<p>OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED TO INSTINCTS:
NEUTER AND STERILE INSECTS.</p>
<p>It has been objected to the foregoing view of the origin of instincts that
"the variations of structure and of instinct must have been simultaneous
and accurately adjusted to each other, as a modification in the one
without an immediate corresponding change in the other would have been
fatal." The force of this objection rests entirely on the assumption that
the changes in the instincts and structure are abrupt. To take as an
illustration the case of the larger titmouse, (Parus major) alluded to in
a previous chapter; this bird often holds the seeds of the yew between its
feet on a branch, and hammers with its beak till it gets at the kernel.
Now what special difficulty would there be in natural selection preserving
all the slight individual variations in the shape of the beak, which were
better and better adapted to break open the seeds, until a beak was
formed, as well constructed for this purpose as that of the nuthatch, at
the same time that habit, or compulsion, or spontaneous variations of
taste, led the bird to become more and more of a seed-eater? In this case
the beak is supposed to be slowly modified by natural selection,
subsequently to, but in accordance with, slowly changing habits or taste;
but let the feet of the titmouse vary and grow larger from correlation
with the beak, or from any other unknown cause, and it is not improbable
that such larger feet would lead the bird to climb more and more until it
acquired the remarkable climbing instinct and power of the nuthatch. In
this case a gradual change of structure is supposed to lead to changed
instinctive habits. To take one more case: few instincts are more
remarkable than that which leads the swift of the Eastern Islands to make
its nest wholly of inspissated saliva. Some birds build their nests of
mud, believed to be moistened with saliva; and one of the swifts of North
America makes its nest (as I have seen) of sticks agglutinated with
saliva, and even with flakes of this substance. Is it then very improbable
that the natural selection of individual swifts, which secreted more and
more saliva, should at last produce a species with instincts leading it to
neglect other materials and to make its nest exclusively of inspissated
saliva? And so in other cases. It must, however, be admitted that in many
instances we cannot conjecture whether it was instinct or structure which
first varied.</p>
<p>No doubt many instincts of very difficult explanation could be opposed to
the theory of natural selection—cases, in which we cannot see how an
instinct could have originated; cases, in which no intermediate gradations
are known to exist; cases of instincts of such trifling importance, that
they could hardly have been acted on by natural selection; cases of
instincts almost identically the same in animals so remote in the scale of
nature that we cannot account for their similarity by inheritance from a
common progenitor, and consequently must believe that they were
independently acquired through natural selection. I will not here enter on
these several cases, but will confine myself to one special difficulty,
which at first appeared to me insuperable, and actually fatal to the whole
theory. I allude to the neuters or sterile females in insect communities:
for these neuters often differ widely in instinct and in structure from
both the males and fertile females, and yet, from being sterile, they
cannot propagate their kind.</p>
<p>The subject well deserves to be discussed at great length, but I will here
take only a single case, that of working or sterile ants. How the workers
have been rendered sterile is a difficulty; but not much greater than that
of any other striking modification of structure; for it can be shown that
some insects and other articulate animals in a state of nature
occasionally become sterile; and if such insects had been social, and it
had been profitable to the community that a number should have been
annually born capable of work, but incapable of procreation, I can see no
especial difficulty in this having been effected through natural
selection. But I must pass over this preliminary difficulty. The great
difficulty lies in the working ants differing widely from both the males
and the fertile females in structure, as in the shape of the thorax, and
in being destitute of wings and sometimes of eyes, and in instinct. As far
as instinct alone is concerned, the wonderful difference in this respect
between the workers and the perfect females would have been better
exemplified by the hive-bee. If a working ant or other neuter insect had
been an ordinary animal, I should have unhesitatingly assumed that all its
characters had been slowly acquired through natural selection; namely, by
individuals having been born with slight profitable modifications, which
were inherited by the offspring, and that these again varied and again
were selected, and so onwards. But with the working ant we have an insect
differing greatly from its parents, yet absolutely sterile; so that it
could never have transmitted successively acquired modifications of
structure or instinct to its progeny. It may well be asked how it is
possible to reconcile this case with the theory of natural selection?</p>
<p>First, let it be remembered that we have innumerable instances, both in
our domestic productions and in those in a state of nature, of all sorts
of differences of inherited structure which are correlated with certain
ages and with either sex. We have differences correlated not only with one
sex, but with that short period when the reproductive system is active, as
in the nuptial plumage of many birds, and in the hooked jaws of the male
salmon. We have even slight differences in the horns of different breeds
of cattle in relation to an artificially imperfect state of the male sex;
for oxen of certain breeds have longer horns than the oxen of other
breeds, relatively to the length of the horns in both the bulls and cows
of these same breeds. Hence, I can see no great difficulty in any
character becoming correlated with the sterile condition of certain
members of insect communities; the difficulty lies in understanding how
such correlated modifications of structure could have been slowly
accumulated by natural selection.</p>
<p>This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be applied
to the family, as well as to the individual, and may thus gain the desired
end. Breeders of cattle wish the flesh and fat to be well marbled
together. An animal thus characterized has been slaughtered, but the
breeder has gone with confidence to the same stock and has succeeded. Such
faith may be placed in the power of selection that a breed of cattle,
always yielding oxen with extraordinarily long horns, could, it is
probable, be formed by carefully watching which individual bulls and cows,
when matched, produced oxen with the longest horns; and yet no one ox
would ever have propagated its kind. Here is a better and real
illustration: According to M. Verlot, some varieties of the double annual
stock, from having been long and carefully selected to the right degree,
always produce a large proportion of seedlings bearing double and quite
sterile flowers, but they likewise yield some single and fertile plants.
These latter, by which alone the variety can be propagated, may be
compared with the fertile male and female ants, and the double sterile
plants with the neuters of the same community. As with the varieties of
the stock, so with social insects, selection has been applied to the
family, and not to the individual, for the sake of gaining a serviceable
end. Hence, we may conclude that slight modifications of structure or of
instinct, correlated with the sterile condition of certain members of the
community, have proved advantageous; consequently the fertile males and
females have flourished, and transmitted to their fertile offspring a
tendency to produce sterile members with the same modifications. This
process must have been repeated many times, until that prodigious amount
of difference between the fertile and sterile females of the same species
has been produced which we see in many social insects.</p>
<p>But we have not as yet touched on the acme of the difficulty; namely, the
fact that the neuters of several ants differ, not only from the fertile
females and males, but from each other, sometimes to an almost incredible
degree, and are thus divided into two or even three castes. The castes,
moreover, do not generally graduate into each other, but are perfectly
well defined; being as distinct from each other as are any two species of
the same genus, or rather as any two genera of the same family. Thus, in
Eciton, there are working and soldier neuters, with jaws and instincts
extraordinarily different: in Cryptocerus, the workers of one caste alone
carry a wonderful sort of shield on their heads, the use of which is quite
unknown: in the Mexican Myrmecocystus, the workers of one caste never
leave the nest; they are fed by the workers of another caste, and they
have an enormously developed abdomen which secretes a sort of honey,
supplying the place of that excreted by the aphides, or the domestic
cattle as they may be called, which our European ants guard and imprison.</p>
<p>It will indeed be thought that I have an overweening confidence in the
principle of natural selection, when I do not admit that such wonderful
and well-established facts at once annihilate the theory. In the simpler
case of neuter insects all of one caste, which, as I believe, have been
rendered different from the fertile males and females through natural
selection, we may conclude from the analogy of ordinary variations, that
the successive, slight, profitable modifications did not first arise in
all the neuters in the same nest, but in some few alone; and that by the
survival of the communities with females which produced most neuters
having the advantageous modification, all the neuters ultimately came to
be thus characterized. According to this view we ought occasionally to
find in the same nest neuter-insects, presenting gradations of structure;
and this we do find, even not rarely, considering how few neuter-insects
out of Europe have been carefully examined. Mr. F. Smith has shown that
the neuters of several British ants differ surprisingly from each other in
size and sometimes in colour; and that the extreme forms can be linked
together by individuals taken out of the same nest: I have myself compared
perfect gradations of this kind. It sometimes happens that the larger or
the smaller sized workers are the most numerous; or that both large and
small are numerous, while those of an intermediate size are scanty in
numbers. Formica flava has larger and smaller workers, with some few of
intermediate size; and, in this species, as Mr. F. Smith has observed, the
larger workers have simple eyes (ocelli), which, though small, can be
plainly distinguished, whereas the smaller workers have their ocelli
rudimentary. Having carefully dissected several specimens of these
workers, I can affirm that the eyes are far more rudimentary in the
smaller workers than can be accounted for merely by their proportionately
lesser size; and I fully believe, though I dare not assert so positively,
that the workers of intermediate size have their ocelli in an exactly
intermediate condition. So that here we have two bodies of sterile workers
in the same nest, differing not only in size, but in their organs of
vision, yet connected by some few members in an intermediate condition. I
may digress by adding, that if the smaller workers had been the most
useful to the community, and those males and females had been continually
selected, which produced more and more of the smaller workers, until all
the workers were in this condition; we should then have had a species of
ant with neuters in nearly the same condition as those of Myrmica. For the
workers of Myrmica have not even rudiments of ocelli, though the male and
female ants of this genus have well-developed ocelli.</p>
<p>I may give one other case: so confidently did I expect occasionally to
find gradations of important structures between the different castes of
neuters in the same species, that I gladly availed myself of Mr. F.
Smith's offer of numerous specimens from the same nest of the driver ant
(Anomma) of West Africa. The reader will perhaps best appreciate the
amount of difference in these workers by my giving, not the actual
measurements, but a strictly accurate illustration: the difference was the
same as if we were to see a set of workmen building a house, of whom many
were five feet four inches high, and many sixteen feet high; but we must
in addition suppose that the larger workmen had heads four instead of
three times as big as those of the smaller men, and jaws nearly five times
as big. The jaws, moreover, of the working ants of the several sizes
differed wonderfully in shape, and in the form and number of the teeth.
But the important fact for us is that, though the workers can be grouped
into castes of different sizes, yet they graduate insensibly into each
other, as does the widely-different structure of their jaws. I speak
confidently on this latter point, as Sir J. Lubbock made drawings for me,
with the camera lucida, of the jaws which I dissected from the workers of
the several sizes. Mr. Bates, in his interesting "Naturalist on the
Amazons," has described analogous cases.</p>
<p>With these facts before me, I believe that natural selection, by acting on
the fertile ants or parents, could form a species which should regularly
produce neuters, all of large size with one form of jaw, or all of small
size with widely different jaws; or lastly, and this is the greatest
difficulty, one set of workers of one size and structure, and
simultaneously another set of workers of a different size and structure; a
graduated series having first been formed, as in the case of the driver
ant, and then the extreme forms having been produced in greater and
greater numbers, through the survival of the parents which generated them,
until none with an intermediate structure were produced.</p>
<p>An analogous explanation has been given by Mr. Wallace, of the equally
complex case, of certain Malayan butterflies regularly appearing under two
or even three distinct female forms; and by Fritz Muller, of certain
Brazilian crustaceans likewise appearing under two widely distinct male
forms. But this subject need not here be discussed.</p>
<p>I have now explained how, I believe, the wonderful fact of two distinctly
defined castes of sterile workers existing in the same nest, both widely
different from each other and from their parents, has originated. We can
see how useful their production may have been to a social community of
ants, on the same principle that the division of labour is useful to
civilised man. Ants, however, work by inherited instincts and by inherited
organs or tools, while man works by acquired knowledge and manufactured
instruments. But I must confess, that, with all my faith in natural
selection, I should never have anticipated that this principle could have
been efficient in so high a degree, had not the case of these neuter
insects led me to this conclusion. I have, therefore, discussed this case,
at some little but wholly insufficient length, in order to show the power
of natural selection, and likewise because this is by far the most serious
special difficulty which my theory has encountered. The case, also, is
very interesting, as it proves that with animals, as with plants, any
amount of modification may be effected by the accumulation of numerous,
slight, spontaneous variations, which are in any way profitable, without
exercise or habit having been brought into play. For peculiar habits,
confined to the workers of sterile females, however long they might be
followed, could not possibly affect the males and fertile females, which
alone leave descendants. I am surprised that no one has advanced this
demonstrative case of neuter insects, against the well-known doctrine of
inherited habit, as advanced by Lamarck.</p>
<p>SUMMARY.</p>
<p>I have endeavoured in this chapter briefly to show that the mental
qualities of our domestic animals vary, and that the variations are
inherited. Still more briefly I have attempted to show that instincts vary
slightly in a state of nature. No one will dispute that instincts are of
the highest importance to each animal. Therefore, there is no real
difficulty, under changing conditions of life, in natural selection
accumulating to any extent slight modifications of instinct which are in
any way useful. In many cases habit or use and disuse have probably come
into play. I do not pretend that the facts given in this chapter
strengthen in any great degree my theory; but none of the cases of
difficulty, to the best of my judgment, annihilate it. On the other hand,
the fact that instincts are not always absolutely perfect and are liable
to mistakes; that no instinct can be shown to have been produced for the
good of other animals, though animals take advantage of the instincts of
others; that the canon in natural history, of "Natura non facit saltum,"
is applicable to instincts as well as to corporeal structure, and is
plainly explicable on the foregoing views, but is otherwise inexplicable—all
tend to corroborate the theory of natural selection.</p>
<p>This theory is also strengthened by some few other facts in regard to
instincts; as by that common case of closely allied, but distinct,
species, when inhabiting distant parts of the world and living under
considerably different conditions of life, yet often retaining nearly the
same instincts. For instance, we can understand, on the principle of
inheritance, how it is that the thrush of tropical South America lines its
nest with mud, in the same peculiar manner as does our British thrush; how
it is that the Hornbills of Africa and India have the same extraordinary
instinct of plastering up and imprisoning the females in a hole in a tree,
with only a small hole left in the plaster through which the males feed
them and their young when hatched; how it is that the male wrens
(Troglodytes) of North America, build "cock-nests," to roost in, like the
males of our Kitty-wrens,—a habit wholly unlike that of any other
known bird. Finally, it may not be a logical deduction, but to my
imagination it is far more satisfactory to look at such instincts as the
young cuckoo ejecting its foster-brothers, ants making slaves, the larvae
of ichneumonidae feeding within the live bodies of caterpillars, not as
specially endowed or created instincts, but as small consequences of one
general law leading to the advancement of all organic beings—namely,
multiply, vary, let the strongest live and the weakest die.</p>
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