In current language the word "instinct" is used for all the hereditary and automatic manifestations of activity, from tropisms to the most complicated outward manifestations of individual memory. Instinctive acts are stereotyped, always the same when responding to stimuli of the same kind, and almost always adapted to their object, although not the result of the previous individual experience. It is impossible to define them more precisely, since they are varied and complex, overlapping one another and often becoming so confused that it is difficult to trace their limits.
We should not, however, put them all in the same category and attribute a common origin to all. Tropic reactions result from the properties of living matter, rhythms presuppose an organic memory, and therefore an ancient or recent apprenticeship ; but this apprenticeship is purely mechanical and is dependent upon the stimuli that produce it.
Apprenticeship has also played a part in the manifestations of species memory which play so great a part in the behavior of articulates, although this has a distinctly higher character, since it was created by the distant ancestors of the individual under the form of a choice between the different possible responses of differential sensitiveness. Choice means still more and has an intellectual character that is even more wonderful in the instinctive manifestations of the individual memory. Endowed with well-developed senses and a nervous system, the animal not only reacts to new necessities with new acts, but it preserves the memory of old sensations, associates the old with the new, and uses them to meet the new emergency. Thus, in an intelligent way, new habits are established, which by heredity are added to the patrimony of instinct, modifying it and forming one of the essential elements of its evolution. It is these instincts, acquired by an intelligent apprenticeship, that Forel said were automatized reasoning, and it is to them particularly that we may apply the conception of certain biologists who define instincts as habits which have become hereditary and automatic. It is probable that all the higher instincts had originally this intellectual quality. This certainly is true of those which originated from more or less slowly acquired habits; but it is apparently also the rule with those originating as mutations; for whether they result from sudden psychic change or follow a sudden organic modification, these instincts must always he preceded by some intelligent period of apprenticeship, during which they are perfected, to pass on to posterity and to take the instinctive character.
Here, then, we are confronted with several classes of instinctive acts, which differ not only in their origin but in intellectual characteristics. No doubt they are linked together by many intermediates, and in the animals which we are considering they often blend the one with the other or even with the reflexes, on account of the profound differentiation of nerve and sensorial centers. Nevertheless it is very difficult to see in them manifestations of a special faculty which under the name instinct we can place on the plane of intelligence. There are certain forms of activity which may justly be called instincts, since they are innate and automatic, but these forms proceeded in diverse ways from the vital energy which is the source of all organic activity ; and the highest, which are at the same time the most striking ones in the animals we are studying, were originally acts which required more or less true intelligence on the part of species and of individuals. Intelligence never played any part in the development of that instinct which attracts nocturnal Lepidoptera toward the light, nor does it play any greater part with the rhythms by which organic memory manifests itself. But it is intelligence that rules by adaptive selection all manifestations of species memory; and it is intelligence, again, that puts together the most complicated mechanisms of instinct in the diverse forms of association and of individual memory.