=Classification of Instincts.= For convenience in treatment the instincts are grouped in classes. Those instincts most closely related to individual survival are called _individualistic_ instincts. Those more closely related to the survival of the group are called _socialistic_. Those individualistic tendencies growing out of periodic changes of the environment may be called _environmental_ instincts. Those closely related to human infancy, adapting and adjusting the child to the world in which he lives, may be called _adaptive_. There is still another group of inherited tendencies connected with sex and reproduction, which are not discussed in this book. We shall give a brief discussion of the instincts falling under these various classes. It must be remembered, however, that the psychology of the instincts is indefinite and obscure. It is difficult to bring the instincts into the laboratory for accurate study. For our knowledge of the instincts we are dependent, for the most part, on general observation. We have had a few careful studies of the very earliest years of childhood. However, although from the theoretical point of view our knowledge of the instincts is incomplete, it is sufficient to be of considerable practical value. =The Individualistic Instincts.= Man's civilized life has covered but a short period of time, only a few hundred or a few thousand years. His pre-civilized life doubtless covered a period of millions of years. The inborn tendencies in us are such as were developed in the long period of savage life. During all of man's life in the time before civilization, he was always in danger. He had many enemies, and most of these enemies had the advantage of him in strength and natural means of defense. Unaided by weapons, he could hardly hold his own against any of the beasts of prey. So there were developed in man by the process of natural selection many inherited responses which we group under the head of _fear_ responses. Just what the various situations are that bring forth these responses has never been carefully worked out. But any situation that suddenly puts an individual in danger of losing his life brings about characteristic reactions. The most characteristic of the responses are shown in connection with circulation and respiration. Both of these processes are much interfered with. Sometimes the action is accelerated, at other times it is retarded, and in some cases the respiratory and circulatory organs are almost paralyzed. Also the small muscles of the skin are made to contract, producing the sensation of the hair standing on end. Just what the original use of all these responses was it is difficult now to work out, but doubtless each served some useful purpose.