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DARWIN, AND AFTER DARWIN
III
POST-DARWINIAN QUESTIONS
ISOLATION AND PHYSIOLOGICAL SELECTION
BY THE SAME AUTHOR.
DARWIN, AND AFTER DARWIN. An Exposition of the Darwinian Theory and a
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2. POST-DARWINIAN QUESTIONS. Edited by Prof. C. Lloyd Morgan. With
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3. POST-DARWINIAN QUESTIONS. ISOLATION AND PHYSIOLOGICAL SELECTION.
Edited by Prof. C. Lloyd Morgan. With portrait of Mr. J. T.
Gulick. 181 pages. Cloth, $1.00.
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THE OPEN COURT PUBLISHING COMPANY,
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[Illustration: Frontispiece--John J. Gulick]
DARWIN, AND AFTER DARWIN
_AN EXPOSITION OF THE DARWINIAN THEORY
AND A DISCUSSION OF
POST-DARWINIAN QUESTIONS_
BY THE LATE
GEORGE JOHN ROMANES, M.A., LL.D., F.R.S.
_Honorary Fellow of Gonville and Caius College, Cambridge_
III
POST-DARWINIAN QUESTIONS
ISOLATION
AND PHYSIOLOGICAL SELECTION
Chicago
THE OPEN COURT PUBLISHING COMPANY
1906
CHAPTER 1. COPYRIGHTED BY
THE OPEN COURT PUBLISHING CO.
1897.
The Lakeside Press
R. R. DONNELLEY & SONS CO., CHICAGO
PREFACE
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."
C. LLOYD MORGAN.
BRISTOL, _May 1897_.
CONTENTS
CHAPTER I.
ISOLATION 1
CHAPTER II.
ISOLATION (_continued_) 28
CHAPTER III.
PHYSIOLOGICAL SELECTION 41
CHAPTER IV.
EVIDENCES OF PHYSIOLOGICAL SELECTION 62
CHAPTER V.
FURTHER EVIDENCES OF PHYSIOLOGICAL SELECTION 81
CHAPTER VI.
A BRIEF HISTORY OF ISOLATION AS A FACTOR IN
ORGANIC EVOLUTION 101
GENERAL CONCLUSIONS 144
APPENDIX A. MR. GULICK'S CRITICISM OF MR. WALLACE'S
VIEWS ON PHYSIOLOGICAL SELECTION 151
APPENDIX B. AN EXAMINATION BY MR. FLETCHER
MOULTON OF MR. WALLACE'S CALCULATION TOUCHING
THE POSSIBILITY OF PHYSIOLOGICAL SELECTION
EVER ACTING ALONE 157
APPENDIX C. SOME EXTRACTS FROM THE AUTHOR'S
NOTE-BOOKS 169
_ISOLATION_
CHAPTER I.
ISOLATION.
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
judice_.
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
evolution.
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
species[4].
[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
death.
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 Delboeuf.
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 Delboeuf," 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 Delboeuf 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 Delboeuf's Mathematical
Law.
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
reasoning.
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
evolution.
(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
greatest.
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.