CHAPTER VIII - INSTINCT
Objections to the Theory of Natural Selection as applied to
Instincts: Neuter and Sterile Insects
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.
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.
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 is it possible to
reconcile this case with the theory of natural selection?
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.
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 characterised 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 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, is 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.
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 commonly 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 Myrmecoeystus, 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.
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 INSTINCT is neuters having the
advantageous modifications, all the neuters ultimately came to be thus
characterised. 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,
whilst those of an intermediate size are scanty in numbers. Formica
lava 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
proportionally 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.
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 size, 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.
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.
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.
I have now explained how, as 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, whilst 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 or 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 hitherto advanced this demonstrative case of
neuter insects, against the well-known doctrine of inherited habit, as
advanced by Lamarck.
Previous section | Next section
