their instinctive reactions to the same stimuli may often vary considerably. It is in this degree that sex enters as a modifying factor into all forms of association. Just as biological variation constantly alters the physical aspects of heredity, so it also constantly alters the inborn psycho-physical dispositions of individuals. The concept of instinct, therefore, leaves as large a place as any sociologist could desire for the influence of selection, of race, of sex and of inborn individual differences in the social life.
While there can be no question but that instinctive reactions, from the psychological point of view, are the basis of the relationships of individuals in society, nevertheless, it is very difficult to say just what proportion of human activities may be regarded as primarily instinctive. It is especially difficult to trace the instinctive element in human institutions as they exist in modern civilized society. It is certainly incorrect to explain anything important in the social life of civilized peoples simply through instinct, on account of the fact that instinctive reactions under such conditions are overlaid with a mass of habits which we term custom and tradition, and are constantly modified or inhibited by many other social factors. On the other hand, it is an equally serious error to ignore the instinctive element, even in the complex conditions of modern life. Even though we can not determine quantitatively the relations between the instinctive and acquired elements in any given social situation, it is important to note that they both exist and that the instinctive is the basis of the acquired. Some tests of the instinctive element in human society can, however, be devised by psychologists and sociologists. In general we may safely regard those activities as instinctive which characterize the species, that is, which are relatively common to all men in all stages of culture. Again, those activities which man shares with the animals below him may, for the most part, be regarded as instinctive. Finally, from the study of the child and the adolescent, the sociologist may also perceive with more or less clearness some of the instinctive elements in human conduct and character.
Another difficulty which confronts the sociologist in tracing concretely the instinctive element in any given social situation is the great complexity of human instincts themselves. It is, of course, a grave psychological error to suppose that there are a number of separate and distinct human instincts which exist side by side without running into each other and which have each a separate function to perform. Rather human instincts, corresponding to the conception of them just given as inborn pathways in the nervous system, continually run into each other and reenforce each other like a network of streams or electric currents. The consequence is that human institutions are generally expressions of a number of instincts combined in various ways, besides being, of course, often built up largely on the basis of acquired traits. It is