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.

Another proof that the pleasure felt in the employment of an organ is
wholly inadequate to account for this employment is to be found in
the fact that the moral greatness of instinct, the point in respect
of which it most commands our admiration, consists in the obedience
paid to its behests, to the postponement of all personal well-being,
and at the cost, it may be, of life itself. If the mere pleasure of
relieving certain glands from overfulness were the reason why
caterpillars generally spin webs, they would go on spinning until
they had relieved these glands, but they would not repair their work
as often as any one destroyed it, and do this again and again until
they die of exhaustion. The same holds good with the other instincts
that at first sight appear to be inspired only by a sensation of
pleasure; for if we change the circumstances, so as to put self-
sacrifice in the place of self-interest, it becomes at once apparent
that they have a higher source than this. We think, for example,
that birds pair for the sake of mere sexual gratification; why, then,
do they leave off pairing as soon as they have laid the requisite
number of eggs? That there is a reproductive instinct over and above
the desire for sexual gratification appears from the fact that if a
man takes an egg out of the nest, the birds will come together again
and the hen will lay another egg; or, if they belong to some of the
more wary species, they will desert their nest, and make preparation
for an entirely new brood. A female wryneck, whose nest was daily