Observe the dog, the birds of different species, the monkeys, the hares, and you find wonderful differences of habit, each adapting the animal differently, but with equal effectiveness, to the life which he in particular is called upon to lead. The ants and bees are notoriously expert in the matter of instinct. They have colonies in which some of the latest principles of social organization seem to find analogues: slavery, sexual regulations, division of labour, centralization of resources, government distribution of food, capital punishment, etc. All this--not to stop upon details which the books on animal life give in great abundance--has furnished grounds for speculation for centuries, and it is only in the last generation that the outlines of a theory of instinct have been filled in with substantial knowledge. A rapid sketch of this theory may be drawn in the following pages. 1. In instinct in general there is a basis of inherited nervous tendency toward the performance of just the sort of action which the instinct exhibits. This nervous tendency shows itself independently of learning by the individual in a great many cases, as in the instinct of sucking by young animals, pecking for food by young fowls, the migrating actions of adult mammals and birds, the courting movements of many varieties of animal species. In all this we have what is called the "perfect" instinct. To be perfect, an instinct must be carried out successfully by the animal when his organism is ready, without any instruction, any model to imitate, any experience to go upon. The "perfect" instincts are entirely congenital or inborn; the nervous apparatus only needs to reach the proper stage of maturity or growth, and forthwith the instinctive action is performed as soon as the external conditions of life are such as to make its performance appropriate and useful. 2. On the other hand, many instincts--indeed, probably the greater number--are not perfect, but "imperfect." Imperfect instincts are those which do not fully equip the animal with the function in question, but only take him part way to the goal. He has a spontaneous tendency to do certain things, such as building a nest, singing, etc.; but he is not able to do these things adequately or perfectly if left to himself from birth. This sort of endowment with imperfect instincts has been the field of some of the most interesting research in animal psychology, and has led to a new view of the relation of instinct to intelligence.