation for the purpose of designating what in all cases is the
same result--namely, a prevention of intercrossing between A and B,
where A is the separated portion and B the rest of the species or kind.

The importance of isolation as against dissimilar forms has always been
fully appreciated by breeders, fanciers, horticulturists, &c., who are
therefore most careful to prevent their pedigree productions from
intercrossing with any other stock. Isolation is indeed, as Darwin has
observed, "the corner-stone of the breeder's art." And similarly with
plants and animals in a state of nature: unless intercrossing with
allied (i.e. dissimilar) forms is prevented, the principle of heredity
is bound to work for uniformity, by blending the dissimilar types in
one: only when there is exclusive breeding of similarly modified forms
can the principle of heredity work in the direction of change--i.e. of

Now, the forms of isolation--or the conditions which may lead to
exclusive breeding--are manifold. One of the most important, as well as
the most obvious, is geographical isolation; and no one questions that
this has been an important factor in the process of evolution, although
opinions still vary greatly as to the degree of its importance in this
respect. At one end of the series we may place the opinion of Mr.
Wallace, who denies that any of what may be termed the evolutionary
effect of geographical isolation is due to "influence exerted by
isolation _per se_." This effect, he says, is to be ascribed exclusively
to the fact that a geographically isolated portion of a species must
always encounter a change of environment, and therefore a new set of
conditions necessitating a new set of adaptations at the hands of
natural selection[2]. At the other end of the series we must place the
opinion of Moritz Wagner, who many years ago published a masterly
essay[3], the object of which was to prove that, in the absence of
geographical isolation (including migration), natural selection would be
powerless to effect any change of specific type. For, he argued, the
initial variations on which the action of this principle depends would
otherwise be inevitably swamped by free intercrossing. Wagner adduced a
large number of interesting facts in support of this opinion; but
although he thus succeeded in enforcing the truth that geographical
isolation is an important aid to organic evolution, he failed to
establish his conclusion that it is an indispensable condition.
Nevertheless he may have been right--and, as I shall presently show, I
believe he was right--in his fundamental premiss, that in the presence
of free intercrossing natural selection would be powerless to effect
divergent evolution. Where he went wrong was in not perceiving that
geographical isolation is not the only form of isolation. Had it
occurred to him that there may be other forms quite as effectual for the
prevention of free intercrossing, his essay could hardly have failed to
mark an epoch in the history of Darwinism. But, on account of this
oversight, he really weakened his main contention, namely, that in the
presence of free intercrossing natural selection must be powerless to
effect divergent evolution. This main contention I am now about to
re-argue. At present, therefore, we have only to observe that Wagner did
it much more harm than good by neglecting to perceive that free
intercrossing may be prevented in many other ways besides by migration,
and by the intervention of geographical barriers.

    [2] _Darwinism_, p. 150.

    [3] _The Darwinian Theory, and the Law of Migration_ (Eng. Trans.,
    Stanford, London, 1873).

In order that we may set out with clearer views upon this matter, I will
make one or two preliminary remarks on the more general facts of
isolation as these are found to occur in nature.

In the first place, it is obvious that isolation admits of degrees: it
may be either total or partial; and, if partial, may occur in numberless
grades of efficiency. This is so manifest that I need not wait to give
illustrations. But now, in the second place, there is another general
fact appertaining to isolation which is not so manifest, and a clear
appreciation of which is so essential to any adequate consideration of
the subject, that I believe the reason why evolutionists have hitherto
failed to perceive the full importance of isolation, is because they
have failed to perceive the distinction which has now to be pointed out.
The distinction is, that isolation may be either discriminate or
indiscriminate. If it be discriminate, the isolation has reference to
the resemblance of the separated individuals to one another; if it be
indiscriminate, it has no such reference. For example, if a shepherd
divides a flock of sheep without regard to their characters, he is
isolating one section from the other indiscriminately; but if he places
all the white sheep in one field, and all the black sheep in another
field, he is isolating one section from the other discriminately. Or, if
geological subsidence divides a species into two parts, the isolation
will be indiscriminate; but if the separation be due to one of the
sections developing, for example, a change of instinct determining
migration to another area, or occupation of a different habitat on the
same area, then the isolation will be discriminate, so far as the
resemblance of instinct is concerned.

With the exception of Mr. Gulick, I cannot find that any other writer
has hitherto stated this supremely important distinction between
isolation as discriminate and indiscriminate. But he has fully as well
as independently stated it, and shown in a masterly way its far-reaching
consequences. Indiscriminate isolation he calls Separate Breeding, while
discriminate isolation he calls Segregate Breeding. For the sake,
however, of securing more descriptive terms, I will coin the words
Apogamy and Homogamy. Apogamy, of course, answers to indiscriminate
isolation, or separate breeding. Homogamy, on the other hand, answers
to discriminate isolation, or segregate breeding: only individuals
belonging to the same variety or kind are allowed to propagate.
Isolation, then, is a genus, of which Apogamy and Homogamy are

    [4] I may here most conveniently define the senses in which all the
    following terms will be used throughout the present
    discussion:--_Species_ of isolation are, as above stated, homogamy
    and apogamy, or isolation as discriminate and indiscriminate.
    _Forms_ of isolation are modes of isolation, such as the
    geographical, the sexual, the instinctive, or any other of the
    numerous means whereby isolation of either species may be secured.
    _Cases_ of isolation are the instances in which any of the forms of
    isolation may be at work: thus, if a group of _n_ intergenerants be
    segregated into five groups, _a_, _b_, _c_, _d_, _e_, then, before
    the segregation there would have been one case of isolation, but
    after the segregation there would be five such cases.

Now, in order to appreciate the unsurpassed importance of isolation as
one of the three basal principles of organic evolution, let us begin by
considering the discriminate species of it, or Homogamy.

To state the case in the most general terms, we may say that if the
other two basal principles are given in heredity and variability, the
whole theory of organic evolution becomes neither more nor less than a
theory of homogamy--that is, a theory of the causes which lead to
discriminate isolation, or the breeding of like with like to the
exclusion of unlike. For the more we believe in heredity and variability
as basal principles of organic evolution, the stronger must become our
persuasion that discriminate breeding leads to divergence of type, while
indiscriminate breeding leads to uniformity. This, in fact, is securely
based on what we know from the experience supplied by artificial
selection, which consists in the intentional mating of like with like
to the exclusion of unlike.

The point, then, which in the first instance must be firmly fastened in
our minds is this:--so long as there is free intercrossing, heredity
cancels variability, and makes in favour of fixity of type. Only when
assisted by some form of discriminate isolation, which determines the
exclusive breeding of like with like, can heredity make in favour of
change of type, or lead to what we understand by organic evolution.

Now the forms of discriminate isolation, or homogamy, are very numerous.
When, for example, any section of a species adopts somewhat different
habits of life, or occupies a somewhat different station in the economy
of nature, homogamy arises within that section. There are forms of
homogamy on which Darwin has laid great stress, as we shall presently
find. Again, when for these or any other reasons a section of a species
becomes in any small degree modified as to form or colour, if the
species happens to be one where any psychological preference in pairing
can be exercised--as is very generally the case among the higher
animals--exclusive breeding is apt to ensue as a result of such
preference; for there is abundant evidence to show that, both in birds
and mammals, sexual selection is usually opposed to the intercrossing of
dissimilar varieties. Once more, in the case of plants, intercrossing of
dissimilar varieties may be prevented by any slight difference in their
seasons of flowering, of topographical stations, or even, in the case of
flowers which depend on insects for their fertilization, by differences
in the instincts and preferences of their visitors.

But, without at present going into detail with regard to these different
forms of discriminate isolation, there are still two others, both of
which are of much greater importance than any that I have hitherto
named. Indeed, these two forms are of such immeasurable importance, that
were it not for their virtually ubiquitous operation, the process of
organic evolution could never have begun, nor, having begun, continued.

The first of these two forms is sexual incompatibility--either partial
or absolute--between different taxonomic groups. If all hares and
rabbits, for example, were as fertile with one another as they are
within their own respective species, there can be no doubt that sooner
or later, and on common areas, the two types would fuse into one. And
similarly, if the bar of sterility could be thrown down as between all
the species of a genus, or all the genera of a family, _not otherwise
prevented from intercrossing_, in time all such species, or all such
genera, would become blended into a single type. As a matter of fact,
complete fertility, both of first crosses and of their resulting
hybrids, is rare, even as between species of the same genus; while as
between genera of the same family complete fertility does not appear
ever to occur; and, of course, the same applies to all the higher
taxonomic divisions. On the other hand, some degree of infertility is
not unusual as between different varieties of the same species; and,
wherever this is the case, it must clearly aid the further
differentiation of those varieties. It will be my endeavour to show that
in this latter connexion sexual incompatibility must be held to have
taken an immensely important part in the differentiation of varieties
into species. But meanwhile we have only to observe that _wherever_ such
incompatibility is concerned, it is to be regarded as an isolating
agency of the very first importance. And as it is of a character purely
physiological, I have assigned to it the name Physiological Isolation;
while for the particular case where this general principle is concerned
in the origination of specific types, I have reserved the name
Physiological Selection.

The other most important form of discriminate isolation to which I have
alluded is Natural Selection. To some evolutionists it has seemed
paradoxical thus to regard natural selection as a form of isolation; but
a little thought will suffice to show that such is really the most
accurate way of regarding it. For, as Mr. Gulick says, "Natural
selection is the exclusive breeding of those better adapted to the
environment: ... it is a process in which the fittest are prevented from
crossing with the less fitted, by the exclusion of the less fitted."
Therefore it is, strictly and accurately, a mode of isolation, where the
isolation has reference to adaptation, and is secured in the most
effectual of possible ways--i.e. by the destruction of all individuals
whose intercrossing would interfere with the isolation. Indeed, the very
term "natural _selection_" shows that the principle is tacitly
understood to be one of isolation, because this name was assigned to the
principle by Darwin for the express purpose of marking the analogy that
obtains between it and the intentional isolation which is practised by
breeders, fanciers, and horticulturists. The only difference between
"natural selection" and "artificial selection" consists in this--that
under the former process the excluded individuals must necessarily
perish, while under the latter they need not do so. But clearly this
difference is accidental: it is in no way essential to the process
considered as a process of discriminate isolation. For, as far as
homogamous breeding is concerned, it can matter nothing whether the
exclusion of the dissimilar individuals is effected by separation or by

Natural selection, then, is thus unquestionably a form of isolation of
the discriminate kind; and therefore, notwithstanding its unique
importance in certain respects, considered as a principle of organic
evolution it is less fundamental--and also less extensive--than the
principle of isolation in general. In other words, it is but a part of a
much larger whole. It is but a particular form of a general principle,
which, as just shown, presents many other forms, not only of the
discriminate, but likewise of the indiscriminate kind. Or, reverting to
the terminology of logic, it is a sub-species of the species Homogamy,
which in its turn is but a constituent part of the genus Isolation.

So much then for homogamy, or isolation of the discriminate order.
Passing on now to apogamy, or isolation of the indiscriminate kind, we
may well be disposed, at first sight, to conclude that this kind of
isolation can count for nothing in the process of evolution. For if the
fundamental importance of isolation in the production of organic forms
be due to its segregation of like with like, does it not follow that any
form of isolation which is indiscriminate must fail to supply the very
condition on which all the forms of discriminate isolation depend for
their efficacy in the causing of organic evolution? Or, to return to our
concrete example, is it not self-evident that the farmer who separated
his stock into two or more parts indiscriminately, would not effect any
more change in his stock than if he had left them all to breed together?

Well, although at first sight this seems self-evident, it is in fact
untrue. For, unless the individuals which are indiscriminately isolated
happen to be a very large number, sooner or later their progeny will
come to differ from that of the parent type, or unisolated portion of
the previous stock. And, of course, as soon as this change of type
begins, the isolation ceases to be indiscriminate: the previous apogamy
has been converted into homogamy, with the usual result of causing a
divergence of type. The reason why progeny of an indiscriminately
isolated section of an originally uniform stock--e.g. of a species--will
eventually deviate from the original type is, to quote Mr. Gulick, as
follows:--"No two portions of a species possess exactly the same average
character, and, therefore, the initial differences are for ever reacting
on the environment and on each other in such a way as to ensure
increasing divergence as long as the individuals of the two groups are
kept from intergenerating[5]." Or, as I stated this principle in my
essay on _Physiological Selection_, published but a short time before
Mr. Gulick's invaluable contributions to these topics:--

    [5] _Divergent Evolution through Cumulative Segregation_ (_Zool.
    Journal, Linn. Soc._, vol. xx. pp. 189-274).

     As a matter of fact, we find that no one individual "is like
     another all in all"; which is another way of saying that a specific
     type may be regarded as the average mean of all its individual
     variations, any considerable departure from this average being,
     however, checked by intercrossing.... Consequently, if from any
     cause a section of a species is prevented from intercrossing with
     the rest of its species, we might expect that new varieties should
     arise within that section, and that in time these varieties should
     pass into new species. And this is just what we do find[6].

    [6] The passage proceeds to show that in view of this consideration
    we have a strong additional reason for rejecting the _a priori_
    dogma that all specific characters must necessarily be useful
    characters. For it is evident that any divergence of specific
    character which is brought about in this way need not present any
    utilitarian significance--although, of course, natural selection
    will ensure that it shall never be deleterious.

The name which I gave to this cause of specific change was Independent
Variability, or variability in the absence of overwhelming
intercrossing. But it now appears to me that this cause is really
identical with that which was previously enunciated by Delbœuf.
Again, in his important essay on _The Influence of Isolation_, Weismann
concludes, on the basis of a large accumulation of facts, that the
constancy of any given specific type "does not arise suddenly, but
gradually, and is established by the promiscuous intercrossing of all
individuals." From which, he says, it follows, that this constancy must
cease so soon as the condition which maintains it ceases--i. e. so soon
as intercrossing (Panmixia) between all individuals ceases, or so soon
as a portion of a species is isolated from its parent stock. To this
principle he assigns the name of Amixia. But Weismann's Amixia differs
from my Independent Variability in several important particulars; and on
this account I have designedly abstained from adopting his term. Here
it is enough to remark that it answers to the generic term Isolation,
without reference to the _kind_ of isolation as discriminate or
indiscriminate, homogamous or apogamous. On the other hand, my
Independent Variability is merely a re-statement of the so-called "Law
of Delbœuf," which, in his own words, is as follows:--

     One point, however, is definitely attained. It is that the
     proposition, which further back we designated paradoxical, is
     rigorously true, A constant cause of variation, however
     insignificant it may be, changes the uniformity [of type] little by
     little, and diversifies it _ad infinitum_. From the homogeneous,
     left to itself, only the homogeneous can proceed; but if there be a
     slight disturbance ["léger ferment"] in the homogeneous, the
     homogeneity will be invaded at a single point, differentiation will
     penetrate the whole, and, after a time--it may be an infinite
     time--the differentiation will have disintegrated it altogether.

In other words, the "Law," which Delbœuf has formulated on
mathematical grounds, and with express reference to the question of
segregate breeding, proves that, no matter how infinitesimally small the
difference may be between the average qualities of an isolated section
of a species compared with the average qualities of the rest of that
species, if the isolation continues sufficiently long, differentiation
of specific type is necessarily bound to ensue. But, to make this
mathematical law biologically complete, it ought to be added that the
time required for the change of type to supervene (supposing apogamy to
be the only agent of change) will be governed by the range of individual
variability which the species in question presents. A highly stable
species (such as the Goose) might require an immensely long time for
apogamy alone to produce any change of type in an isolated portion of
the species, while a highly variable species (such as the Ruff) would
rapidly change in any portion that might be indiscriminately isolated.
It was in order to recognize this additional and very important factor
that I chose the name Independent _Variability_ whereby to designate the
diversifying influence of merely indiscriminate isolation, or apogamy.
Later on Mr. Gulick published his elaborate papers upon the divergence
of type under all kinds of isolation; and retained my term Independent,
but changed Variability into Generation. I point this out merely for the
sake of remarking that his Independent Generation is exactly the same
principle as my Independent Variability, and Delbœuf's Mathematical

Now, while I fully agree with Mons. Giard where he says, in the
introductory lecture of his course on _The Factors of Evolution_[7],
that sufficient attention has not been hitherto given by naturalists to
this important factor of organic evolution (apogamy), I think I have
shown that among those naturalists who have considered it there is a
sufficient amount of agreement. _Per contra_, I have to note the opinion
of Mr. Wallace, who steadily maintains the impossibility of any cause
other than natural selection (i.e. one of the forms of homogamy) having
been concerned in the evolution of species. But at present it is enough
to remark that even Professor Ray Lankester--whose leanings of late
years have been to the side of ultra-Darwinism, and who is therefore
disposed to agree with Mr. Wallace wherever this is logically
possible--even Professor Ray Lankester observes:--

    [7] _Revue Scientifique_, Nov. 23, 1889.

     Mr. Wallace does not, in my judgement, give sufficient grounds for
     rejecting the proposition which he indicates as the main point of
     Mr. Gulick's valuable essay on _Divergent Evolution through
     Cumulative Segregation_. Mr. Gulick's idea is that ... no two
     portions of a species possess exactly the same average character,
     and the initial differences will, if the individuals of the two
     groups are kept from intercrossing, assert themselves continuously
     by heredity in such a way as to ensure an increasing divergence of
     the forms belonging to the two groups, amounting to what is
     recognized as specific distinction. Mr. Gulick's idea is simply the
     recognition of a permanence or persistency in heredity, which,
     _caeteris paribus_, gives a twist or direction to the variations of
     the descendants of one individual as compared with the descendants
     of another[8].

    [8] _Nature_, Oct. 10, 1889, p. 568.

Now we have seen that "Mr. Gulick's idea," although independently
conceived by him, had been several times propounded before; and it is
partly implicated in more than one passage of the _Origin of Species_,
where free intercrossing, or the _absence_ of isolation, is alluded to
as maintaining the _constancy_ of a specific type[9]. Moreover, it is
still more fully recognized in the last edition of the _Variation of
Animals and Plants_, where a paragraph is added for the purpose of
sanctioning the principle in the imperfect form that it was stated by
Weismann[10]. Nevertheless, to Mr. Gulick belongs the credit, not only of
having been the first to conceive (though the last to publish) the
"idea" in question, and of having stated it with greater fullness than
anybody else; but still more of having verified its importance as a
factor of organic evolution.

    [9] e. g. p. 81.

    [10] See Chapter xxiii. vol. ii. p. 262. (Edition of 1888.)

For, in point of fact, Mr. Gulick was led to his recognition of the
principle in question, not by any deductive reasoning from general
principles, but by his own particular and detailed observations of the
land mollusca of the Sandwich Islands. Here there are an immense number
of varieties belonging to several genera; but every variety is
restricted, not merely to the same island, but actually to the same
valley. Moreover, on tracing this fauna from valley to valley, it is
apparent that a slight variation in the occupants of valley 2 as
compared with those of the adjacent valley 1, becomes more pronounced in
the next--valley 3, still more so in 4, &c., &c. Thus it was possible,
as Mr. Gulick says, roughly to estimate the amount of divergence between
the occupants of any two given valleys by measuring the number of miles
between them.

As already stated, I have myself examined his wonderful collection of
shells, together with a topographical map of the district; and therefore
I am in a position to testify to the great value of Mr. Gulick's work in
this connexion, as in that of the utility question previously
considered. The variations, which affect scores of species, and
themselves eventually run into fully specific distinctions, are all more
or less finely graduated as they pass from one isolated region to the
next; and they have reference to changes of form and colour, which in no
one case presents any appearance of utility. Therefore--and especially
in view of the fact that, as far as he could ascertain, the environment
in the different valleys was essentially the same--no one who examines
this collection can wonder that Mr. Gulick attributes the results which
he has observed to the influence of apogamy alone, without any reference
to utility or natural selection.

To this solid array of remarkable facts Mr. Wallace has nothing further
to oppose than his customary appeal to the argument from ignorance,
grounded on the usual assumption that no principle other than natural
selection _can_ be responsible for even the minutest changes of form or
colour. For my own part, I must confess that I have never been so deeply
impressed by the dominating influence of the _a priori_ method as I was
on reading Mr. Wallace's criticism of Mr. Gulick's paper, after having
seen the material on which this paper is founded. To argue that every
one of some twenty contiguous valleys in the area of the same small
island must necessarily present such differences of environment that all
the shells in each are differently modified thereby, while in no one out
of the hundreds of cases of modification in minute respects of form and
colour can any human being suggest an adaptive reason therefor--to argue
thus is merely to affirm an intrinsically improbable dogma in the
presence of a great and consistent array of opposing facts.

I have laid special stress on this particular case of the Sandwich
Islands' mollusca, because the fifteen years of labour which Mr. Gulick
has devoted to their exhaustive working out have yielded results more
complete and suggestive than any which so far have been forthcoming with
regard to the effects of isolation in divergent evolution. But, if space
permitted, it would be easy to present abundance of additional facts
from other sources, all bearing to the same conclusion--namely, that as
a matter of direct observation, no less than of general reasoning, any
unprejudiced mind will concede to the principle of indiscriminate
isolation an important share in the origination of organic types. For as
indiscriminate isolation is thus seen sooner or later to become
discriminate, and as we have already seen that discriminate isolation is
a necessary condition to all or any modification, we can only conclude
that isolation in both its kinds takes rank with heredity and
variability as one of the three basal principles of organic evolution.

       *       *       *       *       *

Having got thus far in the way of generalities, we must next observe
sundry further matters of comparative detail.

1. In any case of indiscriminate isolation, or apogamy, the larger the
bulk of the isolated section the more nearly must its average qualities
resemble those of its parent stock; and, therefore, the less divergence
of character will ensue in a given time from this cause alone. For
instance, if one-fourth of a large species were to be separated from the
other three-fourths (say, by subsidence causing a discontinuity of
area), it would continue the specific characters unchanged for an
indefinitely long time, so far as the influence of such an
indiscriminate isolation is concerned. But, on the other hand, if only
half a dozen individuals were to be thus separated from the rest of
their species, a comparatively short time would be needed for their
descendants to undergo some varietal modification at the hands of
apogamy. For, in this case, the chances would be infinitely against the
average characters of the original half-dozen individuals exactly
coinciding with those of all the rest of their species.

2. In any case of homogamy, however, it is immaterial what proportional
number of individuals are isolated in the first instance. For the
isolation is here discriminate, or effected by the initial difference of
the average qualities themselves--a difference, therefore, which
presupposes divergence as having already commenced, and equally bound to
proceed whether the number of intergenerants be large or small.

It may here be remarked that, in his essay on the _Influence of
Isolation_, Professor Weismann fails to distinguish between the two
kinds of isolation. This essay deals only with one of the many different
forms of isolation--the geographical--and is therefore throughout
concerned with a consideration of diversity as arising from apogamy
alone. But in dealing with this side of the matter Weismann anticipated
both Gulick and myself in pointing out the law of inverse proportion,
which I have stated in the preceding paragraph in what appears to me its
strictly accurate form.

3. Segregate Breeding, or homogamy, which arises under any of the many
forms of discriminate isolation, must always tend to be _cumulative_.
For, again to quote Mr. Gulick, who has constituted this fact the most
prominent as it is the most original feature of his essay, "In the first
place, every new form of Segregation[11] that now appears depends on, and
is superimposed upon, forms of Segregation that have been previously
induced; for when Negative Segregation arises [i. e. isolation due to
mutual sterility], and the varieties of a species become less and less
fertile with one another, the complete infertility that has existed
between them and some other species does not disappear, nor does the
Positive Segregation cease [i. e. any other form of isolation previously
existing].... In the second place, whenever Segregation is directly
produced by some quality of the organism, variations that possess the
endowment in a superior degree will have a larger share in producing the
segregated forms of the next generation, and accordingly the segregative
endowment of the next generation will be greater than that of the
present generation; and so with each successive generation the
segregation will become increasingly complete." And to this it may be
added, in the third place, that where the segregation (isolation) is due
to the external conditions of life under which the organism is placed,
or where it is due to natural selection simultaneously operating in
divergent lines of evolution, the same remarks apply. Hence it follows
that discriminate isolation is, in all its forms, cumulative.

    [11] This term may here be taken as equivalent to Isolation.

4. The next point to be noted is, that the cumulative divergence of type
thus induced can take place only in as many different lines as there are
different _cases_ of isolation. This is a point which Mr. Gulick has not
expressly noticed; but it is one that ought to be clearly recognized.
Seeing that isolation secures the breeding of similar forms by exclusion
(immediate or eventual) of those which are dissimilar, and that only in
as far as it does this can it be a factor in organic evolution, it
follows that the resulting segregation, even though cumulative, can only
lead to divergence of organic types in as many directions as there are
cases of isolation. For any one group of intergenerants only _serial_
transformation is possible, even though the transformation be cumulative
through successive generations in the single line of change. But there
is always a probability that during the course of such _serial
transformation in time_, some other case of isolation may supervene, so
as to divide the previously isolated group of intergenerants into two or
more further isolated groups. Then, of course, opportunity will be
furnished for _divergent transformation in space_--and this in as many
different lines as there are now different homogamous groups.

That this must be so is further evident, if we reflect that the
evolutionary power of isolation depends, not only on the _preventing_ of
intercrossing between the isolated portion of a species and the rest of
that species, but also upon the _permitting_ of intercrossing between
all individuals of the isolated portion, whereby the peculiar average of
qualities which they as a whole present may be allowed to assert itself
in their progeny--or, if the isolation has been from the first
discriminate, whereby the resulting homogamy may thus be allowed to
assert itself. Hence any one case of either species of isolation,
discriminate or indiscriminate, can only give rise to what Mr. Gulick
has aptly called "monotypic evolution," or a chain-like series of types
arising successively in time, as distinguished from what he has called
"polytypic evolution," or an arborescent multiplication of types
arising simultaneously in space.

For example, let us again take the geographical form of isolation. Where
a single small intergenerant group of individuals is separated from the
rest of its species--say, on an oceanic island--_monotypic_ evolution
may take place through a continuous and cumulative course of independent
variation in a single line of change: all the _individuals_ composing
any one given generation will closely resemble one another, although the
_type_ may be progressively altering through a long series of
generations. But if the original species had had two small colonies
separated from itself (one on each of two different islands, so giving
rise to two cases of isolation), then _polytypic_ evolution would have
ensued to the extent of there having been two different lines of
evolution going on simultaneously (one upon each of the two islands
concerned). Similarly, of course, if there had been three or four such
colonies, there would have been three or four divergent lines of
evolution, and so on.

5. In the _cases_ of isolation just supposed there is only one _form_ of
isolation; and it is thus shown that under one form of isolation there
may be as many lines of divergence as there are separate cases of such
isolation. But now suppose that there are two or more forms of
isolation--for instance, that on the same oceanic island the original
colony has begun to segregate into secondary groups under the influence
of natural selection, sexual selection, physiological selection, or any
of the other forms of isolation--then there will be as many lines of
divergent evolution going on at the same time (and here on the same
area) as there are forms of isolation affecting the oceanic colony. And
this because each of the _forms_ of isolation has given rise to a
different _case_ of isolation.

Now, inasmuch as different forms of isolation, when thus superadded one
to another, constitute different cases of isolation, we may lay down the
following general law as applying to all the forms of isolation--namely,
_The number of possible directions in which divergent evolution can
occur, is never greater than, though it may be equal to, the number of
cases of efficient isolation--or the number of efficiently separated
groups of intergenerants._

6. We have now to consider with some care the particular and highly
important form of isolation that is presented by natural selection. For
while this form of isolation resembles all the other forms of the
discriminate kind in that it secures homogamy, there are two points in
which it differs from all of them, and one point in which it differs
from most of them.

Natural selection differs from _all_ the other known forms of isolation
(whether discriminate or indiscriminate) in that it has exclusive
reference to _adaptations_ on the one hand, and, on the other hand,
necessitates not only the elimination, but the destruction of the
excluded individuals. Again, natural selection differs from _most_ of
the other forms of isolation in that, unless assisted by some other
form, it can never lead to polytypic, but only to monotypic evolution.
The first two points of difference are here immaterial; but the last is
one of the highest importance, as we shall immediately perceive.

In nearly all the other forms of isolation, polytypic or divergent
evolution may arise under the influence of that form alone, or without
the necessary co-operation of any other form. This we have already seen,
for example, in regard to geographical isolation, under which there may
be as many different lines of transmutation going on simultaneously as
there are different cases of isolation--say, in so many different
oceanic islands. Again, in regard to physiological isolation the same
remark obviously applies; for it is evident that even upon the same
geographical area there may be as many different lines of transmutation
going on simultaneously as there are cases of this form of isolation.
The bar of mutual sterility, whenever and wherever it occurs, must
always render polytypic evolution possible. And so it is with almost all
the other forms of isolation: that is to say, one _form_ does not
necessarily require the assistance of another _form_ in order to create
an additional _case_ of isolation. But it is a peculiarity of natural
selection, considered as a form of isolation, that it does necessarily
require the assistance of some other form before it can give rise to an
additional case of isolation; and therefore before it can give rise to
any _divergence_ of character in ramifying lines, as distinguished from
_transformation_ of characters in a single line. Or, in other words,
natural selection, when acting alone, can never induce polytypic
evolution, but only monotypic.

That this important conclusion is a necessary deduction from the theory
of natural selection itself, a very few words will be enough to show.
For, according to the theory, survival of the fittest is a form of
isolation which acts through utility, by _destroying_ all the
individuals whom it fails to isolate. Hence it follows that survival of
the fittest is a form of isolation which, if acting alone, cannot
_possibly_ effect divergent evolution. For, in the first place, there is
nothing in this form of isolation to ensure that the fitter individuals
should fail to interbreed with the less fit which are able to survive;
and, in the second place, in all cases where the less fit are not
sufficiently fit to be suffered to breed, they are exterminated--i. e.
not permitted to form a distinct variety of their own. If it be said
that survival of the fittest may develop simultaneously two or more
lines of _useful_ change, the answer is that it can only do this if each
of the developing varieties is isolated from the others by some
_additional form_ of isolation; for, if not, there can be no
commencement of utilitarian _divergence_, since whatever number of
utilitarian changes may be in course of simultaneous development, they
must in this case be all blended together in a single line of specific
transmutation. Nay, even if specific divergence has actually been
commenced by natural selection when associated with some other form of
homogamy, if the latter should afterwards be withdrawn, natural
selection would then be unable to maintain even so much divergence of
character as may already have been attained: free intercrossing between
the two collateral, and no longer isolated branches, would ensure their
eventual blending into a common stock. Therefore, I repeat, natural
selection, when acting alone, can never induce polytypic evolution, but
only monotypic.

Now I regret to say that here, for the first and only time throughout
the whole course of the present treatise, I find myself in seeming
opposition to the views of Darwin. For it was the decidedly expressed
opinion of Darwin that natural selection _is_ competent to effect
polytypic, or divergent, evolution. Nevertheless, I believe that the
opposition is to a large extent only apparent, or due merely to the fact
that Darwin did not explicitly state certain considerations which
throughout his discussion on "divergence of character" are seemingly
implied. But, be this as it may, I have not even appeared to desert his
leadership on a matter of such high importance without having duly
considered the question in all its bearings, and to the utmost limit of
my ability. Moreover, about two years after the publication of my first
paper[12] upon the subject, Mr. Gulick followed, at somewhat greater
length, in the same line of dissent. Like all the rest of his work, this
is so severely logical in statement, as well as profoundly thought out
in substance, that I do not see how it is possible for any one to read
impartially what he has written, and then continue to hold that natural
selection, if unassisted by any other form of isolation, can possibly
effect divergence of character--or polytypic as distinguished from
monotypic evolution[13].

    [12] _Zool. Journal Lin. Soc._, vol. xix. pp. 337-411.

    [13] _Ibid._, vol. xx. pp. 202-212.

I may here quote from Mr. Gulick's paper three propositions, serving to
state three large and general bodies of observable fact, which
severally and collectively go to verify, with an overwhelming mass of
evidence, the conclusion previously reached on grounds of general

     The facts of geographical distribution seem to me to justify the
     following statements:--

     (1) A species exposed to different conditions in the different
     parts of the area over which it is distributed, is not represented
     by divergent forms when free interbreeding exists between the
     inhabitants of the different districts. In other words, Diversity
     of Natural Selection without Separation does not produce divergent

     (2) We find many cases in which areas, corresponding in the
     character of the environment, but separated from each other by
     important barriers, are the homes of divergent forms of the same or
     allied species.

     (3) In cases where the separation has been long continued, and the
     external conditions are the most diverse in points that involve
     diversity of adaptation, there we find the most decided divergences
     in the organic forms. That is, where Separation and Divergent
     Selection have long acted, the results are found to be the

     The 1st and 3rd of these propositions will probably be disputed by
     few, if by any. The proof of the 2nd is found wherever a set of
     closely allied organisms is so distributed over a territory that
     each species and variety occupies its own narrow district, within
     which it is shut by barriers that restrain its distribution while
     each species of the environing types is distributed over the whole
     territory. The distribution of terrestrial molluscs on the Sandwich
     Islands presents a great body of facts of this kind.


ISOLATION (_continued_).

I will now recapitulate the main doctrines which have been set forth in
the foregoing chapter, and then proceed to consider the objections which
have been advanced against them.

It must be remembered that by isolation I mean exactly what Mr. Gulick
does by "Segregation," and approximately what Professor Weismann does by
"Amixia "--i. e. the prevention of intercrossing.

Isolation occurs in very many forms besides the geographical, as will be
more fully shown at the end of this chapter; and in all its forms it
admits of degrees.

It also occurs in two very different species or kinds--namely,
discriminate and indiscriminate. These I have called respectively
Homogamy and Apogamy. This all-important distinction has been clearly
recognized by Mr. Gulick, as a result of his own thought and
observation, independently of anything that I have published upon the

In view of this distinction Isolation takes rank with Heredity and
Variability as one of the most fundamental principles of organic
evolution. For, if these other two principles be granted, the whole
theory of descent resolves itself into an inquiry touching the causes,
forms, and degrees of Homogamy.

Save in cases where very large populations are concerned, apogamy must
sooner or later give rise _per se_ to homogamy, owing to the Law of
Delbœuf, which is the principle that I have called Independent
Variability, and Gulick has called Independent Generation. But of course
this does not hinder that under apogamy various other causes of homogamy
are likely to arise--in particular natural selection.

That natural selection differs from most of the other forms of isolation
in not being capable of causing _divergent_ or _polytypic_ evolution
must at once become evident, if we remember that the only way in which
isolation of any form can cause such evolution is by partitioning a
given group of intergenerants into two or more groups, each of which is
able to survive as thus separated from the other, and so to carry on the
evolution in divergent lines. But the distinguishing peculiarity of
natural selection, considered as a form of isolation, is that it effects
the isolation _by killing off all the individuals which it fails to
isolate_: consequently, this form of isolation differs from other forms
in prohibiting the possibility of any ramification of a single group of
intergenerants into two or more groups, for the purpose of carrying on
the evolution in divergent lines. Therefore, under this form of
isolation alone, evolution must proceed, palm-like, in a single line of
growth. So to speak, the successive generations continuously ascend to
higher things on the steps supplied by their own "dead selves"; but in
doing so they must climb a single ladder, no rung of which can be
allowed to bifurcate in the presence of the uniformity secured _for that
generation_ by the free intercrossing of the most fit. Even though
beneficial variations may arise in two or more directions
simultaneously, and all be simultaneously selected by survival of the
fittest, the effect of free intercrossing (in the absence of any other
form of isolation) will be to fuse all these beneficial variations into
one common type, and so to end in _monotypic_ evolution as before. In
order to secure _polytypic_ evolution, intercrossing between the
different beneficial variants which may arise must be prevented; and
there is nothing to prevent such intercrossing in the process of natural
selection _per se_. In order that the original group of intergenerants
should be divided and sub-divided into two or more groups of
intergenerants, some additional form of isolation must necessarily
supervene--when, of course, polytypic evolution will result. And, as Mr.
Gulick has shown, the conclusion thus established by deductive reasoning
is verified inductively by the facts of geographical distribution.

How, then, are we to account for the fact that Darwin attributed to
natural selection the power to cause divergence of character? The answer
is sufficiently simple. _He does so by tacitly invoking the aid of some
other form of homogamy in every case._ If we carefully read pp. 86-97 of
the _Origin of Species_, where this subject is under consideration, we
shall find that in every one of the arguments and illustrations which
are adduced to prove the power of natural selection to effect
"divergence of character," he either pre-supposes or actually names some
other form of homogamy as the originating cause of the diversity that
is afterwards presented to natural selection for further
intensification. To give only one example. At the starting-point of the
whole discussion the priority of such other forms of homogamy is assumed
in the following words:--

     But how, it may be asked, can any analogous principle [to that of
     diversity caused by artificial selection] apply in nature? I
     believe it can and does apply most efficiently (though it was a
     long time before I saw how), from the simple circumstance that the
     more diversified the descendants from any one species become in
     structure, constitution, and habits, by so much will they be better
     enabled to seize on many and widely diversified places in the
     polity of nature, and so be enabled to increase in numbers.

Now, without question, so soon as segregate breeding in two or more
lines of homogamy has been in any sufficient degree determined by some
"change of structure, constitution, or habits," natural selection will
forthwith proceed to increase the divergence in as many different lines
as there are thus yielded discriminately isolated sections of the
species. And this fact it must have been that Darwin really had before
his mind when he argued that diversification of character is caused by
natural selection, through the benefit gained by the diversified forms
being thus "enabled to increase in number." Nevertheless he does not
expressly state the essential point, that although diversification of
character, _when once begun_, is thus _promoted_ by natural selection,
which forthwith proceeds to cultivate each of the resulting branches,
yet diversification of character can never be _originated_ by natural
selection. The change of "structure," of "constitution," of "habits," of
"station," of geographical area, of reciprocal fertility, and so
on--this change, _whatever_ it may have been, must clearly have been
antecedent to any operation of natural selection through the benefit
which arose from the change. Therefore the change must in all cases have
been due, in the first instance, to some other form of isolation than
the superadded form which afterwards arose from superior fitness in the
possession of superior benefit--although, so long as the prior form of
isolation endured, or continued to furnish the necessary condition to
the co-operation of survival of the fittest, survival of the fittest
would have continued to increase the divergence of character in as many
ramifying lines as there were thus given to its action separate cases of
isolation by other means.

In short, as divergence of character must in all cases be due to a
prevention of intercrossing, and as in the process of natural selection
there is, _ex hypothesi_, nothing to prevent the intercrossing until the
divergence has already arisen, to suppose that natural selection alone
can have caused the divergence, is to suppose that natural selection can
have caused the conditions of its own activity, which is absurd.

Seeing, then, that even in cases where any "benefit" arises from
divergence of character, such benefit can arise only after the
divergence has already commenced, and seeing that on this as on other
accounts previously mentioned it is plainly impossible to attribute the
origin of such divergence to natural selection, we find that natural
selection must be in all cases assisted by some other form of isolation,
if it is to be concerned in polytypic as distinguished from monotypic
evolution. But this does not hinder that, when it is so assisted,
natural selection may become--and, I believe, does become--the most
efficient of all the forms of isolation in promoting divergence of
character. For, in the first place, of all the forms of isolation
natural selection is probably the most energetic in promoting monotypic
evolution; so that under the influence of such isolation monotypic
evolution probably advances more rapidly than it does under any other
form of isolation. In the second place, when polytypic evolution has
been begun by any of these other forms of isolation, and natural
selection then sets to work on each of the resulting branches, although
natural selection is thus engaged in as many different acts of monotypic
evolution as there are thus separate cases supplied to it by these other
forms of isolation, the joint result of all these different acts is to
hurry on the polytypic evolution which was originally started by the
other forms of isolation. So to speak, natural selection is the forcing
heat, acting simultaneously on each of the separate branches which has
been induced to sprout by other means; and in thus rapidly advancing the
growth of all the branches, it is still entitled to be regarded as the
most important _single_ cause of diversification in organic nature,
although we must henceforth cease to regard it as in any instance the
_originating_ cause--or even so much as the _sustaining_ cause.

So much by way of summary and recapitulation. I will now briefly
consider the only objections which, so far as I can see, admit of being
brought against the foregoing doctrine of Isolation as held by Mr.
Gulick and myself. These possible objections are but two in
number--although but one of them has been hitherto adduced. This,
therefore, I will take first.

Mr. Wallace, with his customary desire to show that natural selection is
everywhere of itself capable of causing organic evolution, seeks to
minimize the swamping effects of free intercrossing, and the consequent
importance of other forms of isolation. His argument is as follows.

Alluding to the researches of Mr. J. A. Allen, and others, on the amount
of variation presented by individuals of a species in a state of nature,
Mr. Wallace shows that, as regards any given part of the animal under
consideration, there is always to be found a considerable range of
individual variation round the average mean which goes to constitute the
specific character of the type. Thus, for example, Mr. Allen says of
American birds, "that a variation of from fifteen to twenty per cent. in
general size, and an equal degree of variation in the relative size of
different parts, may be ordinarily expected among specimens from the
same species and sex, taken at the same locality, while in some cases
the variation is even greater than this." Now, Mr. Wallace is under the
impression that these facts obviate the difficulty which arises from the
presence of free intercrossing--the difficulty, that is, against the
theory of natural selection when natural selection is supposed to have
been the exclusive means of modification. For, as he says, "if less size
of body would be beneficial, then, as half the variations in size are
above and half below the mean or existing standard of the species, there
would be ample beneficial variations"; and similarly with regard to
longer or shorter legs, wings, tails, &c., darker or lighter colour, and
so on through all the parts of any given organism.

Well, although I have no wish at all to disparage the biological value
of these actual measurements of the range of individual variation, I
must point out that they are without any value at all in the connexion
which Mr. Wallace adduces them. We did not require these measurements to
tell us the broad and patent fact that "no being on this earthly ball is
like another all in all"--or, in less Tennysonian words, that as regards
every specific structure there is a certain amount of individual
variability round an average mean. Indeed, in my own paper on
_Physiological Selection_--against which Mr. Wallace is here specially
arguing--I expressly said, as previously remarked, "that a specific type
may be regarded as _the average mean of all individual variations_." The
fact of such individual variability round a specific mean has always
been well known to anatomists; it constitutes one of the basal pillars
of the whole Darwinian theory; and is besides a matter of universal
recognition as regards human stature, features, and so forth. The value
of Mr. Allen's work consists in accurately measuring the _amount_ or
_range_ of individual variation; but the question of its amount or range
is without relevancy in the present connexion. For the desirability of
isolation as an aid to natural selection even where monotypic evolution
is concerned, does not arise with any reference to the amount or range
of variation: it arises with reference to the _number_ of variations
which are--or are not--_similar_ and _simultaneous_. If there be a
sufficient number which are both similar and simultaneous, the
desirability of any co-operating form of isolation is correspondingly
removed, because natural selection may then have sufficient material
wherewith to overcome the adverse influence of free intercrossing, and
so of itself to produce monotypic evolution. Now, variations may be
numerous, similar, and simultaneous, either on account of some common
cause acting on many individuals at the same time, or on account of the
structures in question being more or less variable round a specific
mean. In the latter case--which is the only case that Mr. Allen's
measurements have to do with--the law of averages will of course
determine that half the whole number of variations in any given
structure, in any given generation, will be above the mean line. But,
equally of course, no one has ever denied that where, for either of
these reasons, natural selection is provided with sufficient material,
it is correspondingly capable of improving the specific type without the
assistance of any other form of homogamy; so to speak, they protect
themselves by their very numbers, and their superiority over others
leads to their survival and accumulation. But what is the result? _The
result can only be monotypic evolution._ No matter how great the number,
or how great the range, of variations round an average specific mean,
out of such material natural selection can never produce _polytypic_
evolution: it may _change_ the type to any extent during successive
generations, and in a single line of change; but it cannot _branch_ the
type, unless some other form of homogamy intervenes. Therefore, when Mr.
Wallace adduces the well-known fact that all structures vary more or
less round a specific mean as proof that natural selection need not be
incommoded by free intercrossing, but can of itself produce all the
known phenomena of specific evolution, he fails to perceive that his
argument refers only to one aspect of such evolution (viz. the
transformation of species in time), and does not apply to the aspect
with which alone my paper on _Physiological Selection_ was concerned
(viz. the multiplication of species in space).

The same thing may be shown in this way. It is perfectly obvious that
where the improvement of type in a linear series is concerned (monotypic
evolution), free intercrossing, far from being a hindrance to the
process, _is the very means by which the process is accomplished_.
Improvement here ascends by successive steps, in successive generations,
simply _because_ of the general intercrossing of the generally most fit
with the result that the species, _as a whole_, gradually becomes
transformed into another species, _as a whole_. Therefore, it would be
mere fatuity in any one to adduce free intercrossing as a "difficulty"
against natural selection alone being competent to produce evolution of
this kind. But where the kind of evolution is that whereby the species
is _differentiated_--where it is required, for instance, to produce
different structures in different portions of the species, such as the
commencement of a fighting spur on the wing of a duck, or _novel_
characters of any sort in different groups of the species--free
intercrossing is no longer a condition to, but an absolute preventive
of, the process; and, therefore, unless checked as between each portion
of the species by some form of homogamy other than natural selection,
it must effectually inhibit any _segregation_ of specific types, or
divergence of character.

Hence it is that, while no Darwinian has ever questioned the power of
unaided selection to cause _improvement of character in successive
generations_, in common now with not a few other Darwinians I have
emphatically denied so much as the abstract possibility of selection
alone causing a _divergence of character in two or more simultaneous
lines of change_.

And, although these opposite views cannot be reconciled, I am under the
impression that they do admit of being explained. For I take them to
indicate a continued failure to perceive the all-important distinction
between evolution as monotypic and polytypic. Unless one has fully
grasped this distinction, and constantly holds it in mind, he is not in
a position to understand the "difficulty" in question; nor can he avoid
playing fast and loose with natural selection as possibly the sole cause
of evolution, and as necessarily requiring the co-operation of some
other cause. But if he once clearly perceives that "evolution" is a
logical genus, of which the monotypic and the polytypic forms are
species, he will immediately escape from his confusion, and find that
while the monotypic form may be caused by natural selection alone the
polytypic form can never be so caused.

       *       *       *       *       *

The second difficulty which I have to mention as at first sight
attaching to the views of Mr. Gulick and myself on the subject of
Isolation is, that in an isolated section of a species Mr. Francis
Galton's law of regression in the average character of offspring to the
typical character of the group through reversion or atavism (_Natural
Inheritance_, p. 97) must have the effect of neutralizing the
segregative influence of mere apogamy. That such, however, cannot be the
case has been well shown by Mr. Gulick in his paper on _Intensive
Segregation_. Without at all disputing the validity of Mr. Galton's law,
he proves that "it can hold in full force only where there is free
crossing, otherwise no divergent race could ever be formed by any amount
of selection and independent breeding[14]." This is so self-evident that
I need not quote his demonstration of the point.

    [14] _Zool. Journal Lin. Soc._, vol. xxiii. p. 313.

       *       *       *       *       *

In conclusion, then, and having regard to the principle of isolation as
a whole, or in all the many and varied forms in which this principle
obtains, I trust that I have redeemed the promise with which I set
out--viz. to show that in relation to the theory of descent this
principle is of an importance second to no other, not even excepting
heredity, variability, and the struggle for existence. This has now been
fully shown, inasmuch as we have clearly seen that the importance of the
struggle for existence, and consequent survival of the fittest, arises
just because survival of the fittest is a form, and a very stringent
form, of isolation; while, as regards both heredity and variability, we
are now in a position to see that the more fully we recognize their
supreme importance as principles concerned in organic evolution, the
more must we also recognize that any rational theory of such evolution
becomes, in the last resort, a theory of the different modes in which
efficient isolation can be secured. For, in whatever degree the process
of organic evolution has been dependent upon heredity with variability,
in that degree must it also have been dependent upon the means of
securing homogamy, whereby alone the force of heredity can be made to
expend itself in the innumerable directions of progressive change,
instead of continually neutralizing the force of variability by
promiscuous intercrossing.



So far we have been concerned with the principle of Isolation in
general. We have now to consider that form of isolation which arises in
consequence of mutual infertility between the members of any group of
organisms and those of all other similarly isolated groups occupying
simultaneously the same area.

       *       *       *       *       *

Against the view that natural selection is a sufficient explanation of
the origin of species, there are two fatal difficulties: one, the
contrast between natural species and domesticated varieties in respect
of cross-sterility; the other, the fact that natural selection cannot
possibly give rise to polytypic as distinguished from monotypic
evolution. Now it is my belief that the theory of physiological
selection fully meets both these difficulties. Indeed I hold this to be
undeniable in a formal or logical sense: the only question is as to the
evidence which can be adduced for the theory in a practical or
biological sense. Therefore in this chapter, where the theory has first
of all to be stated, I shall restrict the exposition as much as possible
to the former, leaving for subsequent consideration the biological

The following is a brief outline sketch of this theory[15].

    [15] _See Nineteenth Century_, January, 1887, pp. 61, 62.

Of all parts of those variable objects which we call organisms, the most
variable is the reproductive system; and the variations may carry with
them functional changes, which may be either in the direction of
increased or of diminished fertility. Consequently variations in the way
of greater or less fertility frequently take place, both in plants and
animals; and probably, if we had adequate means of observing this point,
we should find that there is no one variation more common. But of course
where infertility arises--whether as a result of changed conditions of
life, or, as we say, spontaneously--it immediately becomes extinguished,
seeing that the individuals which it affects are less able (if able at
all) to propagate and to hand on the variation. If, however, the
variant, while showing some degree of infertility with the parent form,
continues to be as fertile as before when mated with similar variants,
under these circumstances there is no reason why such differential
fertility should not be perpetuated.

Stated in another form this suggestion enables us to regard many, if not
most, species as the records of variations in the reproductive systems
of their ancestors. When variations of a non-useful kind occur in any of
the other systems or parts of organisms, they are, as a rule,
immediately extinguished by intercrossing. But whenever they arise in
the reproductive system in the way here suggested, they tend to be
preserved as new natural varieties, or incipient species. At first the
difference would only be in respect of the reproductive systems; but
eventually, on account of independent variation, other differences would
supervene, and the variety would take rank as a true species.

Now we must remember that physiological isolation is not like those
other forms of isolation (e.g. geographical) which depend for their
occurrence on accidents of the environment, and which may therefore take
place suddenly in a full degree of completeness throughout a large
section of a species. Physiological isolation depends upon distinctive
characters belonging to organisms themselves; and it would be opposed to
the whole theory of descent with progressive modification to imagine
that absolute sterility usually arises, in a single generation between
two sections of a perfectly fertile species. Therefore evolutionists
must believe that in most, if not in all cases--could we trace the
history, say of any two species, which having sprung from a single
parent stock on a common area, are now absolutely sterile with one
another--we should find that this mutual sterility had been itself a
product of gradual evolution. Starting from complete fertility within
the limits of a single parent species, the infertility between
derivative or divergent species, _at whatever stage in their evolution
this began to occur_, must usually at first have been well-nigh
imperceptible, and thenceforth have proceeded to increase stage by

But, if it be true that physiological isolation between genetically
allied groups must usually itself have been the product of a gradual
evolution; and if, when fully evolved, it constitutes a condition of the
first importance to any further differentiation of these groups (by
preventing fusion again into one group, more or less resembling the
original parent form), do we not perceive at least a strong probability
that in the lower stages of its evolution such mutual infertility must
have acted as a segregating influence between the diverging types, in a
degree proportional to its own development? The importance of mutual
sterility as a condition to divergent evolution is not denied, _when
this sterility is already present in an absolute degree_; and we have
just seen that, before it can have attained to this absolute degree _it
must presumably, and as a rule, itself have been the subject of a
gradual development_. Does it not therefore become, on merely antecedent
grounds, in a high degree probable, that from the moment of its
inception this isolating agency must have played the part of a
segregating cause, in a degree proportional to that of its completeness
as a physiological character?

Whoever answers this question in the affirmative will have gone most of
the way towards accepting, on merely antecedent grounds, the theory of
physiological selection. And therefore it is that I have begun this
statement of the theory by introducing it upon these grounds, thereby
hoping to show how extremely simple--how almost self-evident--is the
theory which it will now be my endeavour to substantiate. I may here add
that the theory was foreshadowed by Mr. Belt in 1874[16], clearly
enunciated in its main features by Mr. Catchpool in 1884[17], and very
fully thought out by Mr. Gulick during a period of about fifteen years,
although he did not publish until a year after the appearance of my own
paper in 1886[18].

    [16] _Nicaragua_, p. 207.

    [17] _Nature_, vol. xxxi. p. 4.

    [18] _Zool. Journal, Lin. Soc._, vol. xix. pp. 337-411 (1886); and
    for Mr. Gulick's papers, _ibid._, vol. xx. pp. 189-274 (1887), vol.
    xxiii. pp. 312-380 (1889). Mr. Gulick has recently drawn my
    attention, in a private letter, to the fact that as early as 1872 a
    paper of his was read at the British Association, bearing the title
    _Diversity of Evolution under one set of External Conditions_, and
    that here the principle of physiological segregation is stated.
    Although it does not appear that Mr. Gulick then appreciated the
    great importance of this principle, it entitles him to claim

I must next proceed to state some of the leading features of
physiological selection in further detail.

It has already been shown that Darwin clearly perceived that the very
general occurrence of some degree of infertility between allied species
cannot possibly be attributed to the _direct_ agency of natural
selection. His explanation was that the slight structural modifications
entailed by the transformation of one specific type into another, so
react upon the highly delicate reproductive system of the changing type
as to render it in some degree infertile with its parent type. Now the
theory of physiological selection begins by traversing this view. It
does not, however, deny that in _some_ cases the morphological may be
the prior change; but it strenuously denies that this must be so in
_all_ cases. Indeed, according to my statement in 1886, the theory
inclines to the view that, _as a rule_, the physiological change is
prior. At the same time, the theory, as I have always stated it,
maintains that it is immaterial whether, "in the majority of instances,"
the physiological change has been prior to the morphological, or vice
versa; since in either case the physiological change will equally make
for divergence of character.

To show this clearly the best way will be to consider the two cases
separately, taking first that in which the physiological change has
priority. In this case our theory regards any morphological changes
which afterwards supervene as due to the independent variability which
will sooner or later arise under the physiological isolation thus
secured. But to whatever causes the subsequent morphological changes may
be due, the point to notice is that they are as a general rule,
consequent upon the physiological change. For in whatever _degree_ such
infertility arises between two sections of a species occupying the same
area, in that _degree_ is their interbreeding prevented, and, therefore,
opportunity is given for a subsequent divergence of type, whether by the
influence of independent variability alone, or also by that of natural
selection, as now acting more or less independently on each of the
partially separated groups. In short, all that was said in the foregoing
chapters with respect to isolation in general, here applies to
physiological isolation in particular; and by supposing such isolation
to have been the prior change, we can as well understand the subsequent
appearance of morphological divergence on continuous areas, as in other
forms of isolation we can understand such divergence on discontinuous
areas, seeing that even a moderate degree of cross-infertility may be as
effectual for purposes of isolation as a high mountain-chain, or a
thousand miles of ocean.

Here, then, are two sharply-defined theories to explain the very general
fact of there being some greater or less degree of cross-infertility
between allied species. The older, and hitherto current theory,
supposes the cross-infertility to be but an _accident_ of specific
divergence, which, therefore, has nothing to do with _causing_ the
divergence. The newer theory, on the other hand, supposes the
cross-infertility to have often been a necessary _condition_ to the
divergence having begun at all. Let us now consider which theory has
most evidence in its favour.

First of all we have to notice the very general occurrence of the fact
in question. For when we include the infertility of hybrids, as well as
first crosses, the occurrence of some degree of infertility between
allied species is so usual that Mr. Wallace recommends experiments to
ascertain whether careful observation might not prove, even of species
which hybridize, "that such species, when crossed with their near
allies, do always produce offspring which are more or less sterile
_inter se_[19]." This seems going too far, but nevertheless it is the
testimony of a highly competent naturalist to the very general
occurrence of an association between the morphological differentiation
of species and the fact of a physiological isolation. Now I regard it as
little short of self-evident that this general association between
mutual infertility and innumerable secondary, or relatively variable
morphological distinctions, is due to the former having been an original
and a necessary condition to the occurrence of the latter, in cases
where intercrossing has not been otherwise prevented.

    [19] _Darwinism_, p. 169.

The importance of physiological isolation, _when once fully developed_,
cannot be denied, for it is evident that if such isolation could be
suddenly destroyed between two allied species occupying a common area,
they would sooner or later become fused into a common type--supposing,
of course, no other form of isolation to be present. The necessity then
for this physiological form of isolation in _maintaining_ a specific
differentiation which has been already _attained_ cannot be disputed.
Yet it has been regarded as "Darwinian heresy" to suggest that it can
have been of any important service _during the process of attainment_,
or while the specific differentiation is being advanced, and this
notwithstanding that the physiological change must presumably have
developed _pari passu_ with the morphological, and notwithstanding that
in countless cases the former is associated with every conceivable
variety of the latter.

Again, why should the physiological change be thus associated with
_every conceivable variety_ of morphological change? Throughout the
length and breadth of both vegetable and animal kingdoms we find this
association, in the great majority of cases, where new species arise.
Therefore, on the supposition that in all such cases the physiological
change has been adventitiously induced by the morphological changes, we
have to face an apparently unanswerable question--Why should the
reproductive mechanism of all organic beings have been thus arranged, as
it were, to change in immediate response to the very slightest
alteration in the complex harmony of "somatic" processes, which now more
than ever is recognized as exercising so comparatively little influence
on the _hereditary_ endowments of this mechanism? Consider the
difference between a worm and the bird that is eating it, an oak tree
and the gall-insect that is piercing it: are we to suppose that in all
cases, no matter how greatly the types differ, they must agree in this,
that when any parts of these complex structures change, ever so
slightly, the reproductive system is almost certain to be adventitiously
affected, yet always thus affected in the same peculiar way?

If it be answered that the reproductive system is known to be very
sensitive to slight changes in the external conditions of life, the
answer proves too much. For though this is true, yet our opponents must
acknowledge that the reproductive system is not so sensitive, _in this
particular respect_, as their interpretation of the origin of specific
infertility requires. The proof of this point is overwhelming, for there
is the evidence from the entire range of our domesticated productions,
both vegetable and animal. Here the amount of structural change, which
has been slowly accumulated by artificial selection, is often much
greater in amount, and incomparably more rapid, than that which has been
induced between allied species by natural selection; and yet there is
scarcely any indication of the reproductive system having been affected
in the particular way that our opponents' theory requires. There are
many instances of its having been affected in sundry other ways
(chiefly, however, without any accompanying morphological change); but
among all the thousands of our more or less enormously modified
artificial types, there is scarcely one instance of such a peculiar
sexual relation between the modified descendants of a common type as so
usually obtains between allied species in nature. Yet in all other
respects evolutionists are bound to believe that the process of
modification has been in both cases strictly analogous. Why then this
conspicuous difference with respect to the reproductive system?

The answer is simple. It has never been the object of breeders or of
horticulturists to select variations in the direction of
cross-infertility, for the swamping effects of intercrossing are much
more easily and rapidly prevented by artificial isolation. Consequently,
although they have been able to modify natural types in so many
directions and in such high degrees with regard to _morphology_, there
has been no accompanying physiological modification of the kind
required. But in nature there is no such thing as artificial, i.e.
intentional, isolation. Consequently, on common areas it must usually
happen that those changes of morphology which are associated with
cross-infertility are the only ones which can arise. Hence the very
remarkable contrast between our domesticated varieties and natural
species with regard to cross-infertility is just what the present theory
would expect, or, indeed, require. But on any other theory it has
hitherto remained inexplicable.

In particular, the contrast in question has constituted one of the main
difficulties with which the theory of natural selection has hitherto had
to contend, not only in the popular mind, but also in the judgement of
naturalists, including the joint-authors of the theory themselves. Thus
Darwin says:--

     The fertility of varieties is, with reference to my theory, of
     equal importance with the sterility of species, for it seems to
     make a broad and clear distinction between varieties and

    [20] _Origin of Species_, p. 136.

And Mr. Wallace says:--

     One of the greatest, or perhaps we may say the greatest, of all the
     difficulties in the way of accepting the theory of natural
     selection as a complete explanation of the origin of species, has
     been the remarkable difference between varieties and species in
     respect of fertility when crossed[21].

    [21] _Darwinism_, p. 152.

Now, in view of this conspicuous contrast, Darwin suggested that species
in a state of nature "will have been exposed during long periods of time
to more uniform conditions than have domesticated varieties, and [that]
this may well make a wide difference in the result." Now we have to
remember that species, living and extinct, are numbered by millions, and
represent every variety of type, constitution, and habits; is it
probable, then, that this one peculiarity of the reproductive system
should be due, in so many cases, to some merely incidental effect
produced on that system by uniform conditions of life? Again, _ex
hypothesi_, at the time when a variety is first forming, the influence
exercised by uniform conditions of life (whatever in different cases
this may happen to be) cannot be present as regards that variety: yet
this is just the time when its infertility with the parent (or allied)
form is most likely to have arisen; for it is just then that the nascent
variety would otherwise have been most liable to extinction by free
intercrossing--even supposing that in the presence of such intercrossing
the variety could ever have come into existence at all.

Mr. Wallace meets the difficulty by arguing that sterility between
allied species may have been brought about by the direct influence of
natural selection. But, as previously remarked, this view is expressly
opposed to that of Darwin, who held that Wallace's contention is

It will be seen, then, that both Darwin, and Wallace, fully recognize
the necessity of finding some explanation of the infertility of allied
species, over and above the mere reaction of morphological
differentiation on the physiology of the reproductive system, and they
both agree in suggesting additional causes, though they entirely
disagree as to what these causes are. Now, the theory of physiological
selection likewise suggests an additional cause--or, rather, a new
explanation--and one which is surely the most probable. For what is to
be explained? The very general association of a certain physiological
peculiarity with that amount of morphological change which distinguishes
species from species, of whatever kind the change may be, and in
whatever family of the animal or vegetable kingdom it may occur. Well,
the theory of physiological selection explains this very general
association by the simple supposition that, at least in a large number
of cases, it was the physiological peculiarity which first of all led to
the morphological divergence, by interposing the bar of sterility
between two sections of a previously uniform species; and by thus
isolating the two sections one from another, started each upon a
subsequently independent course of divergent evolution.

Or, to put it in another way, if the occurrence of this physiological
peculiarity has been often the only possible means of isolating two
sections of a species occupying a common area, and thus giving rise to a
divergence of specific type (as obviously _must_ have been the case
wherever there was an absence of any other form of isolation), it is
nothing less than a necessary consequence that many allied species
should now present the physiological peculiarity in question. Thus the
association between the physiological peculiarity and the morphological
divergence is explained by the simple hypothesis, that the former has
acted as a necessary condition to the occurrence of the latter. In the
absence of other forms of isolation, the morphological divergence could
not have taken place at all, had not the physiological peculiarity
arisen; and hence it is that we now meet with so many cases where such
divergence is associated with this peculiarity.

       *       *       *       *       *

So far we have been considering the physiological change as historically
the prior one. Here, at first sight, it may seem that the segregative
power of physiological selection must end; for it may well seem
impossible that the physiological change can ever be necessary for the
divergence of morphological varieties into true species in cases where
it has _not_ been the prior change, but has only set in after
morphological changes have proceeded far enough to have already
constituted definite varieties. A little thought, however, will show
that physiological selection is quite as potent a condition to the
differentiation of species when it occurs after varietal divergence has
begun, as it is when it occurs before the divergence--and hence that it
really makes no difference to the theory of physiological selection
whether, in particular cases, the cross-infertility arises before or
after any structural or other modifications with which it is

For the theory does not assert that all varieties have been due to
physiological selection. There are doubtless many other causes of the
origin of varieties besides cross-infertility with parent forms; but, as
a general rule, it does not appear that they are by themselves capable
of carrying divergence beyond a merely varietal stage. In order to carry
divergence to the stage of producing _species_, it appears to be a
general condition that, sooner or later, cross-infertility should
arise--seeing that, when varieties do succeed in becoming species, we
almost invariably find that, as a matter of fact, cross-infertility has
arisen. Hence, if cross-infertility has thus usually been a necessary
condition to a varietal divergence becoming specific, it can make no
material difference when the incipient infertility arose.

It may be asked, however, whether I suppose that, when the physiological
change is subsequent, it is directly _caused_ by change of structure,
size, colour, &c., or that it arises, so to speak, accidentally, from
other causes which may have affected the sexual system in the required
way. To this question I may briefly reply, that, looking to the absence
of any influence exercised on the reproductive systems of our
domesticated plants and animals by the great and varied changes which so
many of these forms present, it would seem that among natural varieties
such closely analogous changes are presumably not the usual causes of
the physiological change, even where the latter are subsequent to the
former. Nevertheless, I do not deny that in some of these cases changes
of structure, size, colour, &c., may be the causes of the physiological
change by reacting on the sexual system in the required way. But in
such cases free intercrossing will have prevented the perpetuation of
any morphological changes, save those which have the power of so
reacting on the reproductive system as to produce the physiological
change, and thus to protect themselves against the full and adverse
power of free intercrossing. We know that slight or initial changes of
structure, colour, &c., frequently occur as varieties, and yet that on
common areas very few of these varieties become distinct species: free
intercrossing prevents any such further divergence of character. But if
in the course of many such abortive attempts, as it were, to produce a
new species, nature happens to hit upon a structural or a colour
variation which is capable of reacting on the sexual system in the
particular way required, then this variation will be enabled to protect
itself against free intercrossing in proportion to its own development.
Or, in other words, the more it develops as a morphological change, the
more will it increase the physiological change; while the more the
physiological change is thus increased, the more will it in turn promote
the morphological. By such action and reaction the development of each
furthers the development of the other, till from an almost imperceptible
variety, apparently quite fertile with its parent form, there arises a
distinct species absolutely sterile with its parent form. In such cases,
therefore, it is still the physiological conditions which have
_selected_ the particular morphological changes capable of so reacting
on the reproductive system as to produce cross-infertility, and thus to
protect themselves against the destructive power of free intercrossing.
So to speak, free intercrossing is always on the watch to level down
any changes which natural selection, or any other cause of varietal
divergence, may attempt to produce; and therefore, in order to
produce--or to increase--such divergence in the absence of any other
form of isolation, natural selection must hit upon such changes of
structure, form, or colour, as are so correlated with the reproductive
system as to create the physiological isolation that is required.

To show how the principle of selective fertility may be combined with
what apparently is the most improbable form of isolation for this
purpose--the geographical--I quote the following suggestion made by
Professor Lloyd Morgan in his _Animal Life and Intelligence_:--

     Suppose two divergent local varieties were to arise in adjacent
     areas, and were subsequently (by stress of competition or by
     geographical changes) driven together into a single area.... If
     their unions be fertile, the isolation will be annulled by
     intercrossing--the two varieties will form one mean or average
     variety. But if the unions be infertile, the isolation will be
     preserved, and the two varieties will continue separate. Suppose
     now, and the supposition is by no means an improbable one, that
     this has taken place again and again in the evolution of species;
     then it is clear that those varietal forms which had continued to
     be fertile together would be swamped by intercrossing; while those
     varietal forms which had become infertile would remain isolated.
     Hence, in the long run, isolated forms occupying a common area
     would be infertile, (p. 107.)

If then cross-sterility may thus arise even in association with
geographical isolation, may it not also arise in its absence? And may it
not thus give rise to the differentiation of varieties on account of
this physiological isolation alone?

Only two further points need be mentioned to make this statement of
physiological selection as complete as the present _résumé_ of its main
principles requires.

The first is, that, as Mr. Wallace remarks, "every species has come into
existence coincident both in space and time with a pre-existing and
closely allied species." I regard this as important evidence that
physiological selection is one of the natural causes concerned. For the
general fact implied is that every species has come into existence on an
area occupied by its parent type, and therefore under circumstances
which render it imperative that intercrossing with that type should be
prevented. In the case of monotypic evolution by natural selection
alone, intercrossing with the parent type is prevented through the
gradual extinction of that type by successive generations of the
developing type. But in the case of polytypic evolution, intercrossing
with the parent type can only be prevented by some form of isolation
other than natural selection; and here it is evident that
cross-infertility with the parent type must be as efficient to that end
as any other form of isolation that can be imagined. Consequently we
might almost have expected beforehand that in a large proportional
number of cases cross-infertility should have been the means employed.
And the fact that this is actually the case so far corroborates the only
theory which is able to explain it.

The second point is this.

It appears to be comparatively rare for any cause of specific divergence
to prove effectual on common areas, unless it sooner or later becomes
associated with some degree of cross-infertility. But through this
association, the segregating influence of both the causes concerned is,
as Mr. Gulick has shown, greatly increased. For instance, if the
segregating influence of some degree of cross-infertility be associated
with that of any other form of isolation, then, not only will the two
segregating influences be added, but multiplied together. And thus, by
their mutual action and reaction, divergent evolution is promoted at a
rapidly increasing rate.

I will now summarize the main points of the theory of physiological
isolation in a categorical form.

1. If no other form of isolation be present, specific divergence can
only take place when some degree of cross-infertility has previously
arisen between two or more sections of a species.

2. When such cross-infertility has arisen it may cause specific
divergence, either (_a_) by allowing independent variability in each of
the physiologically isolated groups; (_b_) by becoming associated with
any other cause of differentiation already operating; or (_c_) by both
these means combined.

3. As some degree of cross-infertility generally obtains between allied
species, we are justified in concluding that this has been the most
frequent--or, at any rate, the most effective--kind of isolation where
the origin of species is concerned; and therefore the kind with which,
in the case of species-formation, natural selection, or any other cause
of specific divergence, has been most usually associated.

4. Where varietal divergence has begun in the absence of
cross-infertility, such divergence seems, as a general rule, to have
been incapable of attaining to a specific value.

5. Therefore, in the vast majority of such cases, it must have been
those varietal changes of structure, size, colour, &c., which happened
to have afterwards been assisted by the reproductive change that were on
this account _selected_ as successful candidates for specific

6. It follows, that it makes no difference to the general theory of
physiological selection in what proportion of cases the physiological
change has been the initial change; for, whether prior or subsequent to
the varietal changes with which it becomes associated, its presence has
been equally important as a condition to specific divergence.

7. When physiological isolation becomes associated with natural
selection, or any other form of homogamy, the segregative power of both
is augmented. Moreover, so great is the augmentation that even very
moderate degrees of physiological isolation--themselves capable of
effecting little or nothing--become very powerful when associated with
moderate degrees of any other kind of homogamy, and vice versa.

8. The theory of physiological selection effectually explains the
divergent evolution of specific types and the cross-infertility of such
types when evolved.

       *       *       *       *       *

To prevent, if possible, the continuance of certain misunderstandings
with regard to my original statement of the new theory, let me here
disclaim some views which have been assigned to me. They are:

1. That the theory of physiological selection is opposed to the theory
of natural selection. Far from this being so, it is--at all events in my
own opinion--a very important aid to it, in preventing free
intercrossing on a common area, and thus allowing divergent evolution to
occur within that area.

2. That, in advancing the theory of physiological selection as "an
additional suggestion on the origin of species," I wish to represent it
as being the originating cause of _all_ species. What I hold is, that
all species must have owed their origin to _isolation_, in some form or
other; but that as physiological selection is only one among many other
forms of isolation (including natural selection), and as it can only act
on common areas, a large number of species must have been formed without
its aid.

3. That I imagine physiological varieties always to arise
"sporadically," or as merely individual "sports" of the reproductive
system. On the contrary, I expressly stated that this is _not_ the way
in which I suppose the "physiological variation" to arise, when giving
origin to a new species; but that it arises, whenever it is effectual,
as a "collective variation" affecting a number of individuals
simultaneously, and therefore characterizing "a whole race, or strain."

4. That I suppose physiological selection always to act alone. This I
have never supposed. The essential point is, not that the physiological
isolation is unassociated with other forms of isolation, but that unless
associated with some degree of physiological isolation, no one of the
other forms is capable of originating species on common areas with any
approach to frequency. This proposition is the essence of the new
theory, and I take it to be proved, not only by general deductive
reasoning which shows that it _must_ be so, but also by the fact of an
otherwise inexplicable association between specific divergence on common
areas and some more or less considerable degree of mutual infertility.



I will now give an outline sketch of the evidences in favour of the
theory which has been set forth in the preceding chapter, stating first
what is the nature of the verification which it requires.

The theory is deduced from a highly general association between
distinctive specific characters of _any_ kind and a relatively constant
specific character of a _particular_ kind--namely, sexual exclusiveness.
For it is from this highly general association that the theory infers
that this relatively constant specific character has been at least one
of the needful conditions to the development of the other specific
characters with which it is found associated. Hence the necessary
verification must begin by showing the strength of the theory on these
merely deductive, or antecedent, grounds. It may then proceed to show
how far the facts of organic nature corroborate the theory in other and
independent ways.

First, let it be carefully observed that here we have to do only with
the _fact_ of selective fertility, and with its _consequences_ as
supposed by the theory: we have nothing to do either with its _causes_
or its _degrees_. Not with its causes, because in this respect the
theory of physiological selection is in just the same position as that
of natural selection: it is enough for both if the needful variations
are provided, without its being incumbent on either to explain the
causes which produce them. Not with its degrees, because, in the first
place, it can only be those degrees of variation which in particular
cases are supposed adequate to induce specific divergence, that fall
within the scope of the theory; and because, in the second place,
degrees which are adequate only to induce--or to assist in inducing,
_varietal_ divergence, must always tend to increase, or pass into higher

_Antecedent Standing of the Theory._

The antecedent standing or logical basis of the theory has already been
in large measure displayed in the preceding chapter; for it was
impossible to state the theory without thereby showing in how
considerable a degree it is self-evident. A brief recapitulation is
therefore all that is here necessary.

It has been shown that divergent or polytypic evolution on common areas
is inexplicable by natural selection alone. Hence the question arises:
What form of isolation has, under such circumstances, rendered possible
divergent evolution? In answer to this question the theory of
physiological selection suggests that variations in the reproductive
function occur in such a way as to isolate more or less perfectly from
each other different sections of a species. While cross-fertility
remains unimpaired among the members of each section, there is more or
less cross-infertility when members of either section mate with those of
the other. Thus a physiological barrier is interposed between the two
sections; and any divergences of structure, colouring, or instinct
arising in the members of either section will not in any way be affected
by such divergences as arise among the members of the other.

In support of this suggestion, it has been shown in the preceding
chapter that the very general association of cross-infertility with
specific differentiation points most strongly to the inference that the
former has usually been an indispensable condition to the occurrence of
the latter. It cannot be denied that in many cases the specific
distinction is now maintained by means of that sexual isolation which
cross-infertility confers: it is therefore probable that such isolation
has been instrumental in securing its initial attainment.

This probability is strengthened by the observed fact that the general
association in question is conspicuously absent in the case of
domesticated varieties, notwithstanding that their multitudinous and
diverse varietal characters usually equal, and frequently surpass,
specific characters in their degrees of divergence.

Since, then, it would seem to be impossible for divergent evolution on
common areas to take place in the absence of some mode of isolation;
since cross-infertility appears to be the only possible mode under the
given circumstances; and since among domesticated varieties, where
isolation is otherwise secured by artificial means, cross-infertility is
usually absent, the logical foundations of the theory of physiological
selection would seem to be securely laid.

We may therefore pass to more special lines of evidence.

_Evidence from Geographical Distribution._

Darwin has adduced very good evidence to show that large areas,
notwithstanding the disadvantages which (on his theory) must arise from
free intercrossing, are what he terms better manufactories of species
than smaller areas, such as oceanic islands. On the other hand, as a
matter of fact, oceanic islands are comparatively rich in peculiar
species. These two statements, however, are not incompatible. Smaller
areas are, as a rule, rich in peculiar species relatively to the number
of their inhabitants; but it does not follow that they are rich in
species as contrasted with larger areas containing very many more
inhabitants. Therefore, the rules are that large areas turn out an
absolutely greater number of specific types than small areas; although,
relatively to the number of individuals or amount of population, the
small areas turn out a larger number of species than the large areas.

Now, these two complementary rules admit of being explained as Darwin
explains them. Small and isolated areas are rich in species relatively
to the amount of population, because, as we have before seen, this
population has been permitted to develop an independent history of its
own, shielded from intercrossing with parent forms, and from competition
with exotic forms; while, at the same time, the homogamy thus secured,
combined with change of environment, will give natural selection an
improved chance of finding new points of departure for its operation. On
the other hand, large and continuous areas are favourable to the
production of numerous species, first, because they contain a large
population, thus favouring the occurrence of numerous variations; and,
secondly, because the large area furnishes a diversity of conditions in
its different parts, as to food, climate, attitude, &c., and thus so
many different opportunities for the occurrence of sundry forms of
homogamy. Now, it is obvious that of all these sundry forms of homogamy,
physiological selection must have what may be termed a first-rate
opportunity of assisting in the manufacture of species on large areas.
For not only is it upon large and continuous areas that the antagonistic
effects of intercrossing are most pronounced (and, therefore, that the
influence of physiological selection must be most useful in the work of
species-making); but here also the diversity in the external conditions
of life, which the large area supplies to different parts of the
extensive population, cannot fail to furnish physiological selection
with a greater abundance of that particular variation in the
reproductive system on which its action depends. Again, and of still
more importance, on large areas there are a greater _number_ of species
already differentiated from one another as such; thus a greater number
of already sexually differentiated forms are presented for further
differentiation at the hands of physiological selection. For all these
reasons, therefore, we might have expected, upon the new theory, that
large and continuous areas would be good manufactories of species.

Again, Darwin has shown that not only large areas, but likewise
"dominant" genera within those areas, are rich in species. By dominant
genera he meant those which are represented by numerous individuals, as
compared with other genera inhabiting the same area. This general rule
he explains by the consideration that the qualities which first led to
the form being dominant must have been useful; that these would be
transmitted to the otherwise varying offspring; and, therefore, that
when these offspring had varied sufficiently to become new species, they
would still enjoy their ancestral advantages in the struggle for
existence. And this, doubtless, is in part a true explanation; but I
also think that the reason why dominant genera are rich in species, is
chiefly because they everywhere present a great number of individuals
exposed to relatively great differences in their conditions of life: or,
in other words, that they furnish the best raw material for the
manufacture of species by physiological selection, as explained in the
last paragraph. For, if the fact of dominant genera being rich in
species is to be explained _only_ by natural selection, it appears to me
that the useful qualities which have already led to the dominance of the
ancestral type ought rather to have proved inimical to its splitting up
into a number of subordinate types. If already so far "in harmony with
its environment" as to have become for this reason dominant, one would
suppose that there is all the more reason for its not undergoing change
by the process of natural selection. Or, at least, I do not see why the
fact of its being in an unusual degree of harmony with its environment
should in itself constitute any unusual reason for its modification by
survival of the fittest. On the other hand, as just observed, I do very
plainly see why such a reason is furnished for the modifying influence
of physiological selection.

Let us next turn to another of Darwin's general rules with reference to
distribution. He took a great deal of trouble to collect evidence of the
two following facts, namely, (1) that "species of the larger genera in
each country vary more frequently than the species of the smaller
genera"; and (2) that "many of the species included within the larger
genera resemble varieties in being very closely, but unequally, related
to each other, and in having restricted ranges[22]." By larger genera he
means genera containing many species; and he accounts for these general
facts by the principle, "that where many species of a genus have been
formed, on an average many are still forming." But _how_ forming? If we
say by natural selection alone, we should expect to find the
multitudinous species differing from one another in respect of features
presenting well-marked adaptive meanings; yet this is precisely what we
do not find. For Darwin's argument here is that "in large genera the
amount of difference between the species is often exceedingly small, so
that in this respect the species of the larger genera resemble varieties
more than do the species of the smaller genera." Therefore the argument,
while undoubtedly a very forcible one in favour of the fact of
_evolution_, appears to me scarcely consistent with the view of this
evolution being due solely to natural selection. On the other hand, the
argument tells strongly (though unconsciously) in favour of
physiological selection. For the larger a genus, or the greater the
number of its species, the greater must be the opportunity for the
occurrence of that particular kind of variation on which the principle
of physiological selection depends. The species of a genus may be
regarded as so many varieties which have already been separated from one
another physiologically; therefore each of them may now constitute a new
starting-point for a further and similar separation--particularly as, in
virtue of their previous segregation, many are now exposed to different
conditions of life. Thus, it seems to me, we can well understand why it
is that genera already rich in species tend to grow richer; while such
is not the case in so great a degree with genera that are poor in
species. Moreover, we can well understand that, multiplication of
species being as a rule, and in the first instance, determined by
changes in the reproductive system, wherever a large number of new
species are being turned out, the secondary differences between them
should be "often exceedingly small"--a general correlation which, so far
as I can see, we are not able to understand on the theory of natural

    [22] _Origin of Species_, pp. 44, 45.

The two subsidiary facts, that very closely allied species have
restricted ranges, and that dominant species are rich in varieties, both
seem to tell more in favour of physiological than of natural selection.
For "very closely allied species" is but another name for species which
scarcely differ from one another at all except in their reproductive
systems; and, therefore, the more restricted their ranges, the more
certainly would they have become fused by intercrossing with one
another, had it not been for the barrier of sterility imposed by the
primary distinction. Or rather, I should say, had it not been for the
original occurrence of this barrier, these now closely-allied species
could never have become species. Again, that dominant species should be
rich in varieties is what might have been expected; for the greater the
number of individuals in a species, the greater is the chance of
variations taking place in all parts of the organic type, and
particularly in the reproductive system, seeing that this system is the
most sensitive to small changes in the conditions of life, and that the
greater the number of individuals composing a specific type, the more
certainty there is of some of them encountering such changes. Hence, the
richness of dominant species in varieties is, I believe, mainly due to
the greater opportunity which such species afford of some degree of
cross-infertility arising between their constituent members.

Here is another general fact, also first noticed by Darwin, and one
which he experiences some difficulty an explaining on the theory of
natural selection. He says:--

     In travelling from north to south over a continent, we generally
     meet at successive intervals with closely-allied or representative
     species, evidently filling the same place in the economy of the
     land. These representative species often meet and interlock, and as
     one becomes rarer and rarer, the other becomes more and more
     frequent, till the one replaces the other. But if we compare these
     species where they intermingle, they are generally as absolutely
     distinct from each other in every detail of structure as are
     specimens taken from the metropolis of each.... In the
     intermediate region, having intermediate conditions of life, why do
     we not now find closely-linking intermediate varieties? This
     difficulty for a long time quite confounded me. But I think it can
     in large part be explained[23].

    [23] _Origin of Species_, ed. 6, pp. 134, 135.


His explanation is that, "as the neutral territory between two
representative species is generally narrow in comparison with the
territory proper to each, ... and as varieties do not essentially differ
from species, the same rule will probably apply to both; and, therefore,
if we take a varying species inhabiting a very large area, we shall have
to adapt two varieties to two large areas, and a third variety to a
narrow intermediate zone." It is hence argued that this third or
intermediate variety, on account of its existing in lesser numbers, will
probably be soon overrun and exterminated by the larger populations on
either side of it. But how is it possible "to adapt two varieties to two
large areas, and a third [transitional] variety to a narrow intermediate
zone," in the face of free intercrossing on a continuous area? Let _A_,
_B_, and _C_ represent the three areas in question. According to the
argument, variety _A_ passes first into variety _B_, and then into
variety _C_, while variety _B_ eventually becomes exterminated by the
inroads both from _A_ and _C_. But how can all this have taken place
with nothing to prevent intercrossing throughout the entire area _A_,
_B_, _C_? I confess that to me it seems this argument can only hold on
the supposition that the analogy between varieties and species extends
to the reproductive system; or, in a sense more absolute than the
argument has in view, that "varieties do not essentially differ from the
species" which they afterwards form, but from the first show some degree
of infertility towards one another. And, if so, we have of course to do
with the principles of physiological selection.

That in all such cases of species-distribution these principles have
played an important part in the species-formation, appears to be
rendered further probable from the suddenness of transition on the area
occupied by contiguous species, as well as from the completeness of
it--i. e. the absence of connecting forms. For these facts combine to
testify that the transition was originally due to that particular change
in the reproductive systems of the forms concerned, which still enables
those forms to "interlock" without intercrossing. On the other hand,
neither of these facts appears to me compatible with the theory of
species-formation by natural selection alone.

But this leads us to another general fact, also mentioned by Darwin, and
well recognized by all naturalists, namely, that closely allied species,
or species differing from one another in trivial details, usually occupy
contiguous areas; or, conversely stated, that contiguity of geographical
position is favourable to the appearance of species closely allied to
one another. Now, the large body of facts to which I here allude, but
need not at present specify, appear to me to constitute one of the
strongest of all my arguments in favour of physiological selection.
Take, for instance, a large continental area, and follow across it a
chain of species, each link of which differs from those on either side
of it by the minute and trivial distinctions of a secondary kind, but
all the links of which differ from one another in respect of the primary
distinction, so that no one member of the series is perfectly fertile
with any other member. Can it be supposed that in every case this
constant primary distinction has been superinduced by the secondary
distinctions, distributed as they are over different parts of all these
kindred organisms, and yet nowhere presenting any but a trifling amount
of morphological change?

For my own part, I cannot believe--any more than Darwin could
believe--that all these numerous, diverse, and trivial changes have
always had the accidental effect of inducing the same peculiar change in
the reproductive system, and so producing it without any reference to
the process of specific divergence. Nor can I believe, as Darwin
incidentally and provisionally suggested, that prolonged exposure to
uniform conditions of life have so generally induced an equally
meaningless result. I can only believe that all the closely allied
species inhabiting our supposed continent, and differing from one
another in so many and such divers points of small detail, are merely so
many records of the fact that selective fertility has arisen among their
ancestry, and has thus given as many opportunities for the occurrence of
morphological differentiations as it has furnished cases of efficient
isolation. Of course, I do not deny that many, or probably most, of
these trivial morphological differentiations have been produced by
natural selection on account of their utility: I merely deny that they
could have been so produced on this common area, but for the sexual
isolation with which every distinct set of them is now found to be

_Evidence from Topographical Distribution of Species._

By topographical distribution I mean the distribution of organisms with
reference to comparatively small areas, as distinguished from larger
regions with reference to which the term geographical distribution is

It will be at once apparent that a study of the topographical
distribution of organic types is of even more importance for us than a
study of their geographical distribution. For while the former study is
conducted, as it were, with a low power of our observing microscope, the
latter is conducted with a high power. The larger facts of geographical
distribution yield, indeed, all the general characters which we might
expect them to yield, on the theory that divergence of specific types on
common areas has been in chief part determined by physiological
conditions. But for the purpose of testing this theory in a still more
exacting manner, it is of the first importance to consider the more
detailed facts of topographical distribution, since we here come to
closer quarters with the problem of specific differentiation. Therefore,
as we have already considered this problem under the most general points
of view, we will now consider it under more special points of view.

       *       *       *       *       *

It is self-evident, as we have seen in the preceding section, that the
greater the number of individuals of the same species on a given area,
the less must be the power of natural selection to split that species
into two or more allied types; because, the more crowded the population,
the greater must be the uniformitarian effect of free intercrossing.
This obvious fact has been insisted upon by several previous writers on
Darwinism; and the only reason why it has not been recognized by all
naturalists is that so few of them have observed the all-important
distinction between monotypic and polytypic evolution. The denser the
population, and therefore the greater the intercrossing and the severer
the struggle for existence within the species, the better will it be for
_transmutation_ of the species by natural selection; but the worse it
will be for _differentiation_ of the species by this form of homogamy.
On the other hand, if physiological selection be entertained as a form
of homogamy, the denser the population, the better opportunity it will
have of differentiating the species, first, because a greater number of
individuals will be present in which the physiological change may arise,
and, secondly, because, if it does arise, the severity of the struggle
for existence will _then_ give natural selection a better chance of
acting rapidly and effectually on each of the isolated sections.

Hence, where the question is whether selective fertility has played any
large or general part in the differentiation of specific types, the best
criterion we can apply is to ascertain whether it is a general rule that
closely allied species occur in intimate association, so that their
individual members constitute, as it were, a single population, or, on
the other hand, whether they occur rather on different sides of
physical barriers. If they occur intimately associated, the form of
homogamy to which their differentiation was due must have presumably
been the physiological form; whereas, if they are proved to be
correlated with physical barriers, the form of homogamy which was
concerned in their differentiation must presumably have been the
geographical form.

Now, at first this consideration was a trouble to me, because Moritz
Wagner had strenuously argued--and supported his argument by a
considerable wealth of illustration--that allied species are always
found correlated with physical barriers or discontinuous areas.
Weismann's answer, indeed, had shown that Wagner's statement was much
too general: nevertheless, I was disappointed to find that so much could
be said in favour of the geographical (or topographical) form of
isolation where closely allied species are concerned. Subsequently,
however, I read the writings of Nägeli on this subject, and in them I
find a very different state of matters represented.

Seeing as clearly as Wagner that it is impossible under any
circumstances for natural selection to cause specific _differentiation_
unless assisted by some other forms of homogamy, but committing the same
oversight as Wagner and Weismann in supposing that the only other form
of homogamy in nature is geographical isolation, Nägeli, with great
force of reasoning, and by many examples, founded his argument against
the theory of natural selection on the ground that in the vegetable
kingdom closely allied species are most frequently found in intimate
association with one another, not, that is to say, in any way isolated
by means of physical barriers. This argument is everywhere logically
intact; and, as he sustains it by a large knowledge of topographical
botany, his indictment against natural selection as a cause of specific
_differentiation_ appeared to be insurmountable. And, in point of fact,
it _was_ insurmountable; so that the whole problem of the origin of
species by _differentiation on common areas_ has hitherto been left in
utter obscurity. Nor is there now any escape from this obscurity, unless
we entertain the "supplementary factor" of selective fertility. And,
apparently, the only reason why this has not been universally
recognized, is because Darwinians have hitherto failed to perceive the
greatness of the distinction between the _differentiation_ and the
_transmutation_ of species; and hence have habitually met such
overwhelming difficulties as Nägeli presented by an illogical
confounding of these two totally distinct things.

But if the idea of selective fertility had ever occurred to Nägeli as a
form of segregation which gives rise to specific differentiation, I can
have no doubt that so astute and logical a thinker would have perceived
that his whole indictment against natural selection was answered. For it
is incredible that he should not have perceived how this physiological
form of homogamy (supposing it to arise _before_ or _during_, and not
_after_ the specific differentiation) would perform exactly the same
function on a continuous area, as he allowed that "isolation" does on a
discontinuous one.

However, be this as it may, there cannot be any question touching the
immense value of his facts and arguments as evidence in favour of
physiological selection--albeit this evidence was given unconsciously,
or, as it were, prophetically. Therefore I will here quote a few
examples of both, from his paper _Du Développement des Espèces

    [24] _Archives des Sciences physiques et naturelles_ (Genève), vol.
    liii. (1875), pp. 211-236.

After stating the theory of natural selection, he says that if the
theory is (of itself) a true explanation of the origin (or divergence)
of specific forms, it ought to follow that

     two closely allied forms, derived the one from the other, would
     necessarily occupy two different geographical areas [or
     topographical stations], since otherwise they would soon become
     blended. Until they had already become sufficiently consolidated as
     distinct species to render mutual intercrossing highly improbable,
     they could not be intermingled without disadvantage [to
     differentiation]. Had Darwin endeavoured to support his hypothesis
     by facts, he would, at least in the vegetable kingdom, have found
     little to favour his cause. I can cite many hundreds of cases, in
     which species in every stage of development have been found closely
     mingling with one another, and not in any way isolated. Therefore,
     I do not think that one can rightly speak of natural selection in
     the Darwinian sense in the vegetable kingdom; and, in my
     estimation, there is a great difference between the formation of
     species by nature and the production of stock by a breeder.... (p.

     Of the two kinds of distribution (i. e. growing apart and growing
     together), Synoicy (or growing together) is by far the most usual
     in nature. I reckon that out of a hundred allied vegetable forms,
     at least ninety-five would be found to be synoical (p. 219).

This is a most important point. That so enormous a proportion of
vegetable species should have originated in intimate association with
their parent or sister types, is clearly unintelligible on the theory of
natural selection alone; there obviously _must_ be some other form of
homogamy which, whether or not in all places _associated_ with natural
selection, is the primary condition to the differentiation. Such I hold
with Nägeli, is a logical necessity; and this whether or not I am right
in believing the other form of homogamy in question to be selective
fertility. But I go further and say, Surely there can be no rational
question that this other form of homogamy must have been, at any rate as
a highly general rule, the one which I have assigned. For how is it that
in these ninety-five per cent. of cases, where vegetable species are
growing intimately associated with their nearest allies, there is no
hybridizing, or blending and relapsing to the original undifferentiated
types? We know well the answer. These are fully differentiated species,
and, as such, are protected from mutual intercrossing by the barrier of
mutual sterility. But now, if this bar is thus necessary for preserving
the specific distinctions when they have been fully developed, much more
must it have been so to admit of their development; or, otherwise
stated, since we know that this barrier is associated with "synoical"
species, and since we clearly perceive that were it withdrawn these
species would soon cease to exist, can we reasonably doubt that their
existence (or origin) is due to the previous erection of this barrier?
If synoical species were comparatively rare, the validity of such
reasoning might be open to question; or, even if we should not doubt it
in such cases, at any rate we might well doubt the importance or extent
of selective fertility as a factor in the origination of species. But
the value of Nägeli's writings on the present subject consists in
showing that synoical species constitute so overwhelming a majority of
the vegetable kingdom, that here, at all events, it appears impossible
to rate too highly the importance of the principle I have called
physiological selection.



_Evidence from Topographical Distribution of Varieties._

In the last section we have considered the topographical distribution of
closely allied _species_. I now propose to go still further into matters
of detail, by considering the case of natural _varieties_. And here we
come upon a branch of our inquiry where we may well expect to meet with
the most crucial tests of our theory. For if it should appear that these
nascent species more or less resemble fully developed species in
presenting the feature of cross-infertility, the theory would be
verified in the most direct and conclusive manner possible. These
nascent species may be called embryo species, which are actually in
course of differentiation from their parent-type; and therefore, if they
do not exhibit the feature in relation to that type which the present
theory infers to be necessary for the purposes of differentiation, the
theory must be abandoned. On the other hand, if they do exhibit this
feature, it is just the feature which the theory predicted as one that
would be found highly characteristic of such embryo types.
Contrariwise, the theory of natural selection can have no reason to form
any such anticipation; or rather its anticipation would necessarily
require to be the exact opposite. For, according to this theory, the
cross-infertility of allied species is due, either to correlation with
morphological changes which are being produced by the selection, or
else, as Darwin supposed, to "prolonged exposure to uniform conditions
of life"; and thus, in either case, the sterility variation ought to be,
as a general rule at all events, subsequent to the specific
differentiation, and, according to Darwin's view, _long_ subsequent.
Thus we ought not to find that the physiological change is ever, on any
large or general scale, the initial change; nor ought we to find that it
is, on any such scale, even so much as a contemporary change: there
ought, in fact, to be no constant or habitual association between
divergence of embryo-types and the concurrence of cross-infertility.

Now, it will be my endeavour to prove that there is an extraordinarily
general association between _varietal_ divergence and cross-infertility,
_wherever common areas are concerned_; and in as far as this can be
proved, I take it that the evidence will make wholly in favour of
physiological selection as the prime condition to specific divergence,
while at the same time they will make no less wholly, _and quite
independently_, against natural selection as the unaided cause of such

I shall begin with some further quotations from Nägeli.

     Species may be synoical at all stages of relationship. We come
     across varieties, scarcely distinguishable from one another,
     growing in the same locality (as, for example, the _Cirsium
     heterophyllum_, with smooth or jagged leaves, the _Hieracium
     sylvaticum_, with or without caulinary leaves); again, we meet
     other varieties more accentuated (as the _H. hoppeanum_, with under
     ligules of white or red, the _Campanula_, with white or lilac
     flowers, &c.), other varieties even more marked, which might almost
     be elevated to the rank of species (_Hieracium alpinum_, with hairs
     and glands, and the new form _H. holadenium_, which has only
     glands, _Campanula rotundifolia_ with smooth and hairy leaves), or
     forms still more distinct, up to well-defined species. I could
     enumerate endless examples at all stages.

     It will be seen that in my definition of synoicy I do not mean to
     assert that _all_ allied forms are invariably found together, but
     that they are much more often seen in groups than singly. Take, for
     instance, nine forms closely related (_A_ to _I_). _A_, _E_, _H_
     will be found side by side at one point, _B_, _D_ at another, _C_,
     _F_ at a third, &c. These facts are plainly opposed to the theory
     of isolation and amixia, and make, on the contrary, in favour of
     the social development of species (_loc. cit._, p. 221).

Not to multiply quotations to the same general effect, I will supply but
one other, referring to a particular case.

     At one spot (_Rothwand_) much exposed to the sun, and difficult of
     access, I remarked two closely allied forms, so nearly related to
     _H. villosum_ that this would seem to be an intermediary form
     between the two. One of these (_H. villosissimum_) is distinguished
     by its tongue and thick pubescence, its tolerably large capitula,
     and by the lengthened and separated scales of the involucrum; the
     other, on the contrary (_H. elongatum_), is less pubescent, has
     smaller capitula, and more compact scales on the involucrum than
     _H. villosum_. Both are finally distinguishable from the type by
     their longer stalks, which are more decidedly aphyllous, and by
     their later flowering. At the spot where I found them the two forms
     were closely intermingled, and each was represented by a
     considerable number of plants. I did not find them anywhere else on
     the mountain, nor could I find at the spot where these were growing
     a single specimen of the true _H. villosum_, nor a single hybrid
     from these two.

     I concluded that these two new forms had, by joining their forces,
     expelled the _H. villosum_ from its primitive abode, but had not
     succeeded in displacing one another. As to their origin, they had
     evidently developed in two different directions from a common point
     of departure, namely _H. villosum_. They had succeeded, not only in
     separating themselves from the original form, but also in
     preventing any intermediary form from interposing. I thought myself
     therefore justified in considering this as a case of varieties
     which have come into existence subsequently to the Glacial epoch.
     The morphological characteristics of the three forms are
     sufficiently distinct for them to be designated as species by a
     good many writers. They are better defined than some of MM. Frolich
     and Fries' weaker species, and as well defined as some of MM. Koch
     and Grisebach's (p. 222).

Now it is clear, without comment, that all this is exactly as it ought
to be, if allied species have been differentiated on common areas by
selective fertility. For if, as Nägeli elsewhere says, "one meets forms
in nature associated with one another, and severally distinguished by
every possible degree of differentiation," not only as Nägeli adds, does
this general fact lead to the inference that species are (usually)
developed when plants grow intimately associated together; but as
certainly it leads to the further inference that such development must
be due to a prior development of cross-infertility between the diverging
varietal forms, cross-infertility which is therefore afterwards so
characteristic of the allied species, when these are found, in their
fully differentiated condition, still occupying the same area in large
and intimately mingled populations.

To my mind there could not be any inference more strongly grounded than
this, because, with the one exception of the physiological form, no
other form of homogamy can be conceived which shall account for the
origin and permanence of these synoical varieties, in all degrees of
differentiation up to well-defined synoical species. Least of all, as we
have seen, can natural selection alone have had anything to do with such
a state of matters; while, as we have likewise seen, in all its details
it is exactly the state of matters which the theory of physiological
selection requires.

Nevertheless, although this inference is so strongly grounded, we ought
to remember that it is only an inference. In order fully to verify the
theory of physiological selection, we ought to prove by experiment the
fact of cross-infertility between these synoical varieties, as we learn
that it afterwards obtains between synoical species. It is to be
regretted that the theory of physiological selection did not occur to
the mind of Nägeli, because he would then, no doubt, have ascertained
this by actual experiment. As it is, the great value of his observations
goes no further than establishing a strong presumption, that it _must_
be selective fertility which causes the progressive differentiation of
synoical varieties; and also that, if so, this _must_ be the principal
factor in the differentiation of vegetable species, seeing that some
ninety-five per cent. are of synoical origin.

_Evidence from Experimental Research._

My paper on _Physiological Selection_ pointed out that the whole theory
would have to stand or fall with the experimental proof of the presence
or the absence of cross-infertility between varieties of the same
species growing on common areas. From the facts and considerations which
we have hitherto been dealing with, it did indeed appear to me that
there was the strongest conceivable ground for inferring that
cross-infertility between such varieties would be found by experiment to
be a phenomenon of highly general occurrence--amply sufficient ground to
prove that allied species on common areas for the most part owed their
origin to this character of mutual sterility, and not vice versa as
previously supposed. At that time I was not aware that any experiments
had been made in this direction. Soon after the paper was published,
however, my attention was directed to a laborious research which had
been directed to this very point, and carried on for more than thirty
years, by M. Jordan[25]. This had not attracted the general notice which
it undoubtedly deserved; and I have since ascertained that even Darwin
began to look into it only a few months before his death.

    [25] _Remarques sur le fait de l'existence en société à l'état
    sauvage des espèces végétales affines et sur d'autres faits relatifs
    à la question de l'espèce_, par Alexis Jordan; lues au congrès de
    l'Association Française pour l'Avancemeat des Sciences, 2^me
    session, Lyon, séance de 28 Août, 1873.

Having devoted his life to closely observing in divers stations
multitudes of different species of plants--annuals and perennials,
bulbous and aquatic, trees and shrubs--M. Jordan has been able to
satisfy himself, and the French school of botanists to which this line
of observation has given rise, that in most cases (or "nearly
everywhere"), when a Linnean species is indigenous to a country and is
there of common occurrence, this species within that district is
represented by more or less numerous and perfectly constant varieties.
These varieties are constituted by such minute differences of
morphological character that their very existence eluded the
observation of botanists, until M. Jordan began to search specially for
them as the special objects of his scrutiny. Moreover, these varieties
of a Linnean species occupy common areas, and there grow in intimate
association with one another, or as M. Jordan says, "_pêle-mêle_." So
far, be it noticed, Jordan was proceeding on exactly the same lines as
Nägeli; only he carried his observations over a still wider range of
species on the one hand, and into a still minuter search for varieties
on the other. But the all-important point for us is, that he further
proceeded to test by experiment the physiological relations between
these morphological varieties; and found, in many hundreds of cases,
that they not only came true to seed (i. e. are hereditary and not
merely climatic), but likewise cross-sterile _inter se_. For these
reasons, M. Jordan, who is opposed to the theory of evolution, regards
all such varieties as separately created species; and the inspiring
motive of his prolonged investigations has been a desire to multiply
these proofs of creative energy. But it clearly makes no difference, so
far as evolutionists are concerned with them, whether all this multitude
of sexually isolated forms be denominated species or varieties.

The points which are of importance to evolutionists--and of the first
order of importance in the present connexion--may be briefly summarized
as follows:--

(1) The research embraces large numbers of species, belonging to very
numerous and very varied orders of plants; (2) in the majority of
cases--although not all--indigenous species which are of common
occurrence present constant varieties; (3) these varieties,
nevertheless, may be morphologically so slight as to be almost
imperceptible; (4) they occupy common areas and grow in intimate
association; (5) although many of them have undergone so small an amount
of morphological change, they have undergone a surprising amount of
physiological change; for (6) not only do very many of these varieties
come true to seed; but, (7) when they do, they are always more or less
cross-infertile _inter se_.

Now, it is self-evident that every one of these seven points is exactly
what the theory of physiological selection requires, while there is not
one of them which it does not require. For if the theory be sound, we
should expect to find large numbers of species belonging to numerous and
varied orders of plants presenting constant varieties on common areas;
we should expect this to be a highly general, though not a universal,
rule; and we should expect it to apply only to species which are
indigenous. Moreover, we should expect these varieties, although but
slightly differentiated morphologically, to present a great
differentiation physiologically--and this in the special direction of
selective fertility, combined, of course, with heredity.

On the other hand, as I have said, this catalogue of evidences leaves
nothing to be supplied. It gives us all the facts--and no more than all
the facts--which my paper on _Physiological Selection_ anticipated as
the eventual result of a prolonged experimental research. And if I have
to regret my ignorance of these facts when that paper was published, at
any rate it now furnishes the best proof that my anticipations were not
guided by the results of a verification which had already been supplied.
These anticipations were deduced exclusively from the theory itself, as
representing what _ought_ to be the case if the theory were true; and, I
must confess, if I had then been told that they had already been
realized--that it had actually been found to be a general rule that
endemic species present constant and hereditary varieties, intimately
commingled on common areas, morphologically almost indistinguishable,
but physiologically isolated by selective fertility--I should have felt
that the theory had been verified in advance. For there are only two
alternatives: either these things are due to physiological selection, or
else they are due--as M. Jordan himself believes--to special creation.
Which is equivalent to saying that, for evolutionists, the facts must be
held to verify the former theory in as complete a manner as it is
logically possible for the theory to be verified.

_Evidence from Prepotency._

We have now to consider the bearing of what is called "prepotency" on
the theory of physiological selection.

Speaking of the vast number of species of Compositae, Darwin says:--

     There can be no doubt that if the pollen of all these species could
     be simultaneously or successively placed on the stigma of any one
     species, this one would elect with unerring certainty its own
     pollen. This elective capacity is all the more wonderful, as it
     must have been acquired since the many species of this great group
     of plants branched off from a common progenitor.

Darwin is here speaking of elective affinity in its fully developed
form, as absolute cross-sterility between fully differentiated species.
But we meet with all lower degrees of cross-infertility--sometimes
between "incipient species," or permanent varieties, and at other times
between closely allied species. It is then known as "prepotency" of the
pollen belonging to the same variety or species over the pollen of the
other variety or species, when both sets of pollen are applied to the
same stigma. Although in the absence of the prepotent pollen the less
potent will fertilize the seed, yet, such is the appetency for the more
appropriate pollen, that even if this be applied to the stigma some
considerable time after the other, it will outstrip or overcome the
other in fertilizing the ovules, and therefore produce the same result
on the next generation as if it had been applied to the mother plant
without any admixture of the less potent pollen, although in some cases
such incipient degrees of cross-infertility are further shown by the
number or quality of the seeds being fewer or inferior.

Now, in different varieties and in different allied species, all degrees
of such prepotency have been noticed by many observers, from the
faintest perceptible amount up to complete impotency of the alien
pollen--when, of course, there is absolute sterility between the two
varieties or allied species. The inference is obvious. In this graduated
scale of prepotency--beginning with an experimentally almost
imperceptible amount of sexual differentiation between two varieties,
and ending in an absolute partitioning of two allied species--we have
the only remaining fact that is required to complete the case in favour
of the present theory. We are here brought back to the very earliest
stages of physiological differentiation or to the stages which lie
behind Jordan's "Physiological Species"; and therefore, when taken in
conjunction with his results, the phenomena of prepotency may be said to
give us the complete and final demonstration of one continuous
development, which, beginning in an almost imperceptible amount of
cross-infertility, ends in absolute cross-sterility. The "elective
capacity" to which Darwin alludes as having been "acquired" by all the
species of Compositae since they "branched off from a common
progenitor," is thus seen among innumerable other species actually in
process of acquisition; and so we can perfectly well understand, what is
otherwise unintelligible, that closely allied species of plants occur,
in ninety-five per cent. of cases, intimately associated on common
areas, while exhibiting towards one another the character of mutual

But more than this. The importance of the widespread phenomena of
prepotency to the theory of physiological selection does not consist
merely in thus supplying the last link in the chain of evidence touching
the origin of species by selective fertility, or "elective capacity."
These phenomena are of further importance as showing how in plants, at
all events, physiological selection appears to be frequently capable of
differentiating specific types without the necessary assistance of any
other form of homogamy. In my original statement of the theory, I was
careful to insist upon the great value, as differentiating agents, of
even small degrees of other forms of homogamy when co-operating with
physiological selection. But I also stated my belief that in many cases
selective fertility is presumably of itself capable of splitting a
specific type; and the reason why I still believe this is, that I do not
otherwise understand these phenomena of prepotency. I cannot believe
that in all the innumerable cases where they arise, they have been
super-induced by some prior morphological changes going on in some other
part of the organism, or by "prolonged exposure to uniform conditions of
life," on the part of two well-nigh identical forms which have arisen
intimately commingled in exactly the same environment, and under the
operation of a previously universal intercrossing. Even if such a thing
could be imagined as happening occasionally, I feel it difficult to
imagine that it can happen habitually, and yet this view must be held by
those who would attribute prepotency to natural selection.

It must never be forgotten that the relatively enormous changes as to
size, structure, habit, &c., which are presented by our domesticated
plants as results of artificial selection, do not entail the
physiological character of cross-sterility in any degree, save possibly
in some small number of cases. Although in wild species any
correspondingly small percentage of cases (where natural selection
happens to hit upon parts of the organism modifications of which produce
the physiological change by way of correlation) would doubtless be the
ones to survive on common areas, still it is surely incredible that such
an accidental association between natural selection and
cross-infertility is so habitually the means of specific differentiation
as the facts of prepotency (together with the observations of Jordan
and Nägeli) would necessarily demand.

Moreover, this view of the matter is still further corroborated by
certain other facts and considerations. For example, the phenomena of
prepotency (whether as between varieties or between closely allied
species) are found to occur when the two forms occupy a common area,
i.e. are growing intermingled with one another. Therefore, but for this
physiological differentiation, there could be absolutely nothing to
prevent free intercrossing. Yet the fact that hybrids are so
comparatively rare in a state of nature--a fact which Sir Joseph Hooker
has pointed out to me as otherwise inexplicable--proves the efficacy of
even a low degree of such differentiation in preventing the
physiologically-differentiated forms from intercrossing. Even in cases
where there is no difficulty in producing artificial hybrids or mongrels
between species or varieties growing on common areas, it is perfectly
astonishing what an extremely small percentage of the hybrid or mongrel
forms are found to occur in nature. And there can be no question that
this is due to the very efficient manner in which prepotency does its
work--efficient, I mean, from the point of view of the new theory; for
upon any other theory prepotency is a meaningless phenomenon, which,
notwithstanding its frequent occurrence, plays no part whatever in the
process of organic evolution.

I attach considerable importance to the phenomena of prepotency in view
of the contrast which is presented between plants and animals in the
relation of their species to physical barriers. For animals--and
especially the higher animals--appear to depend for their specific
differentiations upon such barriers much more than in the case with
plants. This is no more than we should expect; for, in accordance with
our theory, selective fertility is not so likely to work alone in the
case of the higher animals which mate together, as in plants which are
fertilized through the agency of wind or insects. In the former case
there is no opportunity given for the first rise of cross-infertility,
in the form of prepotency; and even where selective fertility has gained
a footing in other ways, the chances against the suitable mating of
"physiological complements" must be much greater than it is in the
latter case. Hence, among the higher animals, selective fertility ought
much more frequently to be found in association with other forms of
homogamy than it is among plants. And this is exactly what we find. Thus
it seems to me that this contrast between the comparative absence and
presence of physical barriers, where allied species of plants and of
higher animals are respectively concerned, is entitled to be taken as a
further corroboration of our theory. For while it displays exactly such
a general correlation as this theory would expect, the correlation is
one which cannot possibly be explained on any other theory. It is just
where physiological selection can be seen to have the best opportunity
of acting (viz. in the vegetable kingdom) that we find the most
unequivocal evidence of its action; while, on the other hand, it is just
where it can be seen to have the least opportunity of asserting itself
(viz. among the higher animals) that we find it most associated with,
and therefore assisted by, other forms of homogamy, i. e. not only
geographical isolation, but also by sexual preference in pairing, and
the several other forms of homogamy, which Mr. Gulick has shown to arise
in different places as the result of intelligence.

_Evidence from Special Cases._

Hitherto I have been considering, from the most general point of view,
the most widespread facts and broadest principles which serve to
substantiate the theory of physiological selection. I now pass to the
consideration of one of those special cases in which the theory appears
to have been successfully applied.

Professor Le Conte has adduced the fossil snails of Steinheim as serving
to corroborate the theory of physiological selection[26].

    [26] _Evolution and its Relations to Religious Thought_, &c. pp.

The facts are these. The snail population of this lake remain for a long
time uniform and unchanged. Then a small percentage of individuals
suddenly began to vary as regards the form of their shells, and this in
two or three directions at the same time, each affected individual,
however, only presenting one of the variations. But after all these
variations had begun to affect a proportionally large number of
individuals, some individuals occur in which two or more of the
variations are blended together, evidently, as Weismann says, by
intercrossing of the varieties so blended. Later still, both the
separate varieties and their blended progeny became more and more
numerous, and eventually a single blended type, comprising in itself all
the initial varieties, supplanted the parent form. Then another long
period of stability ensued until another eruption of new variations took
place; and these variations, after having affected a greater and greater
number of individuals, eventually blended together by intercrossing and
supplanted their parent form. So the process went on, comparatively
short periods of variation alternating with comparatively long periods
of stability, the variations, moreover, always occurring suddenly in
crops, then multiplying, blending together, and in their finally blended
type eventually supplanting their parent form.

Now, the remarkable fact here is that whenever the variations arose,
they only intercrossed between themselves, they did not intercross with
their parent form; for, if they had, not only could they never have
survived (having been at first so few in number and there having been no
geographical barriers in the small lake), but we should have found
evidence of the fact in the half-bred progeny. Moreover, natural
selection can have had nothing to do with the process, because not only
are the variations in the form of the shells of no imaginable use in
themselves; but it would be preposterous to suppose that at each of
these "variation periods" several different variations should always
have occurred simultaneously, all of which were of some hidden use,
although no one of them ever occurred during any of the prolonged
periods of stability. How, then, are we to explain the fact that the
individuals composing each crop of varieties, while able to breed among
themselves, never crossed with their parent form? These varieties, each
time that they arose, were intimately commingled with their parent
form, and would certainly have been reabsorbed into it had intercrossing
in that direction been possible. With Professor Le Conte, therefore, I
conclude that there is only one conceivable answer to this question.
Each crop of varieties must have been _protected from intercrossing with
their parent form_.

They must have been the result of a variation, which rendered the
affected individuals sterile with their parent form, whilst leaving them
fertile amongst themselves. The progeny of these individuals would then
have dispersed through the lake, physiologically isolated from the
parent population, and especially prone to develop secondary variations
as a direct result of the primary variation. Thus, as we might expect,
two or three variations arose simultaneously, as expressions of so many
different lines of family descent from the original or physiological
variety; these were everywhere prevented from intercrossing with their
parent form, yet capable of blending whenever they or their
ever-increasing progeny happened to meet. Thus, without going into
further details, we are able by the theory of physiological selection to
give an explanation of all these facts, which otherwise remain

       *       *       *       *       *

In view of the evidence which has now been presented, I will now ask
five questions which must be suitably answered by critics of the theory
of physiological selection.

1. Can you doubt that the hitherto insoluble problem of inter-specific
sterility would be solved, supposing cross-infertility were proved to
arise before or during the process of specific differentiation, instead
of after that process had been fully completed?

2. Can you doubt, after duly considering the circumstances under which
allied species of plants have been differentiated--viz. in ninety-five
per cent. of cases intimately commingled on common areas, and therefore
under identical environments--that cross-infertility _must_ have arisen
before or during the specific differentiation?

3. Can you doubt, after duly considering the facts of prepotency on the
one hand and those of Jordan's physiological varieties on the other,
that cross-infertility _does_ arise before or during the specific

4. If you cannot express a doubt upon any of these points, can you
explain why you refuse to accept the theory of the origin of species by
means of physiological selection, together with the explanation which
this theory affords of the continued cross-fertility of domesticated

5. Supposing this theory to be true, can you conceive of any other
classes of facts which, either quantitatively or qualitatively, could
more directly or more effectually prove its truth than those which have
now been adduced?

On these five heads I entertain no doubt. I am convinced that the theory
of physiological selection is the only one that can explain the facts of
inter-specific sterility on the one hand, and, on the other hand, the
contrast which these facts display to the unimpaired fertility of our
domesticated varieties.

In conclusion, it seems desirable once more to insist that there is no
antagonism or rivalry between the theories of natural and of
physiological selection. For which purpose I will quote the final
paragraph of my original paper.

     So much, then, for the resemblances and the differences between the
     two theories. It only remains to add that the two are
     complementary. I have already shown some of the respects in which
     the newer theory comes to the assistance of the older, and this in
     the places where the older has stood most in need of assistance. In
     particular, I have shown that segregation of the fit entirely
     relieves survival of the fittest from the difficulty under which it
     has hitherto laboured of explaining why it is that sterility is so
     constantly found between species, while so rarely found between
     varieties which differ from one another even more than many
     species; why so many features of specific distinction are useless
     to the species presenting them; and why it is that incipient
     varieties are not obliterated by intercrossing with parent forms.
     Again, we have seen that physiological selection, by preventing
     such intercrossing, enables natural selection to promote diversity
     of character, and thus to evolve species in ramifying branches
     instead of in linear series--a work which I cannot see how natural
     selection could possibly perform unless thus aided by physiological
     selection. Moreover, we have seen that although natural selection
     alone could not induce sterility between allied types, yet when
     this sterility is given by physiological selection, the forms which
     present it would be favoured in the struggle for existence; and
     thus again the two principles are found playing, as it were, into
     each other's hands. And here, as elsewhere, I believe that the
     co-operation enables the two principles to effect very much more in
     the way of species-making than either of them could effect if
     working separately. On the one hand, without the assistance of
     physiological selection, natural selection would, I believe, be all
     but overcome by the adverse influences of free
     intercrossing--influences all the more potent under the very
     conditions which are required for the multiplication of species by
     divergence of character. On the other hand, without natural
     selection, physiological selection would be powerless to create any
     differences of specific type, other than those of mutual sterility
     and trivial details of structure, form, and colour--differences
     wholly without meaning from a utilitarian point of view. But in
     their combination these two principles appear to me able to
     accomplish what neither can accomplish alone--namely, a full and
     satisfactory explanation of the origin of species.



This historical sketch must begin with a consideration of Darwin's
opinions on the subject; but as these were considerably modified from
time to time during a period of thirty years by the publications of
other naturalists, it will be impossible to avoid cross-references as
between his writings and theirs. It may also be observed that the _Life
and Letters of Charles Darwin_ was not published until the year 1887, so
that the various opinions which I shall quote from the letters, and
which show some considerable approximation in his later years to the
views which have been put forward by Mr. Gulick and myself, were not
before us at the time when our papers were read.

The earliest allusion that I can find to geographical isolation in the
writings of Darwin occurs in a correspondence with Sir Joseph Hooker, as
far back as 1844. He there says:--

     I cannot give my reasons in detail; but the most general conclusion
     which the geographical distribution of all organic beings appears
     to me to indicate is, that isolation is the chief concomitant or
     cause of the appearance of _new_ forms (I well know there are some
     staring exceptions)[27].

    [27] _Life and Letters_, vol. ii. p. 28.

And again:--

     With respect to original creation or production of new forms, I
     have said that isolation appears the chief element[28].

    [28] _Ibid._

Next, in the earlier editions of the _Origin of Species_ this view is
abandoned, and in its stead we meet with the opinion that geographical
isolation lends a certain amount of assistance to natural selection, by
preventing free intercrossing. But here we must note two things. First,
the distinction between monotypic and polytypic evolution is not
defined. Secondly, the levelling effect of free intercrossing in nature,
and hence its antagonism to divergence of character by natural
selection, is not sufficiently recognized; while, on the other hand, and
in consequence of this, the importance of isolation as a factor of
evolution is underrated--not only in its geographical, but likewise in
all its other forms.

Taking these two points separately, the only passages in Darwin's
writings, so far at least as I can find, in which any distinction is
drawn between evolution as monotypic and polytypic, are those in which
he deals with a somewhat analogous distinction between artificial
selection as intentional and unconscious. He says, for example:--

     In the case of methodical selection, a breeder selects for some
     definite object, and if the individuals be allowed freely to
     intercross, his work will completely fail. But when many men,
     without intending to alter the breed, have a nearly common
     standard of perfection, and all try to procure and breed from the
     best animals, improvement surely but slowly follows from this
     unconscious process of selection, notwithstanding that there is no
     separation of selected individuals. Thus it will be under

    [29] _Origin of Species_, p. 80, 6th ed. (1872).

Here we have what may perhaps be regarded as a glimmering of the
distinction between monotypic and polytypic evolution. But that it is
only a glimmering is proved by the immediately ensuing sentences, which
apply this analogy of unconscious selection _not_ to the case of
monotypic, _but_ to that of polytypic evolution. So likewise, in the
succeeding discussion on "divergence of character," the analogy is again
resorted to for the purpose of showing how polytypic evolution may occur
in nature.

Thus far, then, it may be said that we have scarcely so much as a
glimmering of the distinction between monotypic and polytypic evolution;
and as the same discussion (with but a few verbal alterations) runs
through all the editions of the _Origin_, it may well be asked why I
should have alluded to such passages in the present connexion. Well, I
have done so because it is apparent that, during the last years of his
life, the distinction between selection as "methodical" and
"unconscious" enabled Darwin much more clearly to perceive that between
evolution as monotypic and polytypic. Thus in 1868 he wrote to Moritz
Wagner (who, as we shall presently see, entirely failed to distinguish
between monotypic and polytypic evolution), expressing his belief--

     That in many large areas all the individuals of the same species
     have been slowly modified, in the same manner, for instance, as the
     English racehorse has been improved, that is, by the continued
     selection of the fleetest individuals, without any separation. But
     I admit that by this process two or more new species could hardly
     be formed within the same limited area[30].

    [30] _Life and Letters_, vol. iii. p. 158.

Again, in 1876 he wrote another letter to Wagner, in which the following
passage occurs:--

     I believe that all the individuals of a species can be slowly
     modified within the same district, in nearly the same manner as man
     effects by what I have called the process of unconscious selection.
     I do not believe that one species will give birth to two or more
     new species as long as they are mingled together within the same

    [31] _Ibid._ p. 159.

Two years later he wrote to Professor Semper:--

     There are two different classes of cases, it appears to me, viz.
     those in which species becomes slowly modified in the same country,
     and those cases in which a species splits into two, or three, or
     more new species; and, in the latter case, I should think nearly
     perfect separation would greatly aid in their "specification," to
     coin a new word[32].

    [32] _Ibid._ p. 160.

Now, these passages show a very much clearer perception of the
all-important distinction between monotypic and polytypic evolution than
any which occur in the _Origin of Species_; and they likewise show that
he was led to this perception through what he supposed to be a somewhat
analogous distinction between "unconscious" and "methodical" selection
by man. The analogy, I need hardly say, is radically unsound; and it is
a curious result of its unsoundness that, whereas in the _Origin of
Species_ it is adduced to illustrate the process of polytypic evolution,
as previously remarked, in the letters above quoted we find it adduced
to illustrate the process of monotypic evolution. But the fact of this
analogy being unsound does not affect the validity of the distinction
between monotypic and polytypic evolution to which it led Darwin, in his
later years, so clearly to express[33].

    [33] The analogy is radically unsound because unconscious selection
    differs from methodical selection only in the _degree_ of
    "separation" which it effects. These two forms of selection do not
    necessarily differ from one another in regard to the _number_ of
    characters which are being simultaneously diversified; for while it
    may be the object of methodical selection to breed for modification
    of a single character alone, it may, on the other hand, be the
    result of unconscious selection to diversify an originally uniform
    stock, as Darwin himself observes with regard to horse-breeding. The
    real distinction between monotypic and polytypic evolution is, not
    at all with reference to the _degree_ of isolation (i. e. _amount_
    of "separation"), but to the _number of cases_ in which any
    efficient degree of it occurs (i. e. whether in but a single case,
    or in two or more cases).

Turning next to the second point which we have to notice, it is easy to
show that in the earlier editions of his works Darwin did not
sufficiently recognize the levelling effects of free intercrossing, and
consequently failed to perceive the importance of isolation (in any of
its forms) as a factor of organic evolution. This may be most briefly
shown by quoting his own more matured opinion upon the subject. Thus,
with reference to the swamping effects of intercrossing, he wrote to Mr.
Wallace in 1867 as follows:--

     I must have expressed myself atrociously: I meant to say exactly
     the reverse of what you have understood. F. Jenkin argued in the
     _North British Review_ against single variations being perpetuated,
     and has convinced me, though not in quite so broad a manner as here
     put. I always thought individual differences more important; but I
     was blind, and thought that single variations might be preserved
     much oftener than I now see is possible or probable. I mentioned
     this in my former note merely because I believed that you had come
     to a similar conclusion, and I like much to be in accord with you.
     I believe I was mainly deceived by single variations offering such
     simple illustrations, as when man selects [i.e. isolates][34].

    [34] _Life and Letters_, vol. iii. pp. 157-8.

Again, somewhere about the same time, he wrote to Moritz Wagner:--

     Although I saw the effects of isolation in the case of islands and
     mountain-ranges, and knew of a few instances of rivers, yet the
     greater number of your facts were quite unknown to me. I now see
     that, from the want of knowledge, I did not make nearly sufficient
     use of the views which you advocate[35].

    [35] _Ibid._ pp. 157-8.

Now it would be easy to show the justice of these self-criticisms by
quoting longer passages from earlier editions of the _Origin of
Species_; but as this, in view of the above passages, is unnecessary, we
may next pass on to another point.

The greatest oversight that Wagner made in his otherwise valuable essays
on geographical isolation, was in not perceiving that geographical
isolation is only one among a number of other forms of isolation: and,
therefore, that although it is perfectly true, as he insisted, that
polytypic evolution cannot be effected by natural selection alone, it is
very far from true, as he further insisted, that _geographical_
isolation is the only means whereby natural selection can be assisted in
this matter. Hence it is that, when Darwin said he had not himself "made
nearly sufficient use" of geographical isolation as a factor of specific
divergence, he quite reasonably added that he could not go so far as
Wagner did in regarding such isolation as a condition, _sine qua non_,
to divergent evolution in all cases. Nevertheless, he adds the
important words, "I almost wish I could believe in its importance to the
same extent with you; for you well show, in a manner which never
occurred to me, that it removes many difficulties and objections." These
words are important, because they show that Darwin had come to feel the
force of the "difficulties and objections" with regard to divergent
evolution being possible by means of natural selection alone, and how
readily they could be removed by assuming the assistance of isolation.
Hence, it is much to be deplored that Wagner presented a single kind of
isolation (geographical) as equivalent to the principle of isolation in
general. For he thus failed to present the complete--and, therefore, the
true--philosophy of the subject to Darwin's mind; and in this, as in
certain other respects which I shall notice later on, served rather to
confuse than to elucidate the matter as a whole.

To sum up. Although in his later years, as shown by his correspondence,
Darwin came to recognize more fully the swamping effects of free
intercrossing, and the consequent importance of "separation" for the
prevention of these effects, and although in this connexion he likewise
came more clearly to distinguish between the "two cases" of monotypic
and polytypic evolution, it is evident that he never worked out any of
these matters--"thinking it prudent," as he wrote with reference to them
in 1878, "now I am growing old, to work at easier subjects[36]."
Therefore he never clearly saw, on the one hand, that free
intercrossing, far from constituting a "difficulty" to _monotypic_
evolution by natural selection, is the very means whereby natural
selection is in this case enabled to operate; or, on the other hand,
that, in the case of _polytypic_ evolution, the "difficulty" in question
is so absolute as to render such evolution, by natural selection alone,
absolutely impossible. Hence, although in one sentence of the _Origin of
Species_ he mentions three forms of isolation (besides the geographical
form) as serving in some cases to assist natural selection in causing
"divergence of character" (i. e. polytypic evolution[37]), on account of
not perceiving how great and how sharp is the distinction between the
two kinds or "cases" of evolution, he never realized that, where "two or
more new species" are in course of differentiation, _some_ form of
isolation other than natural selection must _necessarily_ be present,
whether or not natural selection be likewise so. The nearest approach
which he ever made to perceiving this necessity was in one of his
letters to Wagner above quoted, where, after again appealing to the
erroneous analogy between monotypic evolution and "unconscious
selection," he says:--"But I admit that by this process (i. e.
unconscious selection) two or more new species could hardly be formed
within the same limited area: some degree of separation, if not
indispensable, would be highly advantageous; and here your facts and
views will be of great value." But even in this passage the context
shows that by "separation" he is thinking exclusively of _geographical_
separation, which he rightly enough concludes (as against Wagner) need
certainly not be "indispensable." Had he gone a step further, he must
have seen that separation, _in some form or another, is_ "indispensable"
to polytypic evolution. Instead of taking this further step, however,
two years later he wrote to Semper as follows:--

    [36] _Life and Letters_, vol. iii. p. 161.

    [37] Page 81. The three forms of isolation mentioned are, "from
    haunting different stations, from breeding at slightly different
    seasons, or from the individuals of each variety preferring to pair

     I went as far as I could, perhaps too far, in agreement with Wagner
     [i. e. in the last edition of the _Origin of Species_]; since that
     time I have seen no reason to change my mind; but then I must add
     that my attention has been absorbed on other subjects[38].

    [38] _Life and Letters_, vol. iii. p. 159.

And he seems to have ended by still failing to perceive that the
explanation which he gives of "divergence of character" in the _Origin
of Species_, can only hold on the unexpressed assumption that free
intercrossing is in some way prevented at the commencement, and
throughout the development, of each diverging type.

Lastly, we have to consider Darwin's opinion touching the important
principle of "Independent Variability." This, it will be remembered, is
the principle which ensures that when a portion (not too large) of a
species is prevented from interbreeding with the rest of the species,
sooner or later a divergence of type will result, owing to the fact that
the average qualities of the separated portion at the time of its
separation cannot have been exactly the same as the average qualities of
the specific type as a whole. Thus the state of Amixia, being a state of
what Mr. Gulick calls Independent Generation, will of itself--i.e. even
if unassisted by natural selection--induce divergence of type, in a
ratio that has been mathematically calculated by Delbœuf.

Darwin wrote thus to Professor Weismann in 1872:--

     I have now read your essay with very great interest. Your view of
     the origin of local races through "Amixia" is altogether new to me,
     and seems to throw an important light on an obscure question[39].

    [39] _Life and Letters_, vol. iii. p. 155.

And in the last edition of the _Variation of Animals and Plants_ he adds
the following paragraph:--

     This view may throw some light on the fact that the domestic
     animals which formerly inhabited the several districts in Great
     Britain, and the half-wild cattle lately kept in several British
     parks, differed slightly from one another; for these animals were
     prevented from wandering over the whole country and intercrossing,
     but would have crossed freely within each district or park[40].

    [40] _Variation_, &c., vol. ii. p. 262.

Now, although I allow that Darwin never attributed to this principle of
Amixia, or Independent Variability, anything like the degree of
importance to which, in the opinion of Delbœuf, Gulick, Giard, and
myself, it is entitled, the above passage appears to show that, as soon
as the "view" was clearly "suggested" to his mind, he was so far from
being unfavourably disposed towards it, that he added a paragraph to the
last edition of his _Variation_ for the express purpose of countenancing
it. Nevertheless, later on the matter appears to have entirely escaped
his memory; for in 1878 he wrote to Semper, that he did "not see at all
more clearly than I did before, from the numerous cases which he
[Wagner] has brought forward, how and why it is that a long isolated
form should almost always become slightly modified[41]." I think this
shows entire forgetfulness of the principle in question, because, if
the latter is good for explaining the _initial_ divergence of type as
between separated stocks of "domesticated animals," much more must it be
competent to explain the _further_ divergence of type which is "almost
always" observable in the case of "a long isolated form" under nature.
The very essence of the principle being that, when divergence of type
has once begun, this divergence must _ipso facto_ proceed at an
ever-accelerating pace, it is manifestly inconsistent to entertain the
principle as explaining the first commencement of divergence, and then
to ignore it as explaining the further progress of divergence. Hence, I
can only conclude that Darwin had forgotten this principle altogether
when he wrote his letter to Semper in 1878--owing, no doubt, as he says
in the sentence which immediately follows, to his having "not attended
much of late years to such questions."

    [41] _Life and Letters_, vol. iii. p. 161.

       *       *       *       *       *

So much, then, for Darwin's opinions. Next in order of time we must
consider Moritz Wagner's essays on what he called the "Law of
Migration[42]." The merit of these essays was, first, the firm expression
of opinion upon the swamping effects of free intercrossing; and, second,
the production of a large body of facts showing the importance of
geographical isolation in the prevention of these effects, and in the
consequent differentiation of specific types. On the other hand, the
defect of these essays was, first, not distinguishing between evolution
as monotypic and polytypic; and, second, not perceiving that
geographical isolation is only one among a number of other forms of
isolation. From these two radical oversights--which, however, were
shared by all other writers of the time, with the partial exception of
Darwin himself, as previously shown--there arose the following and most
lamentable errors.

    [42] _Die Darwin'sche Theorie und das Migrationsgesetz_ (1868):
    _Ueber den Einfluss der geographischen Isolirung_, &c. (1870).

Over and over again Moritz Wagner insists, as constituting the
fundamental doctrine of his attempted reform of Darwinism, that
evolution by natural selection is impossible, unless natural selection
be assisted by geographical isolation, in order to prevent the swamping
effects of intercrossing[43]. Now, if instead of "evolution" he had said
"divergence of type," and if instead of "geographical isolation" he had
said "prevention of intercrossing," he would have enunciated the general
doctrine which it has been the joint endeavour of Mr. Gulick and myself
to set forth. But by not perceiving that "evolution" is of two radically
different kinds--polytypic and monotypic--he entirely failed to perceive
that, while for one of its kinds the _prevention_ of intercrossing is an
absolute necessity, for the other of its kinds the _permission_ of
intercrossing is a necessity no less absolute. And, again, in missing
the fact that geographical isolation is but one of the many ways
whereby intercrossing may be prevented, he failed to perceive that, even
as regards the case of polytypic evolution, he greatly erred in
representing this one form of isolation as being universally a necessary
condition to the process. The necessary condition to this process is,
indeed, the prevention of intercrossing _by some means or another_; but
his unfortunate insistence on geographical separation as the only
possible means to this end--especially when coupled with his no less
unfortunate disregard of monotypic evolution--caused him to hinder
rather than to advance a generalization which he had only grasped in
part. And this generalization is, as now so repeatedly stated, that
while the form of isolation which we know as natural selection depends
for its action upon the intercrossing of all the individuals which it
isolates (i. e. selects), when acting alone it can produce only
monotypic evolution; but that when it is supplemented by any of the
other numerous forms of isolation, it is furnished with the necessary
condition to producing polytypic evolution--and this in as many lines of
divergent change as there may be cases of this efficient separation.

    [43] For instance, speaking of common, or continuous areas, he
    says:--"In this case a constant variety, or new species, cannot be
    produced, because the free crossing of a new variety with the old
    unaltered stock will always cause it to revert to the original type;
    in other words, will destroy the new form. The formation of a real
    variety, which Darwin, as we know, regards as the commencement of a
    new species, will only succeed when a few individuals, having
    crossed the barrier of their habitat, are able to separate
    themselves for a long time from the old stock." And the last
    sentence, given as a summary of his whole doctrine, is--"The
    geographical isolation of the form, a necessary consequence of
    migration, is the cause of its typical character."

Nevertheless, while we must lament these shortcomings on the part of
Wagner, we ought to remember that he rendered important services in the
way of calling attention to the swamping effects of free intercrossing,
and, still more, in that of showing the high importance of geographical
isolation as a factor of organic evolution. Therefore, although in an
elaborate criticism of his views Weismann was easily able to dispose of
his generalizations in the imperfect form that they presented, I do not
think it was just in Weismann to remark, "if Wagner had confined himself
to the statement that geographical isolation materially assists the
process of natural selection, and thus also promotes the origination of
new species, he would have met with little or no opposition; but then,
of course, in saying this much, he would not have been saying anything
new." No doubt, as I have just shown, he _ought_ thus (as well as in
other and still more important respects not perceived by Prof. Weismann)
to have limited his statement; but, had he done so, it does not follow
that he would not have been saying anything new. For, in point of fact,
in as far as he said what was true, he did say a great deal that was
also new. Thus, most of what he said of the _principle of separation_
(apogamy) was as new as it was true, although, as we have seen, he said
it to very little purpose on account of his identifying this principle
as a whole with that of but one of its forms. Again, notwithstanding
this great error, or oversight, he certainly showed of the particular
form in question--viz. geographical isolation--that it was of
considerably _more_ importance than had previously been acknowledged.
And this was so far a valuable contribution to the general theory of

       *       *       *       *       *

Prof. Weismann's essay, to which allusion has just been made[44], was,
however, in all respects a great advance upon those of Wagner. It was
not only more comprehensive in its view of the whole subject of
geographical isolation, but likewise much more adequate in its general
treatment thereof. Its principal defects, in my judgement, were, first,
the inordinately speculative character of some of its parts, and,
second, the restriction of its analysis to but one form of isolation--a
defect which it shares with the essays of Wagner, and in quite as high a
degree. Furthermore, although this essay had the great merit of
enunciating the principle of Amixia, it did so in a very inefficient
manner. For not only was this principle adduced with exclusive reference
to _geographical_ isolation, but even in regard to this one kind of
isolation it was presented in a highly inconsistent manner, as I will
now endeavour to show.

    [44] _Ueber den Einfluss der Isolirung auf die Artbildung_ (1872).

Weismann was led to perceive the principle in question by the
consideration that new specific characters, when they first appear, do
not all appear together in the same individuals: they appear one in one
individual, another in another, a third in a third, &c.; and it is only
in the course of successive generations that they all become blended in
the same individuals by free intercrossing. Hence, the eventually
emerging constant or specific type is the resultant of all the
transitory or varietal types, when these have been fused together by
intercrossing. From which Weismann deduces what he considers a general
law--namely, that "the constancy of a specific type does not arise
suddenly, but gradually; and it is established by the promiscuous
crossing of all individuals[45]." From which again it follows, that this
constancy must cease so soon as the condition which maintains it
ceases--i. e. so soon as free intercrossing is prevented by the
geographical isolation of a portion of the species from its parent

    [45] _Loc. cit._, p. 43.

Now, to begin with, this statement of the principle in question is not a
good statement of it. There was no need while stating the doctrine that
separation induces differentiation, to found the doctrine on any such
highly speculative basis. In point of fact, there is no real evidence
that specific types do attain their constancy in the way supposed; nor,
for the purposes of the doctrine in question, is it necessary that there
should be. For this doctrine does not need to show how the constancy has
been _attained_; it only has to show that the constancy is _maintained_
by free intercrossing, with the result that when free intercrossing is
_by any means_ prevented, divergence of character ensues. In short, the
correct way of stating the principle is that which has been adopted by
Delbœuf and Gulick--namely, the average characters of a separated
portion of a species are not likely to be the same as those of the whole
species; with the result that divergence of type will be set up in the
separated portion by intercrossing within that portion. Or the principle
may be presented as I presented it under the designation of "Independent
Variability"--namely, "a specific type may be regarded as the average
mean of all individual variations, any considerable departure from this
average mean being, however, checked by intercrossing," with the result
that when intercrossing is prevented between a portion of a species and
the rest of the species, "this population is permitted to develop an
independent history of its own, shielded from intercrossing with its
parent form[46]."

    [46] _Physiological Selection_, pp. 348, 389.

Not only, however, is Weismann's principle of "Amixia" thus very
differently stated from that of my "Independent Variability" (apogamy),
or Gulick's "Independent Generation"; but, apparently owing to this
difference of statement, the principle itself is not the same. In
particular, while Weismann holds with us that when new characters arise
in virtue of the mere prevention of intercrossing with parent forms
these new characters will be of non-utilitarian kind[47], he appears to
think that divergence of character under such circumstances is not
likely to go on to a _specific_ value. Now, it is of importance to
observe why he arrives at this conclusion, which is not only so
different from that of Delbœuf, Gulick, and myself, but apparently so
inconsistent with his own recognition of the diversifying effect of
"Amixia" as regards the formation of _permanent varieties_. For, as we
have already seen while considering Darwin's views on this same
principle of "Amixia," it is highly inconsistent to recognize its
diversifying effect up to the stage of constituting fixed varieties, and
then not to recognize that, so much divergence of character having been
already secured by the isolation alone, much more must further
divergence continue, and continue at an ever accelerating pace--as
Delbœuf and Gulick have so well shown. What, then, is the explanation
of this apparent inconsistency on Weismann's part? The explanation
evidently is that, owing to his erroneous statement of the principle, he
misses the real essence of it. For, in the first place, he does not
perceive that this essence consists in an initial difference of average
characters on the part of the isolated colony as compared with the rest
of their species. On the contrary, he loses himself in a maze
of speculation about all species having had what he calls
"variation-periods," or eruptions of general variability alternating
with periods of repose--both being as unaccountable in respect of their
causation as they are hypothetical in respect of their occurrence. From
these speculations he concludes, that isolation of a portion of a
species will then only lead to divergence of character when the
isolation happens to coincide with a "variation-period" on the part of
the species as a whole, and that the divergence will cease so soon as
the "variation-period" ceases. Again, in the second place as previously
remarked, equally with Wagner whom he is criticizing, he fails to
perceive that _geographical_ isolation is not the only kind of
isolation, or the only possible means to the prevention of free
intercrossing. And the result of this oversight is, that he thinks
amixia can act but comparatively seldom upon sufficiently small
populations to become a factor of much importance in the differentiation
of species. Lastly, in the third place, owing to his favourite
hypothesis that all species pass through a "variation-period," he
eventually concludes that the total amount of divergence of type
producible by isolation alone (even in a small population) can never be
greater than that between the extremes of variation which occur within
the whole species at the date of its partition (p. 75). In other words,
the possibility of change due to amixia alone is taken to be limited by
the range of deviation from the general specific average, as manifested
by different individual variations, before the species was divided. Thus
the doctrine of amixia fails to recognize the law of Delbœuf, or the
_cumulative_ nature of divergence of type when once such divergence
begins in a separated section. Therefore, in this all-important--and,
indeed, essential--respect, amixia differs entirely from the principle
which has been severally stated by Delbœuf, Gulick, and myself.

    [47] _Loc. cit._, p. 54.

Upon the whole, then, we must say that although Professor Weismann was
the first to recognize the diversifying influence of merely
indiscriminate isolation _per se_ (apogamy), he did so only in part. He
failed to distinguish the true essence of the principle, and by
overlaying it with a mass of hypothetical speculation, concealed even
more of it than he revealed.

       *       *       *       *       *

The general theory of Isolation, as independently worked out by Mr.
Gulick and myself, has already been so fully explained, that it will
here be sufficient merely to enumerate its more distinguishing features.
These are, first, drawing the sharpest possible line between evolution
as monotypic and polytypic; second, showing that while for the former
the peculiar kind of isolation which is presented by natural selection
suffices of itself to _transform_ a specific type, in order to work for
the latter, or to _branch_ a specific type, natural selection must
necessarily be assisted by some other kind of isolation; third, that
even in the absence of natural selection, other kinds of isolation may
be sufficient to effect specific divergence through independent
generation alone; fourth, that, nevertheless, natural selection, where
present, will always accelerate the process of divergence; fifth, that
monotypic evolution by natural selection depends upon the _presence_ of
intercrossing, quite as much as polytypic evolution (whether with or
without natural selection) depends upon the _absence_ of it; sixth,
that, having regard to the process of evolution throughout all taxonomic
divisions of organic nature, we must deem the physiological form of
isolation as the most important, with the exception only of natural

The only difference between Mr. Gulick's essays and my own is, that, on
the one hand, he has analyzed much more fully than I have the various
forms of isolation; while, on the other hand, I have considered much
more fully than he has the particular form of physiological isolation
which so frequently obtains between allied _species_. This particular
form of physiological isolation I have called "physiological selection,"
and claim for it so large a share in the differentiation of specific
types as to find in it a satisfactory explanation of the contrast
between natural species and artificial varieties in respect of

       *       *       *       *       *

Mr. Wallace, in his _Darwinism_, has done good service by enabling all
other naturalists clearly to perceive how natural selection alone
produces monotypic evolution--namely, through the free intercrossing of
all individuals which have not been eliminated by the isolating process
of natural selection itself. For he very lucidly shows how the law of
averages must always ensure that in respect of any given specific
character, half the individuals living at the same time and place will
present the character above, and half below its mean in the population
as a whole. Consequently, if it should ever be of advantage to a
species that this character should undergo either increase or decrease
of its average size, form, colour, &c., there will always be, in each
succeeding generation, a sufficient number of individuals--i. e. half of
the whole--which present variations in the required direction, and which
will therefore furnish natural selection with abundant material for its
action, without the need of any other form of isolation. It is to be
regretted, however, that while thus so clearly presenting the fact that
free intercrossing is the very means whereby natural selection is
enabled to effect monotypic evolution, he fails to perceive that such
intercrossing must always and necessarily render it impossible for
natural selection to effect polytypic evolution. A little thought might
have shown him that the very proof which he gives of the necessity of
intercrossing where the _transmutation_ of species is concerned,
furnishes, measure for measure, as good a proof of the necessity of its
absence where the _multiplication_ of species is concerned. In justice
to him, however, it may be added, that this distinction between
evolution as monotypic and polytypic (with the important consequence
just mentioned) still continues to be ignored also by other well-known
evolutionists of the "ultra-Darwinian" school. Professor Meldola, for
example, has more recently said that in his opinion the "difficulty from
intercrossing" has been in large part--if not altogether--removed by Mr.
Wallace's proof that natural selection alone is capable of effecting
[monotypic] evolution; while he regards the distinction between
monotypic and polytypic evolution as mere "verbiage[48]."

    [48] _Nature_, vol. xliii. p. 410, and vol. xliv. p. 29.

It is in relation to my presentment of the impossibility of natural
selection alone causing polytypic evolution, that Mr. Wallace has been
at the pains to show how the permission of intercrossing (panmixia) is
necessary for natural selection in its work of causing monotypic
evolution. And not only has he thus failed to perceive that the
"difficulty" which intercrossing raises against the view of natural
selection being of itself capable of causing polytypic evolution in no
way applies to the case of monotypic; but as regards this "difficulty,"
where it does apply, he says:--

     Professor G. J. Romanes has adduced it as one of the difficulties
     which can alone be overcome by his theory of physiological

    [49] _Darwinism_, p. 143.

This, however, is a misapprehension. I have by no means represented that
the difficulty in question can alone be overcome by this theory. What I
have represented is, that it can be overcome by any of the numerous
forms of isolation which I named, and of which physiological selection
is but one. And although, _where common areas are concerned_, I believe
that the physiological form of isolation is the most important form,
this is a very different thing from entertaining the supposition which
Mr. Wallace here assigns to me.

       *       *       *       *       *

I may take this opportunity of correcting a somewhat similar
misunderstanding which has been more recently published by Professor W.
A. Herdman, of Liverpool; and as the case which he gives is one of
considerable interest in itself, I will quote his remarks in extenso. In
his _Opening Address to the Liverpool Biological Society_, Professor
Herdman said:--

     Some of you will doubtless remember that in last year's address,
     while discussing Dr. Romanes' theory of physiological selection, I
     quoted Professor Flemming Jenkin's imaginary case of a white man
     wrecked upon an island inhabited by negroes, given as an
     illustration of the supposed swamping effect by free intercrossing
     of a marked variety with the parent species. I then went on to say
     in criticism of the result at which Jenkin arrived, viz. that the
     characteristics of the white man would be stamped out by
     intercrossing with the black:--

     "Two influences have, I think, been ignored, viz. atavism, or
     reversion to ancestral characters, and the tendency of the members
     of a variety to breed with one another. Keeping to the case
     described above, I should imagine that the numbers of intelligent
     young mulattoes produced in the second, third, fourth, and few
     succeeding generations would to a large extent intermarry, the
     result of which would be that a more or less white aristocracy
     would be formed on the island, including the king and all the chief
     people, the most intelligent men and the bravest warriors. Then
     atavism might produce every now and then a much whiter
     individual--a reversal to the characteristics of the ancestral
     European--who, by being highly thought of in the whitish
     aristocracy, would have considerable influence on the colour and
     other characteristics of the next generation. Now such a white
     aristocracy would be in precisely the same circumstances as a
     favourable variety competing with its parent species," &c.

     You may imagine then my pleasure when, a few months after writing
     the above, I accidentally found, in a letter[50] written by the
     celebrated African traveller Dr. David Livingstone to Lord
     Granville, and dated "Unyanyembe, July 1st, 1872," the following

    [50] In Appendix to H. M. Stanley's _How I found Livingstone_, 2nd
    ed. London, 1872, p. 715.

     "About five generations ago, a white man came to the highlands of
     Basañgo, which are in a line east of the watershed. He had six
     attendants, who all died, and eventually their headman, called
     Charura, was elected chief by the Basañgo. In the third generation
     he had sixty able-bodied spearmen as lineal descendants. This
     implies an equal number of the other sex. They are very light in
     colour, and easily known, as no one is allowed to wear coral beads
     such as Charura brought except the royal family. A book he brought
     was lost only lately. The interest of the case lies in its
     connexion with Mr. Darwin's celebrated theory on the 'origin of
     species,' for it shows that an improved variety, as we whites
     modestly call ourselves, is not so liable to be swamped by numbers
     as some have thought."

     Here we have a perfect fulfilment of what I last year, in ignorance
     of this observation of Livingstone's, predicted as being likely to
     occur in such a case. We have the whitish aristocracy in a dominant
     condition, and evidently in a fair way to spread their
     characteristics over a larger area and give rise to a marked
     variety, and it had clearly struck Livingstone fourteen years
     before the theory of physiological selection had been heard of,
     just as it must strike us now, as an instance telling strongly
     against the "swamping" argument as used by Flemming Jenkin and

Here we have a curious example of one writer supporting the statements
of another, while appearing to be under the impression that he is
controverting those statements. Both Professor Herdman's imaginary case,
and its realization in Livingstone's account, go to show "the tendency
of the members of a variety to breed with one another." This is what I
have called "psychological selection," and, far from "ignoring" it, I
have always laid stress upon it as an obviously important form of
isolation or _prevention_ of free intercrossing. But it is a form of
isolation which can only occur in the higher animals, and, therefore,
the whole of Professor Herdman's criticism is merely a restatement of my
own views as already published in the paper which he is criticizing.
For all that his argument goes to prove is, first, the necessity for
_some_ form of isolation if the overwhelming effects of intercrossing
are to be obviated; and, secondly, the manifest consequence that where
the psychological form is unavailable (as in many of the lower animals
and in all plants), some other form must be present if divergent
evolution is taking place on a common area.

       *       *       *       *       *

Seeing that so much misunderstanding has been shown with reference to my
views on "the swamping effects of intercrossing," and seeing also that
this misunderstanding extends quite as much to Mr. Gulick's views as to
my own, I will here supply brief extracts from both our original papers,
for the double purpose of showing our complete agreement, and of leaving
it to be judged whether we can fairly be held responsible for the
misunderstanding in question. After having supplied these quotations, I
will conclude this historical sketch by considering what Mr. Wallace has
said in reply to the views therein presented. I will transcribe but a
single passage from our papers, beginning with my own.

     Any theory of the origin of species in the way of descent must be
     prepared with an answer to the question, Why have species
     _multiplied_? How is it that, in the course of evolution, species
     have not simply become transmuted in linear series instead of
     ramifying into branches? This question Mr. Darwin seeks to answer
     "from the simple circumstance that the more diversified the
     descendants from any one species becomes in structure,
     constitution, and habits, by so much will they be better enabled to
     seize on many and widely diversified places in the economy of
     nature, and so be enabled to increase in numbers." And he proceeds
     to illustrate this principle by means of a diagram, showing the
     hypothetical divergence of character undergone by the descendants
     of seven species. Thus, he attributes divergence of character
     exclusively to the influence of natural selection.

     Now, this argument appears to me unassailable in all save one
     particular; but this is a most important particular: the argument
     wholly ignores the fact of intercrossing with parent forms.
     Granting to the argument that intercrossing with parent forms is
     prohibited, and nothing can be more satisfactory. The argument,
     however, sets out with showing that it is in limited areas, or in
     areas already overstocked with the specific form in question, that
     the advantages to be derived from diversification will be most
     pronounced. It is where they "jostle each other most closely" that
     natural selection will set a premium upon any members of the
     species which may depart from the common type. Now, inasmuch as
     this jostling or overcrowding of individuals is a needful condition
     to the agency of natural selection in the way of diversifying
     character, must we not feel that the general difficulty from
     intercrossing previously considered is here presented in a special
     and aggravated form? At all events, I know that, after having duly
     and impartially considered the matter, to me it does appear that
     unless the swamping effects of intercrossing with the parent form
     on an overcrowded area is in some way prevented to begin with,
     natural selection could never have any material supplied by which
     to go on with. Let it be observed that I regard Mr. Darwin's
     argument as perfectly sound where it treats of the divergence of
     _species_, and of their further divergence into _genera_; for in
     these cases the physiological barrier is known to be already
     present. But in applying the argument to explain the divergence of
     individuals into varieties, it seems to me that here, more than
     anywhere else, Mr. Darwin has strangely lost sight of the
     formidable difficulty in question; for in this particular case so
     formidable does the difficulty seem to me, that I cannot believe
     that natural selection alone could produce any divergence of
     specific character, so long as all the individuals on an
     overcrowded area occupy that area together. Yet, if any of them
     quit that area, and so escape from the unifying influence of free
     intercrossing, these individuals also escape from the conditions
     which Mr. Darwin names as those that are needed by natural
     selection in order to produce divergence. Therefore, it appears to
     me that, under the circumstances supposed, natural selection alone
     could not produce divergence; the most it could do would be to
     change the whole specific type in some one direction, and thus
     induce transmutation of species in a linear series, each succeeding
     member of which might supplant its parent form. But in order to
     secure _diversity_, _multiplication_, or _ramification_ of species,
     it appears to me obvious that the primary condition required is
     that of preventing intercrossing with parent forms at the origin of
     each branch, whether the prevention be from the first absolute, or
     only partial.

Now for Mr. Gulick, a portion of whose more lengthy discussion of the
subject, however, is all that I need quote:--

     Having found that the evolution of the fitted is secured through
     the prevention of crossing between the better fitted and the less
     fitted, can we believe that the evolution of a special race,
     regularly transmitting a special kind of fitness, can be realized
     without any prevention of crossing with other races that have no
     power to transmit that special kind of fitness? Can we suppose that
     any advantage, derived from new powers that prevent severe
     competition with kindred, can be permanently transmitted through
     succeeding generations to one small section of the species while
     there is free crossing equally distributed between all the families
     of the species? Is it not apparent that the terms of this
     supposition are inconsistent with the fundamental laws of heredity?
     Does not inheritance follow the lines of consanguinity; and when
     consanguinity is widely diffused, can inheritance be closely
     limited? When there is free crossing between the families of one
     species, will not any peculiarity that appears in one family either
     be neutralized by crosses with families possessing the opposite
     quality, or, being preserved by natural selection, while the
     opposite quality is gradually excluded, will not the new quality
     gradually extend to all the branches of the species; so that, in
     this way or in that, increasing divergence of form will be

     If the advantage of freedom from competition in any given variation
     depends on the possession, in some degree, of new adaptations to
     unappropriated resources, there must be some cause that favours the
     breeding together of those thus specially endowed, and interferes
     in some degree with their crossing with other variations, or,
     failing this, the special advantage will in succeeding generations
     be lost. As some degree of Independent Generation is necessary for
     the continuance of the advantage, it is evident that the same
     condition is necessary for the accumulation through Natural
     Selection of the powers on which the advantage depends. The
     advantage of divergence of character cannot be retained by those
     that fail to retain the divergent character; and divergent
     character cannot be retained by those that are constantly crossing
     with other kinds; and the prevention of free crossing between those
     that are equally successful is in no way secured by Natural

So much, then, as expressive of Mr. Gulick's opinion upon this subject.
To exactly the same effect Professor Lloyd Morgan has recently published
his judgement upon it thus:--

     That perfectly free intercrossing, between any or all of the
     individuals of a given group of animals, is, so long as the
     characters of the parents are blended in the offspring, fatal to
     divergence of character, is undeniable. Through the elimination of
     less favourable variations, the swiftness, strength, and cunning of
     a race may be gradually improved. But no form of elimination can
     possibly differentiate the group into swift, strong, and cunning
     varieties, distinct from each other, so long as all three varieties
     freely interbreed, and the characters of the parents blend in the
     offspring. Elimination may and does give rise to progress in any
     given group, _as a group_; it does not and cannot give rise to
     differentiation and divergence, so long as interbreeding with
     consequent interblending of characters be freely permitted. Whence
     it inevitably follows, as a matter of simple logic, that where
     divergence has occurred, intercrossing and interbreeding must in
     some way have been lessened or prevented. Thus a new factor is
     introduced, that of _isolation_ or _segregation_. And there is no
     questioning the fact that it is of great importance. Its
     importance, indeed, can only be denied by denying the swamping
     effects of intercrossing, and such denial implies the tacit
     assumption that interbreeding and interblending are held in check
     by some form of segregation. The isolation explicitly denied is
     implicitly assumed[51].

    [51] _Animal Life and Intelligence_, pp. 98, 99 (1890-1891).

Similarly, and still more recently, Professor Le Conte writes:--

     It is evident, then, as Romanes claims, that natural selection
     alone tends to _monotypic_ evolution. Isolation of some sort seems
     necessary to _polytypic_ evolution. The tree of evolution under the
     influence of natural selection alone grows palm-like from its
     terminal bud. Isolation was necessary to the starting of lateral
     buds, and thus for the profuse ramification which is its most
     conspicuous character[52].

    [52] _The Factors of Evolution_ (1891).

In order to complete this historical review, it only remains to consider
Mr. Wallace's utterances upon the subject.

It is needless to say that he stoutly resists the view of Weismann,
Delbœuf, Gulick, and myself, that specific divergence can ever be
due--or, as I understand him, even so much as assisted--by this
principle of indiscriminate isolation (apogamy). It will be remembered,
however, that Mr. Gulick has adduced certain general principles and
certain special facts of geographical distribution, in order to prove
that apogamy eventually leads to divergence of character, provided that
the isolated section of the species does not contain any very large
number of individuals. Now, Mr. Wallace, without making any reference to
this argument of Mr. Gulick, simply states the reverse--namely, that, as
a matter of fact, indiscriminate isolation is not found to be
associated with divergence of character. For, he says, "there is an
entire absence of change, where, if this were a _vera causa_, we should
expect to find it[53]." But the only case which he gives is that of

    [53] _Darwinism_, p. 151.

This, he says, furnishes "an excellent test case, for we know that it
[Ireland] has been separated from Britain since the end of the glacial
epoch: ... yet hardly one of its mammals, reptiles, or land molluscs has
undergone the slightest change[54]." Here, however, Mr. Wallace shows
that he has failed to understand "the views of those who, like Mr.
Gulick, believe isolation itself to be a cause of modification of
species"; for it belongs to the very essence of these views that the
efficiency of indiscriminate isolation as a "_vera causa_" of organic
evolution varies inversely with the number of individuals (i. e. the
size of the species-section) exposed to its influence. Therefore, far
from being "an excellent test case," the case of Ireland is
unsatisfactory. If we are in search of excellent test cases, in the
sense intended by Mr. Wallace, we ought not to choose a large island,
which from the time of its isolation must have contained large bulks of
each of the geographically separated species concerned: we ought to
choose cases where as small a number as possible of the representatives
of each species were in the first instance concerned. And, when we do
this, the answer yielded by any really "excellent test case" is

    [54] _Ibid._

No better test case of this kind has ever been furnished than that of
Mr. Gulick's land-shells, which Mr. Wallace is specially considering in
the part of his book where the sentence above quoted occurs. How, then,
does he meet this case? He meets it by assuming that in all the numerous
adjacent valleys of a small island there must be as many differences of
environment, each of which is competent to induce slight varietal
changes on the part of its occupants by way of natural selection,
although in no one case can the utility of these slight changes be
surmised. Now, against this explanation there are three overwhelming
considerations. In the first place, it is purely gratuitous, or offered
merely in order to save the hypothesis that there _can_ be no other
cause of even the most trivial change in species than that which is
furnished by natural selection. In the second place, as Mr. Gulick
writes to me in a private letter, "if the divergence of Sandwich Island
land molluscs is wholly due to exposure to different environments, as
Mr. Wallace argues on pages 147-150, then there must be completely
occult influences in the environment that vary progressively with each
successive mile. This is so violent an assumption that it throws doubt
on any theory that requires such support." In the third place, the
assumption that the changes in question must have been due to natural
selection, is wholly incompatible with the facts of isolation
elsewhere--namely, in those cases where (as in that of Ireland) a large
section of species, instead of a small section, has been
indiscriminately isolated. Mr. Wallace, as we have seen, inadvertently
alludes to these "many other cases of isolation" as evidence against
apogamy being _per se_ a cause of specific change. But although, for
the reason above stated, they are without relevancy in this respect,
they appear to me fatal to the explanation which he gives of specific
changes under apogamy where only small sections of species are
concerned. For example, can it be rationally maintained that there are
more differences of environment between every two of the many contiguous
valleys of a small island, such as Mr. Gulick describes, than there are
in the incomparably larger area of the whole of Ireland? But, if not,
and if natural selection is able to work such "occult" wonders in each
successive mile on the Sandwich Islands, why has it so entirely lost
this magic power in the case of Ireland--or in the "many other cases of
isolation" to which Mr. Wallace refers? On his theory there is no
coherent answer to be given to this question, while on our theory the
answer is given in the very terms of the theory itself. The facts are
plainly just what the theory requires that they should be; and
therefore, if they were not as they are, the theory would be deprived of
that confirmation which it now derives from them.

Thus, in truth, though in an opposite way, the case of Ireland is, as
Mr. Wallace says, "an excellent test case," when once the theory of
apogamy as a "_vera causa_" of specific change is understood; and the
effect of applying the test is fully to corroborate this theory, while
at the same time it as fully negatives the other. For the consideration
whereby Mr. Wallace seeks to explain the inactivity of natural selection
in the case of Ireland is not "coherent." What he says is, "That changes
have not occurred through natural selection, is perhaps due to the less
severe struggle for existence, owing to the smaller number of competing
species[55]." But even with regard to molluscs alone, there is a greatly
larger number of species in Ireland than occurs in any one valley of the
Sandwich Islands; while if we have regard to all the other classes of
animal life, comparison entirely fails.

    [55] _Loc. cit._, p. 151.

Much more to the point are certain cases which were adduced long ago by
Weismann in his essay previously considered. Nevertheless, although this
essay was published as far back as 1872, and, although it expressly
deals with the question of divergence of character through the mere
prevention of intercrossing (Amixia), Mr. Wallace nowhere alludes to
these cases _per contra_, which are so much more weighty than his own
"test case" of Ireland. Of such are four species of butterflies,
belonging to three genera[56], which are identical in the polar regions
and in the Alps, notwithstanding that the sparse Alpine populations have
been presumably separated from their parent stocks since the glacial
period; or of certain species of fresh water crustaceans (_Apus_), the
representatives of which are compelled habitually to form small isolated
colonies in widely separated ponds, and nevertheless exhibit no
divergence of character, although apogamy has probably lasted for
centuries. These cases are unquestionably of a very cogent nature, and
appear of themselves to prove that apogamy alone is not invariably
capable of inducing divergence--at any rate, so rapidly as we might
expect. There appears, however, to be another factor, the presence or
absence of which makes a great difference. This as stated in the text,
is the degree in which a specific type is stable or unstable--liable or
not liable to vary. Thus, for example, the Goose is what Darwin calls an
"inflexible" type as compared with most other domesticated birds.
Therefore, if a lot of geese were to be indiscriminately isolated from
the rest of their species, the probability is that in a given time their
descendants would not have diverged from the parent type to such an
extent as would a similar lot of ducks under similar circumstances: the
more stable specific type would require a longer time to change under
the influence of apogamy alone. Now, the butterflies and crustaceans
quoted by Weismann may be of a highly stable type, presenting but a
small range of individual variability; and, if so, they would naturally
require a long time to exhibit any change of type under the influence of
apogamy alone. But, be this as it may, Weismann himself adduces these
cases merely for the sake of showing that there are cases which seem to
tell against the general principle of modification as due to apogamy
alone--i.e. the general principle which, under the name amixia, he is
engaged in defending. And the conclusion at which he himself arrives is,
that while it would be wrong to affirm that apogamy _must_ in all cases
produce divergence, we are amply justified in affirming that in many
cases it _may_ have done so; while there is good evidence to prove that
in not a few cases it _has_ done so, and therefore should be accepted
as one of the factors of organic evolution[57].

    [56] Namely, _Lycaena denzelii_, _L. pheretes_, _Argynnis pales_,
    _Erebia mante_.

    [57] Since the above was written, I have heard of some cases which
    seem to present greater difficulties to our theory than those above
    quoted. These refer to some of the numerous species of land mollusca
    which inhabit the isolated rocks near Madeira (Dezertas). My
    informant is Dr. Grabham, who has himself investigated the matter,
    and reports as follows:--

    "It is no uncommon thing to meet with examples of the same species,
    sub-fossil, recent, and living upon one spot, and presenting no
    variation in the long record of descent." Then, after naming these
    examples, he adds, "All seem to vary immediately on attaining new
    ground, assuming many aspects in different districts."

    Unquestionably these statements support, in a very absolute manner,
    Mr. Wallace's opinion, while making directly against my own. It is
    but fair, however, to add that the cases are not numerous (some
    half-dozen at the most, and all within the limits of a single
    genus), and that, even in the opinion of my informant himself, the
    facts have not hitherto been sufficiently investigated for any
    decisive judgement to be formed upon them.

My view from the very first has been that variations in the way of
cross-infertility are of frequent occurrence (how, indeed, can they be
otherwise, looking to the complex conditions that have to be satisfied
in every case of full fertility?); and, therefore, however many of such
variations are destined to die out, whenever one arises, "under suitable
conditions," "it must inevitably tend to be preserved as a new natural
variety, or incipient species." Among the higher animals--which are
"comparatively few in number"--I think it probable that some slight
change of form, colour, habit, &c., must be usually needed either to
"superinduce," or, which is quite a different thing, to _coincide_ with
the physiological change But in the case of plants and the lower
invertebrata. I see no reason for any frequent concomitance of this
kind; and therefore believe the physiological change to be, "as a
general rule," the primordial change. At the same time, I have always
been careful to insist that this opinion had nothing to do with "the
essence of physiological selection"; seeing that "it was of no
consequence" to the theory in what proportional number of cases the
cross-sterility had begun _per se_, had been superinduced by
morphological changes, or only enabled to survive by happening to
coincide with any other form of homogamy. In short, "the essence of
physiological selection" consists in _all_ cases of the diversifying
_effect_ of cross-infertility, whensoever and howsoever it may happen in
particular cases to have been _caused_.

Thus I emphatically reaffirm that "from the first I have always
maintained that it makes no essential difference to the theory _in what
proportional number of cases_ they [the physiological variations] have
arisen 'alone in an otherwise undifferentiated species'"; therefore,
"even if I am wrong in supposing that physiological selection can _ever_
act alone, the _principle_ of physiological selection, as I have stated
it, is not thereby affected. And this principle is, as Mr. Wallace has
re-stated it, 'that some amount of infertility characterizes the
distinct varieties which are in process of differentiation into
species'--infertility whose absence, 'to obviate the effects of
intercrossing, may be one of the _usual_ causes of their failure to
become developed into distinct species.'"

These last sentences are quoted from the correspondence in _Nature_[58],
and to them Mr. Wallace replied by saying, "if this is not an absolute
change of front, words have no meaning"; that "if this is 'the whole
essence of physiological selection,' then physiological selection is but
a re-statement and amplification of Darwin's views"; that such a "change
of front" is incompatible, not only with my term "physiological
selection," but also with my having "acknowledged that Mr. Catchpool had
'very clearly put forward the theory of physiological selection'"; and
much more to the same effect.

    [58] Vol. xliii. p. 127.

Now, to begin with, it is due to Mr. Catchpool to state that his only
publication upon this subject is much too brief to justify Mr.
Wallace's, inference, that he supposes variations in the way of
cross-infertility always to arise "alone in an otherwise
undifferentiated species." What Mr. Catchpool's opinion on this point
may be, I have no knowledge; but, whatever it is, he was unquestionably
the first writer who "clearly stated the leading principles" of
physiological selection, and this fact I am very glad to have
"acknowledged." In my correspondence with Mr. Wallace, however, I not
only named Mr. Catchpool: I also named--and much more prominently--Mr.
Gulick. For even if I were to grant (which I am far indeed from doing)
that there was any want of clearness in my own paper touching the point
in question, I have now repeatedly shown that it is simply impossible
for any reader of Mr. Gulick's papers to misunderstand _his_ views with
regard to it. Accordingly, I replied to Mr. Wallace in _Nature_ by

     Not only have I thus from the first fully recognized the sundry
     other causes of specific change with which the physiological
     variations may be associated; but Mr. Gulick has gone into this
     side of our common theory much more fully, and elaborately
     calculated out the high ratio in which the differentiating agency
     of any of these other causes must be increased when assisted by--i.
     e. associated with--even a moderate degree of the selective
     fertility, and vice versa. Therefore, it is simply impossible for
     Mr. Wallace to show that "our theory" differs from his in this
     respect. Yet it is the only respect in which his reply alleges any
     difference. (Vol. xliii. p. 127.)

I think it is to be regretted that, in his answer to this, Mr. Wallace
alludes only to Mr. Catchpool, and entirely ignores Mr. Gulick--whose
elaborate calculations above alluded to were communicated to the
Linnaean Society by Mr. Wallace himself in 1887.

The time has now come to prove, by means of quotations, that I have from
the first represented the "principle," or "essence," of physiological
selection to consist in selective fertility furnishing a needful
condition to specific differentiation, in at least a large proportional
number of allied species which afterwards present the reciprocal
character of cross-sterility; that I have never represented variations
in the way of this selective fertility as necessarily constituting the
initial variations, or as always arising "alone, in an otherwise
undifferentiated species"; and that, although I have uniformly given it
as my opinion that these variations do _in some cases_ thus arise
(especially among plants and lower invertebrata), I have as uniformly
stated "that it makes no difference to the theory in what proportional
number of cases they have done so"--or even if, as Mr. Wallace supposes,
they have never done so in any case at all[59]. These statements (all of
which are contradictory of the only points of difference alleged) have
already been published in my article in the _Monist_ of October, 1890.
And although Mr. Wallace, in his reply to that article, ignores my
references to the "original paper," it is scarcely necessary to quote
the actual words of the paper itself, since the reader who is further
interested in this controversy can readily refer to it in the _Journal
of the Linnaean Society_ (vol. xix. pp. 337-411).

    [59] This refers to what I understand Mr. Wallace to say in the
    _Nature_ correspondence is the supposition on which his own theory
    of the origin of species by cross-infertility is founded. But in the
    original statement of that theory itself, it is everywhere
    "supposed" that when species are originated by cross-infertility,
    the _initial_ change _is_ the physiological change. In his original
    statement of that theory, therefore, he literally went further than
    I had gone in my "original paper," with reference to supposing the
    physiological change to be the initial change. I do not doubt that
    this is due to some oversight of expression; but it is curious that,
    having made it, he should still continue his endeavour to fix
    exactly the same oversight upon me.

Having arrived at these results with regard to the theory of Isolation
in general and of Physiological Isolation in particular, I arrive also
at the end of this work. And if, while dealing with the post-Darwinian
period, I have imparted to any general reader the impression that there
is still a great diversity of expert opinion; I must ask him to note
that points with reference to which disagreement still exists are but
very subordinate to those with regard to which complete agreement now
prevails. The noise of wrangling disputations which has so filled the
camp of evolutionists since the death of their captain, is apt to hide
from the outside world the solid unanimity that prevails with regard to
all the larger and more fundamental questions, which were similarly the
subjects of warfare in the past generation. Indeed, if we take a fair
and general view of the whole history of Darwinism, what must strike us
as the really significant fact is the astonishing unanimity which has
been so rapidly attained with regard to matters of such immeasurable
importance. It is now but little more than thirty years since the
publication of the _Origin of Species_; and in that period not only have
all naturalists unequivocally embraced the doctrine of descent
considered as a fact; but, in one degree or another, they have all as
unequivocally embraced the theory of natural selection considered as a
method. The only points with regard to which any difference of opinion
still exist, have reference to the precise causation of that mighty
stream of events which, under the name of organic evolution, we have now
all learnt to accept as scientifically demonstrated. But it belongs to
the very nature of scientific demonstration that, where matters of great
intricacy as well as of high generality are concerned, the process of
demonstration must be gradual, even if it be not always slow. It is only
by the labours of many minds working in many directions that, in such
cases, truth admits of being eventually displayed. Line upon line,
precept upon precept, here a little and there a little--such is the
course of a scientific revelation; and the larger the subject-matter,
the more subtle and the more complex the causes, the greater must be the
room for individual differences in our reading of the book of Nature.
Now, if all this be true, must we not feel that in the matter of organic
evolution the measure of agreement which has been attained is out of all
proportion to the differences which still remain--differences which,
although of importance in themselves, are insignificant when compared
with those which once divided the opinions of not a few still living
men? And if we are bound to feel this, are we not bound further to feel
that the very intensity of our disputations over these residual matters
of comparative detail, is really the best earnest that can be given of
the determination of our quest--determination which, like that of our
fathers, cannot fail to be speedily rewarded by the discovery of truth?

Nevertheless, so long as this noise of conflict is in the Senate, we
cannot wonder if the people are perplexed. Therefore, in conclusion, I
may ask it to be remembered exactly what are the questions--and the only
questions--which still divide the parties.

Having unanimously agreed that organic evolution is a fact and that
natural selection is a cause, or a factor in the process, the primary
question in debate is whether natural selection is the only cause, or
whether it has been assisted by the co-operation of other causes. The
school of Weismann maintain that it is the only cause; and therefore
deem it worse than useless to search for further causes. With this
doctrine Wallace in effect agrees, excepting as regards the particular
case of the human mind. The school of Darwin, on the other hand--to
which I myself claim to belong--believe that natural selection has been
to a considerable extent supplemented by other factors; and, therefore,
although we further believe that it has been the "main" factor, we agree
with Darwin himself in strongly reprobating all attempts to bar _a
priori_ the progress of scientific investigation touching what, if any,
these other factors may be. Lastly, there are several more or less
struggling schools, chiefly composed of individual members who agree
with each other only to the extent of holding that the causal agency of
natural selection is not so great as Darwin supposed. The Duke of
Argyll, Mr. Mivart and Mr. Geddes may be named in this connexion;
together with the self-styled neo-Lamarckians, who seek to magnify the
Lamarckian principles at the expense of the distinctively Darwinian.

This primary difference of opinion leads deductively to certain
secondary differences. For if a man starts with the premiss that natural
selection must necessarily be the "exclusive" cause of organic
evolution, he is likely to draw conclusions which another man would not
draw who starts with the premiss that natural selection is but the
"main" cause. Of these subordinate differences the most important are
those which relate to the possible transmission of acquired characters,
to the necessary (or only general) utility of specific characters, and
to the problem touching the inter-sterility of allied species. But we
may well hope that before another ten years shall have passed, even
these still outstanding questions will have been finally settled; and
thus that within the limits of an ordinary lifetime the theory of
organic evolution will have been founded and completed in all its parts,
to stand for ever in the world of men as at once the greatest
achievement in the history of science, and the most splendid monument of
the nineteenth century.

In the later chapters of the foregoing treatise I have sought to
indicate certain matters of general principle, which many years of study
specially devoted to this great movement of contemporary thought have
led me to regard as almost certainly sound in themselves, and no less
certainly requisite as complements of the Darwinian theory. I will now
conclude by briefly summarizing these matters of general principle in
the form of twelve sequent propositions. And, in doing so, I may ask it
to be noticed that the system which these propositions serve to express
may now claim, at the least, to be a strictly logical system. For the
fact that, not merely in its main outlines, but likewise in its details,
it has been independently constructed by Mr. Gulick, proves at any rate
this much; seeing that, where matters of such intricacy are concerned,
nothing but accurate reasoning from a common foundation of _data_ could
possibly have yielded so exact an agreement. The only difference between
us is, that Mr. Gulick has gone into much further detail than I have
ever attempted in the way of classifying the many and varied forms of
isolation; while I have laid more special stress upon the physiological
form, and found in it what appears to me a satisfactory solution of "the
greatest of all the difficulties in the way of accepting the theory of
natural selection as a complete explanation of the origin of
species"--namely, "the remarkable difference between varieties and
species when crossed."

















I have received from Mr. Gulick the results of his consideration of Mr.
Wallace's criticism. As these results closely resemble those which I
have myself reached, and as they were independently worked out on the
other side of the globe, I deem it desirable to publish them here for
the sake of comparison.

In his covering letter Mr. Gulick writes:--

     Mr. Wallace has most certainly adopted the fundamental principles
     of our theory, and in an arbitrary way attempted to claim the
     results produced by these principles as the effects of natural
     selection. He takes our principles, which in the previous chapter
     he has combated; but he makes such disjointed use of them that I am
     not willing to recognize his statement as an intelligible
     exposition of our theory.... I have endeavoured to indicate at what
     points Mr. Wallace has deserted his own principles, and at what
     points he has failed to make the best use of ours. To bring out
     these points distinctly has been no easy task; but if you regard
     this paper on _The Preservation and Accumulation of
     Cross-infertility_ as giving any help in elucidating the true
     principles, and in showing Mr. Wallace's position in regard to
     them, I shall be satisfied. Please make any use of it that may seem
     desirable, and then forward it to Professor Dana.

The following is a general summary of Mr. Gulick's results:--

     Mr. Wallace's criticism of the theory of Physiological Selection is
     unsatisfactory; (l) because he has accepted the fundamental
     principle of that theory on pages 173-9, in that he maintains that
     without the cross-infertility the incipient species there
     considered would be swamped; (2) because he assumes that
     physiological selection pertains simply to the infertility of first
     crosses, and has nothing to do with the infertility of mongrels and
     hybrids; (3) because he assumes that infertility between first
     crosses is of rare occurrence between species of the same genus,
     ignoring the fact that in many species of plants the pollen of the
     species is pre-potent on the stigma of the same species when it has
     to compete with the pollen of other species of the same genus; (4)
     because he not only ignores Mr. Romanes' statement that
     cross-infertility often affects "a whole race or strain," but he
     gratuitously assumes that the theory of Physiological Selection
     excludes this "racial incompatibility" (which Mr. Romanes maintains
     is the more probable form), and bases his computation on the
     assumption that the cross-infertility is not associated with any
     other form of segregation; (5) because he claims to show that "all
     infertility not correlated with some _useful_ variation has a
     constant tendency to effect its own elimination," while his
     computation only shows that, if the cross-infertility is not
     associated with some form of _positive_ segregation, it will
     disappear[60]; and (6) because he does not observe that the positive
     segregation may be secured by the very form of the physiological
     incompatibility.... Without here entering into any computation, it
     is evident that, e.g. the prepotency of pollen of each kind with
     its own kind, if only very slight, will prevent cross-fertilization
     as effectually as a moderate degree of instinctive preference in
     the case of an animal.

    [60] "Positive segregation" is Mr. Gulick's term for forms of
    homogamy other than that which is due to selective fertility. Of
    these other, or "positive" forms, natural selection is one; but as
    it is far from being the _only_ one, the criticism points out that
    utility is not the _only_ conserving principle with which selective
    fertility may be associated.

The paper likewise indicates a point which, in studying Mr. Wallace's
theory, I have missed. It will be remembered that the only apparent
difference between his theory and mine has been shown to consist in
this--that while I was satisfied to state, in a general way, that
natural selection is probably able to increase a selective fertility
which has already been begun by other causes, Mr. Wallace has sought to
exhibit more in detail the precise conditions under which it can do so.
Now, Mr. Gulick shows that the particular conditions which Mr. Wallace
describes, even if they do serve to promote an increase of
cross-infertility, are conditions which preclude the possibility of
natural selection coming into play at all. So that if, under these
particular conditions, a further increase of cross-infertility does take
place, it does not take place in virtue of natural selection. To me it
appears that this criticism is sound; and, if so, it disposes of even
the one very subordinate addition to our theory which Mr. Wallace
"claims" as the most "distinctive" part of his.

The following is the criticism in question:--

     On pages 173-186 Mr. Wallace maintains that "Natural selection is,
     in some probable cases at all events, able to accumulate variations
     in infertility between incipient species" (p. 174); but his
     reasoning does not seem to me conclusive. Even if we grant that the
     increase of this character [cross-infertility] occurs by the steps
     which he describes, _it is not a process of accumulation by natural
     selection_. In order to be a means of cumulative modification of
     varieties, races, or species, selection, whether artificial or
     adaptational [i.e. natural], must preserve certain forms of an
     intergenerating stock, to the exclusion of other forms of the same
     stock. Progressive change in the size of the occupants of a
     poultry-yard may be secured by raising only bantams the first, only
     common fowls the second, and only Shanghai fowls the third year;
     but this is not the form of selection that has produced the
     different races of fowls. So in nature, rats may drive out and
     supplant mice; but this kind of selection modifies neither rats
     nor mice. On the other hand, if certain variations of mice prevail
     over others, through their superior success in escaping their
     pursuers, then modification begins. Now, turning to page 175, we
     find that, in the illustrative case introduced by Mr. Wallace, the
     commencement of infertility between the incipient species is in the
     relations to each other of two portions of a species that are
     locally segregated from the rest of the species, and partially
     segregated from each other by different modes of life. These two
     local varieties, being by the terms of his supposition better
     adapted to the environment than the freely interbreeding forms in
     other parts of the general area, increase till they supplant these
     original forms. Then, in some limited portion of the general area,
     there arise two still more divergent forms, with greater mutual
     infertility, and with increased adaptation to the environment,
     enabling them to prevail throughout the whole area. The process
     here described, if it takes place, is not modification by natural

On the other hand, it _is_ modification by physiological selection. For,
among the several other forms of isolation which are called
into requisition, the physiological (i.e. ever accumulating
cross-infertility) is supposed to play an important part. That the
modification is not modification by natural selection may perhaps be
rendered more apparent by observing, that in as far as _any_ other mode
of isolation is involved or supposed, so far is the _possible_ agency of
natural selection eliminated _as between the two or more otherwise
isolated sections of a species_; and yet it is modes of isolation other
than that furnished by natural selection (i.e. perishing of the less
fit), that Mr. Wallace here supposes to have been concerned--including,
as I have before shown, the physiological form, to which, indeed, he
really assigns most importance of all. Or, as Mr. Gulick states the
matter in his independent criticism:--

     In the supposed case pictured by Mr. Wallace, the principle by
     which the two segregating forms are kept from crossing, and so are
     eventually preserved as permanently distinct forms, is no other
     than that which Mr. Romanes and myself have discussed under the
     terms Physiological Selection and Segregate Fecundity. Not only is
     Mr. Wallace's exposition of the divergence and the continuance of
     the same in accord with these principles which he has elsewhere
     rejected, but his whole exposition is at variance with his own
     principle, which, in the previous chapter, he vigorously maintains
     in opposition to my statement that many varieties and species of
     Sandwich Island land molluscs have arisen, while exposed to the
     same environment, in the isolated groves of the successive valleys
     of the same mountain range. If he adhered to his own theory, "the
     greater infertility between the two forms in one portion of the
     area" would be attributed to a difference between the _environment_
     presented in that portion and that presented in the other portions;
     and the difficulty would be to consistently show how this greater
     infertility could continue unabated when the varieties thus
     characterized spread beyond the environment on which the character
     depends. But, without power to continue, the process which he
     describes would not take place. Therefore, in order to solve the
     problem of the _origin_ and _increase_ of infertility between
     species, he tacitly gives up his own theory, and adopts not only
     the theory of Physiological Selection but that of Intensive
     Segregation[61] through Isolation, though he still insists on
     calling the process natural selection; for on page 183 he says, "No
     form of infertility or sterility between the individuals of a
     species can be increased by natural selection unless correlated
     with some useful variation, while all infertility not so correlated
     has a constant tendency to effect its own elimination." Even this
     claim he seems to unwittingly abandon when on page 184 he says:
     "The moment it [a species] becomes separated either by geographical
     or selective isolation, or by diversity of station or of habits,
     then, while each portion must be kept fertile _inter se_, there is
     nothing to prevent infertility arising between the two separated

    [61] By Intensive Segregation Mr. Gulick means what I have called
    Independent Variability.

The criticism proceeds to show yet further inconsistencies and
self-contradictions in Mr. Wallace's treatment of this subject; but it
now seems needless to continue. Nor, indeed, should I have quoted this
much but for the sake of so fully justifying my own criticism by showing
the endorsement which it has received from a completely independent



We have seen that the only important point of difference between Mr.
Wallace's more recent views and my own on the problem of inter-specific
sterility, has reference to the question whether variations in the way
of cross-infertility can _ever_ arise and act "alone, in an otherwise
undifferentiated species," or whether they can _never_ so arise and act.
It is Mr. Wallace's opinion that, even if they ever do arise alone, at
all events they can never act in differentiating a specific type, seeing
that the chances against their suitable mating must be so great: only if
they be from the first associated with some other form of homogamy,
which will have the effect of determining their suitable mating, does he
think that they can act in the way supposed by our theory of "selective
fertility"[62]. On the other hand, as previously and frequently stated,
I have so strong a belief in the segregating power of physiological
selection, or selective fertility, that I do not think it is necessary
for this principle to be _always___ associated with some other form of
homogamy. From the first, indeed, I have laid great stress (as, also,
has Mr. Gulick) on the re-enforcing influence which association with any
other form of homogamy must exercise upon the physiological form, and
vice versa; but I have also said that, in my opinion, the physiological
form may in many cases be able to act entirely alone, or without
assistance derived from any other source. The question here is, as we
have already so fully seen, a question of but secondary importance;
since, whether or not the physiological form of homogamy ever acts
alone, even Mr. Wallace now allows, or rather argues, that it acts in
combination--and this so habitually, as well as with so much effect,
that it constitutes a usual condition to the origination of species.
Nevertheless, although the only relevancy of his numerical computation
of chances--whereby he thinks that he overturns my theory _in toto_--is
such relevancy as it bears to this question of secondary importance, I
have thought it desirable to refer the question, together with Mr.
Wallace's views upon it, to the consideration of a trained

    [62] His sentence, "all fertility not correlated with some _useful_
    variation has a constant tendency to effect its own elimination,"
    still further restricts the possible action of physiological
    selection to cases where at least one of the other forms of homogamy
    with which it is associated is natural selection. Or, in other
    words, it is represented that physiological selection must always be
    associated with natural selection, even if it be likewise associated
    with any other form of exclusive breeding. But as this further
    limitation appears to me self-evidently unjustifiable (seeing that
    utility is not the only possible means of securing effective
    isolation) I here neglect it, and take the wider ground marked out
    above. It is needless to say that this is giving Mr. Wallace every
    possible advantage, by not holding him to his still narrower ground.

As this "subordinate question" depends entirely on numerical
computations involving the doctrine of chances, I should first of all
like to remark, that in reference to biological problems of the kind now
before us, I do not myself attach much importance to a merely
mathematical analysis. The conditions which such problems involve are so
varied and complex, that it is impossible to be sure about the validity
of the _data_ upon which a mathematical analysis is founded.
Nevertheless, for the sake of meeting these criticisms upon their own
ground, I will endeavour to show that, even as mathematical
calculations, they are quite untrustworthy. And, in order to do this
effectually, I will quote the results of a much more competent, as well
as a much more thorough, inquiry. I applied to Mr. Moulton for this
purpose, not only because he is one of the ablest mathematicians of my
acquaintance; but also because his interest in biology, and his
knowledge of Darwinian literature, render him well fitted to appreciate
exactly, and in all their bearings, the questions which were submitted
to his consideration. I need only add that his examination was
completely independent, and in no way influenced by me. Having
previously read my paper on _Physiological Selection_, Mr. Gulick's
paper on _Divergent Evolution_, and Mr. Wallace's book on _Darwinism_,
he was in possession of all the materials; and I merely requested the
favour of his opinion upon the whole case from a mathematical point of
view. The following is his reply; and I give it _in extenso_, because it
serves to place in another light some of the general considerations
which it has already been my endeavour to present[63].

    [63] In our _Nature_ correspondence of 1890-1891, Mr. Wallace
    remarked: "If Dr. Romanes will carefully work out numerically (as I
    have attempted to do) a few cases showing the preservative and
    accumulative agency of pure physiological selection within an
    otherwise undifferentiated species, he will do more for his theory
    than volumes of general disquisition or any number of assertions
    that it _does_ possess this power." Several months before this was
    written I had already in my hands Mr. Moulton's letter, with its
    accompanying calculations.

After some introductory remarks on Mr. Wallace's "adoption of the theory
of physiological selection pure and simple," and "the pure caricature of
it which he puts forward as" mine, the letter proceeds thus:--

     The reason why it is so easy to attack your theory is that it is so
     easy to confuse the survival of an _individual_ with the survival
     of a _peculiarity_ of _type_. No one has ever said that an
     _individual_ is _assisted_ by the possession of selective
     fertility: that is a matter which cannot affect his chance of
     _life_. Nor has any one said that the possession of selective
     fertility in an _individual_ will _of itself_ increase the chance
     of his having _progeny_ that will survive, and in turn become the
     progenitors of others that will survive. Taken by itself, the fact
     that an _individual_ is capable of fertility with some only of the
     opposite sex lessens the chance of his having progeny. Whether or
     not he is more or less favourably situated than his _confreres_ for
     the battle of life must be decided by the _total sum_ of his
     peculiarities; and the question whether or not this selective
     fertility will be a hindrance must be decided by considerations
     depending on the other peculiarities associated with it.

     But when we come to consider the survival or permanence of a _type_
     or _peculiarity_, the case is quite different. It then becomes not
     only a favourable circumstance, but, in my opinion, almost a
     necessary condition, that the peculiarity should be associated with
     selective fertility[64].

    [64] As, for example, in the case of sexuality in general. It is not
    to the advantage of such individual male Arthropoda as perish after
    the performance of the sexual act that they should perform it; but
    its performance is necessary for the perpetuation of their
    species.--G. J. R.

     Take the case of the Jews. I don't think that intermarriage with
     other nations would lessen their fertility, or diminish the number
     of their progeny; nor is there any reason to think that this
     progeny would be unequal to the struggle for existence. But no one
     doubts that the abandonment of their voluntary isolation (which
     operates so far as this is concerned as a selective fertility),
     would lead to the disappearance of the familiar Jewish type. All
     the world would get some of it; but as a whole it would be

     Now although no doubt Wallace would admit all this, he fails to
     give it the weight it ought to have. In discussing the question of
     its operation he considers too exclusively the case of the

     Of course, a type can only be perpetuated through the medium of
     individuals, and all that his argument amounts to is, that
     selective fertility would be so fatal to individuals that _no_ type
     which presents it could be formed or perpetuated--a conclusion
     which is not only absurd in itself, but contradicted by his own
     subsequent adoption of your theory. Besides, apart from
     calculations (with which I will deal when I write next), such
     reasoning brings its own refutation. Selective fertility is not in
     the same category as some of the other influences to which an
     important share has been ascribed in the formation of the existing
     types. _It exists as a recognized phenomenon._ Hence all these
     numerical proofs that it would lead to extinction, because it is so
     disadvantageous to the possessor, prove too much. They would show
     that the degree of selective fertility which so frequently
     characterizes species is a most onerous gift; and that, were it not
     present, there would be a vastly increased chance of fertility,
     which would render the races fitter and lead to their increased
     survival. Why then has it not been got rid of?

     The two answers which no doubt would be given seem to me to support
     rather than to make against your theory. In the first place,
     Wallace might say that this infertility is an advantage because it
     keeps pure a type which is specially fitted to its surroundings, as
     shown by its continued existence. But if this be so, and it is
     necessary to protect the _developed_ type, how much more necessary
     to protect the _incipient_ type! In the second place, he might say
     that this selective fertility is not so disadvantageous when the
     species has been formed, because the individual can choose his mate
     from his like; whereas, when it is beginning to be formed, he must
     mate blindly, or without what you call "psychological selection."
     But this seems to me to be wholly inapplicable to at least half the
     animal, and to all the vegetable kingdom. Moreover, with regard to
     the other half of the animal kingdom, it merely raises the
     question,--How soon will such an incipient type recognize itself?
     Seeing it is probable that many families [broods] will belong to
     the same [incipient] type, I should not be surprised if it were
     found that this sexual recognition and preference sets in very

     But this leads me to the question of your letter. I understand you
     to want me to examine and criticize the attempted numerical
     arguments against or for your theory. Now it seems to me that it
     will be best to take, in the first instance, the vegetable kingdom,
     and with regard to it I cannot see how there can be any numerical
     argument against the theory. For we often have species side by side
     with others nearly allied, but much more numerous. The condition of
     these is precisely analogous to that of your incipient species.
     They are exposed to fertilization from, say, ten times as numerous
     individuals of the allied species. They reject this in favour of
     that from the relatively few individuals of their own. Yet the two
     species are in competition. I could go through the numerical
     arguments of your assailant word for word, applying them to such a
     case as this, and they would triumphantly show that the specific
     fertility of the rarer kind would lead to its certain extinction.
     Yet we know that this is not so.

     Indeed, the too triumphant character of the logic used against you
     seems to me to be capable of being turned to your use. If
     cross-infertility is so intensely disadvantageous to the
     individuals presenting it, it cannot have been _that_ which made
     these individuals and their progeny survive. It is therefore a
     burden which they have carried. But we find that it is more or less
     present in all the closely allied types that occur on common areas:
     therefore it must be a necessary feature in the formation of such
     types; for it cannot be an accident that it is present in so many.
     In other words, it must be the price which the individual and his
     progeny pay for their formation into a type. And this is your
     theory pure and simple.

     The more I consider the matter, the more I feel that it is
     impossible to decide as to the sufficiency of selective fertility
     to explain the formation of species, if we consider merely the
     effect it would have on the number of individuals, as contrasted
     with what it would be if no such peculiarity had developed itself.
     Indeed, I may say that on pondering over the matter I have come to
     the conclusion, that mere fertility is probably a comparatively
     unimportant factor in the preservation of the species, after a
     certain sufficient degree of fertility is attained. I do not wish
     to be misunderstood. To a certain point fertility is not only
     advantageous but necessary, in order to secure survival of the
     type; but I feel that little reliance can be placed on calculations
     based on the numerical co-efficient of fertility (i. e. the ratio
     of the number of offspring to the number of parents) in determining
     the relative chance of type-survival.

     Take, for instance, the oak tree. It produces thousands of acorns,
     almost the whole of which die without producing any progeny. Have
     we any reason to believe that if the number of acorns borne by oak
     trees were diminished, even so much as to one-tenth, the race of
     oaks would perish? It may of course be said that, if all other
     things are equal, the probabilities of survival must be increased
     by increased fertility of this kind; but I feel convinced that when
     numerical fertility has attained to a high point in circumstances
     in which actual increase of the race cannot take place to any
     substantial extent, the numerical value of this fertility sinks
     down into a factor of the second or third order of importance--that
     is to say, into the position of a factor whose effects are only to
     be considered when we have duly allowed for the full effects of all
     the main factors. Until we have done that, we gain little or
     nothing in the way of accuracy of conclusion by taking into
     consideration the minor factors. It may be very well to neglect the
     effect of the attraction of Jupiter in our early researches on the
     motion of the Moon; and our doing so will not prevent the results
     being approximate and having considerable value, because we are
     retaining the two main factors that establish the motion, viz. the
     effects of the Earth and the Sun. But if we exclude the effect of
     one of these main factors, our results would be worthless; and it
     would not be rendered substantially less so by the fact that we had
     taken Jupiter into account in arriving at them.

     You must not imagine, however, that I think it wholly profitless to
     see whether there would be any substantial effect on numerical
     fertility were _selective_ fertility to manifest itself. But if we
     want to derive any assistance from calculation, it must be by
     applying it with a good deal more precision and definiteness than
     anything that Wallace shows. And, in the first place, it is useless
     to confuse the vegetable and animal kingdoms. In the former you
     have union unaffected by choice; in the latter, so far at all
     events as the higher animals are concerned, you have "psychological
     selection." In order to give you a specimen of what can safely be
     done by calculation if you take a problem of sufficient
     definiteness, I have chosen the case of a flowering plant in which
     a certain proportion of the race have developed the peculiarity of
     being sterile with the remainder, while retaining the normal
     fertility of the race in unions among themselves. In order to give
     the greatest advantage to your critics, I have assumed that such
     flowers as possess the peculiarity are not self-fertilizable; for
     it is clear that if we suppose that they are self-fertilizable, the
     fertility need be very slightly affected.

     As I have excluded self-fertilization, it is necessary, if we are
     to get any trustworthy results, that one should consider the mode
     in which fertilization will be produced. I have taken the case of
     fertilization by insects, and have assumed that each flower is
     visited a certain number of times by insects during the period when
     fertilization is possible; and, further, that the insects which
     visit it have on the average visited a certain number of flowers of
     the same species before they came there. Of course nothing but
     observation can fix these latter numbers; but I should not be
     surprised at finding that they are of considerable magnitude[65]. In
     order to make the results a little more intelligible, I have
     grouped them under the numbers which represent the average number
     of flowers that an insect visits in a journey. This is a little
     more than twice as great as the number which represents the number
     of flowers he has on the average visited before coming to the
     individual whose fertility we are considering.

    [65] In this anticipation Mr. Moulton is right. The well-known
    botanist, Mr. Bennett, read a most interesting paper on the subject
    before the British Association in 1881. His results have since been
    corroborated by other observers. In particular, Mr. R. M. Christy
    has recorded the movements of 76 insects while visiting at least
    2,400 flowers. (_Entomologist_, July 1883, and _Zool. Journal Lin.
    Soc._, August 1883.) The following is an analysis of his results. In
    the case of butterflies, in twelve observations on nearly as many
    species, there are recorded altogether 99 visits to fifteen species
    of flowers; and of these 99 visits 94 were constant to the same
    species, leaving only 5 visits to any other, or second species. In
    the case of the hive-bee, there were 8 individuals observed: these
    visited altogether 258 flowers, and all the visits paid by the same
    individual were paid to the same species in each of the eight cases.
    Lastly, as regards bumble-bees, there were altogether observed 55
    individuals belonging to four species. These paid altogether 1751
    visits to 94 species of flowers. Of these 1751 visits, 1605 were
    paid to one species, 131 to two species, 16 to three, 6 to four, and
    1 to five. Adding all these results together, we find that 75
    insects (butterflies and bees) visited 117 species of flowers: of
    these visits, 1957 were constant to one species of flower; 136 were
    paid also to a second species, 16 also to a third, 6 also to a
    fourth, and 1 also to a fifth. Or, otherwise stated, while 1957 were
    absolutely constant, from such absolute constancy there were only
    159 deviations. Moreover, if we eliminate three individual humble
    bees, which paid nearly an equal number of visits to two species
    (and, therefore, would have ministered to the work of physiological
    selection almost as well as the others), the 159 deviations become
    reduced to 72, or about four per cent. of the whole.--G. J. R.

     I send you the formula and the calculation on which it is based in
     an Appendix; but as I know you have a holy horror of algebraical
     formulae, I give you here a few numerical results.

     The cases I have worked out are those in which the number of
     insects visiting each flower is 5, or 10, or 15; and I have also
     taken 5, 10, and 15, to represent the number of flowers which an
     insect visits each journey. This makes nine cases in all; and I
     have applied these to two instances--viz. one in which one-fifth of
     the whole race have developed cross-infertility, and the other in
     which one-tenth only have done so. Taking first the instance where
     one-fifth have developed the peculiarity, I find that if on the
     average five insects visit a flower, and each insect on the average
     visits five flowers on a journey, the fertility is diminished by
     about one-tenth. If, however, the average number of flowers the
     insect visits is ten, the reduction of fertility is less than one
     per cent. And it becomes inappreciable if the average number is
     fifteen. If on the average ten insects visit each flower, then, if
     each insect visits on the average five flowers on a journey, the
     reduction of fertility is a little over one per cent.; but if it
     visits ten or fifteen the reduction is inappreciable. If fifteen
     insects visit the flower on an average, then, if these insects on
     the average visit five or more flowers on a journey, the reduction
     of fertility is inappreciable.

     By the term inappreciable I mean that it is not substantially
     greater than one-tenth of one per cent.--i.e. not more than

     Of course, if the proportion of individuals acquiring the
     peculiarity is less, the effect on the fertility under the above
     hypothesis will be greater; and it will not be counteracted so
     fully unless the number of insect visits is larger, or unless the
     insects visit more flowers on a journey. Thus if only one-tenth of
     the race have developed the peculiarity, then, if each flower is
     visited on the average by five insects who visit five flowers on
     each trip, the fertility will be reduced about one-third. If,
     however, the insects visit on the average ten flowers per trip, it
     will be only diminished about one-tenth; and if they visit fifteen
     on each trip, it will be only diminished about one-fortieth. If in
     the same case we suppose that each flower receives ten insect
     visits, then, if the insects visit on an average five flowers per
     trip, the fertility will be diminished about one-eighth. If they
     visit ten on a trip, it will be diminished about one-hundredth, and
     the diminution is inappreciable if they visit fifteen on a trip.
     Similarly, if a flower receives fifteen insect visits, the
     diminution is about one-twenty-fifth, if insects visit on the
     average five flowers on a trip; and is inappreciable if they visit
     ten or fifteen.

     These figures will show you that it is exceedingly possible that a
     peculiarity like this, the effect of which at first sight would
     seem to be so prejudicial to fertility, may in fact have little or
     no influence upon it; and if you set against this the overwhelming
     importance of such a peculiarity in segregating the type so as to
     give it a chance of becoming a fixed species, you will, I think,
     feel that your hypothesis has nothing to fear from a numerical

     I have not examined the case of fertilization by other means; nor
     have I examined the case of fertilization in animals, where
     psychological selection can come in. To obtain any useful results,
     one would have to consider very carefully the circumstances of each
     case; and at present, at all events, I do not think it would be
     useful to do so. Nor have I attempted to show the converse of the
     problem--viz. the effect of swamping where cross-fertilization is
     possible. I shall be very glad to examine any one of these cases if
     you want me to do so; but I should prefer to leave it until I hear
     from you again.

     If you contrast the results that I have given above with those
     given on pages 181 to 183 of Wallace's book, you will see the
     enormous difference. His calculations can only apply to the animal
     kingdom in those cases in which there is only a union between one
     individual of each sex; and before you can deal with the question
     of such animals, you will have to take into consideration many
     elements besides that of mere fertility, if you wish to get any
     tolerably accurate result[66].

    [66] Here follows the Appendix presenting the calculations on which
    the above results are founded; but it seems unnecessary to reproduce
    it on the present occasion.--G. J. R.

The above analysis leaves nothing to be added by me. But, in conclusion,
I may once more repeat that the particular point with which it is
concerned is a point of very subordinate importance. For even if Mr.
Wallace's computation of chances had been found by Mr. Moulton to have
been an adequate computation--and, therefore, even if it had been thus
proved that physiological homogamy must always be associated with some
other form of homogamy in order to produce specific divergence--still
the importance of selective fertility as a factor of organic evolution
would not have been at all diminished. For such a result would merely
have shown that, not only "in many cases" (as I originally said), but
actually in all cases, the selective fertility which I hold to have been
so generally concerned in the differentiation of species has required
for this purpose the co-operation of some among the numerous other forms
of homogamy. But inasmuch as, by hypothesis, no one of these other or
co-operating factors would of itself have been capable of effecting
specific divergence in any of the cases where its association with
selective fertility is concerned, the mathematical proof that such an
association is _always_--and not merely _often_--necessary, would not
have materially affected the theory of the origin of species by means of
physiological selection. We have now seen, however, that a competent
mathematical treatment proves the exact opposite; and, therefore, that
Mr. Wallace's criticism fails even as regards the very subordinate point
in question.



_Bearing of Weismannism on Physiological Selection._--If in view of
other considerations I could fully accept Professor Weismann's theory of
heredity, it would appear to me in no small measure to strengthen my own
theory of physiological selection. For Weismann's theory supposes that
all changes of specific type must have their origin in variations of a
continuous germ-plasm. But _the more the origin of species is referred
directly to variations arising in the sexual elements, the greater is
the play given to the principles of physiological selection_[67]; while,
on the other hand, the less standing-ground is furnished to the theory
that cross-infertility between allied species is due to "external
conditions of life," "prolonged exposure to uniform change of
conditions," "structural modifications re-acting on the sexual
functions"; or, in short, that "somatogenetic" changes of any kind can
of themselves induce the "blastogenetic" change of cross-infertility
between progeny of the same parental stock.

    [67] _Doctrine of Descent and Darwinism_, Eng. trans. p. 139.

_Cross-infertility and Diversity of Life._--Observe that one great
consequence of duly recognizing the importance of intercrossing is
indefinitely to raise our estimate of the part played by the principle
of cross-infertility in diversifying organic nature. For whenever in any
line of descent the bar of sterility arises, there the condition is
given for a new crop of departures (species of a genus); and when genera
are formed by the occurrence of this bar, there natural selection and
all other equilibrating causes are supplied with new material for
carrying on adaptational changes in new directions. Thus, owing to
cross-infertility, all these causes are enabled to work out numberless
adaptations in many directions (i. e. lines of descent) simultaneously.

_Cross-infertility and Stability._--The importance of sterility as a
diagnostic feature is obvious if we consider that more than any other
feature it serves to give _stability_ to the type; and unless a type is
stable or constant, it cannot be ranked as a species. That Darwin
himself attributes the highest importance to this feature as diagnostic,
see _Forms of Flowers_, pp. 58, 64.

_Cross-infertility and Specific Differentiation._--In their elaborate
work on the many species of the genus Hieracium, Nägeli and Peter are
led to the general conclusion that the best defined species are always
those which display absolute sterility _inter se_; while the species
which present most difficulty to the systematist are always those which
most easily hybridize. Moreover, they find, as another general rule
applicable to the whole genus, that there is a constant correlation
between inability to hybridize and absence of intermediate varieties,
and, conversely, between ability to hybridize and the presence of such

_Cross-infertility in Domesticated Cattle._--Mr. J. W. Crompton, who has
had a large experience as a professional cattle-breeder, writes to me
(March 2, 1887)--

     "That form of barrenness, very common in some districts, which
     makes heifers become what are called 'bullers'--that is,
     irregularly in 'season,' wild, and failing to conceive--is
     certainly produced by excess of iron in their drinking-water, and I
     suspect also by a deficiency of potash in the soil."

He also informs me that pure white beasts of either sex are so well
known by experienced breeders to be comparatively infertile together,
that they are never used for breeding purposes, so that "in some parts
of the country, where a tendency to sterility had become so confirmed in
the white race that they utterly died out," only the coloured breeds are
now to be found. He goes on to say that if "a lot of white heifers were
put to a lot of white bulls, I think you would probably get a fertile
breed of pure white cattle.... I think, in short, that domestication has
produced just what your theory suggests, a new variety inclined to prove
sterile with its parent stock."

Commenting on the origin of domesticated cattle, Professor Oscar Schmidt
remarks (_Doctrine of Descent_, p. 139)--

     "Rütimeyer's minute researches on domestic cattle have shown that,
     in Europe at least, three well-defined species of the diluvial
     period have contributed to their formation--_Bos primigenius_,
     _longifrons_, and _frontosus_. These species once lived
     geographically separate, but contemporaneously; and they and their
     specific peculiarities have perished, to rise again in our domestic
     races. These races breed together with unqualified fertility. In
     the form of skull and horns they recall one or other of the extinct
     species; but collectively they constitute a new main species. That
     from their various breeds, the three or any one of the aboriginal
     species would ever emerge in a state of pristine purity, would be
     an utterly ludicrous assertion."

Now, seeing that these "aboriginal species," although living
"contemporaneously," were "geographically separate," we can well
understand that their divergence of type from a common ancestor did not
require, as a condition to their divergence, that any cross-sterility
should have arisen between them. The geographical isolation was enough
to secure immunity from mutual intercrossing, and therefore, as our
present theory would have expected as probable, morphological divergence
occurred without any corresponding physiological divergence, as must
almost certainly have been the case if such polytypic evolution had
occurred on a common area. Indeed, one of the two lines of experimental
verification of our theory consists in selecting cases where nearly
allied species are separated by geographical barriers, and proving that,
in such cases, there is no cross-sterility.

_Fertility of Domesticated Varieties._--Some writers have sought to
explain the contrast between domesticated varieties and natural species
in respect of fertility when crossed, by the consideration that it is
only those natural species which have proved themselves so far flexible
as to continue fertile under changed conditions of life that can have
ever allowed themselves to become domesticated. But although this
condition may well serve to explain the unimpaired fertility under
domestication of such species as for this very reason have ever become
domesticated, I fail to see how it explains the further and altogether
different fact, that this fertility continues unimpaired between all the
newly differentiated morphological types which have been derived from
the original specific type. It is one thing that this type should
continue fertile after domestication: it is quite another thing that
fertility should continue as between all its modified descendants, even
although the amount of modification may extend much further than that
which usually obtains between different natural species.

_Testing for Cross-infertility_ among varieties growing on the same area
is a much more crucial line of verification than testing for unimpaired
fertility between allied species which occupy different areas, because
while in the former case we are dealing with "incipient species" with a
view to ascertaining whether the divergence which they have already
undergone is accompanied by physiological isolation, in the latter case
we can never be sure that two allied species, which are now widely
disconnected geographically, have always been so disconnected. They may
both have originated on the same area; or one may have diverged from the
other before it migrated from that area; or even if, when it migrated,
it was unchanged, and if in its new home it afterwards split into two
species by physiological selection, the newer species would probably
prove infertile, not only with its parent type, but also with its
grand-parent in any other part of the world.

_Seebohm on Isolation._--Seebohm is so strongly influenced by the
difficulty from "the swamping effects of free intercrossing," that he is
driven by it to adopt Asa Gray's hypothesis of variations as
teleological. Indeed, he goes as far as Wagner, for he maintains that in
no case can there be divergence or multiplication of species without
isolation. He makes the important statement that "the more the
geographical distribution of birds is studied, the more doubtful it
seems to be that any species of bird has ever been differentiated
without the aid of geographical isolation" (_Charadriidae_, p. 17). If
this is true, it makes in favour of physiological selection by showing
the paramount importance of the swamping effects of intercrossing, and
consequent importance of isolation. But it makes against physiological
selection by showing that the geographical form of isolation is
sufficient to explain all the cases of specific differentiation in
birds. But I must remember that the latter point rests largely on
negative inference, and that birds, owing to their highly locomotive
habits, are the class of animals where physiological selection is likely
to be most handicapped.

_Herbert on Hybridization._--Herbert tells us that when he first
astonished the Horticultural Society by laying before them the results
of his experiments on hybridization, his brother botanists took serious
alarm. For it appeared to them that this "intermixture of species would
confuse the labours of botanists, and force them to work their way
through a wilderness of uncertainty." Therefore he was bluntly told by
several of these gentlemen, "I do not thank you for your mules." Now,
although naturalists have travelled far and learnt much since those
days, it appears to me that a modern evolutionist might still turn to
the horticulturist with the same words. For assuredly he has no reason
to thank the horticulturist for his mules, until he has found a
satisfactory answer to the question why it is that natural species
differ so profoundly as regards their capacity for hybridizing.

_Advance on Herbert's Position._--- If it be said that all my work
amounts to showing what Herbert said long ago--viz. that the only true
or natural distinction between organic types is the sexual
distinction--I answer that my work does much more than this. For it
shows that the principle of sterility is the main condition to the
differentiation, not merely of species and genera, but also to the
evolution of adaptations everywhere, in higher as well as in lower
taxonomic divisions. Moreover, even though naturalists were everywhere
to consent to abandon specific designations, and, as Herbert advises, to
"entrench themselves behind genera," there would still remain the facts
of what are now called specific differences (of the secondary or
morphological kind), and by whatever name these are called, they alike
demand explanation at the hands of the evolutionist.

_Fritz Müller on Cross-infertility._--Fritz Müller writes, "Every plant
requires, for the production of the strongest possible and most prolific
progeny, a certain amount of difference between male and female elements
which unite. Fertility is diminished as well when this degree is too low
(in relatives too closely allied) as when it is too high (in those too
little related)." Then he adds, as a general rule, "Species which are
wholly sterile with pollen of the same stock, and even with pollen of
nearly allied stocks, will generally be fertilized very readily by the
pollen of another species. The self-sterile species of the genus
Abutilon, which are, on the other hand, so much inclined to
hybridization, afford a good example of this theory, which appears to be
confirmed also by Lobelia, Passiflora, and Oncidium" (_American
Naturalist_, vol. viii, pp. 223-4, 1874).

_Different groups of plants exhibit remarkable differences in the
capability of their constituent species to hybridize._--In so far as
these differences have reference only to first crosses, they have no
bearing either for or against my theory. Only in so far as the
differences extend to the production of fertile hybrids does any
question arise for me. First of all, therefore, I must ascertain whether
(or how far) there is any correlation between groups whose species
manifest aptitude to form first crosses, and groups where first crosses
manifest aptitude to produce fertile hybrids. Next, whatever the result
of this inquiry should be, if I find that certain natural groups of
plants exhibit comparatively well-marked tendencies to form fertile
hybrids, the question will arise, Are these tendencies correlated with
_paucity_ of species? If they are, the fact would make strongly in
favour of physiological selection. For the fact would mean that in these
natural groups, owing to "the nature of the organisms" included under
them, less opportunity is given to physiological selection in its work
of differentiating specific types than is given by other natural groups
where the nature of the organism renders them more prone to mutual
sterility. But in prosecuting this branch of verification, I must
remember to allow for possibilities of differential degrees of
geographical isolation in the different groups compared.

On this subject Focke writes me as follows:--"In a natural group
(family, order, genus) showing considerable variability in the structure
of the flower, we may expect to find [or do find] a greater number of
mules than in a group whose species are only distinguished by
differences in the shape of the leaves, or in growth, &c. I do not
know, however, which in this connexion of things is the cause and which
the effect. A useful ancestral structure of the flower may be conserved
by an otherwise varying progeny, on condition that the progress of
diversity be not disturbed by frequent intercrossings. [Therefore, if
this condition be satisfied, the structure of the flower in different
members of the group will continue constant: here the cause of
_constancy_ in the flower (however much variability there may be in the
leaves, &c.) is its original _inability_ to hybridize.] On the other
hand, in species or groups ready to hybridize [or capable of
hybridizing], the fixation of a new specific type will require some
change in the structure of the flower, and a change considerable enough
to alter the conditions of fertilization. [Here the reason of the
_in_constancy of the flower in different members of the group is the
original _aptitude_ of their ancestral forms to hybridize.] Perhaps
there is something in this suggestion, but certainly there are other
efficient physiological relations, which are at present unknown. Your
theory of physiological selection may serve to explain many difficult

_The Importance of Prepotency._--A. Kerner shows by means of his own
observations on sundry species of plants which hybridize in the wild
state, that they do so very much more frequently if both, or even if
only one of the parent forms be rare in the neighbourhood. This fact can
only be explained by supposing that, even in species most prone to
hybridizing under Nature, there is some degree of prepotency of pollen
of the same species over that of the other species; so that where both
species are common, it is correspondingly rare that the foreign pollen
gets a chance. But if there were no prepotency, the two species would
blend; and this Kerner supposes must actually take place wherever two
previously separated species, thus physiologically circumstanced, happen
to be brought together. (Kerner's paper is published in _Oester. Bot.
Zeitschrift_, XXI, 1871, where he alludes to sundry other papers of his
own advocating similar views.)

The relation of these observations to Jordan's _espèces affines_ is
obvious. We have only to suppose that some such slight and constant
difference characterizes the sexual elements of these allied varieties
as demonstrably characterizes their morphology, and we can understand
how pollen-prepotency would keep the forms distinct--such forms,
therefore, being so many records of such prepotency.

Both from Kerner's work, and still more from that of Jordan and Nägeli,
I conclude that (at all events in plants) prepotency is the way in which
physiological selection chiefly acts. That is to say, _sudden_ and
_extreme_ variations in the way of sexual incompatibility are probably
rare, as compared with some degree of prepotency. According as this
degree is small or great so will be the amount of the corresponding
separation. This view would show that in plants the principle of
physiological selection is one of immensely widespread influence,
causing (on the same areas) more or less permanent varieties much below
specific rank. And when we remember on how delicate a balance of
physiological conditions complete correspondency of pollen to ovules
depends, we may be prepared to expect that the phenomenon of prepotency
is not of uncommon occurrence.

_Self-fertilization and Variability._--It occurred to Count Berg Sagnitz
that, if physiological selection is a true principle in nature,
vegetable species in which self-fertilization obtains ought to be more
rich in constant varieties than are species in which cross-fertilization
rules. For, although even in the latter case physiological isolation may
occasionally arise, it cannot be of such habitual or constant occurrence
as it must be in the former case. Acting on this idea, Count Berg
Sagnitz applied himself to ascertain whether there is any general
correlation between the habit of self-fertilization and the fact of
high variability; and he says that in all the cases which he has
hitherto investigated, the correlation in question is unmistakable.

_Additional Hypothesis concerning Physiological Selection._--In
reciprocal crosses _A_ × _B_ is often more fertile than _B_ × _A_. If
hybrid _AB_ is more fertile with _A_, and hybrid _BA_ with _B_, than
vice versa, there would be given a good analogy on which to found the
following hypothesis.

Let _A_ and _B_ be two intergenerating groups in which segregate
fecundity is first beginning. Of the hybrids, _AB_ will be more fertile
with _A_, and _BA_ with _B_, than vice versa. The interbreeding of _AB_
with _A_ will eventually modify sexual characters of _A_ by assimilating
it to those of _AB_, while the interbreeding of _BA_ with _B_ will
similarly modify sexual characters of _B_ by assimilating it to those of
_BA_. Consequently, _A_ will become more and more infertile with _B_,
while _B_ becomes more and more infertile with _A_. Fewer and fewer
hybrids will thus be produced till mutual sterility is complete.

To sustain this hypothesis it would be needful to prove experimentally,
(1) that hybrid forms _AB_ are more fertile with _A_ than with _B_,
while hybrid forms _BA_ are more fertile with _B_ than with _A_ [or, it
may be possible that the opposite relations would be found to obtain,
viz. that _AB_ would be more fertile with _B_, and _BA_ with _A_]; (2)
that, if so, effect of intercrossing _AB_ with _A_ is to make progeny
more fertile with _A_ than with _B_, while effect of intercrossing _BA_
with _B_ is to make progeny more fertile with _B_ than with _A_.

Such experiments had best be tried with species where there is already
known to be a difference of fertility between reciprocal crosses (e.g.
Matthiola annua and M. glabra, see _Origin of Species_, p. 244).



ALLEN, Mr. J. A., on variation under nature, 34.

Amixia, 12-28, 110-115, 117-133.

Apogamy, 5, 6, 10, 18, 28.


BELT, on physiological selection, 44.

BERG SAGNITZ, Count, on self-fertilization and variability, 177.

Breeding, separate and segregate, 5.

Butterflies of polar regions and Alps, 133.


CATCHPOOL, Mr., on physiological selection, 44, 137.

Cross-infertility, 46;
  and varietal divergence, 82;
  and diversity of life, 169;
  and stability, 170;
  and specific differentiation, 170;
  in domesticated cattle, 170;
  testing for, 172;
  Fritz Müller on, 174.


_Darwin_, Charles, on isolation, 2, 106;
  on diversity under nature, 31;
  on the fertility of varieties, 50;
  on the origin of cross-infertility, 51;
  on distribution, 68;
  on prepotency, 89;
  on geographical isolation, 101, 108;
  on methodical selection, 102;
  on modification in large areas, 103;
  on the swamping effects of intercrossing, 105;
  on independent variability, 109;
  on domestic animals, 110.

DELBŒUF, law of independent variability, 13.

Differentiation under natural selection, 37.

Diversity of life and cross-infertility, 169.

Domesticated cattle and cross-infertility, 170, 172.


Evidences of physiological selection, 62.

Evolution, monotypic and polytypic, 21, 75, 102, 107, 112, 129.

Experimental research in physiological selection, 85.


Fertility of domesticated varieties, 172.

FOCKE, Herr, on hybridization, 175.


GALTON, Mr. Francis, law of regression, 39.

General conclusions, 144.

Geographical distribution and physiological selection, 65.

GIARD, M., on apogamy, 14.

GRABHAM, Dr., on mollusca of Madeira, 135.

GULICK, Rev. J., on natural Selection as a mode of isolation, 9;
  on divergence, 11;
  on segregate breeding, 19;
  on geographical distribution, 27;
  on the prevention of intercrossing, 127;
  on Mr. Wallace's criticisms, 151.


HERBERT, on hybridization, 173;
  advance on his position, 174.

HERDMAN, Prof., on physiological isolation, 123.

Historical sketch of opinions on isolation, 101.

Homogamy, 5, 6;
  forms of, 7, 19, 29.

Hybridization, HERBERT on, 173;
  in plants, 175.

Hypothesis, additional, concerning physiological selection, 178.


Independent variability, 12-29.

Isolation, defined, 2;
  forms of, 3, 6;
  geographical, 3;
  discriminate and indiscriminate, 5;
  physiological, 9, 41, 58;
  its importance, 39;
  sketch of opinions on, 101;
  general conclusions, 144;
  SEEBOHM on, 173.


JORDAN, M., on cross sterile varieties of plants, 86;
  his researches summarized, 87.


KERNER, Prof. A., on prepotency, 176.


LANKESTER, Prof. Ray, on divergent evolution, 15.

LE CONTE, Prof., on fossil snails of steinheim, 95;
  on isolation, 129.

LIVINGSTONE, Dr. David, Quoted, 123.


MELDOLA, Prof., on difficulty from intercrossing, 121.

Misunderstandings of Physiological selection, 59.

Monotypic evolution, see Evolution.

MORGAN, Prof. Lloyd, on sterility, 56;
  on isolation, 128.

MOULTON, Mr. Fletcher, an examination of Mr. Wallace's calculations on
physiological selection, 157.

MÜLLER, Fritz, on cross-infertility, 174.


NÄGELI, on isolation, 76;
  on synoicy, 78, 82.

Natural selection, a form of discriminate isolation, 9, 10, 23;
  leads to monotypic evolution, 24-29;
  difficulties of, 41, 51.


Panmixia, 12.

Physiological selection, 9, 41;
  summarized, 58;
  misunderstandings of, 59;
  evidences of, 81-119;
  and Weismannism, 169;
  additional hypothesis, 178.

Polytypic evolution, see Evolution.

Prepotency, 89; importance of, 176.


SCHMIDT, Prof. Oscar, on domesticated cattle, 171.

SEEBOHM on isolation, 173.

Segregation, 28.

Selection, physiological, see Physiological selection.

Self-fertilization and variability, 177.

Snails of Sandwich Islands, 16, 130;
  fossil of Steinheim, 95.

Specific differentiation and cross-infertility, 170.

Stability and cross-infertility, 170.

Synoicy, 78.


Topographical distribution and physiological selection, 74;
  of varieties, 81.

Transformation, serial and divergent, 21, 121.


Variability and self-fertilization, 177.

Variation in birds, 34.

Varieties, topographical distribution of, 81.


WAGNER, Maritz, 3;
  on geographical isolation, 76;
  quoted, 103;
  law of migration, 111.

WALLACE, Mr. A. R., 3, 17;
  quoted, 34, 47, 51, 57,130-136;
  criticized by Gulick, 152.

WEISMANN, Prof., on geographical isolation, 76, 114-118.

Weismannism and physiological selection, 169.



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Transcriber's Notes

In paragraph 4 of page 171 "peculiarites" has been corrected to

Variable spacing in the following abbreviation was left as it was in the
original: "i. e." (22 instances) and "i.e." (14 instances).

Different hyphenation patterns were left as in the original text:

  prepotent (1 instance)    pre-potent (1 instance)
  presupposes (1)           pre-supposes (1)
  reacting(5)               re-acting (1)
  restatement (1)           re-statement (2)
  superinduced (2)          super-induced (1)

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