ORGANIC EVOLUTION.
It is difficult to realize the wealth, the variety, the diversity, of
"animal life." Even if we endeavour to pass in review all that we have
seen in woodland and meadow, in pond or pool, in the air, on the earth,
in the waters, in temperate or tropical regions; even when we try to
remember the results of all anatomical and microscopic investigation
displaying new wonders and new diversities hidden from ordinary and
unaided vision; even when we call to mind the multifarious contents,
recent and fossil, of all the natural history museums we have ever
visited, and throw in such mental pictures as we have formed of all the
diverse adaptations we have read about or heard described;--even so we
cannot but be conscious that not one-tenth, not one-hundredth, part of
the diversity and variety of animal life has passed before our mental
vision even in sample. It is said that our greatest living poet once,
when a young man, left his companions to gaze into the waters of a
clear, still pool. "What an imagination God has!" he said, as he
rejoined his friends. Fit observation for the poet, whose sensitive
nature must be keenly alive to the varied endowments which Nature has
lavishly showered upon her animate children.
Certain it is that words, mere words, can never present, though they may
aid in recalling, an adequate picture of either the wealth or the beauty
of animal life. Fortunately for those who visit London (and who nowadays
does not?), we have, in our national collection in South Kensington, the
means of getting some insight into the wealth of life. And much is being
done there to aid the imagination and to facilitate study for those who
are not professed students. Many of the birds are now to be seen set in
their natural surroundings, with their life-history illustrated. Our
frontispiece is taken from one of these cases. And this admirable system
will, no doubt, so far as space permits, be extended; and, perhaps,
dramatic incidents may be introduced, like those (notably in the life of
heron and hawk) which form so marked a feature in the little museum at
Exeter. Anything which leads us to understand the life of animals, and
to go forth and study it for ourselves, has an educational value.
In our National Museum, again, much is being wisely done to illustrate
the diversity and variety of structure and the principles that underlie
them. Observe, as you enter the central hall, the case containing
stuffed specimens of ruffs (_Machetes pugnax_). Among the young autumn
birds there is not much difference between males and females, the male
being distinguished chiefly by its somewhat larger size. Nor do the old
birds, male and female, differ much during the winter months. But in
pairing-time, May and June, the females are somewhat richer in colour;
while the males not only don the ruff to which the bird owes its popular
name, but develop striking colour-tints. Among different individuals it
will be seen that the colour-variation is tolerably wide; but the same
individual keeps strictly, we are told, in successive seasons, to the
same summer dress. Note, next, in a bay to the right, the great variety
of form, ornamentation, and colouring among the molluscan shells there
exhibited. Observe that the rich colours are often hidden during life by
the dull epidermis. Half an hour's attentive study of these varied
molluscan forms will give a better idea of the beauty and diversity of
these life-products than pages of mere description.
Pass on, too, to note, in a further bay to the right, the extraordinary
modifications of the antenna, or feeler, in insects. There is the long,
whip-like form in the locust; the clubbed whip in the ant-lion and the
butterfly; the feathered form in certain moths and flies; the hooked
form characteristic of the sphinx-moths; the many-leaf form in the
lamellicorn beetles, like the cockchafer; and the feathered plate of
other beetles. Equally wonderful are the diverse developments of the
mouth-organs of insects, the spiral tube of the butterfly or moth, the
strong jaws of the great beetles, the lancets of the gnat, the
sucking-disc of the fly,--all of them special modifications of the same
set of structures. Then, in the same bay, note some of the striking
differences between the males and females of certain insects. In some
there is an extraordinary difference in size (e.g. the locust
_Xiphocera_, and the moth _Attacus_); in others, like the stag-beetle,
it is the size of the jaws that distinguishes the males; in others,
again, the most notable differences are in the length, development, or
complexity of the antennæ, or feelers; in some beetles the males have
great horns on the head or thorax; while in many butterflies it is in
richness of colour that the difference chiefly lies--the brilliant green
of the _Ornithoptera_ there exhibited contrasting strongly with the
sober brown of his larger mate.
The fact that the special characteristics of the male, which we have
seen to be variable in the ruff, are also variable among insects, is
well exemplified in the case of the stag-beetle, in some males of which
the mandibles are far larger than in others. This is shown in Fig. 22,
which is copied from the series displayed in the British Museum, by the
kind permission of Professor Flower.
[Illustration: Fig. 22.--Variations in the size of, and especially in
the head and mandibles of, the male stag-beetle (_Lucanus cervus_).
(From an exhibit in the British Natural History Museum.)]
Crossing the hall to where the vertebrate structures are displayed, the
development of hair, of feathers, of teeth, the modifications of the
skull and of legs, wings, and fins are being exemplified. Note here and
elsewhere the special adaptations of structure, of which we may select
two examples. The first is that seen in the _Balistes_, or trigger-fish.
The anterior dorsal fin is reduced to three spines, of which that which
lies in front is a specially modified weapon of defence, while that
which follows it is the so-called trigger. These two are so hinged to
the underlying interspinous bones and so related to each other that,
when once the defensive spine in front is erected, it cannot be forced
down until the trigger is lowered. The second example of special
adaptation is well displayed in specimens of the mud-tortoise _Trionyx_.
Between the last vertebra of the neck and the first fixed vertebra of
the dorsal series is a beautiful hinge-joint, enabling the neck to be
bent back, S-fashion, when the creature withdraws its head within the
carapace. These are only one or two particular instances of what any one
who will visit the National Museum may see for himself admirably
displayed and illustrated.
No one can, one would suppose, pass through the galleries in Cromwell
Road and remain quite insensible to the beauties of animal life. Beauty
of form and beauty of colour are conspicuously combined in many species
of birds and insects. And much of this colour-beauty and splendid
iridescence is known to be due to minute scales, to thin films of air or
fluid, and to microscopically fine lines developed upon scales or
feathers. But there is one phase of beauty which cannot be exhibited in
the museum--the beauty that comes of life as opposed to death. For this
we must go out into the free air of nature, where the animals not only
have lived, but are still instinct with the glow of life, and where the
silence of the museum galleries is replaced by the song of birds and the
hum of insect-wings.
How have this wealth, this diversity, this beauty, this manifold
activity, which we summarize under the term "animal life," been
produced?
If we answer this question in a word--the word "evolution"[CI]--we must
remember that this word merely expresses our belief in a general fact;
and we must not forget that many questions remain behind, all centering
round that little question, to which an adequate answer is so difficult
to give, the question--How? Reduced to its simplest expression, the
doctrine of evolution merely states that the animal world as it exists
to-day is naturally developed out of the animal world as it existed
yesterday, and will in turn develop into the animal world as it shall
exist to-morrow. This is the central belief of the evolutionist. No
matter what moment in the past history of life you select, the life at
that moment was in the act of insensibly passing from the previous
towards a future condition. Then at once arises the question--Does life
remain the same yesterday, to-day, and to-morrow? A thousand indubitable
facts at once make answer--No! Underlying the law of continuity there is
a law of change. Life to-day is not what it was yesterday, nor will it
be to-morrow the same as to-day. What, then, is the nature of this
change? If it be replied that the change must be either for the better
or the worse, we shall have to answer the further question--Better or
worse in what respects?
Let us narrow our view from the contemplation of life as a whole to the
more particular consideration of an organism as one of its constituent
units. The individual life of that organism depends on (some would say
consists in) its ceaseless adaptation to surrounding circumstances. The
circumstances remaining the same, or only varying within constant
limits, the adaptation may be _more_ or _less_ perfect. A change in the
direction of more perfect adaptation will be a change for the better, a
tendency to less perfect adaptation will be a change for the worse.
But the relation of an organism to its circumstances or environment is
itself subject to change. The environment itself may alter, or the
organism may be brought into relation with a new environment. We have to
consider not only the changes in an organism in the direction of more or
less perfect adaptation to its environment, but also changes in the
environment. These changes are in the direction of increased simplicity
or of increased complexity. So that we may say that the modification of
life is in the direction of more or of less complete adaptation to
simpler or to more complex conditions. Where the adaptation advances to
more complex conditions, we speak of elaboration; where it retrogrades
to less complex conditions, we speak of degeneration; but both fall
under the head of evolution in its more general sense. Viewed as a
whole, there can be little doubt that the general tendency of evolution
is towards more complete adaptation to more diverse and complex
environment. And this tendency is accompanied by a general increase of
differentiation and of integration; of differentiation whereby the
constituent elements of life, whether cells, tissues, organs, organisms,
or groups of organisms, become progressively more specialized and more
different from one another; of integration whereby these elements become
progressively more interdependent one on the other. We may conveniently
sum up the tendency towards more perfect adaptation to more complex
circumstances in the word _progress_; the tendency to differentiation in
the word _individuality_; and the tendency to integration in the word
_association_.
Nobody now doubts the propositions thus briefly summarized, and it is
therefore unnecessary to bring forward evidence in their favour.
We may pass, then, to the question--How? Evolution being continuity,
associated with change, tending in certain directions, and accompanied
by certain processes, how has it been effected? What are its methods?
_Natural Selection._
Natural selection claims a foremost place. We have already devoted a
chapter to its consideration. Animals vary; more are born than can
survive to procreate their kind; hence a struggle for existence, in
which the weaker and less adapted are eliminated, the stronger and
better adapted surviving to continue the race.
It is scarcely possible to over-estimate what Darwin's labour and genius
have done for the study of animal life. Through Darwin's informing
spirit, biology has become a science. But now we must be on our guard.
So long as natural selection was winning its way to acceptance, every
application of the theory had to be made with caution, and was subjected
to keen, if sometimes ignorant, criticism. Now there is, perhaps, some
danger lest it should suffer the Nemesis of triumphant creeds, and be
used blindly as a magic formula.
First, we should be careful not to use the phrase, "of advantage to the
species," vaguely and indefinitely, but should in all cases endeavour
clearly to indicate wherein lies the particular advantage, and how its
possession enables the organism to escape elimination; next, we must
remember that the advantage must be immediate and present, prospective
advantage being, of course, inoperative; then we must endeavour to show
that the advantage is really sufficient to decide the question of
elimination or non-elimination; lastly, we must distinguish between
indiscriminate and differential destruction, between mere numerical
reduction by death or otherwise and selective elimination.
(1) In illustration of the first point, we may select a passage from the
writings of even so great a biologist as Professor Weismann. As is well
known, Professor Weismann believes that senility and death are no part
of the natural heritage of animal life, but have been introduced among
the metazoa on utilitarian grounds. In his earlier papers, he attributed
the introduction of death, and the tissue-degeneration that precedes it,
to the direct action of natural selection.[CJ] More lately, he
attributes it to the cessation of selection.[CK] Concerning this later
view, we shall have somewhat to say presently; we may now consider the
former as an example of too indefinite a use of such phrases as "of
advantage to the species." "Worn-out individuals," says Professor
Weismann, "are not only valueless to the species, but they are even
harmful, for they take the places of those which are sound. Hence, by
the operation of natural selection, the life of our hypothetically
immortal individual would be shortened by the amount which was useless
to the species. It would be reduced to a length which would afford the
most favourable conditions of existence of as large a number as possible
of vigorous individuals at the same time." This may be so, but, as it
stands, the _modus operandi_ is not given, and is not obvious. We start
with a hypothetically immortal metazoon. Barring accidents, it will go
on existing indefinitely. But you cannot bar accidents for an indefinite
time; hence, the longer the individual lives, the more defective and
crippled it becomes. There is neither natural decay nor natural death
here. The organism is gradually crippled through accident and injury.
But the crippled individuals are harmful to the species, because they
take the places of those which are sound. Therefore, says Professor
Weismann, natural decay and death step in to take them off before they
have time to become cripples. Now, the point I wish to notice is that
there is no definite statement how or why natural decrepitude should
thus be introduced. We must remember that it is not until a late stage
in evolution that, through the association of its members, groups of
organisms compete with other groups. In the earlier stages, when we must
suppose decrepitude and death to arise on Professor Weismann's
hypothesis, the law of the struggle for existence is--each for himself
against all. The question, therefore, is--What advantage _to the
individual_ is there in natural decay and death to enable it, through
the possession of these attributes, to escape elimination? Surely none
as such. At the same time, it is quite conceivable that natural decay
and death may be the penalty the individual has to pay for increased
strength and vitality in the early stages of life. This, probably, was
Professor Weismann's meaning. But, if so, it would surely have been
better to state the matter in such a way as to lay the chief stress on
the really important feature, and to say that, through natural
selection, those individuals have survived which exhibited predominant
strength and vitality for a shortened period, even at the expense of
natural decay and death. The increased life-power, not the seeds of
decay and death, was that which natural selection picked out for
survival, or rather that which elimination allowed to survive.
In such ways--a short life with heightened activity being of advantage
to some forms, a more prolonged existence at a lower level of vitality
being essential to others--natural selection may have determined in some
degree the relative longevity of different organisms. That it caused the
introduction of senility as a preparation for death is a less tenable
hypothesis.
And here we may note, in passing, that in using the phrase, "of
advantage to the race or species," we must steadily bear in mind the
fact that it is with _individuals_ that the process of elimination
deals. In the individual it is that every modification must make good
its claim to existence and transmission. Where the principle of
association for mutual benefit obtains, as in the case of social
insects, it is still the individual that must resist elimination.
Self-sacrifice, whether conscious or unconscious, must not be carried so
far as to lead to the elimination of the self-sacrificing individual,
for in this event it cannot but defeat its own ends. Within these
limits, self-sacrifice is of advantage, as in the case of parental
self-sacrifice, in that it enables certain other individuals to escape
elimination. We should endeavour, then, not to use the phrase, "of
advantage to the species," vaguely and indefinitely, but to indicate in
what particular ways certain individuals are to be so advantaged as to
escape the Nemesis of elimination.
(2) The second point that I mentioned above scarcely needs
exemplification. That the advantage which enables an organism to escape
elimination must be present and existent, not merely prospective, is
obvious. Still, the mistake is sometimes made. I have heard it stated
that feathers were evolved for the sake of flight. But clearly, unless
the wing sprang into existence already sufficiently developed for
flight, this would be impossible. The same is true of the first stages
of many structures which could not be of service for the purpose and use
to which they were subsequently turned. Not impossibly, the earliest
"wings" were for diving, and flight was, so to speak, an after-thought.
Undoubtedly, structures which have been fostered under the wing of one
form of advantage have been subsequently applied to new purposes, and
fostered through new modes of adaptation. Teeth, for example, are
probably modified scales, such as are found in the thorn-back skate. But
the early development of these scales could have had no reference to
their future application to purposes subservient to alimentation.
Again, such and such a structure is sometimes spoken of as a "prevision
against emergencies." In his interesting and valuable work on "The
Colours of Animals," for example, Mr. E. B. Poulton says, "Dimorphism
[in the larvæ of butterflies and moths] is also valuable in another way:
the widening range of a species may carry it into countries in which one
of its forms may be especially well concealed, while in other countries
the other form may be more protected. Thus a dimorphic form is more
fully provided against emergencies than one with only a single form."
And after giving, as an example, the fact that the convolvulus hawk-moth
has a browner and a greener form of caterpillar, of which the browner is
more prevalent under European conditions, and the greener under those
which obtain in the Canary Islands, Mr. Poulton adds, "This result
appears to have been brought about by the ordinary operation of natural
selection, leading to the extermination of the less-protected variety."
Now, I do not mean for one moment to imply that so careful and able a
naturalist as Mr. Poulton believes that any character has been evolved
through natural selection in prevision for future emergencies. But I do
think that his statement is open to this criticism.
(3) It is sometimes said, in bold metaphor, that natural selection is
constantly on the watch to select any modification, however slight,
which is of advantage to the species. And it is true that elimination is
ceaselessly operative. But it is equally certain that the advantage must
be of sufficient value to decide the question whether its possessor
should be eliminated or should escape elimination. If it does not reach
this value, Natural Selection, watch she never so carefully, can make no
use of it. Elimination need not, however, be to the death; exclusion
from any share in continuing the species is sufficient. To breed or not
to breed, that is the question. Any advantage affecting this essential
life-function will at once catch the eye of a vigilant natural
selection. But it must be of sufficient magnitude for the machinery of
natural selection to deal with. That machinery is the elimination of a
certain proportion of the individuals which are born. Which shall be
eliminated, and which shall survive, depends entirely on the way in
which the individuals themselves come out in life's competitive
examination. The manner in which that examination is conducted is often
rude and coarse, too rough-and-ready to weigh minute and infinitesimal
advantages.
What must be the value of a favourable or advantageous modification to
decide the question of elimination, to make it an _available advantage_,
must remain a matter of conjecture. It will vary with the nature and the
pressure of the eliminative process. And perhaps it is scarcely too much
to say that, at present, we have not observational grounds on which to
base a reliable estimate in a single instance. We must not let our
conviction of its truth and justice blind us to the fact that natural
selection is a logical inference rather than a matter of direct
observation. A hundred are born, and two survive; the ninety-eight are
eliminated in the struggle for existence; we may therefore infer that
the two escaped elimination in virtue of their possession of certain
advantageous characters. There is no flaw in the logic that has thus
convinced the world that natural selection is a factor in evolution. But
by what percentage of elimination-marks the second of the two successful
candidates beats the senior on the list of failures we do not know. We
can only see that, on the hypothesis of natural selection, it must have
been sufficiently appreciable to determine success or failure.
(4) And then, to come to our fourth point, we must remember that, apart
from the differentiating process of elimination, there is much
fortuitous destruction. A hundred are born, and but two survive. But of
the ninety-eight which die, and fail to procreate, how many are
eliminated, how many are fortuitously destroyed, we do not find it easy
to say. And indiscriminate destruction gets rid of good, bad, and
indifferent alike. It is a mistake to say that of the hundred born the
two survivors are necessarily the very best of the lot. It is quite
possible that indiscriminate destruction got rid of ninety of all sorts,
and left only ten subject to the action of a true elimination. "In the
majority of birds," says Professor Weismann, "the egg, as soon as it is
laid, becomes exposed to the attacks of enemies; martens and weasels,
cats and owls, buzzards and crows, are all on the look out for it. At a
later period, the same enemies destroy numbers of the helpless young,
and in winter many succumb in the struggle against cold and hunger, or
to the numerous dangers which attend migration over land and
sea--dangers which decimate the young birds." There is here, first, a
certain amount of fortuitous destruction; secondly, some selection
applied to the eggs; thirdly, a selection among the very young
nestlings; and, fourthly, a selection among the young migratory birds.
What may be the proportion of elimination to destruction at each stage
it is difficult to say. Among the eggs and fry of fishes fortuitous
destruction probably very far outbalances the truly differentiating
process.
_Panmixia and Disuse._
We may now pass on to consider shortly some of the phenomena of
degeneration, and the dwindling or disappearance of structures which are
no longer of use.
Many zoologists believe, or until lately have believed, that disuse is
itself a factor in the process. Just as the well-exercised muscle is
strengthened, so is the neglected muscle rendered weak and flabby. Until
recently it was generally held that the effects of such use or disuse
are inherited. But now Professor Weismann has taught us, if not to doubt
ourselves, at least to admit that doubt is permissible. On the older
view, the gradual dwindling of unused parts was readily comprehensible.
But now, if Professor Weismann is right, we must seek another
explanation of the facts; and, in any case, we may be led to recognize
other factors (than that of disuse alone) in the process.
Professor Weismann regards panmixia, or free intercrossing, when the
preserving influence of natural selection is suspended, as the efficient
cause of a reduction or deterioration in the organ concerned. And Mr.
Romanes had, in England, drawn attention to the fact that the "cessation
of natural selection" would lead to some dwindling of the organ
concerned, since it was no longer kept up to standard. In illustration
of his panmixia, Professor Weismann says, "A goose or duck must possess
strong powers of flight in the natural state, but such powers are no
longer necessary for obtaining food when it is brought into the
poultry-yard, so that a rigid selection of individuals with
well-developed wings at once ceases among its descendants. Hence, in the
course of generations, a deterioration of the organs of flight must
necessarily ensue, and the other members and organs of the bird will be
sensibly affected."[CL] And, again, "As at each stage of retrogressive
transformation individual fluctuations always occur, a continued decline
from the original degree of development will inevitably, although very
slowly, take place, until the last remnant finally disappears."[CM] Now,
I think it can be shown that panmixia, or the cessation of selection,
alone cannot affect much reduction. It can only affect a reduction from
the "survival-mean" to the "birth-mean." This was referred to in the
chapter on "Heredity and the Origin of Variations," but may be again
indicated. Suppose the number of births among wild ducks be represented
by the number nine, of which six are eliminated through imperfections in
the organs of flight. Let us place the nine in order of merit in this
respect, as is done in the table on p. 172. The average wing-power of
the nine will be found in No. 5, there being four ducks with superior
wing-power (1-4), and four with inferior wing-power (6-9). The
birth-mean will therefore be at the level of No. 5, as indicated to the
left of the table. But if six ducks with the poorest wings be
eliminated, only three survive. The average wing-power will now be found
in No. 2, one duck being superior and one inferior to it in this
respect. It is clear that this survival-mean is at a level of higher
excellence than the birth-mean. Now, when the ducks are placed in a
poultry-yard, selection in the matter of flight ceases, and, since all
nine ducks survive, the survival-mean drops to the birth-mean. We may
variously estimate this retrogression; but it cannot be a large
percentage--I should suppose, in the case under consideration, one or
two per cent. at most. But Professor Weismann says, "A _continued_
decline from the original degree of development must inevitably take
place." It is not evident why such decline should continue. If
variations continue in the same proportion as before, the birth-mean
will be preserved, since there are as many positive or favourable
variations above the mean as there are negative or unfavourable
variations below the mean. A continuous decline must result from a
preponderance of negative over positive variations, and for this some
other principle, such as atavism, or reversion to ancestral characters,
must be called in. But in the case of so long-established and stable an
organ as that of flight, fixed and rendered constant through so many
generations, it is hardly probable that reversion would be an important
factor. Mr. Galton has calculated that among human-folk the offspring
inherits one-fourth from each parent, one-sixteenth from each
grandparent, leaving one-fourth to be contributed by more remote
ancestors. There is no doubt, however, that among domesticated animals
reversion occurs to characters which have been lost for many
generations. But we should probably have to go a very long way back in
the ancestry of wild ducks for any marked diminution in wing-power. It
must be remembered that, in the case of the artificial selection of
domesticated animals, man has been working against and not with the
stream of ancestral tendency. Reversion in their case is towards a
standard which was long maintained and had become normal before man's
interference. Reversion in domesticated ducks should therefore be
towards the greater wing-power of their normal ancestry before
domestication, not in the direction of lessened wing-power and
diminished wing-structure. The whole question of reversion is full of
interest, and needs further investigation.
In the dwindling of disused structures, Mr. Romanes has suggested
"failure of heredity" as an efficient cause. I find it difficult,
however, to distinguish this failure of heredity from the effects of
disuse. To what other cause is the failure of heredity due? If natural
selection has intervened to hasten this failure, this can only be
because the failure is advantageous, since it permits the growth-force
to be applied more advantageously elsewhere. And this involves a
different principle. Even so it is difficult to exclude the possibility
(to put it no stronger) that the diversion of growth-force from a less
useful to a more useful organ is in part due to the use of the one and
the disuse of the other. But of disuse Mr. Romanes says, "There is the
gravest possible doubt lying against the supposition that any really
inherited decrease is due to the inherited effects of disuse." We may
fairly ask Mr. Romanes, therefore, to explain to what cause the failure
of heredity is due. In any case, Professor Weismann and his school are
not likely to accept this failure of heredity as an efficient factor in
the process. Nor is Professor Weismann likely to fall back upon any
innate tendency to degeneration. Unless, therefore, some cause be shown
why the negative variations should be prepotent over the positive
variations, we must, I think, allow that unaided panmixia cannot affect
any great amount of reduction.
In this connection we may notice Professor Weismann's newer view of the
introduction of bodily mortality. He says, "The problem is very easily
solved if we seek assistance from the principle of panmixia. As soon as
natural selection ceases to operate upon any character, structural or
functional, it begins to disappear. As soon, therefore, as the
immortality of somatic [body-] cells became useless, they would begin to
lose this attribute."[CN] Even granting that panmixia could continuously
reduce the size of ducks' wings, it is not easy to see how it could get
rid of immortality. The essence of the idea of panmixia is that, when
the natural selection which has raised an organ to a high functional
level, and sustains it there, ceases or is suspended, the organ drops
back from its high level. But on Professor Weismann's hypothesis,
immortality has neither been produced nor is it sustained by natural
selection. How, therefore, the cessation of selection can cause the
disappearance of immortality--a character with which natural selection
has had nothing whatever to do--Professor Weismann does not explain. He
seems to be using "panmixia" in the same vague way that, in his previous
explanation, he used "natural selection."
If panmixia alone cannot, to any very large extent, reduce an organ no
longer sustained by natural selection, to what efficient cause are we to
look? Mr. Romanes has drawn attention to the reversal of selection as
distinguished from its mere cessation. When an organ is being improved
or sustained by selection, elimination weeds out all those which have
the organ in an ill-developed form. Under a reversal of selection,
elimination will weed out all those which possess the organ well
developed. In burrowing animals, the eyes may have been reduced in size,
or even buried beneath the skin, through a reversal of selection. The
tuco-tuco (_Ctenomys_), a burrowing rodent of South America, is
frequently blind. One which Darwin kept alive was in this condition, the
immediate cause being inflammation of the nictitating membrane. "As
frequent inflammation of the eyes," says Darwin, "must be injurious to
any animal, and as eyes are certainly not necessary to animals having
subterranean habits, a reduction in their size, with the adhesion of the
eyelids and growth of fur over them, might in such cases be an
advantage; and, if so, natural selection would aid the effect of
disuse."[CO] Granting that the inflammation of the eyes is a sufficient
disadvantage to lead to elimination, such cases may be assigned to the
effects of a reversal of selection.
Perhaps the best instances of the reversal of selection are to be found
in the insects of wind-swept islands, in which, as we have already seen
(p. 81), the power of flight has been gradually reduced or even done
away with. Such instances are, however, exceptional. And one can hardly
suppose that such reversal of selection can be very far-reaching in its
effects, at least, through any direct disadvantage from the presence of
the organ. One can hardly suppose that the presence of an eye in a
cave-dwelling fish[CP] could be of such direct disadvantage as to lead
to the elimination of those members which still possess this structure.
But may it not be of indirect disadvantage? May not this structure be
absorbing nutriment which would be more advantageously utilized
elsewhere? This is Darwin's principle of economy. Granting its
occurrence, is it effective? We may put the matter in this way: The
crustacea which have been swept into a dark cave may be divided into
three classes so far as fortuitous variations of eyes and antennæ are
concerned. First, those which preserve eyes and antennæ in the original
absolute and relative proportion and value; secondly, those in which,
while the eyes remain the same, the antennæ are longer and more
sensitive; thirdly, those in which, while the antennæ are longer and
more sensitive, the eyes are reduced in size and elaboration. According
to the principle of economy, the third class have sufficient advantage
over the first and second to enable them to survive and escape the
elimination which removes those with fully developed eyes. It may be so.
We cannot estimate the available advantage with sufficient accuracy to
deny it. But we may fairly suppose that, in general, it is only where
the useless organ in question is of relatively large size, and where
nutriment is deficient, that economy of growth is an important factor.
We may here note the case of the hermit crab as one which exemplifies
degeneration through the reversal of natural selection. This animal, as
is well known, adopts an empty whelk-shell or other gasteropod shell as
its own. The hinder part of the body which is thus thrust into the shell
loses its protective armour, and is quite soft. Professor Weismann seems
to regard this loss of the hardened cuticle as due entirely to panmixia.
If what has been urged above has weight, this explanation cannot be
correct. No amount of promiscuous interbreeding of crabs could reduce
the cuticle to a level indefinitely below that of any of the
interbreeding individuals. But it is clear that an armour-sheathed
"tail" would be exceedingly ill adapted to thrusting into a whelk-shell.
Hence there would, by natural selection, be an adaptation to new needs,
involving not the higher development of cuticle, but the reverse. So far
as the cuticle is concerned, it is a case of reversed selection. Whether
this reversal alone will adequately account for the facts is another
matter.
Mr. Herbert Spencer has made a number of observations and measurements
of the jaws of pet dogs, which lead him to conclude that there has been
a reduction in size and muscular power due to disuse. The creatures
being fed on sops, have no need to use to any large extent the
jaw-muscles. In this case, he argues, the principle of economy is not
likely to be operative, since the pampered pet habitually overeats, and
has therefore abundant nutriment and to spare to keep up the jaws. It is
possible, however, that artificial selection has here been a factor.
There may have been a competition among the old ladies who keep such
pets to secure the dear little dog that never bites, while the nasty
little wretch that does occasionally use his jaws for illegitimate
purposes may have been speedily eliminated. Pet dogs are, moreover, a
pampered, degenerate, and for the most part unhealthy race, often
deteriorated by continued in-breeding, so that we must not build too
much on Mr. Spencer's observations, interesting as they undoubtedly are.
There is one feature about the reduction of organs which must not be
lost sight of. They are very apt to persist for a long time as remnants
or vestiges. The pineal gland is the vestigial remnant of a structure
connected with the primitive, median, or pineal eye. The whalebone
whales and the duck-bill platypus have teeth which never cut the gum and
are of no functional value. With regard to these, it may be asked--If
disuse leads to the reduction of unused structures, how comes it that it
has not altogether swept away these quite valueless structures? In
considering this point, we must notice the unfortunate and misleading
way in which disuse is spoken of as if it were a positive determinant,
instead of the mere absence of free and full and healthy exercise. Few
will question the fact that in the individual, if an organ is to be kept
up to its full standard of perfection, it must be healthily and
moderately exercised; and that, if not so exercised, it will not only
cease to increase in size, but will tend to degenerate. The healthy,
functionally valuable tissue passes into the condition of degenerate,
comparatively useless tissue. Now, those who hold that the inheritance
of functional modifications is still a tenable hypothesis, carry on into
the history of the race that which they find to hold good in the history
of the individual. They believe that, in the race, the continued
functional activity of an organ is necessary for the maintenance of the
integrity and perfection of its structure, and that, if not so
exercised, the organ will inevitably tend to dwindle to embryonic
proportions and to degenerate. The healthy, functionally valuable tissue
passes at last into the condition of degenerate, comparatively useless
tissue. The force of heredity will long lead to the production in the
embryo of the structure which, in the ancestral days of healthy
exercise, was to be of service to the organism. At this stage of life
the conditions have not changed. The degeneration sets in at that period
when the ancestral use is persistently denied. There is no reason why
"disuse" should in all cases remove all remnants of a structure; but if
the presence of the degenerate tissue is a source of danger to the
organism which possesses it, that organism will be eliminated, and those
(1) which possess it in an inert, harmless form, or (2) in which it is
absent, will survive. Thus natural selection (which will fall under Mr.
Romanes's reversed selection) will step in--will in some cases reduce
the organ to a harmless and degenerate rudiment, and in others remove
the last vestiges of the organ.
On the whole, even taking into consideration the effects of panmixia, of
reversed selection, and of the principle of economy, the reduction of
organs is difficult to explain, unless we call into play "disuse" as a
co-operating factor.
_Sexual Selection, or Preferential Mating._
It is well known that, in addition to and apart from the primary sexual
differences in animals, there are certain secondary characters by which
the males, or occasionally the females, are conspicuous. The antlers of
stags, the tail of the peacock, the splendid plumes of the male bird of
paradise, the horns or pouches of lizards, the brilliant frilled crest
of the newt, the gay colours of male sticklebacks, the metallic hues of
male butterflies, and the large horns or antennæ of other
insects,--these and many other examples which will at once occur to the
reader are illustrations of the fact.
As a contribution towards the explanation of this order of phenomena,
Darwin brought forward his hypothesis of sexual selection, of which
there are two modes. In the first place, the males struggle together for
their mates; in this struggle the weakest are eliminated; those
possessed of the most efficient weapons of offence and defence escape
elimination. In the second place, the females are represented as
exercising individual choice, and selecting (in the true sense of the
word) those mates whose bright colours, clear voices, or general
strength and vigour render them most pleasing and attractive. For this
mode I shall employ the term "preferential mating." Combining these two
in his summary, Darwin says, "It has been shown that the largest number
of vigorous offspring will be reared from the pairing of the strongest
and best-formed males, victorious in contests over other males, with the
most vigorous and best-nourished females, which are the first to breed
in the spring. If such females select the more attractive and, at the
same time, vigorous males, they will rear a larger number of offspring
than the retarded females, which must pair with the less vigorous and
less attractive males. So it will be if the more vigorous males select
the more attractive and, at the same time, healthy and vigorous females;
and this will especially hold good if the male defends the female, and
aids in providing food for the young. The advantage thus gained by the
more vigorous pairs in rearing a larger number of offspring has
apparently sufficed to render sexual selection efficient."[CQ]
With regard to the first of the two modes, little need be said. There
can be no question that there are both elimination by battle and
elimination by competition in the struggle for mates. It is well known
that the emperor moth discovers his mate by his keen sense of smell
residing probably in the large, branching antennæ. There can be little
doubt that, if an individual is deficient in this sense, or
misinterprets the direction in which the virgin female lies, he will be
unsuccessful in the competition for mates; he will be eliminated from
procreation. And it is a familiar observation of the poultry-yard that
the law of battle soon determines which among the cock birds shall
procreate their kind. The law of battle for mates is, indeed, an
established fact among many animals, especially those which are
polygamous, and the elimination of the unfit in this respect is a
logical necessity.
It is when we come to the second of the two modes, that which involves
selection proper, that we find differences of opinion among naturalists.
Darwin, as we have seen, suggested that those secondary sexual
characters which can be of no value in aiding their possessor to escape
elimination by combat result from the preferential choice of the female,
the female herself remaining comparatively unaffected. But Mr. Wallace
made an exceedingly valuable suggestion with regard to these
comparatively dull colours of the female. He pointed out that
conspicuousness (unless, as we have seen, accompanied by some protective
character, such as a sting or a bitter taste) increased the risk of
elimination by enemies. Now, the males, since they are generally the
stronger, more active, and more pugnacious, could better afford to run
this risk than their mates. They could to some extent take care of
themselves. Moreover, when impregnation was once effected, the male's
business in procreation was over. Not so the female; she had to bear the
young or to lay the eggs, often to foster or nourish her offspring. Not
only were her risks greater, but they extended over a far longer period
of time. Hence, according to Mr. Wallace, the dull tints of the females,
as compared with those of the males, are due to natural selection
eliminating the conspicuous females in far greater proportion than the
gaudy males.
There is clearly no reason why this view should not be combined with
Darwin's; preferential mating being one factor, natural elimination
being another factor; both being operative at the same time, and each
contributing to that marked differentiation of male and female which we
find to prevail in certain classes of the animal kingdom.
But Mr. Wallace will not accept this compromise. He rejects preferential
mating altogether, or, in any case, denies that through its agency
secondary sexual characters have been developed. He admits, of course,
the striking and beautiful nature of some of these characters; he admits
that the male in courtship takes elaborate pains to display all his
finery before his would-be mate; he admits that the "female birds may be
charmed or excited by the fine display of plumage by the males;" but he
concludes that "there is no proof whatever that slight differences in
that display have any effect in determining their choice of a
partner."[CR]
How, then, does Mr. Wallace himself suppose that these secondary sexual
characters have arisen? His answer is that "ornament is the natural
outcome and direct product of superabundant health and vigour," and is
"due to the general laws of growth and development."[CS] At which one
rubs one's eyes and looks to the title-page to see that Mr. Wallace's
name is really there, and not that of Professor Mivart or the Duke of
Argyll. For, if the plumage of the argus pheasant and the bird of
paradise is due to the general laws of growth and development, why not
the whole animal? If Darwin's sexual selection is to be thus superseded,
why not Messrs. Darwin and Wallace's natural selection?
Must we not confess that Mr. Wallace, for whose genius I have the
profoundest admiration, has here allowed himself to confound together
the question of origin and the question of guidance or direction?
Natural selection by elimination and sexual selection through
preferential mating are, supposing them to be _veræ causæ_, guiding or
selecting agencies. Given the variations, however caused, these agencies
will deal with them, eliminating some, selecting others, with the
ultimate result that those specially fitted for their place in nature
will survive. Neither the one nor the other deals with the origin of
variations. That is a wholly different matter, and constitutes the
leading biological problem of our day. Mr. Wallace's suggestion is one
which concerns the origin of variations, and as such is worthy of
careful consideration. It does not touch the question of their guidance
into certain channels or the maintenance of specific standards.
Concerning this Mr. Wallace is silent or confesses ignorance. "Why, in
allied species," he says, "the development of accessory plumes has taken
different forms, we are unable to say, except that it may be due to that
individual variability which has served as the starting-point for so
much of what seems to us strange in form or fantastic in colour, both in
the animal and vegetable world."[CT] It is clear, however, that
"individual variability" cannot be regarded as a _vera causa_ of the
maintenance of a specific standard--a standard maintained _in spite of_
variability.
The only directive agency (apart from that of natural selection) to
which Mr. Wallace can point is that suggested by Mr. Alfred Tylor, in an
interesting, if somewhat fanciful, posthumous work on "Coloration in
Animals and Plants," "namely, that diversified coloration follows the
chief lines of structure, and changes at points, such as the joints,
where function changes." But even if we admit that coloration-bands or
spots originate at such points or along such lines--and the
physiological rationale is not altogether obvious--even if we admit that
in butterflies the spots and bands usually have reference to the form of
the wing and the arrangement of the nervures, and that in highly
coloured birds the crown of the head, the throat, the ear-coverts, and
the eyes have usually distinct tints, still it can hardly be maintained
that this affords us any adequate explanation of the _specific_
colour-tints of the humming-birds, or the pheasants, or the Papilionidæ
among butterflies. If, as Mr. Wallace argues, the immense tufts of
golden plumage in the bird of paradise owe their origin to the fact that
they are attached just above the point where the arteries and nerves for
the supply of the pectoral muscles leave the interior of the body, are
there no other birds in which similar arteries and nerves are found in a
similar position? Why have these no similar tufts? And why, in the birds
of paradise themselves, does it require four years (for it takes so long
for the feathers of the male to come to maturity) ere these nervous and
arterial influences take effect upon the plumage? Finally, one would
inquire how the colour is determined and held constant in each species.
The difficulty of the Tylor-Wallace view, even as a matter of origin, is
especially great in those numerous cases in which the colour is
determined by delicate lines, thin plates, or thin films of air or
fluid.[CU]
Under natural selection, as we have seen, the development of colour is
fostered under certain conditions. The colour is either protective,
rendering the organism inconspicuous amid its normal surroundings, or it
is of warning value, advertising the organism as inedible or dangerous,
or, in the form of recognition-marks, it is of service in enabling the
members of a species to recognize each other. Now, in the case of both
warning colour and recognition-marks, their efficacy depends upon the
perceptual powers of animals. Unless there be a rapidly acquired and
close association of the quality we call nastiness with the quality we
call gaudiness (though, for the animal, there is no such _isolation_ of
these qualities as is implied in our words [CV]), such that the sight of
the gaudy insect suggests that it will be unpleasant to eat, the
gaudiness will be of no avail. And if there is any truth in the doctrine
of mimicry, the association is particular. It is not merely that bright
colours are suggestive of a nasty taste. The insect-eating birds
associate nastiness especially with certain markings and
coloration--"the tawny _Danais_, the barred _Heliconias_, the blue-black
_Euplæas_, and the fibrous _Acræas_;" and this is proved by the fact
that sweet insects mimicking these particular forms are thereby
protected.
So, too, with recognition-marks. If the bird or the mammal have not
sufficient perceptive powers to distinguish between the often not very
different recognition-marks, of what service can they be?
Recognition-marks and mimicry seem, therefore, to show that in the
former case many animals, and in the latter the insect-eating birds,
mammals, lizards, and other animals concerned, have considerable powers
of perception and association.
Among other associations are those which are at the base of what I have
termed preferential mating. We must remember how deeply ingrained in the
animal nature is the mating instinct. _We_ may find it difficult to
distinguish closely allied species. But the individuals of that species
are led to mate together by an impelling instinct that is so well known
as to elicit no surprise. Instinct though it be, however, the mating
individuals must recognize each other in some way. The impulse that
draws them together must act through perceptual agency. It is not
surprising, therefore, to find, when we come to the higher animals,
that, built upon this basis, there are well-marked mating preferences.
And this, as we have before pointed out, following Wallace, is an
efficient factor in segregation. Let us, however, hear Mr. Wallace
himself in the matter.
There is, he says,[CW] "a very powerful cause of isolation in the mental
nature--the likes and dislikes--of animals; and to this is probably due
the fact of the rarity of hybrids in a state of nature. The differently
coloured herds of cattle in the Falkland Islands, each of which keeps
separate, have been already mentioned. Similar facts occur, however,
among our domestic animals, and are well known to breeders. Professor
Low, one of the greatest authorities on our domesticated animals, says,
'The female of the dog, when not under restraint, makes selection of her
mate, the mastiff selecting the mastiff, the terrier the terrier, and so
on.' And again, 'The merino sheep and the heath sheep of Scotland, if
two flocks are mixed together, each will breed with its own variety.'
Mr. Darwin has collected many facts illustrating this point.[CX] One of
the chief pigeon-fanciers in England informed him that, if free to
choose, each breed would prefer pairing with its own kind. Among the
wild horses in Paraguay those of the same colour and size associate
together; while in Circassia there are three races of horses which have
received special names, and which, when living a free life, almost
always refuse to mingle and cross, and will even attack one another. In
one of the Faröe Islands, not more than half a mile in diameter, the
half-wild native black sheep do not readily mix with imported white
sheep. In the Forest of Dean and in the New Forest the dark and pale
coloured herds of fallow deer have never been known to mingle; and even
the curious ancon sheep, of quite modern origin, have been observed to
keep together, separating themselves from the rest of the flock when put
into enclosures with other sheep. The same rule applies to birds, for
Darwin was informed by the Rev. W. D. Fox that his flocks of white and
Chinese geese kept distinct. This constant preference of animals for
their like, even in the case of slightly different varieties of the same
species, is evidently a fact of great importance in considering the
origin of species by natural selection, since it shows us that, so soon
as a slight differentiation of form or colour has been effected,
isolation will at once arise by the selective association of the animals
themselves."
Mr. Wallace thus allows, nay, he lays no little stress on, preferential
mating, and his name is associated with the hypothesis of
recognition-marks. But he denies that preferential mating, acting on
recognition-marks, has had any effect in furthering a differentiation of
form or colour. He admits that so soon as a slight differentiation of
form or colour has been effected, segregation will arise by the
selective association of the animals themselves; but he does not admit
that such selective association can carry the differentiation further.
Now, it is clear that mating preferences must be either fixed or
variable. If fixed, how can differentiation occur in the same flock or
herd? And how can selective association be a means of isolation? Or,
granting that differentiation has occurred, if the mating preferences
are then stereotyped, all further differentiation, so far as colour and
form are concerned, will be rendered impossible; for divergent
modifications, not meeting the stereotyped standard of taste, will for
that reason fail to be perpetuated. We must admit, then, that these
mating preferences are subject to variation. And now we come to the
central question with regard to sexual selection by means of
preferential mating. What guides the variation along special lines
leading to heightened beauty? This, I take it, is the heart and centre
of Mr. Wallace's criticism of Darwin's hypothesis. Sexual selection of
preferential mating involves a standard of taste; that standard has
advanced from what we consider a lower to what we consider a higher
æsthetic level, not along one line, but along many lines. What has
guided it along these lines?
Not as in any sense affording a direct answer to this question, but for
illustrative purposes, we may here draw attention to what seems to be a
somewhat parallel case, namely, the development of flowers through
insect agency. In his "Origin of Species," Darwin contended that flowers
had been rendered conspicuous and beautiful in order to attract insects,
adding, "Hence we may conclude that, if insects had not been developed
on the earth, our plants would not have been decked with beautiful
flowers, but would have produced only such poor flowers as we see on our
fir, oak, nut, and ash trees, on grasses, docks, and nettles, which are
all fertilized through the agency of the wind." "The argument in favour
of this view," says Mr. Wallace,[CY] who quotes this passage, "is now
much stronger than when Mr. Darwin wrote;" and he cites with approval
the following passage from Mr. Grant Allen's "Colour-Sense:" "While man
has only tilled a few level plains, a few great river-valleys, a few
peninsular mountain slopes, leaving the vast mass of earth untouched by
his hand, the insect has spread himself over every land in a thousand
shapes, and has made the whole flowering creation subservient to his
daily wants. His buttercup, his dandelion, and his meadowsweet grow
thick in every English field. His thyme clothes the hillside; his
heather purples the bleak grey moorland. High up among the Alpine
heights his gentian spreads its lakes of blue; amid the snows of the
Himalayas his rhododendrons gleam with crimson light. Even the wayside
pond yields him the white crowfoot and the arrowhead, while the broad
expanses of Brazilian streams are beautified by his gorgeous
water-lilies. The insect has thus turned the whole surface of the earth
into a boundless flower-garden, which supplies him from year to year
with pollen or honey, and itself in turn gains perpetuation by the baits
that it offers to his allurement."[CZ]
Mr. Grant Allen is perfectly correct in stating that the insect has
produced all this beauty. It is the result of insect choice, a genuine
case of selection as contrasted with elimination. And when we ask in
this case, as we asked in the case of the beautiful colours and forms of
animals, what has guided their evolution along lines which lead to such
rare beauty, we are given by Mr. Wallace himself the answer, "The
preferential choice of insects." If these insects have been able to
produce through preferential selection all this wealth of floral beauty
(not, indeed, for the sake of the beauty, but incidentally in the
practical business of their life), there would seem to be no _a priori_
reason why the same class and birds and mammals should not have been
able to produce, through preferential selection, all the wealth of
animal beauty.
It should be noted that the answer to the question is in each case a
manifestly incomplete one. For if we say that these forms of beauty,
floral and animal, have been selected through animal preferences, there
still remains behind the question--How and why have the preferences
taken these _æsthetic_ lines? To which I do not see my way to a
satisfactory answer, though some suggestions in the matter will be made
in a future chapter.[DA] At present all we can say is this--to be
conspicuous was advantageous, since it furthered the mating of flowers
and animals. To be diversely conspicuous was also advantageous. As Mr.
Wallace says, "It is probably to assist the insects in keeping to one
flower at a time, which is of vital importance to the perpetuation of
the species, that the flowers which bloom intermingled at the same
season are usually very distinct, both in form and colour."[DB] But
conspicuousness is not beauty. And the question still remains--From what
source comes this tendency to beauty?
Leaving this question on one side, we may state the argument in favour
of sexual selection in the following form: The generally admitted
doctrine of mimicry involves the belief that birds and other
insect-eating animals have delicate and particular perceptual powers.
The generally received doctrine of the origin of flowers involves the
belief that their diverse forms and markings result from the selective
choice of insects. There are a number of colour and form peculiarities
in animals that cannot be explained by natural selection through
elimination. There is some evidence in favour of preferential mating or
selective association. It is, therefore, permissible to hold, as a
provisional hypothesis, that just as the diverse forms of flowers result
from the preferential choice of insects, so do the diverse secondary
sexual characters of animals result, in part at least, from the
preferential choice of animals through selective mating.
If this be admitted, then the elaborate display of their finery by male
birds, which Mr. Wallace does admit, may fairly be held to have a value
which he does not admit. For if preferential mating is _à priori_
probable, such display may be regarded as the outcome of this mode of
selection. At the same time, it may be freely admitted that more
observations are required. In a recent paper, "On Sexual Selection in
Spiders of the Family _Attidæ_,"[DC] by George W. and Elizabeth G.
Peckham, a full, not to say elaborate, description is given of the
courtship, as they regard it, of spiders. The "love-dances" and the
display of special adornments are described in detail. And the
observers, as the result, be it remembered, of long and patient
investigation and systematic study, come to the conclusion that female
spiders exercise selective choice in their mates. And courtship must be
a serious matter for spiders, for if they fail to please, they run a
very serious risk of being eaten by the object of their attentions. Some
years ago I watched, on the Cape Flats, near Capetown, the courtship of
a large spider (I do not know the species). In this case the antics were
strange, and, to me, amusing; but they seemed to have no effect on the
female spider, who merely watched him. Once or twice she darted forward
towards him, but he, not liking, perhaps, the gleam in her eyes,
retreated hastily. Eventually she seemed to chase him off the field.
We must remember how difficult it is to obtain really satisfactory
evidence of mating preferences in animals. In most cases we must watch
the animals undisturbed, and very rarely can we have an opportunity of
determining whether one particular female selects her mate out of her
various suitors. We watch the courtship in this, that, or the other
case. In some we see that it is successful; in others that it is
unsuccessful. How can we be sure that in the one case it was through
fully attaining, in the other through failing to reach, the standard of
taste? And yet it is evidence of this sort that Mr. Wallace demands.
After noting the rejection by the hen of male birds which had lost their
ornamental plumage, he says, "Such cases do not support the idea that
males with the tail-feathers a trifle longer, or the colours a trifle
brighter, are generally preferred, and that those which are only a
little inferior are as generally rejected,--and this is what is
absolutely needed to establish the theory of the development of these
plumes by means of the choice of the female."[DD] If Mr. Wallace
requires direct observational evidence of this kind, I do not suppose he
is likely to get any large body of it. But one might fairly ask him what
body of direct observational evidence he has of natural selection. The
fact is that direct observational evidence is, from the nature of the
processes involved, almost impossible to produce in either case. Natural
selection is an explanation of organic phenomena reached by a process of
logical inference and justified by its results. It is not claimed for
the hypothesis of selective mating that it has a higher order of
validity.
_Use and Disuse._
As we have already seen, biologists are divided into two schools, one of
which maintains that the effects of use and disuse[DE] have been a
potent factor in organic evolution; the other, that the effects of use
and disuse are restricted to the individual. My own opinion is that we
have not a sufficient body of carefully sifted evidence to enable us to
dogmatize on the subject, one way or the other. But, the position of
strict equilibrium being an exceedingly difficult and some would have us
believe an undesirable attitude of mind, I may add that I lean to the
view that use and disuse, if persistent and long-continued, take effect,
not only on the individual, but also on the species.
It is scarcely necessary to give examples of the kind of change which,
according to the Lamarckian school, are wrought by use and disuse. Any
organ persistently used will have a tendency, on this view, to become in
successive generations more and more adapted to its functional work. To
give but one example. It is well known that certain hoofed creatures are
divisible into two groups--first, those which, like the horse, have in
each limb one large and strong digit armed with a solid hoof; and,
secondly, those which, like the ox, have in each limb two large digits,
so that the hoof is cloven or split. It is also well known that the
ancestral forms from which both horse-group and ox-group are derived
were possessed of five digits to each limb. Professor Cope regards the
differentiation of these two groups as the result of the different modes
of use necessitated by different modes of life. "The mechanical effect,"
he says, "of walking in the mud is to spread the toes equally on
opposite sides of the middle line. This would encourage the equal
development of the digits on each side of the middle line, as in the
cloven-footed types. In progression on hard ground the longest toe (the
third) will receive the greatest amount of shock from contact with the
earth."[DF] Hence the solid-hoofed types. Here, then, the middle digit
in the horse-group, or two digits in the ox-group, having the main
burden to bear, increase through persistent use, while the other digits
dwindle through disuse.[DG]
On the other hand, one who holds the opposite view will say--I do not
believe that use and disuse have had anything whatever to do with the
matter. Fortuitous variations in these digits have taken place. The
conditions have determined which variations should be preserved. In the
horse, variations in the direction of increase of functional value of
the mid digit, and variations in the simultaneous decrease of the
functional value of the lateral digits, have been of advantage, and have
therefore survived the eliminating process of natural selection.
Now, since it is quite clear, in this and numberless similar cases, that
we can explain the facts either way, it is obviously not worth while to
spend much time or ingenuity in devising such explanations. They are not
likely to convince any one worth convincing. What we need is (1) crucial
cases which can only be explained one way or the other; or (2) direct
observation or experiment leading to the establishment of one hypothesis
or the other (or both).
1. Crucial cases are very difficult to find. We cannot exclude the
element of use or disuse, for on both hypotheses it is essential. The
difference is that one school says the organ is developed in the species
_by_ use; the other school says it is developed _for_ use. What we must
seek is, therefore, the necessary exclusion of natural selection; and
that is not easy to prove, in any case, to a Darwinian. If it can be
shown that there exist structures which are of use, but not of vital
importance (that is to say, which have not what I called above the
_available advantage_ necessary to determine the question of elimination
or not-elimination), then we are perhaps able to exclude the influence
of natural selection. I think, if anywhere, such cases are to be found
in faculties and instincts;[DH] and as such they must be considered in a
later chapter. I will, however, here cite one case in illustration of my
meaning.
We have seen that certain insects are possessed of warning colours,
which advertise their nastiness to the taste. Birds avoid these bright
but unpleasant insects, and though there is some individual learning,
there seems to be an instinctive avoidance of these unsavoury morsels.
There is hesitation before tasting; and one or two trials are sufficient
to establish the association of gaudiness and nastiness. Moreover, Mr.
Poulton and others have shown that, under the stress of keen hunger,
these gaudy insects may be eaten, and apparently leave no ill effects.
Birds certainly instinctively avoid bees and wasps; and yet the sting of
these insects can seldom be fatal. It is, therefore, improbable that
nastiness or even the power of stinging can have been an eliminating
agency. In the development of the instinctive avoidance, natural
selection through elimination seems to be excluded, and the inheritance
of individual experience is thus rendered probable. As before pointed
out, it is not enough to say that a nasty taste or a sting in the gullet
is disadvantageous; it must be shown that the disadvantage has an
eliminating value. From my experiments (feeding frogs on nasty
caterpillars, and causing bees to sting chickens), I doubt the
eliminating value in this case. Hence elimination by natural selection
seems, I repeat, to be excluded, and the inheritance of individual
experience rendered probable.