_If we observe the behaviour of the lower animals, we find two sorts of response to stimulation: the one points to the working of an inherited nervous mechanism, the other depends upon nervous connections formed during the life-time of the individual._ The second year’s bird builds the nest of its species, though it has never built a nest before; the cage-reared migrant beats its wings against the bars at the approach of winter, though it has never taken flight to the southward. Here is behaviour that we must refer to innate nervous tendencies, to the working of an inherited nervous mechanism. If, on the other hand, the parent birds come to the window-sill and take crumbs from our hand, we are in presence of behaviour of the second type. The difference, in the broad, is clear enough; only we must not press it too far. The second year’s bird, we say, builds the nest of its species; but one nest is never quite like another; something will depend upon the situation. Contrariwise, the birds would not come to the window if they had not an innate attraction to food, or a natural boldness of disposition, or a native tendency to flock with their fellows. The two sorts of behaviour can be distinguished; but they are likely to enter together, though in unequal degree, into one and the same performance. =Instinct=, _now, is the general name for these innate tendencies to behaviour_. The word explains nothing; it is the business of science to find out what the inherited nervous mechanisms are, and how they work; but though nothing is explained, we are helped by the term toward a _classification_ of the facts of behaviour. All of man’s conduct will be instinctive, for example, that can be shown to issue from innate nervous tendencies; and further, all of man’s conduct will be _in so far_ instinctive as innate nervous tendencies can be shown to have a share in producing it. _How large a part, then, does instinct, in this sense, play in the life of man?_ Not a question that can be answered offhand! For you might argue, as has been argued, that because man is the most flexible and adaptable and teachable of all animals, because he lives in all climates and thrives in the most varied conditions of life, therefore he has but few instincts. Or you might argue that, since man has undergone more change and has progressed further than any other animal; since his evolutionary history, though not longer in time, is richer in biological incident than that of the other animals; therefore he must have a great variety of instincts, or at any rate a great variety of inherited nervous mechanisms that help to guide and shape his conduct. What are the facts?