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 robbed of the egg she laid in it, continued to lay a new one, which grew smaller and smaller, till, when she had laid her twenty-ninth egg, she was found dead upon her nest. If an instinct cannot stand the test of self-sacrifice—if it is the simple outcome of a desire for bodily gratification—then it is no true instinct, and is only so called erroneously.