Birds with and without feet adapted for climbing live in trees; so also do monkeys with and without flexible tails, squirrels, sloths, pumas, &c. Mole-crickets dig with a well-pronounced spade upon their fore-feet, while the burying-beetle does the same thing though it has no special apparatus whatever. The mole conveys its winter provender in pockets, an inch wide, long and half an inch wide within its cheeks; the field-mouse does so without the help of any such contrivance. The migratory instinct displays itself with equal strength in animals of widely different form, by whatever means they may pursue their journey, whether by water, land, or air. It is clear, therefore, that instinct is in great measure independent of bodily organisation. Granted, indeed, that a certain amount of bodily apparatus is a _sine quâ non_ for any power of execution at all—as, for example, that there would be no ingenious nest without organs more or less adapted for its construction, no spinning of a web without spinning glands—nevertheless, it is impossible to maintain that instinct is a consequence of organisation. The mere existence of the organ does not constitute even the smallest incentive to any corresponding habitual activity. A sensation of pleasure must at least accompany the use of the organ before its existence can incite to its employment. And even so when a sensation of pleasure has given the impulse which is to render it active, it is only the fact of there being activity at all, and not the special characteristics of the activity, that can be due to organisation. The reason for the special mode of the activity is the very problem that we have to solve. No one will call the action of the spider instinctive in voiding the fluid from her spinning gland when it is too full, and therefore painful to her; nor that of the male fish when it does what amounts to much the same thing as this. The instinct and the marvel lie in the fact that the spider spins threads, and proceeds to weave her web with them, and that the male fish will only impregnate ova of his own species.