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From intelligent behavior, instinctive behavior differs in that it is neither learned by the individual nor, in its main features, much improved by learning. Nevertheless, like other structures and functions, it is strengthened and perfected as the animal matures; and the fact that the animal may attempt clumsily to perform some instinctive activity before maturation is complete may give the impression that competence in its execution depends upon learning. Yet these first tentative efforts, these preliminary interactions with the environment that in many cases seem necessary to perfect the instinct, are perhaps not to be sharply distinguished from learning in the ordinary sense of the word.

Accordingly, we must recognize, as Hebb (1953) pointed out, that the special effect of environmental stimulation that we call learning plays an essential part in perfecting all so-called innate activities; just as learning itself would be impossible without a large endowment of innate capacities. All behavior, both instinctive and acquired, is dependent upon both heredity and the environment; in the former, the individual's experience is supplementary to the hereditary endowment, whereas in learned behavior the innate endowment is overshadowed by the individual's experience. The difference between instinctive and learned activities reduces itself, in the final analysis, to the relative weight of these two factors that jointly determine all behavior. In the absence of either, animal activity is hardly possible.

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That instinctive behavior is adaptive or adjusted to promote the welfare of the individual and the perpetuation of its species is a conclusion that follows from prevailing views on the origin and mode of transmission of instincts. Since they are held to grow and change by successive genetic mutations, and are transmitted through the germ plasm rather than passed by example or instruction from generation to generation, only instincts beneficial to the species are likely to arise and to escape elimination by natural selection.

In conclusion, we might say that an instinctive activity consists of a series of coordinated acts into which an animal enters as a whole; it is transmitted genetically rather than by example or instruction; and it conforms rather closely to a pattern common to a whole population of animals and is, as a rule, essential to their continued prosperity.

The Two Phases of Instinctive Behavior

Instinctive activity is set in motion by some impulse, tension, or drive, which may be wholly internal or spontaneous in origin or may be awakened or strengthened by external stimuli acting upon an innate foundation or propensity. However initiated, the whole activity of an animal that in general follows innate patterns may conveniently be divided into two stages or phases, the appetitive and the consummatory. In the second phase, the animal completes some act of value to itself or its species, such as seizing and swallowing food, making a nest or shelter, performing stereotyped courtship rites, or sexual union with its mate. These consummatory acts are, as a rule, carried out in rather definite circumstances, with a sequence of movements that permits slight variation. The animal's whole organization, bodily and psychic, is set to perform these acts in a predetermined way.

But before an animal can seize and swallow food, it must find or overtake it; before it can dig a burrow or build a nest, it must choose a suitable site or select proper materials from the great variety that the locality may afford; before it can court a mate, it must find one.

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In these and similar cases, the seeking and acquiring of the appropriate object or situation is known as appetitive behavior. This questing and selecting amid the endlessly variable conditions of the external world, with all its surprises and perils, demands greater plasticity that the consummatory act, which, rigid as it is, cannot be successfully performed