regarded as directed against the _principle_ of what Professor Weismann
calls "continuity." On the contrary, it appears to be self-evident that
this principle must be accepted in some degree or another by every one,
whether Darwinians, Neo-Darwinians, Lamarckians, Neo-Lamarckians, or
even the advocates of special creation. Yet, to hear or to read some of
the followers of Weismann, one can only conclude that, prior to his
publications on the subject, they had never thought about it at all.
These naturalists appear to suppose that until then the belief of
Darwinians was, that there could be no hereditary "continuity" between
any one organic type and another (such, for instance, as between Ape and
Man), but that the whole structure of any given generation must be due
to "gemmules" or "somato-plasm," derived exclusively from the preceding
generation. Nothing can show more ignorance, or more thoughtlessness,
with regard to the whole subject. The very basis of the general theory
of evolution is that there must always have been a continuity in the
material substance of heredity since the time when the process of
evolution began; and it was not reserved for our generation, or even for
our century, to perceive the special nature of this material substance
in the case of sexual organisms. No, the real and the sole question,
where Weismann's theory of heredity is concerned, is simply this--Are we
to hold that this material substance has been _absolutely_ continuous
"since the first origin of sexual propagation," always occupying a
separate "sphere" of its own, at all events to the extent of never
having been modified by the body substance in which it resides
(Lamarckian factors); _or_, are we to hold that this "germ-plasm,"
"stirp," or "formative-material," has been but _relatively_ continuous,
so as to admit of some amount of commerce with body-substance, and
therefore to admit of acquired characters, when sufficiently long
continued as such, eventually becoming congenital? If this question be
answered in the latter sense, of course the further question arises as
to the _degree_ of such commerce, or the _time_ during which acquired
characters must continue to be acquired in successive generations before
they can sufficiently impress themselves on the substance of heredity to
become congenital. But this is a subordinate question, and one which, in
the present state of our information, it seems to me almost useless to
speculate upon. My own opinion has always been the same as that of Mr.
Galton; and my belief is that eventually both Weismann and his followers
will gravitate into it. It was in order to precipitate this result as
far as possible that I wrote the _Examination_. If it ever should be
accomplished, Professor Weismann's elaborate theory of evolution will
have had its bases removed.
SECTION II
_UTILITY_
CHAPTER VII.
CHARACTERS AS ADAPTIVE AND SPECIFIC.
One of the great changes which has been wrought in biological science by
the Darwinian theory of natural selection, consists in its having
furnished an intelligible explanation of the phenomena of _adaptation_.
Indeed, in my opinion, this is the most important function which this
theory has had to perform; and although we still find systematic
zoologists and systematic botanists who hold that the chief merit of
Darwin's work consists in its having furnished an explanation of the
origin of _species_, a very little consideration is enough to show that
such an idea is but a survival, or a vestige, of an archaic system of
thought. So long as species were regarded as due to separate acts of
creation, any theory which could explain their production by a process
of natural evolution became of such commanding importance in this
respect, that we cannot wonder if in those days the principal function
of Darwin's work was held to be what the title of that work--_The Origin
of Species by means of Natural Selection_--itself serves to convey. And,
indeed, in those days this actually was the principal function of
Darwin's work, seeing that in those days the _fact_ of evolution
itself, as distinguished from its _method_, had to be proved; and that
the whole proof had to stand or fall with the evidence which could be
adduced touching the mutability of species. Therefore, without question,
Darwin was right in placing this issue as to the stability or
instability of species in the forefront of his generalizations, and
hence in constituting it the title of his epoch-making book. But
nowadays, when the fact of evolution has been sufficiently established,
one would suppose it self-evident that the theory of natural selection
should be recognized as covering a very much larger field than that of
explaining the origin of _species_--that it should be recognized as
embracing the whole area of organic nature in respect of _adaptations_,
whether these happen to be distinctive of species only, or of genera,
families, orders, classes, and sub-kingdoms. For it follows from the
general fact of evolution that species are merely arbitrary divisions,
which present no deeper significance from a philosophical point of view
than is presented by well-marked varieties, out of which they are in all
cases believed to have arisen, and from which it is often a matter of
mere individual taste whether they shall be separated by receiving the
baptism of a specific name. Yet, although naturalists are now
unanimously agreed that what they classify as species are nothing
more than pronounced--and in some greater or less degree
permanent--varieties, so forcible is the influence of traditional modes
of thought, that many zoologists and botanists still continue to regard
the origin of species as a matter of more importance than the origin of
adaptations. Consequently, they continue to represent the theory of
natural selection as concerned, primarily, with explaining the origin of
species, and denounce as a "heretic" any one who regards the theory as
primarily a theory of the origin and cumulative development of
adaptations--whether structural or instinctive, and whether the
adaptations are severally characteristic of species only or of any of
the higher taxonomic divisions. Indeed, these naturalists appear to deem
it in some way a disparagement of the theory to state that it is,
primarily, a theory of adaptations, and only becomes secondarily a
theory of species in those comparatively insignificant cases where the
adaptations happen to be distinctive of the lowest order of taxonomic
division--a view of the matter which may fitly be compared to that of an
astronomer who should define the nebular hypothesis as a theory of the
origin of Saturn's rings. It is indeed a theory of the origin of
Saturn's rings; but only because it is a theory of the origin of the
entire solar system, of which Saturn's rings form a part. Similarly, the
theory of natural selection is a theory of the entire system of organic
nature in respect of adaptations, whether these happen to be distinctive
of particular species only, or are common to any number of species.
Now the outcry which has been raised over this definition of the theory
of natural selection is a curious proof of the opposition which may be
furnished by habitual modes of thought to an exceedingly plain matter of
definition. For, I submit, that no one can deny any of the following
propositions; nor can it be denied that from these propositions the
foregoing definition of the theory in question follows by way of
necessity. The propositions are, first, that natural selection is taken
to be the agency which is mainly, if not exclusively, concerned in the
evolution of adaptive characters: secondly, that these characters, when
evolved, are in some cases peculiar to single species only, while in
other cases, and in process of time, they become the common property of
many species: thirdly, that in cases where they are peculiar to single
species only, they constitute at all events one of the reasons (or even,
as the ultra-Darwinians believe, the only reason) why the particular
species presenting them have come to be species at all. Now, these being
the propositions on which we are all agreed, it obviously follows, of
logical necessity, that the theory in question is primarily one which
explains the existence of adaptive characters wherever these occur; and,
therefore, whether they happen to be restricted to single species, or
are common to a whole group of species. Of course in cases where they
are restricted to single species, the theory which explains the origin
of these particular adaptations becomes also a theory which explains the
origin of these particular species; seeing that, as we are all agreed,
it is in virtue of such particular adaptations that such particular
species exist. Yet even in these cases the theory is, primarily, a
theory of the adaptations in virtue of which the particular species
exists; for, _ex hypothesi_, it is the adaptations which condition the
species, not the species the adaptations. But, as just observed,
adaptations may be the common property of whole groups of species; and
thus the theory of natural selection becomes a theory of the origin of
genera, of families, of orders, and of classes, quite as much as it is a
theory of the origin of species. In other words, it is everywhere a
theory of adaptations; and it is only where the adaptations happen to be
restricted to single species that the theory therefore and incidentally
becomes also a theory of the particular species which presents them.
Hence it is by no means the same proposition to affirm that the theory
of natural selection is a theory of the origin of species, and that it
is a theory of the origin of adaptations, as some of my critics have
represented it to be; for these two things are by no means conterminous.
And in as far as the two propositions differ, it is perfectly obvious
that the latter is the true one.
Possibly, however, it may be said--Assuredly natural selection is a
theory of the origin (i.e. cumulative development) of adaptations; and,
no less assuredly, although species owe their origin to such
adaptations, there is now no common measure between these two things,
seeing that in numberless cases the same adaptations are the common
property of numberless species. But, allowing all this, we must still
remember that in their _first beginnings_ all these adaptations must
have been distinctive of, or peculiar to, some one particular species,
which afterwards gave rise to a whole genus, family, order, or class of
species, all of which inherited the particular adaptations derived from
this common ancestor, while progressively gaining additional adaptive
characters severally distinctive of their subsequently diverging lines
of descent. So that really all adaptive characters must originally have
been specific characters; and therefore there is no real distinction to
draw between natural selection as a theory of species and as a theory of
adaptations.
Well, if this objection were to be advanced, the answer would be
obvious. Although it is true that every adaptive character which is now
common to a group of species must originally have been distinctive of a
single parent species, it by no means follows that in its first
beginning as a specific character it appeared in the fully developed
form which it now presents as a generic, family, ordinal, or yet higher
character. On the contrary, it is perfectly certain that in the great
majority of instances such cannot possibly have been the case; and the
larger the group of species over which any particular adaptive character
now extends, the more evidently do we perceive that this character must
itself have been the product of a gradual evolution by natural selection
through an innumerable succession of species in branching lines. The
wing of a bird, for example, is an adaptive structure which cannot
possibly have ever appeared suddenly as a merely specific character: it
must have been slowly elaborated through an incalculable number of
successive species, as these branched into genera, families, and orders
of the existing class. So it is with other class distinctions of an
adaptive kind; and so, in progressively lessening degrees, is it with
adaptive characters of an ordinal, a family, or a generic value. That is
to say, in _all_ cases where an adaptive structure is common to any
considerable group of species, we meet with clear evidence that the
structure has been the product of evolution through the ancestry of
those species; and this evidence becomes increasingly cogent the higher
the taxonomic value of the structure. Indeed, it may be laid down as a
general rule, that the greater the _degree_ of adaptation the greater is
its _diffusion_--both as regards the number of species which present it
now, and the number of extinct species through which it has been handed
down, in an ever ramifying extension and in an ever improving form.
Species, therefore, may be likened to leaves: successive and transient
crops are necessary for the gradual building up of adaptations, which,
like the woody and permanent branches, grow continuously in importance
and efficiency through all the tree of life. Now, in my view, it is the
great office of natural selection to see to the growth of these
permanent branches; and although natural selection has likewise had an
enormously large share in the origination of each successive crop of
leaves--nay, let it be granted to the ultra-Darwinians for the sake of
argument, an exclusive prerogative in this respect--still, in my view,
this is really the least important part of its work. Not as an
explanation of those merely permanent varieties which we call species,
but as an explanation of the adaptive machinery of organic nature, which
has led to the construction both of the animal and vegetable kingdoms in
all their divisions do I regard the Darwinian theory as one of the
greatest generalizations in the history of science.
* * * * *
I have dwelt thus at some length upon a mere matter of definition
because, as we shall now find, although it is but a matter of
definition, it is fraught with consequences of no small importance to
the general theory of descent. Starting from an erroneous definition of
the theory of natural selection as primarily a theory of the origin of
species, both friends and foes of the theory have concluded that the
principle of utility must by hypothesis be of universal occurrence so
far as species are concerned; whereas, if once these naturalists were to
perceive that their definition of the theory is erroneous, they would
likewise perceive that their conclusion cannot follow deductively from
the theory itself. If such a conclusion is to be established at all, it
can only be by other and independent evidence of the inductive kind--to
wit, by actual observation.
Hence we see the importance of starting with an accurate definition of
the theory before proceeding to examine the doctrine of utility as of
universal application to species--a doctrine which, as just stated, has
been habitually and expressly deduced from the theory. This doctrine
occurs in two forms; or, more correctly, there are with reference to
this subject two distinct doctrines, which partly coincide and partly
exclude one another. First, it is held by some naturalists that all
species must necessarily owe their origin to natural selection. And
secondly, it is held by other naturalists, that not only all species,
but likewise all specific characters must necessarily do the same. Let
us consider these two doctrines separately.
The first, and less extensive doctrine, rests on the deduction that
every species must owe its differentiation as a species to the evolution
of at least one adaptive character, which is peculiar to that species.
Although, when thus originated, a species may come to present any
number of other peculiar characters of a non-adaptive kind, these merely
indifferent peculiarities are supposed to hang, as it were, on the peg
supplied by the one adaptive peculiarity; it is the latter which
conditions the species, and so furnishes an opportunity for any number
of the former to supervene. But without the evolution of at least one
adaptive character there could have been no distinct species, and
therefore no merely adventitious characters as belonging to that
species. I will call this the Huxleyan doctrine, because Professor
Huxley is its most express and most authoritative supporter.
The second and more extensive doctrine I will call, for the same reason,
the Wallacean doctrine. This is, as already stated, that it follows
deductively from the theory of natural selection, that not only all
species, but even all the distinctive characters of every species, must
necessarily be due to natural selection; and, therefore, can never be
other than themselves useful, or, at the least, correlated with some
other distinctive characters which are so.
Here, however, I should like to remark parenthetically, that in choosing
Professor Huxley and Mr. Wallace as severally representative of the
doctrines in question, I earnestly desire to avoid any appearance of
discourtesy towards such high authorities.
I am persuaded--as I shall hereafter seek to show Darwin was
persuaded--that the doctrine of utility as universal where species are
concerned, is, in both the above forms, unsound. But it is less
detrimental in its Huxleyan than in its Wallacean form, because it does
not carry the erroneous deduction to so extreme a point. Therefore let
us first consider the doctrine in its more restricted form, and then
proceed, at considerably greater length, to deal with it in its more
extended form.
* * * * *
The doctrine that all _species_ must necessarily be due to natural
selection, and therefore must severally present at least one adaptive
character, appears to me doubly erroneous.
In the first place, it is drawn from what I have just shown to be a
false premiss; and, in the second place, the conclusion does not follow
even from this premiss. That the premiss--or definition of the theory as
primarily a theory of the origin of species--is false, I need not wait
again to argue. That the conclusion does not follow even from this
erroneous premiss, a very few words will suffice to prove. For, even if
it were true that natural selection is primarily a theory of the origin
of species, it would not follow that it must therefore be a theory of
the origin of _all_ species. This would only follow if it were first
shown that the theory is not merely _a_ theory of the origin of species,
but _the_ theory of the origin of species--i.e. that there can be no
further theory upon this subject, or any cause other than natural
selection which is capable of transforming any single specific type.
Needless to say, this cannot be shown by way of deduction from the
theory of natural selection itself--which, nevertheless, is the only way
whereby it is alleged that the doctrine is arrived at[86].
[86] For a full treatment of Professor Huxley's views upon this
subject, see Appendix II.
* * * * *
From the doctrine of utility as advocated by Professor Huxley, we may
now pass on to consider it in the much more comprehensive form advocated
by Mr. Wallace. Of course it is obvious that if the doctrine is
erroneous in its Huxleyan form, much more must it be so in its
Wallacean; and, therefore, that having shown its erroneousness in its
less extended application, there is little need to consider it further
in its more extended form. Looking, however, to its importance in this
more extended application, I think we ought to examine it independently
as thus presented by Mr. Wallace and his school. Let us therefore
consider, on its own merits, the following statement:--It follows
directly from the theory of natural selection that not only all species,
but likewise all specific characters, must be due to natural selection,
and, therefore, must all be of use to the species which present them, or
else correlated with other characters which are so.
It seems worth while to observe, _in limine_, that this doctrine is
contradicted by that of Professor Huxley. For supposing natural
selection to be the only principle concerned in the origin of all
species, it by no means follows that it is the sole agency concerned in
the origin of all specific characters. It is enough for the former
proposition if only some of the characters distinctive of any given
species--nay, as he very properly expresses it, if only one such
character--has been due to natural selection; for it is clear that, as
he adds, "any number of indifferent [specific] characters" may thus have
been furnished with an opportunity, so to speak, of being produced by
causes other than natural selection. Hence, as previously remarked, the
Huxleyan doctrine, although coinciding with the Wallacean up to the
point of maintaining utility as the only principle which can be
concerned in the origin of species, designedly excludes the Wallacean
doctrine where this proceeds to extend any similar deduction to the case
of specific characters[87].
[87] Professor Huxley's views upon this matter are quoted _in
extenso_ in Appendix II.
In the next place, and with special reference to the Wallacean doctrine,
it is of importance to observe that, up to a certain point there is
complete agreement between Darwinists of all schools. We all accept
natural selection as a true cause of the origin of species (though we
may not all subscribe to the Huxleyan deduction that it is necessarily a
cause of the origin of _all_ species). Moreover, we agree that specific
characters are often what is called rudimentary or vestigial; and, once
more, that our inability to detect the use of any given structure or
instinct is no proof that such a structure or instinct is actually
useless, seeing that it may very probably possess some function hitherto
undetected, or possibly undetectable. Lastly, we all agree that a
structure which is of use may incidentally entail the existence of some
other structure which is not of use; for, in virtue of the so-called
principle of correlation, the useless structure may be an indirect
consequence of natural selection, since its development may be due to
that of the useful structure, with the growth of which the useless one
is correlated.
Nevertheless, while fully conceding all these facts and principles to
the Wallacean party, those who think with Professor Huxley--and still
more, of course, those few naturalists who think as I do--are unable
to perceive that they constitute any grounds for holding the doctrine
that all specific _characters_ are, or formerly have been, directly or
indirectly due to natural selection. My own reasons for dissenting from
this Wallacean doctrine are as follows.
* * * * *
From what has just been said, it will be apparent that the question in
debate is not merely a question of fact which can be settled by a direct
appeal to observation. If this were the case, systematic naturalists
could soon settle the question by their detailed knowledge of the
structures which are severally distinctive of any given group of
species. But so far is this from being the case, that systematic
naturalists are really no better qualified to adjudicate upon the matter
than are naturalists who have not devoted so much of their time to
purely diagnostic work. The question is one of general principles, and
as such cannot be settled by appeals to special cases. For example,
suppose that the rest of this chapter were devoted to a mere enumeration
of cases where it appears impossible to suggest the utility of certain
specific characters, although such cases could be adduced by the
thousand, how should I be met at the end of it all? Not by any one
attempting to suggest the utility, past or present, of the characters
named; but by being told that they must all present some _hidden_ use,
must be _vestigial_, or else must be due to _correlation_. By appealing
to one or other of these assumptions, our opponents are always able to
escape the necessity of justifying their doctrine in the presence of
otherwise inexplicable facts. No matter how many seemingly "indifferent
characters" we may thus accumulate, Mr. Wallace and his followers will
always throw upon us the impossible burden of proving the negative, that
these apparently useless characters do _not_ present some hidden or
former use, are _not_ due to correlation, and therefore have _not_ been
produced by natural selection. It is in vain to retort that the burden
of proof really lies the other way, or on the side of those who affirm
that there is utility where no man can see it, or that there is
correlation where no one can detect it. Thus, so far as any appeal to
particular facts is concerned, it does not appear that there is any
_modus vivendi_. Our opinions upon the question are really determined by
the views which we severally take on matters of general principle. The
issue, though it has a biological bearing, is a logical issue, not a
biological one: it turns exclusively on those questions of definition
and deduction with which we have just been dealing.
But although it thus follows that we cannot determine in fact what
proportion of apparently useless characters are or are not really
useful, we may very easily determine in fact what proportion of specific
characters _fail to present any observable evidences of utility_. Yet,
even upon this question of observable fact, it is surprising to note the
divergent statements which have of late years been made by competent
writers; statements in fact so divergent that they can only be explained
by some want of sufficient thought on the part of those naturalists who
are antecedently persuaded that all specific characters must be either
directly or indirectly due to natural selection. Hence they fail to give
to apparently useless specific characters the attention which, apart
from any such antecedent persuasion, they deserve. For example, a few
years ago I incidentally stated in a paper before the Linnaean Society,
that "a large proportional number of specific characters" are of a
trivial and apparently unmeaning kind, to which no function admits of
being assigned, and also stated that Darwin himself had expressly given
utterance to the same opinion. When these statements were made, I did
not anticipate that they would be challenged by anybody, except perhaps,
by Mr. Wallace. And, in order now to show that my innocence at that time
was not due to ignorance of contemporary thought on such matters, a
sentence may here be quoted from a paper which was read at the meeting
of the British Association of the same year, by a highly competent
systematic naturalist, Mr. Henry Seebohm, and soon afterwards
extensively republished. Criticizing adversely my then recently
published paper, he said:--
"I fully admit the truth of this statement; and I presume that few
naturalists would be prepared to deny that 'distinctions of
specific value frequently have reference to structures which are
without any utilitarian significance[88].'"
[88] _Geographical Distribution of the Family Charadriidae_, p. 19.
But since that time the course of Darwinian speculation has been greatly
influenced by the writings of Weismann, who, among other respects in
which he out-darwins Darwin, maintains the doctrine of utility as
universal. In consequence of the influence which these writings have
exercised, I have been more recently and extensively accused of "heresy"
to Darwinian principles, for having stated that "a large proportional
number of specific characters" do not admit of being proved useful, or
correlated with other characters that are useful. Now, observe, we have
here a simple question of fact. We are not at present concerned with the
question how far the argument from ignorance may be held to apply in
mitigation of such cases; but we are concerned only with the question of
fact, as to what proportional number of cases actually occur where we
are _unable to suggest_ the use of specific characters, or the useful
characters with which these apparently useless ones are correlated. I
maintain, as a matter of fact, that the cases in question embrace "a
large proportional number of specific characters." On the other hand, I
am accused of betraying ignorance of species, and of the work of
"species-makers," in advancing this statement; and have been told by Mr.
Wallace, and others of his school, that there is absolutely no evidence
to be derived from nature in support of my views. Well, in the first
place, if this be the case, it is somewhat remarkable that a large body
of competent naturalists, such as Bronn, Broca, Naegeli, Kerner, Sachs,
De Vries, Focke, Henslow, Haeckel, Koelliker, Eimer, Giard, Pascoe,
Mivart, Seebohm, Lloyd Morgan, Dixon, Beddard, Geddes Gulick, and also,
as we shall presently see, Darwin himself, should have fallen into the
same error. And it is further remarkable that the more a man devotes
himself to systematic work in any particular department--whether as an
ornithologist, a conchologist, an entomologist, and so forth--the less
is he disposed to accept the dogma of specific characters as universally
adaptive characters. But, in the second place, and quitting
considerations of mere authority, I appeal to the facts of nature
themselves; and will now proceed, as briefly as possible, to indicate
the result of such an appeal.
For the following reasons, that birds and mammals seem to furnish the
best field for testing the question by direct observation. First, these
classes present many genera which have been more carefully worked out
than is usually the case with genera of invertebrates, or even of
cold-blooded vertebrates. Secondly, they comprise many genera each
including a large number of species, whose habits and conditions of life
are better known than is the case with species belonging to large genera
of other classes. Thirdly, as birds and mammals represent the highest
products of evolution in respect of organization, a more severe test is
imposed than could be imposed elsewhere, when the question is as to the
utility of specific characters; for if these highest products of
organization fail to reveal, in a large proportional number of cases,
the utility of their specific characters, much more is this likely to be
the case among organic beings which stand lower in the scale of
organization, and therefore, _ex hypothesi_, are less elaborate products
of natural selection. Fourthly, and lastly, birds and mammals are the
classes which Mr. Wallace has expressly chosen to constitute his ground
of argument with regard to the issue on which we are now engaged.
It would take far too long to show, even in epitome, the results of this
inquiry. Therefore I will only state the general upshot. Choosing genera
of birds and mammals which contain a large number of species whose
diagnostic characters have been worked out with most completeness, I
restricted the inquiry to specific distinctions of colour, not only for
the sake of having a uniform basis for comparisons, but still more
because it seemed that the argument from our ignorance of possibly
unknown uses could be more successfully met in the case of slight
differences of colour or of shading, than in that of any differences of
structure or of form. Finally, after tabulating all the differences of
colour which are given as diagnostic of each species in a genus, and
placing in one column those which may conceivably be useful, while
placing in another column those of which it appeared inconceivable that
any use could be suggested, I added up the figures in the two columns,
and thus obtained a grand total of all the specific characters of the
genus in respect of colours, separated into the two classes of
conceivably useful and apparently useless. Now, in all cases the
apparently useless characters largely preponderated over the conceivably
useful ones; and therefore I abundantly satisfied myself regarding the
accuracy of my previous statement, that a large proportional number--if
not an actual majority--of specific characters belong to the latter
category.
The following is a brief abstract of these results.
With respect to Birds, a large number of cases were collected wherein
the characters of allied species differ from one another in such minute
respects of colour or shading, that it seemed unreasonable to suppose
them due to any selective value to the birds in question. It is
needless--even if it were practicable on the present occasion--to
adduce this evidence in detail, since an exceedingly good sample of it
may be found in a small book which is specially devoted to considering
the question in its relation to birds. I allude to an essay by Mr.
Charles Dixon, entitled _Evolution without Natural Selection_ (1885). In
this work Mr. Dixon embodies the results of five years' "careful working
at the geographical distribution and variations of plumage of
Palaearctic birds and their allies in various other parts of the world";
and shows, by a large accumulation of facts, not only that there is no
utility to be suggested in reference to the minute or trivial
differences of colouration which he describes; but also that these
differences are usually correlated with isolation on the one hand, or
with slight differences of climate on the other. Now it will be shown
later on that both these agents can be proved, by independent evidence,
capable of inducing changes of specific type without reference to
utility: therefore the correlation which Mr. Dixon unquestionably
establishes between apparently useless (because utterly trivial)
specific distinctions on the one hand, and isolation or climatic change
on the other, constitutes additional evidence to show that the
uselessness is not only apparent, but real. Moreover I have collected a
number of cases where such minute differences of colour between allied
species of birds happen to affect parts of the plumage which are
_concealed_--as for instance, the breast and abdomen of creepers. In
such cases it seems impossible to suggest how natural selection can have
operated, seeing that the parts affected are not exposed to the view
either of enemies or of prey.
Analogous illustrations to any amount may be drawn from Mammals. For
instance, I have worked through the Marsupials with the aid of Mr.
Oldfield Thomas' diagnostic description of their numerous species. Now,
let us take any one of the genera, such as the kangaroos. This comprises
23 species living on an island continent of high antiquity, and not
exposed to the depredations of any existing carnivorous enemies; so that
there is here no present need to vary colour for purposes of protection.
Moreover, in all cases the diagnostic distinctions of colour are so
exceedingly trivial, that even if large carnivora were recently abundant
in Australia, no one could reasonably suggest that the differences in
question would then have been protective. On an average, each of the 23
species presents rather more than 20 peculiarities of shading, which are
quoted as specifically diagnostic. Altogether there are 474 of these
peculiarities distributed pretty evenly among the 23 species; and in no
case can I conceive that utility can be suggested.
* * * * *
Hitherto we have been considering the question of fact, as to whether "a
large proportional number of specific characters" do or do not admit of
having their utility demonstrated, or even so much as plausibly
suggested. In the result, I can only conclude that this question of fact
is really not an open one, seeing that it admits of an abundantly
conclusive answer by any naturalist who will take the trouble to work
through the species of any considerable number of genera in the way
above indicated. But although the question of fact is thus really
closed, there remains a more ultimate question as to its theoretical
interpretation. For, as already pointed out, no matter how great an
accumulation of such facts may be collected, our opponents are always
able to brush them aside by their _a priori_ appeal to the argument from
ignorance. In effect they say--We do not care for any number of
thousands of such facts; it makes no difference to us what "proportional
number" of specific characters fail to show evidence of utility; you are
merely beating the air by adducing them, for we are already persuaded,
on antecedent grounds, that _all_ specific characters _must_ be either
themselves useful, or correlated with others that are, whether or not we
can perceive the utility, or suggest the correlation.
To this question of theoretical interpretation, therefore, we must next
address ourselves. And here, first of all, I should like to point out
how sturdy must be the antecedent conviction of our opponents, if they
are to maintain it in the face of such facts as have just been adduced.
It must be remembered that this antecedent conviction is of a most
uncompromising kind. By its own premisses it is committed to the
doctrine that _all_ specific characters, without a single exception,
_must_ be either useful, vestigial, or correlated. Well, if such be the
case, is it not somewhat astonishing that out of 474 differences of
colour which are distinctive of the 23 species of the genus Macropus, no
single one appears capable of having any utility demonstrated, or indeed
so much as suggested? For even the recent theory that slight differences
of colour, which cannot be conceived as serving any other purpose, may
enable the sexes of the same species quickly to recognize each other, is
not here available. The species of the genus Macropus are more
conspicuously distinguished by differences of size and form than by
these minute differences of colour; and therefore no such use can be
attributed to the latter. And, as previously stated, even within the
order Marsupialia the genus Macropus is not at all exceptional in this
respect; so that by including other genera of the order it would be easy
to gather such apparently indifferent specific characters by the
hundred, without any one of them presenting evidence--or even
suggestion--of utility. How robust therefore is the faith of an _a
priori_ conviction which can stand against such facts as these! What,
then, are the _a priori_ grounds on which it stands? Mr. Wallace, the
great leader of this school of thought, says:--
"It is a necessary deduction from the theory of natural selection,
that none of the definite facts of organic nature, no special
organ, no characteristic form or marking, no peculiarities of
instinct or of habit, no relations between species or between
groups of species, can exist, but which must now be, or once have
been, _useful_ to the individuals or the races which possess
them[89]."
[89] _Contributions to the Theory of Natural Selection_, p. 47
(1870); republished in 1892.
Here, then, we have in brief compass the whole essence of our opponents'
argument. It is confessedly an argument _a priori_, a deduction from the
theory of natural selection, a supposed consequence of that theory which
is alleged to be so necessary that to dispute the consequence is
tantamount to denying the theory from which it is derived. In short, as
before stated, it is a question of theory, not a question of fact: our
difference of opinion is logical, not biological: it depends on our
interpretation of principles, not on our observation of species. It will
therefore be my endeavour to show that the reasoning in question is
fallacious: that it is _not_ a necessary deduction from the theory of
natural selection that no characteristic form or marking, no
peculiarities of instinct or of habit, can exist, but which must now be,
or once have been, useful, or correlated with some other peculiarity
that is useful.
"The tuft of hair on the breast of a wild turkey-cock _cannot be of any
use_, and it is doubtful whether it can be ornamental in the eyes of the
female bird;--indeed, had the tuft appeared under domestication, it
would have been called a monstrosity[90]."
[90] _Origin of Species_, p. 70: italics mine.
As a matter of common sense, unprejudiced by dogma, this appears to be a
perfectly sound judgement; but if Wallace had asked Darwin to prove such
a negative, Darwin could only have replied that it was for Wallace to
prove the affirmative--and thus the issue would have been thrown back
upon a discussion of general principles. Then Wallace would have
said--"The assertion of inutility in the case of any organ or
peculiarity which is not a rudiment or a correlation _is not, and can
never be_, the statement of a fact, but _merely an expression of our
ignorance of its purpose or origin_[91]." Darwin, however, would have
replied:--"Our ignorance of the laws of variation is profound"; and
while, on this account, we ought "to be extremely cautious in
_pretending to decide what structures are now, or have formerly been, of
use to each species_," in point of fact "there can be little doubt that
the tendency to vary in the same manner has _often_ been so strong, that
_all_ individuals of the same _species_ have been similarly modified
_without the aid of any form of selection_[92]."
[91] _Darwinism_, p. 137: italics mine.
[92] _Origin of Species_, p. 72: Mr. Wallace himself quotes this
passage (_Darwinism_, p. 141); but says with regard to it "the
important word 'all' is probably an oversight." In the Appendix
(II), on Darwin's views touching the doctrine of utility I
adduce a number of precisely equivalent passages, derived from
all his different works on evolution, and _every one of them_
presenting "the important word 'all.'"
It will be my endeavour in the following discussion to show that Darwin
would have had an immeasurable advantage in this imaginary debate.
To begin with, Wallace's deductive argument is a clear case of circular
reasoning. We set out by inferring that natural selection is a cause
from numberless cases of observed utility as an effect: yet, when "in a
large proportional number" of cases we fail to perceive any imaginable
utility, it is argued that nevertheless utility must be there, since
otherwise natural selection could not have been the cause.
Be it observed, in any given case we may properly anticipate utility as
_probable_, even where it is not perceived; because there are already so
enormous a number of cases where it is perceived, that, if the principle
of natural selection be accepted at all, we must conclude with Darwin
that it is "the _main_ means of modification." Therefore, in particular
cases of unperceived utility we may take this antecedent probability as
a guide in our biological researches--as has been done with such
brilliant success both by Darwin and Wallace, as well as by many of
their followers. But this is a very different thing from laying down the
universal maxim, that in _all_ cases utility _must_ be present, whether
or not we shall ever be able to detect it[93]. For this universal maxim
amounts to an assumption that natural selection has been the
"_exclusive_ means of modification." That it has been "the main means of
modification" is proved by the generality of the observed facts of
adaptation. That it has been "the exclusive means of modification," with
the result that these facts are universal, cannot be thus proved by
observation. Why, then, is it alleged? Confessedly it is alleged by way
of deduction from the theory of natural selection itself. Or, as above
stated, after having deduced the theory from the facts, it is sought to
deduce the facts from the theory.
[93] See Introductory Chapter, p. 20.
Thus far I have been endeavouring to show that the universality of
adaptation cannot be inferred from its generality, or from the theory of
natural selection itself. But, of course, the case would be quite
different if there were any independent evidence--or rather, let us say,
any logical argument--to show that natural selection is "the exclusive
means of modification." For in this event it would no longer involve
circular reasoning to maintain that all specific characters are likewise
adaptive characters. It might indeed appear antecedently improbable that
no other principle than natural selection can possibly have been
concerned in the differentiation of those relatively permanent varieties
which we call species--that in all the realm of organic nature, and in
all the complexities of living processes, there is no room for any
other influence in the production of change, even of the most trivial
and apparently unmeaning kind. But if there were any good evidence or
logical argument to the contrary, this antecedent presumption would have
to give way; and the certainty that all specific characters are likewise
adaptive characters would be determined by the cogency of such evidence
or argument as could be adduced. In short, we are not entitled to
conclude--and still less does it follow "as a necessary deduction from
the theory of natural selection"--that all the details of specific
differentiation must in every case be either useful, vestigial, or
correlated, _unless it has been previously shown, by independent
evidence, or accurate reasoning, that there is no room for any other
principle of specific change_.
This, apparently, is the central core of the question. Therefore I will
now proceed to consider such arguments as have been adduced to prove
that, other than natural selection, there _can_ have been no "means of
modification." And, after having exhibited the worthlessness of these
arguments, I will devote the next chapter to showing that, as a matter
of observable fact, there _are_ a considerable number of other
principles, which can be proved to be capable of producing such minute
differences of form and colour as "in a large proportional number" of
cases constitute diagnostic distinctions between species and species.
First, then, for the reasons _a priori_--and they are confessedly _a
priori_--which have been adduced to prove that natural selection has
been what in Darwin's opinion it has not been,--"the _exclusive_ means
of modification." Disregarding the Lamarckian factors--which, even if
valid, have but little relation to the present question, seeing that
they are concerned, almost exclusively, with the evolution of _adaptive_
characters--it is alleged that natural selection must occupy the whole
field, because no other principle of change can be allowed to operate in
the presence of natural selection. Now, I fully agree that this
statement may hold as regards any principle of change which is
deleterious; but clearly it does not hold as regards any principle which
is merely neutral. If any one were to allege that specific characters
are frequently detrimental to the species presenting them, he would no
doubt lay himself open to the retort that natural selection could not
allow such characters to persist; or, which amounts to the same thing,
that it _does_ "necessarily follow from the theory of natural selection"
that specific characters can never be in any large number, or in any
large measure, _harmful_ to the species presenting them. But where
the statement is that specific characters are frequently
_indifferent_--again to use Professor Huxley's term--the retort loses
all its relevancy. No reason has ever been shown why natural selection
should interfere with merely indifferent characters, supposing such to
have been produced by any of the agencies which we shall presently have
to consider. Therefore this argument--or rather assertion--goes for
nothing.
The only other argument I have met with on this side of the question is
one that has recently been adduced by Mr. Wallace. He says:--
"One very weighty objection to the theory that _specific_
characters can ever be wholly useless appears to have been
overlooked by those who have maintained the frequency of such
characters, and that is, their almost necessary instability[94]."
[94] _Darwinism_, p. 138.
This argument he proceeds to elaborate at considerable length, but fails
to perceive what appears to me the obvious answer. Provided that the
cause of the useless character is constant, there is no difficulty in
understanding why the character is stable. Utility is not the only
principle that can lead to stability: any other principle must do the
same, provided that it acts for a sufficient length of time, and with a
sufficient degree of uniformity, on all the individuals of a species.
This is a consideration the cogency of which was clearly recognized by
Darwin, as the following quotations will show. Speaking of unadaptive
characters, he says they may arise as merely
"fluctuating variations, which sooner or later become _constant_
through the nature of the organism and of surrounding conditions,
_but not through natural selection_[95]."
[95] _Origin of Species_, p. 176: italics mine, as also in the
following.
Elsewhere we read:--
"Each of the endless variations which we see in the plumage of our
fowls must have had some efficient cause; and if the _same_ cause
were to act _uniformly_ during a long series of generations on
_many_ individuals, _all_ probably would be modified in the same
manner."
As special illustrations of this fact I may quote the following cases
from Darwin's works.
"Dr. Bachman states that he has seen turkeys raised from the eggs
of wild species, lose their metallic tints, and become spotted in
the third generation. Mr. Yarrell many years ago informed me that
the wild ducks bred in St James' Park lost their true plumage
after a few generations. An excellent observer (Mr. Hewitt) ...
found that he could not breed wild ducks true for more than five or
six generations, as they proved so much less beautiful. The white
collar round the neck of the mallard became broader and more
irregular, and white feathers appeared in the duckling's wings
&c.[96]"
[96] _Var._ vol. ii. p. 250.
Now, such cases--to which numberless others might be added--prove that
even the subtle and inconspicuous causes incidental to domestication are
capable of inducing changes of specific character quite as great, and
quite as "stable," as any that in a state of nature are taken to
constitute specific distinctions. Yet there can here be no suggestion of
utility, inasmuch as the change takes place in the course of a few
generations, and therefore without leaving time for natural selection to
come into play--even if it ever could come into play among the sundry
domesticated birds in question.
But the facts of domestication also make for the same conclusion in
another way--namely, by proving that when time enough _has_ been allowed
for the production of useless changes of greater magnitude, such changes
are not infrequently produced. And the value of this line of evidence is
that, great as are the changes, it is impossible that either natural or
artificial selection can have been concerned in their production. It
will be sufficient to give two examples--both with regard to structure.
The first I will render in the words whereby it has already been stated
in my own paper on _Physiological Selection_, because I should like to
take this opportunity of answering Mr. Wallace's objection to it.
"Elsewhere (_Origin of Species_, p. 158) Mr. Darwin points out that
modifications which appear to present obvious utility are often
found on further examination to be really useless. This latter
consideration, therefore, may be said to act as a foil to the one
against which I am arguing, namely, that modifications which appear
to be useless may nevertheless be useful. But here is a still more
suggestive consideration, also derived from Mr. Darwin's writings.
Among our domesticated productions changes of structure--or even
structures wholly new--not unfrequently arise, which are in every
way analogous to the apparently useless distinctions between wild
species. Take, for example, the following most instructive case:--
[Illustration: Fig. 2.--Old Irish Pig, showing jaw-appendages
(after Richardson).]
"'Another curious anomaly is offered by the appendages described by
M. Eudes-Deslongchamps as often characterizing the Normandy pigs.
These appendages are always attached to the same spot, to the
corners of the jaws; they are cylindrical, about three inches in
length, covered with bristles, and with a pencil of bristles rising
out of a sinus on one side; they have a cartilaginous centre with
two small longitudinal muscles; they occur either symmetrically on
both sides of the face, or on one side alone. Richardson figures
them on the gaunt old Irish Greyhound pig; and Nathusius states
that they occasionally appear in all the long-eared races, but are
not strictly inherited, for they occur or fail in the animals of
the same litter. As no wild pigs are known to have analogous
appendages, we have at present no reason to suppose that their
appearance is due to reversion; and if this be so, we are forced to
admit that a somewhat complex, though apparently useless, structure
may be suddenly developed without the aid of selection[97].'"
[97] _Variation_, &c. vol. i. pp. 78-79.
To this case Mr. Wallace objects:--
"But it is expressly stated that they are not constant; they appear
'frequently' or 'occasionally,' they are 'not strictly inherited,
for they occur or fail in animals of the same litter'; and they are
not always symmetrical, sometimes appearing on one side of the face
alone. Now, whatever may be the cause or explanation of these
anomalous appendages, they cannot be classed with 'specific
characters,' the most essential features of which are, that they
_are_ symmetrical, that they _are_ inherited, and that they _are_
constant[98]."
[98] _Darwinism_, pp. 139-40.
But, to begin with, I have not classed these appendages with "specific
characters," nor maintained that Normandy pigs ought to be regarded as
specifically distinct on account of them. What I said was:--
"Now, if any such structure as this occurred in a wild species, and
if any one were to ask what is the use of it, those who rely on the
argument from ignorance would have a much stronger case than they
usually have; for they might point to the cartilage supplied with
muscles, and supporting a curious arrangement of bristles, as much
too specialized a structure to be wholly meaningless. Yet we happen
to know that this particular structure is wholly meaningless[99]."
[99] Mr. Wallace deems the concluding words "rather confident." I
was not, however, before aware that he extended his _a priori_
views on utility to domesticated varieties which are bred for
the slaughter-house. If he now means to indicate that these
appendages are possibly due to natural selection, he is surely
going very far to save his _a priori_ dogma; and in the case
next adduced will have to go further still.
In the next place, is it either fair or reasonable to expect that a
varietal character of presumably very recent origin should be as
strongly inherited--and therefore as constant both in occurrence and
symmetry--as a true specific character, say, of a thousand times its
age? Even characters of so-called "constant varieties" in a state of
nature are usually less constant than specific characters; while, again,
as Darwin says, "it is notorious that specific characters are more
variable than generic,"--the reason in both cases being, as he proceeds
to show, that the less constant characters are characters of more recent
origin, and therefore less firmly fixed by heredity[100]. Hence I do not
understand how Mr. Wallace can conclude, as he does, "that, admitting
that this peculiar appendage is wholly useless and meaningless, the fact
would be rather an argument against specific characters being also
meaningless, because the latter never have the characteristics [i.e.
inconstancy of occurrence, form, and transmission] which this particular
variation possesses[101]." Mr. Wallace can scarcely suppose that when
specific characters first arise, they present the three-fold kind of
constancy to which he here alludes. But, if not, can it be denied that
these peculiar appendages appear to be passing through a phase of
development which all "specific characters" must have passed through,
before they have had time enough to be firmly fixed by heredity[102]?
[100] _Origin of Species_, pp. 122-3.
[101] _Darwinism_, p. 140.
[102] In the next paragraph Mr. Wallace says that the appendages in
question "are apparently of the same nature as the 'sports'
that arise in our domesticated productions, but which, as Mr.
Darwin says, without the aid of selection would soon
disappear." But I cannot find that Mr. Darwin has made any
such statement: what he does say is, that whether or not a
useless peculiarity will soon disappear without the aid of
selection depends upon the nature of the causes which produce
it. If these causes are of a merely transitory nature, the
peculiarity will also be transitory; but if the causes be
constant, so will be the result. Again, the point to be
noticed about this "sport" is, that, unlike what is usually
understood by a "sport," it affects a whole race or breed, is
transmitted by sexual propagation, and has already attained so
definite a size and structure, that it can only be reasonably
accounted for by supposing the continued operation of _some
constant_ cause. This cause can scarcely be correlation of
growth, since closely similar appendages are often seen in so
different an animal as a goat. Here, also, they run in breeds
or strains, are strongly inherited, and more "constant," as
well as more "symmetrical" than they are in pigs. This, at all
events, is the account I have received of them from
goat-breeders in Switzerland.
If, however, even this should be denied, what will be said of the second
case, that of the niata cattle?
"I saw two herds on the northern bank of the Plata.... The forehead
is very short and broad, with the nasal end of the skull, together
with the whole plane of the upper molar-teeth, curved upwards. The
lower jaw projects beyond the upper, and has a corresponding upward
curvature.... The skull which I presented to the College of
Surgeons has been thus described by Professor Owen. 'It is
remarkable from the stunted development of the nasals,
premaxillaries, and fore part of the lower jaw, which is unusually
curved upwards to come into contact with the premaxillaries. The
nasal bones are about one-third the ordinary length, but retain
almost their normal breadth. The triangular vacuity is left between
them and the frontal and lachrymal, which latter bone articulates
with the premaxillary, and thus excludes the maxillary from any
junction with the nasal.' So that even the connexion of some of
the bones is changed. Other differences might be added: thus the
plane of the condyles is somewhat modified, and the terminal edge
of the premaxillaries forms an arch. In fact, on comparison with
the skull of a common ox, scarcely a single bone presents the same
exact shape, and the whole skull has a wonderfully different
appearance[103]."
[103] Darwin, _Variation_, &c., vol. i. pp. 92-4.
[Illustration: Fig. 3.--Drawn from nature. R. Coll. Surg. Mus.]
As I cannot find that this remarkable skull has been figured before, I
have had the accompanying woodcut made in order to compare it with the
skull of a Charsley Forest ox; and a glance is sufficient to show what
"a wonderfully different appearance" it presents.
* * * * *
Now the important points in the present connexion with regard to this
peculiar race of cattle are the following.
Their origin is not known; but it must have been subsequent to the year
1552, when cattle were first introduced to America from Europe, and it
is known that such cattle have been in existence for at least a century.
The breed is very true, and a niata bull and cow invariably produce
niata calves. A niata bull crossed with a common cow, and the reverse
cross, yield offspring having an intermediate character, but with the
niata peculiarities highly conspicuous[104].
[104] _Ibid._ p. 94.
Here, then, we have unquestionable evidence of a whole congeries of very
distinctive characters, so unlike anything that occurs in any other
cattle, that, had they been found in a state of nature, they would have
been regarded as a distinct species. And the highly peculiar characters
which they present conform to all "the most essential features of
specific characters," as these are stated by Mr. Wallace in his
objection to the case of the pig's appendages. That is to say, "they
_are_ symmetrical, they _are_ inherited, and they _are_ constant." In
point of fact, they are _always_ "constant," both as to occurrence and
symmetry, while they are so completely "inherited" that not only does "a
niata bull and cow _invariably_ produce niata calves"; but even when
crossed with other cattle the result is a _hybrid_, "with the niata
character _strongly_ displayed."
Hence, if we were to follow Mr. Wallace's criteria of specific
characters, which show that the pig's appendages "cannot be classed with
specific characters" (or with anything of the nature of specific
characters), it would follow that the niata peculiarities _can_ be so
classed. This, therefore, is a case where he will find all the reasons
which in other cases he takes to justify him in falling back upon the
argument from ignorance. The cattle are half wild, he may urge; and so
the three-fold constancy of their peculiar characters may very well be
due, either directly or indirectly, to natural selection--i.e. they may
either be of some hidden use themselves, or correlated with some other
modifications that are of use: it is, he may say, as in such cases he
often does say, for us to disprove both these possibilities.
Well, here we have one of those rare cases where historical information,
or other accidents, admit of our discharging this burden of proving a
negative. Darwin's further description shows that this customary refuge
in the argument from ignorance is most effectually closed. For--
"When the pasture is tolerably long, these cattle feed as well as
common cattle with their tongue and palate; but during the great
droughts, when so many animals perish on the Pampas, the niata
breed lies under a great disadvantage, and would, if not attended
to, become extinct; for the common cattle, like horses, are able to
keep alive by browsing with their lips on the twigs of trees and on
reeds; this the niatas cannot so well do, as their lips do not
join, and hence they are found to perish before the common cattle.
This strikes me as a good illustration of how little we are able to
judge from the ordinary habits of an animal, on what circumstances,
occurring only at long intervals of time, its rarity or extinction
may depend. It shows us, also, how natural selection would have
determined the rejection of the niata modification, had it arisen
in a state of nature[105]."
[105] Darwin, _Variation_, &c. vol. i. p. 94.
Hence, it is plainly _impossible_ to attribute this modification to
natural selection, either as acting directly on the modified parts
themselves, or indirectly through correlation of growth. And as the
modification is of specific magnitude on the one hand, while it presents
all "the most essential features of specific characters" on the other, I
do not see any means whereby Mr. Wallace can meet it on his _a priori_
principles. It would be useless to answer that these characters,
although conforming to all his tests of specific characters, differ in
respect of being deleterious, and would therefore lead to extermination
were the animals in a wholly wild state; because, considered as an
argument, this would involve the assumption that, apart from natural
selection, only deleterious characters can arise under nature--i. e.
that merely "indifferent" characters can never do so, which would be
absurd. Indeed, I have chosen this case of the niata cattle expressly
because their strongly marked peculiarities _are_ deleterious, and
therefore exclude Mr. Wallace's appeal to the argument from ignorance of
a possible utility. But if even these pronounced and deleterious
peculiarities can arise and be perpetuated with such constancy and
fidelity, much more is this likely to be the case with less pronounced
and merely neutral peculiarities.
It may, however, be further objected that these cattle are not
improbably the result of _artificial_ selection. It may be suggested
that the semi-monstrous breed originated in a single congenital
variation, or "sport," which was isolated and multiplied as a curiosity
by the early settlers. But even if such be the explanation of this
particular case, the fact would not weaken our illustration. On the
contrary, it would strengthen our general argument, by showing an
additional means whereby indifferent specific characters can arise and
become fixed in a state of nature. As it seems to me extremely probable
that the niata cattle did originate in a congenital monstrosity, which
was then isolated and multiplied by human agency (as is known to have
been the case with the "ancon sheep"), I will explain why this tends to
strengthen our general argument.
It is certain that if these animals were ever subject to artificial
isolation for the purpose of establishing their breed, the process must
have ceased a long time ago, seeing that there is no memory or tradition
of its occurrence. Now this proves that, however the breed may have
originated, it has been able to maintain its many and highly peculiar
characters for a number of generations without the help of selection,
either natural or artificial. This is the first point to be clear upon.
Be its origin what it may, we know that this breed has proved capable of
perpetuating itself with uniform "constancy" for a number of generations
after the artificial selection has ceased--supposing such a process ever
to have occurred. And this certain fact that artificial selection, even
if it was originally needed to establish the type, has not been needed
to perpetuate the type, is a full answer to the supposed objection. For,
in view of this fact, it is immaterial what the origin of the niata
breed may have been. In the present connexion, the importance of this
breed consists in its proving the subsequent "stability" of an almost
monstrous form, continued through a long series of generations by the
force of heredity alone, without the aid of any form of selection.
The next point is, that not only is a seeming objection to the
illustration thus removed, but that, if we do entertain the question of
origin, and if we do suppose the origin of these cattle to have been in
a congenital "sport," afterwards multiplied by artificial isolation, we
actually strengthen our general argument by increasing the importance of
this particular illustration. For the illustration then becomes
available to show how indifferent specific characters may sometimes
originate in merely individual sports, which, if not immediately
extinguished by free intercrossing, will perpetuate themselves by the
unaided force of heredity. But this is a point to which we shall recur
in the ensuing chapter.
In conclusion, it is worth while to remark, with regard to Mr. Wallace's
argument from constancy, that, as a matter of fact, utility does not
seem to present any greater power in securing "stability of characters"
than any other cause of like constancy. Thus, for instance, whatever the
causes may have been which have produced and perpetuated the niata breed
of cattle, they have certainly produced a wonderful "stability" of a
great modification in a wonderfully short time. And the same has to be
said of the ducks in St. James' Park, as well as sundry other cases. On
the other hand, when, as in the case of numberless natural species,
modification has been undoubtedly produced by natural selection,
although the modification must have had a very much longer time in which
to have been fixed by heredity, it is often far from being
stable--notwithstanding that Mr. Wallace regards stability as a
criterion of specific characters. Indeed--and this is more suggestive
still--there even seems to be a kind of _inverse_ proportion between the
utility and the stability of a specific character. The explanation
appears to be (_Origin of Species_, pp. 120-2), that the more a specific
character has been forced on by natural selection on account of its
utility, the less time will it have had to become well fixed by heredity
before attaining a full development. Moreover, as Darwin adds, in cases
where the modification has not only been thus "comparatively recent,"
but also "extraordinarily great," the probability is that the parts so
modified must have been very variable in the first instance, and so are
all the more difficult to render constant by heredity. Thus we see that
utility is no better--even if it be so good--a cause of stability in
specific characters, as are the unknown causes of stability in many
varietal characters[106].
[106] Should it be objected that useless characters, according to my
own view of the Cessation of Selection, ought to disappear,
and therefore cannot be constant, the answer is evident. For,
by hypothesis, it is only those useless characters which were
at one time useful that disappear under this principle.
Selection cannot cease unless it was previously present--i.e.
save in cases where the now useless character was originally
due to selection. Hence, in all cases where it was due to any
other cause, the useless character will persist at least as
long as its originating cause continues to operate. And even
after the latter (whatever it may be) has ceased to operate,
the useless character will but slowly degenerate, until the
eventual failure of heredity causes it to disappear _in
toto_--long before which time it may very well have become a
genetic, or some higher, character.
CHAPTER VIII.
CHARACTERS AS ADAPTIVE AND SPECIFIC
(_continued_).
Let us now proceed to indicate some of the causes, other than natural
selection, which may be regarded as adequate to induce such changes in
organic types as are taken by systematists to constitute diagnostic
distinctions between species and species. We will first consider causes
external to organisms, and will then go on to consider those which occur
within the organisms themselves: following, in fact, the classification
which Darwin has himself laid down. For he constantly speaks of such
causes as arising on the one hand, from "changed conditions of life"
and, on the other hand, from "the nature of the organism"--that is, from
internal processes leading to "variations which seem to us in our
ignorance to arise spontaneously."
In neither case will it be practicable to give more than a brief
_resume_ of all that might be said on these interesting topics.
I. _Climate._
There is an overwhelming mass of evidence to prove that the assemblage
of external conditions of life conveniently summarized in the word
Climate, exercise a potent, an uniform, and a permanent influence on
specific characters.
With regard to plants, Darwin adduces a number of facts to show the
effects of climate on wheat, cabbages, and other vegetables. Here, for
example, is what he says with regard to maize imported from America to
Germany:--
"During the first year the plants were twelve feet high, and a few
seeds were perfected; the lower seeds in the ear kept true to their
proper form, but the upper seeds became slightly changed. In the
second generation the plants were from nine to ten feet high, and
ripened their seed better; the depression on the outer side of the
seed had almost disappeared, and the original beautiful white
colour had become duskier. Some of the seeds had even become
yellow, and in their now rounded form they approached the common
European maize. In the third generation nearly all resemblance to
the original and very distinct American parent-form was lost[107]."
[107] _Variation_, &c. vol. i. p. 340.
As these "highly remarkable" changes were effected in but three
generations, it is obvious that they cannot have been dependent on
selection of any kind. The same remark applies to trees. Thus,--
"Mr. Meehan has compared twenty-nine kinds of American trees with
their nearest European allies, all grown in close proximity and
under as nearly as possible the same conditions. In the American
species he finds, with the rarest exceptions, that the leaves fall
earlier in the season, and assume before their fall a brighter
tint; that they are less deeply toothed or serrated; that the buds
are smaller; that the trees are more diffuse in growth and have
fewer branchlets; and, lastly, that the seeds are smaller--all in
comparison with the corresponding European species. Now,
considering that these corresponding trees belong to several
distinct orders, and that they are adapted to widely different
stations, it can hardly be supposed that their differences are of
any special service to them in the New and Old worlds; and, if so,
such differences cannot have been gained through natural selection,
and must be attributed to the long continued action of a different
climate[108]."
[108] _Variation_, &c. vol. ii. p. 271.
These cases, however, I quote mainly in order to show Darwin's opinion
upon the matter, with reference to the absence of natural selection.
For, where the vegetable kingdom is concerned, the fact of climatic
variation is so general, and in its relation to diagnostic work so
important, that it constitutes one of the chief difficulties against
which species-makers have to contend. And the more carefully the subject
is examined the greater does the difficulty become. But, as to this and
other general facts, it will be best to allow a recognized authority to
speak; and therefore I will give a few extracts from Kerner's work on
_Gute und schlechte Arten_.
He begins by showing that geographical (or it may be topographical)
varieties of species are often so divergent, that without a knowledge of
intermediate forms there could be no question as to their being good
species. As a result of his own researches on the subject, he can
scarcely find language strong enough to express his estimate of the
extent and the generality of this source of error. In different parts of
Europe, or even in different parts of the Alps, he has found these
climatic varieties in such multitudes and in such high degrees both of
constancy and divergence, that, after detailing his results, he
finishes his essay with the following remarkable conclusions:--
"Die Wissenchaft geht aber ihren Entwicklungsgang im grossen Ganzen
gerade so, wie die Erkenntniss bei jedem einzelnen Naturforscher.
Fast jeder Botaniker muss seinen Entwicklungsgang durchmachen und
gelangt endlich mehr oder weniger nahe zu demselben Ziele. Die
Ungleichheit besteht nur darin, dass der eine langsamer, der andere
aber rascher bei dem Ziele ankommt. Anfaenglich mueht sich jeder ab,
die Formen in hergebrachter Weise zu gliedern und die 'guten Arten'
herauszulesen. Mit der Erweiterung des Gesichtskreises und mit der
Vermehrung der Anschauungen aber schwindet auch immer mehr der
Boden unter den Fuessen, die bisher fuer unverrueckbar gehaltenen
Grenzen der gut geglaubten Arten stellen sich als eine der Natur
angelegte Zwangsjacke heraus, die Uebcrzeugung, dass die Grenzen,
welche wir ziehen, eben nur kuenstliche sind, gewinnt immer mehr und
mehr die Oberhand, und wer nicht gerade zu den hartgesottenen
Eigensinnigen gehoert, und wer die Wahrheit hoeher stellt als das
starre Festhalten an seinen frueheren Ansichten, geht schliesslich
bewusst oder unbewusst in das Lager derjenigen ueber, in welchem
auch ich mir ein bescheidenes Plaetzchen aufgesucht habe."
By these "hard-boiled" botanists he means those who entertain the
traditional notion of a species as an assemblage of definite
characters, always and everywhere associated together. This notion
(Artsbestaendigkeit) must be entirely abandoned. Summarizing Kerner's
facts for their general results we find that his extensive
investigations have proved that in his numberless kinds of European
plants the following relations frequently obtain. Supposing that there
are two or more allied species, A and B, then A' and B' may be taken to
represent their respective types as found in some particular area. It
does not signify whether A' and B' are geographically remote from, or
close to, A and B; the point is that, whether in respect of temperature,
altitude, moisture, character of soil, &c., there is some difference in
the conditions of life experienced by the plants growing at the
different places. Now, in numberless plants it is found that the typical
or constant peculiarities of A' differ more from those of A than they do
from those of B; while, conversely, the characters of A' may bear more
resemblance to those of B' than they do to those of A--on account of
such characters being due to the same external causes in both cases. The
consequence is that A' might more correctly be classified with B', or
_vice versa_. Another consequence is that whether A and B, or A' and B',
be recorded as the "good species" usually depends upon which has
happened to have been first described.
Such a mere abstract of Kerner's general results, however, can give no
adequate idea of their cogency: for this arises from the number of
species in which specific characters are thus found to change, and even
to _interchange_, with different conditions of life. Thus he gives an
amusing parable of an ardent young botanist, Simplicius, who starts on a
tour in the Tyrol with the works of the most authoritative systematists
to assist him in his study of the flora. The result is that Simplicius
becomes so hopelessly bewildered in his attempts at squaring their
diagnostic descriptions with the facts of nature, that he can only
exclaim in despair--"Sonderbare Flora, diese tirolische, in welcher so
viele characteristische Pflanzen nur schlechte Arten, oder gar noch
schlechter als schlechte Arten, sind." Now, in giving illustrations of
this young man's troubles, Kerner fills five or six pages with little
else than rows of specific names.
Upon the whole, Kerner concludes that the more the subject is studied,
the more convinced must the student become that all distinction between
species as "good" and "bad" vanishes. In other words, the more that our
knowledge of species and of their diagnostic characters increases, the
more do we find that "bad species" multiply at the expense of "good
species"; so that eventually we must relinquish the idea of "good
species" altogether. Or, conversely stated, we must agree to regard as
equally "good species" any and every assemblage of individuals which
present the same peculiarities: provided that these peculiarities do not
rise to a generic value, they equally deserve to be regarded as
"specific characters," no matter how trivial, or how local, they may be.
In fact, he goes so far as to say that when, as a result of experiments
in transplantation from one set of physical conditions to another,
seedlings are found to present any considerable and constant change in
their specific characters, these seedlings are no less entitled to be
regarded as a "good species" than are the plants from which they have
been derived. Probably few systematists will consent to go quite so far
as this; but the fact that Kerner has been led deliberately to propound
such a statement as a result of his wide observations and experiments is
about as good evidence as possible on the points with which we are here
concerned. For even Simplicius would hardly be quite so simple as to
suppose that each one of all the characters which he observes in his
"remarkable flora," so largely composed of "bad or even worse than bad
species," is of utilitarian significance.
Be it noted, however, that I am not now expressing my own opinion. There
are weighty reasons against thus identifying climatic variations with
good species--reasons which will be dealt with in the next chapter.
Kerner does not seem to appreciate the weight of these reasons, and
therefore I do not call him as a witness to the subject as a whole; but
only to that part of it which has to do with the great and general
importance of climatic variability in relation to diagnostic work. And
thus far his testimony is fully corroborated by every other botanist who
has ever attended to the subject. Therefore it does not seem worth while
to quote further authorities in substantiation of this point, such as
Gaertner, De Candolle, Naegeli, Peter, Jordan, &c. For nowadays no one
will dispute the high generality and the frequently great extent of
climatic variation where the vegetable kingdom is concerned. Indeed, it
may fairly be doubted whether there is any one species of plant, whose
distribution exposes it to any considerable differences in its external
conditions of life, which does not present more or less considerable
differences as to its characters in different parts of its range. The
principal causes of such climatic variation appear to be the chemical,
and, still more, the mechanical nature of soil; temperature; intensity
and diurnal duration of light in spring and summer; moisture; presence
of certain salts in the air and soil of marine plants, or of plants
growing near mineral springs; and sundry other circumstances of a more
or less unknown character.
Before closing these remarks on climatic variation in the vegetable
kingdom, prominent attention must be directed to a fact of broad
generality and, in relation to our present subject, of considerable
importance. This is that the same external causes very frequently
produce the same effects in the way of specific change throughout large
numbers of _unrelated_ species--i.e. species belonging to different
genera, families, and orders. Moreover, throughout all these unrelated
species, we can frequently trace a uniform correlation between the
degrees of change and the degrees to which they have been subjected to
the causes in question.
As examples, all botanists who have attended to the subject are struck
by the similarity of variation presented by different species growing on
the same soils, altitudes, latitudes, longitudes, and so forth. Plants
growing on chalky soils, when compared with those growing on richer
soils, are often more thickly covered with down, which is usually of a
white or grey colour. Their leaves are frequently of a bluish-green
tint, more deeply cut, and less veined, while their flowers tend to be
larger and of a lighter tint. There are similarly constant differences
in other respects in varieties growing on sundry other kinds of soils.
Sea-salt has the general effect, on many different kinds of plants, of
producing moist fleshy leaves, and red tints. Experiments in
transplantation have shown that these changes may be induced
artificially; so there can be no doubt as to its being this that and the
other set of external conditions which produces them in nature. Again,
dampness causes leaves to become smoother, greener, less cut, and the
flowers to become darker; while dryness tends to produce opposite
effects. I need not go on to specify the particular results on all kinds
of plants of altitude, latitude, longitude, and so forth. For we are
concerned only with the fact that these two correlations may be regarded
as general laws appertaining to the vegetable kingdom--namely, (A) that
the same external causes produce similar varietal effects in numerous
unallied species of plants; and, (B) that the more these species are
exposed to such causes the greater is the amount of varietal effect
produced--so that, for instance, on travelling from latitude to
latitude, longitude to longitude, altitude to altitude, &c., we may see
greater and greater degrees of such definite and more or less common
varietal changes affecting the unallied species in question. Now these
general laws are of importance for us, because they prove unequivocally
that it is the direct action of external conditions of life which
produce climatic variations of specific types. And, taken in connexion
with the results of experiments in transplantation (which in a single
generation may yield variations similar to those found in nature under
similar circumstances), these general laws still further indicate that
climatic variations are "indifferent" variations. In other words, we
find that changes of specific characters are of widespread occurrence in
the vegetable kingdom, that they are constantly and even proportionally
related to definite external circumstances, but yet that, in as far as
they are climatic, they cannot be attributed to the agency of natural
selection[109].
[109] Since the above paragraphs have been in type, the Rev. G.
Henslow has published his Linnaean Society papers which are
mentioned in the introductory chapter, and which deal in more
detail with this subject, especially as regards the facies of
desert floras.
Turning next to animals, it may first be observed that climatic
conditions do not appear to exercise an influence either so general or
so considerable as in the case of plants. Nevertheless, although these
influences are relatively more effective in the vegetable kingdom than
they are in the animal, absolutely considered they are of high
generality and great importance even in the latter. But as this fact is
so well recognized by all zoologists, it will be needless to give more
than a very few illustrations. Indeed, throughout this discussion on
climatic influences my aim is merely to give the general reader some
idea of their importance in regard to systematic natural history; and,
therefore, such particular cases as are mentioned are selected only as
samples of whole groups of cases more or less similar.
With regard to animals, then, we may best begin by noticing that, just
as in the case of plants, there is good evidence of the same external
causes producing the same effects in multitudes of species belonging to
different genera, families, orders, and even classes. Moreover, we are
not without similarly good evidence of _degrees_ of specific change
taking place in correlation with _degrees_ of climatic change, so that
we may frequently trace a gradual progress of the former as we advance,
say, from one part of a large continent to another. Instances of these
correlations are not indeed so numerous in the animal kingdom as they
are in the vegetable. Nevertheless they are amply sufficient for our
present purposes.
For example, Mr. Allen has studied in detail changes of size and colour
among birds and mammals on the American continent; and he finds a
wonderfully close sliding scale of both, corresponding stage by stage
with gradual changes of climate. Very reasonably he attributes this to
the direct influence of climatic conditions, without reference to
natural selection--as does also Mr. Gould with reference to similar
facts which he has observed among the birds of Australia. Against this
view Mr. Wallace urges, "that the effects are due to the greater or less
need of protection." But it is difficult to believe that such can be the
case where so innumerable a multitude of widely different species are
concerned--presenting so many diverse habits, as well as so many
distinct habitats. Moreover, the explanation seems incompatible with the
_graduated_ nature of the change, and also with the fact that not only
colouration but size, is implicated.
We meet with analogous facts in butterflies. Thus _Lycaena agestis_ not
only presents seasonal variations, (A) and (B); but while (A) and (B)
are respectively the winter and summer forms in Germany, (B) and (C) are
the corresponding forms in Italy. Therefore, (B) is in Germany the
summer form, and in Italy the winter form--the German winter form (A)
being absent in Italy, while the Italian summer form (C) is absent in
Germany. Probably these facts are due to differences of temperature in
the two countries, for experiments have shown that when pupae of sundry
species of moths and butterflies are exposed to different degrees of
temperature, the most wonderful changes of colour may result in the
insects which emerge. The remarkable experiments of Dorfmeister and
Weismann in relation to this subject are well known. More recently Mr.
Merrifield has added to their facts, and concludes that the action of
cold upon the pupae--and also, apparently, upon the larvae--has a
tendency to produce dark hues in the perfect insect[110].
[110] _Trans. Entom. Soc._ 1889, part i. p. 79 _et seq._
But, passing now from such facts of climatic variations over wide areas
to similar facts within small areas, in an important _Memoir on the Cave
Fauna of North America_, published a few years ago by the American
Academy of Sciences, it is stated:--
"As regards change of colour, we do not recall an exception to the
general rule that all cave animals are either colourless or nearly
white, or, as in the case of Arachnida and Insects, much paler than
their out-of-door relatives."
Now, when we remember that these cave faunas comprise representatives of
nearly all classes of the animal kingdom, it becomes difficult, if not
impossible, to imagine that so universal a discharge of colouring can be
due to natural selection. It must be admitted that the only way in which
natural selection could act in this case would be indirectly through the
principle of correlation. There being no light in the caves, it can be
of no advantage to the animals concerned that they should lose their
colour for the sake of protection, or for any other reason of a
similarly direct kind. Therefore, if the loss of colour is to be
ascribed to natural selection, this can only be done by supposing that
natural selection has here acted indirectly through the principle of
correlation. There is evidence to show that elsewhere modification or
loss of colour is in some cases brought about by natural selection, on
account of the original colour being correlated with certain
physiological characters (such as liability to particular diseases,
&c.); so that when natural selection operates directly upon these
physiological characters, it thereby also operates indirectly upon the
correlated colours. But to suppose that this can be the explanation of
the uniform diminution of colour in all inhabitants of dark caves would
be manifestly absurd. If there were only one class of animals in these
caves, such as Insects, it might be possible to surmise that their
change of colour is due to natural selection acting directly upon their
physiological constitutions, and so indirectly upon their colours. But
it would be absurd to suppose that such can be the explanation of the
facts, when these extend in so similar a manner over so many scores of
species belonging to such different types of animal life.
With more plausibility it might be held that the universal discharge of
colour in these cave-faunas is due, not to the presence, but to the
absence of selection--i. e. to the cessation of selection, or panmixia.
But against this--at all events as a full or general explanation--lie
the following facts. First, in the case of Proteus--which has often been
kept for the purposes of exhibition &c., in tanks--the skin becomes dark
when the animal is removed from the cave and kept in the light.
Secondly, deep-sea faunas, though as much exposed as the cave-faunas, to
the condition of darkness, are not by any means invariably colourless.
On the contrary, they frequently present brilliant colouration. Thus it
is evident that if panmixia be suggested in explanation of the
discharge of colouring in cave-faunas, the continuance of colour in
deep-sea faunas appears to show the explanation insufficient. Thirdly,
according to my view of the action of panmixia as previously explained,
no _total_ discharge of colouration is likely to be caused by such
action alone. At most the bleaching as a result of the mere withdrawal
of selection would proceed only to some comparatively small extent.
Fourthly, Mr. Packard in the elaborate _Memoir on Cave Fauna_, already
alluded to, states that in some of the cases the phenomena of bleaching
appear to have been induced within very recent times--if not, indeed,
within the limits of a single generation. Should the evidence in support
of this opinion prove trustworthy, of course in itself it disposes of
any suggestion either of the presence or the absence of natural
selection as concerned in the process.
Nevertheless, I myself think it inevitable that to some extent the
cessation of selection must have helped in discharging the colour of
cave faunas; although for the reasons now given it appears to me that
the main causes of change must have been of that direct order which we
understand by the term climatic.
As regards dogs, the Rev. E. Everest found it impossible to breed Scotch
setters in India true to their type. Even in the second generation no
single young dog resembled its parents either in form or shape. "Their
nostrils were more contracted, their noses more pointed, their size
inferior, and their limbs more slender[111]." Similarly on the coast of
New Guinea, Bosman says that imported breeds of dogs "alter strangely;
their ears grow long and stiff like those of foxes, to which colour they
also incline ... and in three or four broods their barking turns into a
howl[112]."
[111] _Variation_, &c. vol. i. p. 40.
[112] _Variation_, &c. vol. i. p. 40.
Darwin gives numerous facts showing the effects of climate on horses,
cattle, and sheep, in altering, more or less considerably, the
characters of their ancestral stocks. He also gives the following
remarkable case with regard to the rabbit. Early in the fifteenth
century a common rabbit and her young ones were turned out on the island
of Porto Santo, near Madeira. The feral progeny now differ in many
respects from their parent stock. They are only about one-third of the
weight, present many differences in the relative sizes of different
parts, and have greatly changed in colour. In particular, the black on
the upper surface of the tail and tips of the ears, which is so constant
in all other wild rabbits of the world as to be given in most works as a
specific character, has entirely disappeared. Again, "the throat and
certain parts of the under surface, instead of being pure white, are
generally grey or leaden colour," while the upper surface of the whole
body is redder than in the common rabbit. Now, what answer have our
opponents to make to such a case as this? Presumably they will answer
that the case simply proves the action of natural selection during the
best part of 400 years on an isolated section of a species. Although we
cannot say of what use all these changes have been to the rabbits
presenting them, nevertheless we _must_ believe that they have been
produced by natural selection, and therefore _must_ present some hidden
use to the isolated colony of rabbits thus peculiarly situated. Four
centuries is long enough to admit of natural selection effecting all
these changes in the case of so rapidly breeding an animal as the
rabbit, and therefore it is needless to look further for any explanation
of the facts. Such, I say, is presumably the answer that would be given
by the upholders of natural selection as the only possible cause of
specific change. But now, in this particular case it so happens that the
answer admits of being conclusively negatived, by showing that the great
assumption on which it reposes is demonstrably false. For Darwin
examined two living specimens of these rabbits which had recently been
sent from Porto Santo to the Zoological Gardens, and found them coloured
as just described. Four years afterwards the dead body of one of them
was sent to him, and then he found that the following changes had taken
place. "The ears were plainly edged, and the upper surface of the tail
was covered with blackish-grey fur, and the whole body was much less
red; so that under the English climate this individual rabbit has
recovered the proper colour of its fur in rather less than four years!"
Mr. Darwin adds:--
"If the history of these Porto Santo rabbits had not been known,
most naturalists, on observing their much reduced size, their
colour, reddish above and grey beneath, their tails and ears not
tipped with black, would have ranked them as a distinct species.
They would have been strongly confirmed in this view by seeing them
alive in the Zoological Gardens, and hearing that they refused to
couple with other rabbits. Yet this rabbit, which there can be
little doubt would thus have been ranked as a distinct species, as
certainly originated since the year 1420[113]."
[113] _Variation_, &c. vol. i. p. 120.
Moreover, it certainly originated as a direct result of climatic
influences, independent of natural selection; seeing that, as soon as
individual members of this apparently new species were restored to their
original climate, they recovered their original colouration.
As previously remarked, it is, from the nature of the case, an
exceedingly difficult thing to prove in any given instance that natural
selection has not been the cause of specific change, and so finally to
disprove the assumption that it must have been. Here, however, on
account of historical information, we have a crucial test of the
validity of this assumption, just as we had in the case of the niata
cattle; and, just as in their case, the result is definitely and
conclusively to overturn the assumption. If these changes in the Porto
Santo rabbits had been due to the gradual influence of natural selection
guided by inscrutable utility, it is simply impossible that the same
individual animals, in the course of their own individual life-times,
should revert to the specific characters of their ancestral stock on
being returned to the conditions of their ancestral climate. Therefore,
unless any naturalist is prepared to contradict Darwin's statement that
the changes in question amount to changes of specific magnitude, he can
find no escape from the conclusion that distinctions of specific
importance may be brought about by changes of habitat alone, without
reference to utility, and therefore independently of natural selection.
II. _Food._
Although, as yet, little is definitely known on the subject, there can
be no doubt that in the case of many animals differences of food induce
differences of colour within the life-time of individuals, and therefore
independently of natural selection.
Thus, sundry definite varieties of the butterfly _Euprepia caja_ can be
reared according to the different nourishment which is supplied to the
caterpillar; and other butterflies are also known on whose colouring and
markings the food of the caterpillar has great influence[114].
[114] See especially, Koch, _Die Raupen und Schmetterling der
Wetterau_, and _Die Schmetterling des Suedwestlichen
Deutschlands_, whose very remarkable results of numerous and
varied experiments are epitomized by Eimer, _Organic
Evolution_, Eng. Trans. pp. 147-153; also Poulton, _Trans.
Entom. Soc._ 1893.
Again, I may mention the remarkable case communicated to Darwin by
Moritz Wagner, of a species of _Saturnia_, some pupae of which were
transported from Texas to Switzerland in 1870. The moths which emerged
in the following year were like the normal type in Texas. Their young
were supplied with leaves of _Juglans regia_, instead of their natural
food, _J. nigra_; and the moths into which these caterpillars changed
were so different from their parents, both in form and colour, "that
they were reckoned by entomologists as a distinct species[115]."
[115] Mivart, _On Truth_, p. 378.
With regard to mollusks, M. Costa tells us that English oysters, when
turned down in the Mediterranean, "_rapidly_ became like the true
Mediterranean oyster, altered their manner of growth, and formed
prominent diverging rays." This is most probably due to some change of
food. So likewise may be the even more remarkable case of _Helix
nemoralis_, which was introduced from Europe to Virginia a few years
ago. Under the new conditions it varied to such an extent that up to
last year no less than 125 varieties had been discovered. Of these 67,
or more than half, are new--that is, unknown in the native continent of
the species[116].
[116] Cockerell, _Nature_, vol. xli. p. 393.
In the case of Birds, the Brazilian parrot _Chrysotis festiva_ changes
the green in its feathers to red or yellow, if fed on the fat of certain
fishes; and the Indian Lori has its splendid colouring preserved by a
peculiar kind of food (Wallace). The Bullfinch is well known to turn
black when fed on hemp seeds, and the Canary to become red when fed on
cayenne pepper (Darwin). Starting from these facts, Dr. Sauermann has
recently investigated the subject experimentally; and finds that not
only finches, but likewise other birds, such as fowls, and pigeons, are
subject to similar variations of colour when fed on cayenne pepper; but
in all cases the effect is produced only if the pepper is given to the
young birds before their first moult. Moreover, he finds that a moist
atmosphere facilitates the change of colour, and that the ruddy hue is
discharged under the influence either of sunlight or of cold. Lastly, he
has observed that sundry other materials such as glycerine and aniline
dyes, produce the same results; so there can be no doubt that organic
compounds probably occur in nature which are capable of directly
affecting the colours of plumage when eaten by birds. Therefore the
presence of such materials in the food-stuffs of birds occupying
different areas may very well in many cases determine differences of
colouration, which are constant or stable so long as the conditions of
their production are maintained.
III. _Sexual Selection._
Passing on now to causes of specific change which are internal, or
comprised within the organisms themselves, we may first consider the
case of Sexual Selection.
Mr. Wallace rejects the theory of sexual selection _in toto_, and
therefore nothing that can be said under this head would be held by him
to be relevant. Many naturalists, however, believe that Darwin was right
in the large generalization which he published under this title; and in
so far as any one holds that sexual selection is a true cause of
specific modification, he is obliged to believe that innumerable
specific characters--especially in birds and mammals--have been produced
without reference to utility (other, of course, than utility for sexual
purposes), and therefore without reference to natural selection. This is
so obvious that I need not pause to dilate upon it. One remark, however,
may be useful. Mr. Wallace is able to make a much more effective use of
his argument from "necessary instability" when he brings it against the
Darwinian doctrine of sexual selection, than he does when he brings it
against the equally Darwinian doctrine of specific characters in general
not being all necessarily due to natural selection. In the latter case,
it will be remembered, he is easily met by showing that the causes of
specific change other than natural selection, such as food, climate,
&c., may be quite as general, persistent, and uniform, as natural
selection itself; and therefore in this connexion Mr. Wallace's argument
falls to the ground. But the argument is much more formidable as he
brings it to bear against the theory of sexual selection. Here he asks,
What is there to guarantee the uniformity and the constancy of feminine
taste with regard to small matters of embellishment through thousands of
generations, and among animals living on extensive areas? And, as we
have seen in Part 1, it is not easy to supply an answer. Therefore this
argument from the "necessary instability of character" is of
immeasurably greater force as thus applied against Darwin's doctrine of
sexual selection, than it is when brought against his doctrine that all
specific characters need not necessarily be due to natural selection.
Therefore, also, if any one feels disposed to attach the smallest degree
of value to this argument in the latter case, consistency will require
him to allow that in the former case it is simply overwhelming, or in
itself destructive of the whole theory of sexual selection. And,
conversely, if his belief in the theory of sexual selection can survive
collision with this objection from instability, he ought not to feel any
tremor of contact when the objection is brought to bear against his
scepticism regarding the alleged utility of all specific characters. For
assuredly no specific character which is apparent to our eyes can be
supposed to be so refined and complex (and therefore so presumably
inconstant and unstable), as are those minute changes of cerebral
structure on which a psychological preference for all the refined
shadings and many pigments of a complicated pattern must be held
ultimately to depend. For this reason, then, as well as for those
previously adduced, if any one agrees with Darwin in holding to the
theory of sexual selection notwithstanding this objection from the
necessary instability of unuseful embellishments, _a fortiori_ he ought
to disregard the objection altogether in its relation to useless
specific characters of other kinds.
But quite apart from this consideration, which Mr. Wallace and his
followers may very properly say does not apply to them, let us see what
they themselves have made of the facts of secondary sexual
characters--which, of course, are for the most part specific
characters--in relation to the doctrine of utility.
Mr. Wallace himself, in his last work, quotes approvingly a letter which
he received in 1869 from the Rev. O Pickard-Cambridge, as follows:--
"I myself doubt that particular application of the Darwinian theory
which attributes male peculiarities of form, structure, colour, and
ornament to female appetency or predilection. There is, it seems to
me, undoubtedly something in the male organization of a special and
sexual nature, which, of its own vital force, develops the
remarkable male peculiarities so commonly seen, _and of no
imaginable use to that sex_. In as far as these peculiarities show
a great vital power, they point out to us the finest and strongest
individuals of the sex, and show us which of them would most
certainly appropriate to themselves the best and greatest number of
females, and leave behind them the strongest and greatest number of
progeny. And here would come in, as it appears to me, the proper
application of Darwin's theory of Natural Selection; _for the
possessors of greatest vital power being those most frequently
produced and reproduced, the external signs of it would go on
developing in an ever increasing exaggeration_, only to be checked
where it became really detrimental in some respect or other to the
individual[117]."
[117] _Darwinism_, pp.[typo: period missing in scan] 296-7: italics
mine.
Here then the idea is, as more fully expressed by Mr. Wallace in the
context, that all the innumerable, frequently considerable, and
generally elaborate "peculiarities of form, structure, colour, and
ornament," which Darwin attributed to sexual selection, are really due
to "the laws of growth." Diverse, definite, and constant though these
specific peculiarities be, they are all but the accidental or
adventitious accompaniments of "vigour," or "vital power," due to
natural selection. Now, without waiting to dispute this view, which has
already been dealt with in the chapter on Sexual Selection in Part I, it
necessarily follows that "a large proportional number of specific
characters," which, while presenting "no imaginable use," are very much
less remarkable, less considerable, less elaborate, &c., must likewise
be due to this "correlation with vital power." But if the principle of
correlation is to be extended in this vague and general manner, it
appears to me that the difference between Mr. Wallace and myself, with
respect to the principle of utility, is abolished. For of course no one
will dispute that the prime condition to the occurrence of "specific
characters," whether useful or useless, is the existence of some form
which has been denominated a "species" to present them; and this is
merely another way of saying that such characters cannot arise except in
correlation with a general fitness due to natural selection. Or, to put
the case in Mr. Wallace's own words--"This development [of useless
specific characters] will necessarily proceed by the agency of natural
selection [as a necessary condition] _and the general laws which
determine the production of colour and of ornamental appendages_." The
case, therefore, is just the same as if one were to say, for example,
that all the ailments of animals and plants proceed from correlation
with life (as a necessary condition), "and the general laws which
determine the production" of ill-health, or of specific disease. In
short, the word "correlation" is here used in a totally different sense
from that in which it is used by Darwin, and in which it is elsewhere
used by Wallace for the purpose of sustaining his doctrine of specific
characters as necessarily useful. To say that a useless character A is
correlated with a useful one B, is a very different thing from saying
that A is "correlated with vital power," or with the general conditions
to the existence of the species to which it belongs. So far as the
present discussion is concerned, no exception need be taken to the
latter statement. For it simply surrenders the doctrine against which I
am contending.
IV. _Isolation._
It is the opinion of many naturalists who are well entitled to have an
opinion upon the subject, that, in the words of Mr. Dixon, "Isolation
can preserve a non-beneficial as effectually as natural selection can
preserve a beneficial variation[118]." The ground on which this doctrine
rests is thus clearly set forth by Mr. Gulick:--"The fundamental cause
of this seems to lie in the fact that no two portions of a species
possess exactly the same average characters; and, therefore, that the
initial differences are for ever reacting on the environment and on each
other in such a way as to ensure increasing divergence in each
generation, as long as the individuals of the two groups are kept from
intergenerating[119]." In other words, as soon as a portion of a species
is separated from the rest of that species, so that breeding between the
two portions is no longer possible, the general average of characters in
the separated portion not being in all respects precisely the same as it
is in the other portion, the result of in-breeding among all individuals
of the separated portion will eventually be different from that which
obtains in the other portion; so that, after a number of generations,
the separated portion may become a distinct species from the effect of
isolation alone. Even without the aid of isolation, any original
difference of average characters may become, as it were, magnified in
successive generations, provided that the divergence is not harmful to
the individuals presenting it, and that it occurs in a sufficient
proportional number of individuals not to be immediately swamped by
intercrossing. For, as Mr. Murphy has pointed out, in accordance with
Delboeuf's law, "if, in any species, a number of individuals, bearing a
ratio not infinitely small to the entire number of births, are in every
generation born with a particular variation which is neither beneficial
nor injurious, and if it be not counteracted by reversion, then the
proportion of the new variety to the original form will increase till it
approaches indefinitely near to equality[120]." Now even Mr. Wallace
himself allows that this must be the case; and thinks that in these
considerations we may find an explanation of the existence of certain
definite varieties, such as the melanic form of the jaguar, the brindled
or ring-eyed guillemot, &c. But, on the other hand, he thinks that such
varieties must always be unstable, and continually produced in varying
proportions from the parent forms. We need not, however, wait to dispute
this arbitrary assumption, because we can see that it fails, even as an
assumption, in all cases where the superadded influence of isolation is
concerned. Here there is nothing to intercept the original tendency to
divergent evolution, which arises directly out of the initially
different average of qualities presented by the isolated section of the
species, as compared with the rest of that species[121].
[118] _Nature_, vol. xxxiii. p. 100.
[119] _Divergent Evolution through Cumulative Segregation_, Linn.
Journ. Zoology, vol. xx. p. 215.
[120] _Habit and Intelligence_, p. 241.
[121] Allusion may here again be made to the case of the niata
cattle. For here is a case where a very extreme variety is
certainly not unstable, nor produced in varying proportions
from the parent form. Moreover, as we have seen in the
preceding chapter, this almost monstrous variety most probably
originated as an individual sport--being afterwards maintained
and multiplied for a time by artificial selection. Now,
whether or not this was the case, we can very well see that it
may have been. Hence it will serve to illustrate another
possibility touching the origin and maintenance of useless
specific characters. For what is to prevent an individual
congenital variation of any kind (provided it be not harmful)
from perpetuating itself as a "varietal," and eventually,
should offspring become sufficiently numerous, a "specific
character"? There is nothing to prevent this, save panmixia,
or the presence of free intercrossing. But, as we shall see in
the next division of this treatise, there are in nature many
forms of isolation. Hence, as often as a small number of
individuals may have experienced isolation in any of its
forms, opportunity for perpetuation will have been given to
any congenital variations which may happen to arise. Should
any of these be pronounced variations, it would afterwards be
ranked as a specific character. I do not myself think that
this is the way in which indifferent specific characters
_usually_ originate. On the contrary, I believe that their
origin is most frequently due to the influence of isolation on
the average characters of the whole population, as briefly
stated in the text. But here it seems worth while to notice
this possibility of their occasionally arising as merely
individual variations, afterwards perpetuated by any of the
numerous isolating conditions which occur in nature. For, if
this can be the case with a varietal form so extreme as to
border on the monstrous, much more can it be so with such
minute differences as frequently go to constitute specific
distinctions. It is the business of species-makers to search
out such distinctions, no matter how trivial, and to record
them as "specific characters." Consequently, wherever in
nature a congenital variation happens to arise, and to be
perpetuated by the force of heredity alone under any of the
numerous forms of isolation which occur in nature, there will
be a case analogous to that of the niata cattle.
As we shall have to consider the important principle of isolation more
fully on a subsequent occasion, I need not deal with it in the present
connexion, further than to remark that in this principle we have what
appears to me a full and adequate condition to the rise and continuance
of specific characters which need not necessarily be adaptive
characters. And, when we come to consider the facts of isolation more
closely, we shall find superabundant evidence of this having actually
been the case.
V. _Laws of Growth._
Under this general term Darwin included the operation of all unknown
causes internal to organisms leading to modifications of form or
structure--such modifications, therefore, appearing to arise, as he says
"spontaneously," or without reference to utility. That he attributed no
small importance to the operation of these principles is evident from
the last edition of the _Origin of Species_. But as these "laws of
growth" refer to causes confessedly unknown, I will not occupy space by
discussing this division of our subject--further than to observe that,
as we shall subsequently see, many of the facts which fall under it are
so irreconcilably adverse to the Wallacean doctrine of specific
characters as universally adaptive, that in the face of them Mr. Wallace
himself appears at times to abandon his doctrine _in toto_.
CHAPTER IX.
CHARACTERS AS ADAPTIVE AND SPECIFIC
(_continued_).
It must have appeared strange that hitherto I should have failed to
distinguish between "true species" and merely "climatic varieties." But
it will conduce to clearness of discussion if we consider our subject
point by point. Therefore, having now given a fair statement of the
facts of climatic variation, I propose to deal with their theoretical
implications--especially as regards the distinction which naturalists
are in the habit of drawing between them and so-called true species.
First of all, then, what is this distinction? Take, for example, the
case of the Porto Santo rabbits. To almost every naturalist who reads
what has been said touching these animals, it will have appeared that
the connexion in which they are adduced is wholly irrelevant to the
question in debate. For, it will be said that the very fact of the
seemingly specific differentiation of these animals having proved to be
illusory when some of them were restored to their ancestral conditions,
is proof that their peculiar characters are not specific characters; but
only what Mr. Wallace would term "individual characters," or variations
that are not _inherited_. And the same remark applies to all the other
cases which have been adduced to show the generality and extent of
climatic variation, both in other animals and also in plants. Why, then,
it will be asked, commit the absurdity of adducing such cases in the
present discussion? Is it not self-evident that however general, or
however considerable, such merely individual, or non-heritable,
variations may be, they cannot possibly have ever had anything to do
with the origin of _species_? Therefore, is it not simply preposterous
to so much as mention them in relation to the question touching the
utility of specific characters?
Well, whether or not it is absurd and preposterous to consider climatic
variations in connexion with the origin of species, will depend, and
depend exclusively, on what it is that we are to understand by a
species. Hitherto I have assumed, for the sake of argument, that we all
know what is meant by a species. But the time has now come for showing
that such is far from being the case. And as it would be clearly absurd
and preposterous to conclude anything with regard to specific characters
before agreeing upon what we mean by a character as specific, I will
begin by giving all the logically possible definitions of a species.
1. _A group of individuals descended by way of natural generation from
an originally and specially created type._
This definition may be taken as virtually obsolete.
2. _A group of individuals which, while fully fertile_ inter se, _are
sterile with all other individuals--or, at any rate, do not generate
fully fertile hybrids._
This purely physiological definition is not nowadays entertained by any
naturalist. Even though the physiological distinction be allowed to
count for something in otherwise doubtful cases, no systematist would
constitute a species on such grounds alone. Therefore we need not
concern ourselves with this definition, further than to observe that it
is often taken as more or less supplementary to each of the following
definitions.
3. _A group of individuals which, however many characters they share
with other individuals, agree in presenting one or more characters of a
peculiar kind, with some certain degree of distinctness._
In this we have the definition which is practically followed by all
naturalists at the present time. But, as we shall presently see more
fully, it is an extremely lax definition. For it is impossible to
determine, by any fixed and general rule, what degree of distinctness on
the part of peculiar characters is to be taken as a uniform standard of
specific separation. So long as naturalists believed in special
creation, they could feel that by following this definition (3) they
were at any rate doing their best to tabulate very real distinctions in
nature--viz. between types as originally produced by a supernatural
cause, and as subsequently more or less modified (i.e. within the limits
imposed by the test of cross-fertility) by natural causes. But
evolutionists are unable to hold any belief in such real distinctions,
being confessedly aware that all distinctions between species and
varieties are purely artificial. So to speak, they well know that it is
they themselves who create species, by determining round what degrees of
differentiation their diagnostic boundaries shall be drawn. And, seeing
that these degrees of differentiation so frequently shade into one
another by indistinguishable stages (or, rather, that they _always_ do
so, unless intermediate varieties have perished), modern naturalists are
well awake to the impossibility of securing any approach to a uniform
standard of specific distinction. On this account many of them feel a
pressing need for some firmer definition of a species than this
one--which, in point of fact, scarcely deserves to be regarded as a
definition at all, seeing that it does not formulate any definite
criterion of specific distinctness, but leaves every man to follow his
own standards of discrimination. Now, as far as I can see, there are
only two definitions of a species which will yield to evolutionists the
steady and uniform criterion required. These two definitions are as
follows.
4. _A group of individuals which, however many characters they share
with other individuals, agree in presenting one or more characters of a
peculiar and hereditary kind, with some certain degree of distinctness._
It will be observed that this definition is exactly the same as the last
one, save in the addition of the words "and hereditary." But, it is
needless to say, the addition of these words is of the highest
importance, inasmuch as it supplies exactly that objective and rigid
criterion of specific distinctness which the preceding definition lacks.
It immediately gets rid of the otherwise hopeless wrangling over species
as "good" and "bad," or "true" and "climatic," of which (as we have
seen) Kerner's essay is such a remarkable outcome. Therefore
evolutionists have more and more grown to lay stress on the hereditary
character of such peculiarities as they select for diagnostic features
of specific distinctness. Indeed it is not too much to say that, at the
present time, evolutionists in general recognize this character as,
theoretically, indispensable to the constitution of a species. But it is
likewise not too much to say that, practically, no one of our systematic
naturalists has hitherto concerned himself with this matter. At all
events, I do not know of any who has ever taken the trouble to ascertain
by experiment, with regard to any of the species which he has
constituted, whether the peculiar characters on which his diagnoses have
been founded are, or are not, hereditary. Doubtless the labour of
constituting (or, still more, of _re_-constituting) species on such a
basis of experimental inquiry would be insuperable; while, even if it
could be accomplished, would prove undesirable, on account of the chaos
it would produce in our specific nomenclature. But, all the same, we
must remember that this nomenclature as we now have it--and, therefore,
the partitioning of species as we have now made them--has no reference
to the criterion of heredity. Our system of distinguishing between
species and varieties is not based upon the definition which we are now
considering, but upon that which we last considered--frequently coupled,
to some undefinable extent, with No. 2.
5. There is, however, yet another and closer definition, which may be
suggested by the ultra-Darwinian school, who maintain the doctrine of
natural selection as the only possible cause of the origin of species,
namely:--
_A group of individuals which, however many characters they share with
other individuals, agree in presenting one or more characters of a
peculiar, hereditary, and adaptive kind, with some certain degree of
distinctness._
Of course this definition rests upon the dogma of utility as a necessary
attribute of characters _qua_ specific--i.e. the dogma against which the
whole of the present discussion is directed. Therefore all I need say
with reference to it is, that at any rate it cannot be adduced in any
argument where the validity of its basal dogma is in question. For it
would be a mere begging of this question to argue that every species
must present at least one peculiar and adaptive character, because,
according to definition, unless an organic type does present at least
one such character, it is not a specific type. Moreover, and quite apart
from this, it is to be hoped that naturalists as a body will never
consent to base their diagnostic work on what at best must always be a
highly speculative extension of the Darwinian theory. While, lastly, if
they were to do so with any sort of consistency, the precise adaptation
which each peculiar character subserves, and which because of this
adaptation is constituted a character of specific distinction, would
have to be determined by actual observation. For no criterion of
specific distinction could be more vague and mischievous than this one,
if it were to be applied on grounds of mere inference that such and such
a character, because seemingly constant, must "necessarily" be either
useful, vestigial, or correlated.
Such then, as far as I can see, are all the definitions of a species
that are logically possible[122]. Which of them is chosen by those who
maintain the necessary usefulness of all specific characters? Observe,
it is for those who maintain this doctrine to choose their definition:
it is not for me to do so. My contention is, that the term does not
admit of any definition sufficiently close and constant to serve as a
basis for the doctrine in question--and this for the simple reason that
species-makers have never agreed among themselves upon any criterion of
specific distinction. My opponents, on the other hand, are clearly bound
to take an opposite view, because, unless they suppose that there is
some such definition of a species, they would be self-convicted of the
absurdity of maintaining a great generalization on a confessedly
untenable basis. For example, a few years ago I was allowed to raise a
debate in the Biological Section of the British Association on the
question to which the present chapters are devoted. But the debate ended
as I had anticipated that it must end. No one of the naturalists present
could give even the vaguest definition of what was meant by a
species--or, consequently, of a character as specific. On this account
the debate ended in as complete a destruction as was possible of the
doctrine that all the distinctive characters of every species must
necessarily be useful, vestigial, or correlated. For it became
unquestionable that the same generalization admitted of being made, with
the same degree of effect, touching all the distinctive characters of
every "snark."
[122] It is almost needless to say that by a definition as "logical"
is meant one which, while including all the differentiae of
the thing defined, excludes any qualities which that thing may
share in common with any other thing. But by definitions as
"logically possible" I mean the number of separate definitions
which admit of being correctly given of the same thing from
different points of view. Thus, for instance, in the present
case, since the above has been in type the late M.
Quatrefages' posthumous work on _Darwin et ses Precurseurs
Francais_ has been published, and gives a long list of
definitions of the term "species" which from time to time have
been enunciated by as many naturalists of the highest standing
as such (pp. 186-187). But while none of these twenty or more
definitions is logical in the sense just defined, they all
present one or other of the differentiae given by those in the
text.