The antiquated psychology of the Middle Ages (which, however, still numbers many adherents) considered the mental life of man and that of the brute to be two entirely different phenomena; the one it attributed to "reason," the other to "instinct." In harmony with the traditional story of creation, it was assumed that each animal species had received a definite, unconscious psychic force from the Creator at its formation, and that this instinct of each species was just as unchangeable as its bodily structure. Lamarck proved the untenableness of this error in 1809 by establishing the theory of Descent, and Darwin completely demolished it in 1859. He proved the following important theses with the aid of his theory of selection: 1. The instincts of species show individual differences, and are just as subject to modification under the law of _adaptation_ as the morphological features of their bodily structure. 2. These modifications (generally arising from a change of habits) are partly transmitted to offspring by _heredity_, and thus accumulate and are accentuated in the course of generations. 3. _Selection_, both artificial and natural, singles out certain of these inherited modifications of the psychic activity; it preserves the most useful and rejects the least adaptive. 4. The _divergence_ of psychic character which thus arises leads, in the course of generations, to the formation of new instincts, just as the divergence of morphological character gives rise to new species. Darwin's theory of instinct is now accepted by most biologists; Romanes has treated it so ably, and so greatly expanded it in his distinguished work on _Mental Evolution in the Animal World_, that I need merely refer to it here. I will only venture the brief statement that, in my opinion, there are instincts in _all_ organisms--in all the protists and plants as well as in all the animals and in man; though in the latter they tend to disappear in proportion as reason makes progress at their expense.