It may seem altogether surprising and improbable to us that an instinct for death should arise in man, since we are imbued with an instinct of an opposite nature. From the facts that I collected in my sixth chapter, it was to be inferred plainly that the desire of life and the fear of death are manifestations of an instinct deep-rooted in the constitution of man. That instinct is of the same order as the instincts of hunger and thirst, of the need of sleep, of movement and of sexual and maternal love. The devotion and care bestowed on their young by female birds and mammals are known universally. And yet these instincts can be reversed. There is no sacrifice of which the mothers are not capable if it serve to save the life or promote the well-being of their offspring. Such devotion is a manifestation of the maternal instinct, which is one of the strongest instincts known to us. And yet that love, so tender and so absolute, lasts only for the time during which the wants of the young need to be satisfied. As soon as the young begin to be independent, the maternal love changes to indifference or to dislike. At the next breeding-period, maternal love reappears again, so that there is a periodic ebb and flow of the instinct. The new-born babe takes an instinctive delight in the milk of his mother, which seems to him the only good food in the world. As soon as he can show his feelings, his intense satisfaction as he is suckled is plain. But this instinct lasts only during the period of lactation. As soon as the child begins to take different kinds of food, he ceases to be pleased with his mother’s milk, and may dislike it for the remainder of his life. Several adults to whom I have offered human milk would not even taste it, so disgusting did it seem to them. And yet the taste had nothing intrinsically disagreeable in it. Here again is an example of a strong instinct that changes completely. Children often eat to repletion of some kind of substance, and for long afterwards that substance disgusts them instead of being coveted by them. It is said that apprentices to pastry-cooks and makers of sweetmeats are allowed at first to eat as much as they please. They soon come to have a profound dislike for the sweet things that children like so much. A mother who adores her child, or a child who is extremely fond of sweetmeats cannot understand how any mother could dislike her offspring or any apprentice have a distaste for sweets. In the same way, human beings full of the desire for life, believe more easily in eternal life than in the possibility of an instinct of death. And yet the instinct of death seems to lie, in some potential form, deep in the constitution of man. If the cycle of human life followed its ideal course according to physiological function, then the instinct of death would appear in its time, after a normal life and an old age healthy and prolonged.