In many varieties of animals, however, only two instincts are present, self-preservation and reproduction; but in animals that are associated together into herds or packs, a third instinct is developed, the social instinct. When this occurs, the functioning of the self-preservation instinct is greatly modified; the individual no longer owes his existence solely to his power to cope with his environment, but depends mainly upon his ability to keep his place in the herd; and upon the social organisation devolves the task of adaptation and survival. The strayed sheep is soon hunted down, the solitary wolf starves. This is equally true of man, who is also a social animal. The misery of Central Europe, in the breakdown of social organisation following upon the war, has shown us the helplessness of the individual human being and his complete dependence upon herd life. The self-preservation instinct and its ruthless functioning under the law of natural selection has furnished a theme to many moralists and sociologists of the materialistic type, but they are apt to forget that the socialisation of humanity has changed the nature of the problem; the unit of survival is no longer the individual, but the social organisation of which he is a member. The law of self-preservation has given place to the law of group preservation, and the centre of psychic gravity is shifted. The importance of this point cannot be over-estimated in practical psychology. By some psychologists the instinct of nutrition is distinguished from that of self-preservation, but for all practical purposes they are identical. It must be borne in mind, in applying the standards of psychology to the human character, that in the more highly developed types of human being the self-preservation instinct is not fulfilled simply by the continuance of physical life; there is self-preservation of the personality as well as of the bodily existence, and unless a man has adequate scope for self-expression and self-development, he will experience that sense of incompleteness and imperfection characteristic of the repression of an instinct. CHAPTER IX DISEASES OF THE SELF-PRESERVATION INSTINCT The self-preservation instinct, having its source in the sense of individuality, of separateness, is the motive of our self-assertion. It is necessary that each member of a herd should have a certain amount of self-assertiveness in order to maintain his place among his fellows. If, however, this quality is above or below the requisite standard, his survival will be endangered; if, on the one hand, he is lacking in self-assertion, he will not obtain his fair share of the means of life available for the group of which he is a member. On the other hand, if his self-assertion is excessive, it may disrupt the social organisation, and either lead to the extinction of the group, or to his ejection from it.