ring under two widely distinct male
forms. But this subject need not here be discussed.

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.

SUMMARY.

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