cells, and much wax would be saved. Again, from the same cause, it would
be advantageous to the Melipona, if she were to make her cells closer
together, and more regular in every way than at present; for then, as we
have seen, the spherical surfaces would wholly disappear, and would all be
replaced by plane surfaces; and the Melipona would make a comb as perfect
as that of the hive-bee. Beyond this stage of perfection in architecture,
natural selection could not lead; for the comb of the hive-bee, as far as
we can see, is absolutely perfect in economising wax.
Thus, as I believe, the most wonderful of all known instincts, that of the
hive-bee, can be explained by natural selection having taken advantage of
numerous, successive, slight modifications of simpler instincts; natural
selection having by slow degrees, more and more perfectly, led the bees to
sweep equal spheres at a given distance from each other in a double layer,
and to build up and excavate the wax along the planes of intersection. The
bees, of course, no more knowing that they swept their spheres at one
particular distance from each other, than they know what are the several
angles of the hexagonal prisms and of the basal rhombic plates. The motive
power of the process of natural selection having been economy of wax; that
individual swarm which wasted least honey in the secretion of wax, having
succeeded best, and having transmitted by inheritance its newly acquired
economical instinct to new swarms, which in their turn will have had the
best chance of succeeding in the struggle for existence.
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 possibly have originated; cases, in which no intermediate
gradations are known to exist; cases of instinct of apparently 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 parent, and must therefore believe that they
have been acquired by independent acts of 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 my 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 very great
difficulty in this being effected by 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 prodigious difference in this respect between the workers
and the perfect females, would have been far better exemplified by the
hive-bee. If a working ant or other neuter insect had been an animal in
the ordinary state, I should have unhesitatingly assumed that all its
characters had been slowly acquired through natural selection; namely, by
an individual having been born with some slight profitable modification of
structure, this being inherited by its offspring, which again varied and
were again 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 structure which have become correlated to certain ages, and
that short period alone 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 in other breeds, in comparison with
the horns of the bulls or cows of these same breeds. Hence I can see no
real difficulty in any character having become 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. Thus, a well-flavoured vegetable is cooked, and the individual is
destroyed; but the horticulturist sows seeds of the same stock, and
confidently expects to get nearly the same variety; breeders of cattle wish
the flesh and fat to be well marbled together; the animal has been
slaughtered, but the breeder goes with confidence to the same family. I
have such faith in the powers of selection, that I do not doubt that a
breed of cattle, always yielding oxen with extraordinarily long horns,
could be slowly formed by carefully watching which individual bulls and
cows, when matched, produced oxen with the longest horns; and yet no one ox
could ever have propagated its kind. Thus I believe it has been with
social insects: a slight modification of structure, or instinct,
correlated with the sterile condition of certain members of the community,
has been advantageous to the community: consequently the fertile males and
females of the same community flourished, and transmitted to their fertile
offspring a tendency to produce sterile members having the same
modification. And I believe that this process has been repeated, 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 climax 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 or 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 my theory. In the simpler case
of neuter insects all of one caste or of the same kind, which have been
rendered by natural selection, as I believe to be quite possible, different
from the fertile males and females,--in this case, we may safely conclude
from the analogy of ordinary variations, that each successive, slight,
profitable modification did not probably at first appear in all the
individual neuters in the same nest, but in a few alone; and that by the
long-continued selection of the fertile parents which produced most neuters
with the profitable modification, all the neuters ultimately came to have
the desired character. On this view we ought occasionally to find
neuter-insects of the same species, in the same nest, presenting gradations
of structure; and this we do find, even often, considering how few
neuter-insects out of Europe have been carefully examined. Mr. F. Smith
has shown how surprisingly the neuters of several British ants differ from
sometimes be perfectly linked together by individuals taken out of the same
nest: I have myself compared perfect gradations of this kind. It often
happens that the larger or the smaller sized workers are the most numerous;
or that both large and small are numerous, with those of an intermediate
size scanty in numbers. Formica flava has larger and smaller workers, with
some 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 we here 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 had
come to be in this condition; we should then have had a species of ant with
neuters very nearly in the same condition with 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 to find gradations
in important points of structure 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 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 Mr. Lubbock
made drawings for me with the camera lucida of the jaws which I had
dissected from the workers of the several sizes.
With these facts before me, I believe that natural selection, by acting on
the fertile parents, could form a species which should regularly produce
neuters, either all of large size with one form of jaw, or all of small
size with jaws having a widely different structure; or lastly, and this is
our climax of 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 been first formed, as in the case of the driver
ant, and then the extreme forms, from being the most useful to the
community, having been produced in greater and greater numbers through the
natural selection of the parents which generated them; until none with an
intermediate structure were produced.
Thus, 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 insects, on the same
principle that the division of labour is useful to civilised man. As ants
work by inherited instincts and by inherited tools or weapons, and not by
acquired knowledge and manufactured instruments, a perfect division of
labour could be effected with them only by the workers being sterile; for
had they been fertile, they would have intercrossed, and their instincts
and structure would have become blended. And nature has, as I believe,
effected this admirable division of labour in the communities of ants, by
the means of natural selection. But I am bound to confess, that, with all
my faith in this principle, I should never have anticipated that natural
selection could have been efficient in so high a degree, had not the case
of these neuter insects convinced me of the fact. 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 in structure can be
effected by the accumulation of numerous, slight, and as we must call them
accidental, variations, which are in any manner profitable, without
exercise or habit having come into play. For no amount of exercise, or
habit, or volition, in the utterly sterile members of a community could
possibly have affected the structure or instincts of the fertile members,
which alone leave descendants. I am surprised that no one has advanced
this demonstrative case of neuter insects, against the well-known doctrine
of Lamarck.
Summary. -- I have endeavoured briefly in this chapter 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 I can see no difficulty,
under changing conditions of life, in natural selection accumulating slight
modifications of instinct to any extent, in any useful direction. In some
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 has been
produced for the exclusive good of other animals, but that each animal
takes 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.
This theory is, also, strengthened by some few other facts in regard to
instincts; as by that common case of closely allied, but certainly
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 South America lines its nest with
mud, in the same peculiar manner as does our British thrush: how it is
that the male wrens (Troglodytes) of North America, build 'cock-nests,' to
roost in, like the males of our distinct 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.
Chapter VIII
Hybridism
Distinction between the sterility of first crosses and of hybrids --
Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication -- Laws governing the sterility of
hybrids -- Sterility not a special endowment, but incidental on other
differences -- Causes of the sterility of first crosses and of hybrids --
Parallelism between the effects of changed conditions of life and crossing
-- Fertility of varieties when crossed and of their mongrel offspring not
universal -- Hybrids and mongrels compared independently of their fertility