Instinct, an inherited tendency to make certain responses in a definite manner. With this tendency to respond in a definite manner there is present also the physical structure involved in making the response, and the environment is usually such as to act as a stimulating factor. The young duck has an instinct to swim. It is built for the water; its feet are paddles. If water is a part of its environment the duck is bound to swim. Tendency, structure, and environment are the three factors involved. But very often the instinctive response occurs in the face of unfavorable environment; a broody hen sometimes sits without eggs or nest, and a dog with a bone has been known to go through the motions of burying it on a stone floor.
Instincts may be classified as definite and indefinite. The instincts of lower animals are mostly quite definite; those of man are indefinite. Man may be said to have an instinct to imitate, but it is not definite; he may imitate almost anything. The robin has an instinct to build a nest; it is very definite; all robins build nests much alike. Another classification of instincts may be made on the basis of their time of appearance in the life of the individual. Some instincts are present at all times during life; such is the instinct for self-preservation. Other instincts re-appear from time to time as do the nest-building and brooding instincts of birds. Still others are delayed until complete physical development permits the responses they prompt, as in the case of the parental instinct of most animals.
Many responses of lower animals which seem to be the result of reasoning may be due entirely to instinct. The wonderful activities of bees are thought to be purely instinctive. Higher nerve centers such as reasoning involves need not be excited in making instinctive responses. The right nerve connections for such responses are probably inherited. Instincts are therefore racial rather than individual; they have sometimes been called race habits. Whether habits acquired by individuals may be inherited by offspring and thus finally become instinctive is a question which biologists have been unable to answer.
