Caterpillars which have been fed on the leaves of one kind of tree,
have been known to perish from hunger rather than to eat the leaves
of another tree, although this afforded them their proper food,
under a state of nature;[4] and so it is in many other cases.


[3] A remark to much the same effect was made long ago by Hippocrates
and by the illustrious Harvey; for both assert that a young animal
forgets in the course of a few days the art of sucking, and cannot
without some difficulty again acquire it. I give these assertions
on the authority of Dr. Darwin, `Zoonomia,' 1794, vol. i. p. 140.

The power of Association is admitted by everyone. Mr. Bain remarks,
that "actions, sensations and states of feeling, occurring together
or in close succession, tend to grow together, or cohere, in such a way
that when any one of them is afterwards presented to the mind, the others
are apt to be brought up in idea."[5] It is so important for our purpose
fully to recognize that actions readily become associated with other actions
and with various states of the mind, that I will give a good many instances,
in the first place relating to man, and afterwards to the lower animals.
Some of the instances are of a very trifling nature, but they are as
good for our purpose as more important habits. It is known to everyone
how difficult, or even impossible it is, without repeated trials, to move
the limbs in certain opposed directions which have never been practised.
Analogous cases occur with sensations, as in the common experiment
of rolling a marble beneath the tips of two crossed fingers, when it
feels exactly like two marbles. Everyone protects himself when falling
to the ground by extending his arms, and as Professor Alison has remarked,
few can resist acting thus, when voluntarily falling on a soft bed.
A man when going out of doors puts on his gloves quite unconsciously;