Finally, we need to note that natural selection does not necessitate in any mechanical sense certain conduct on the part of individuals or groups. Rather, natural selection marks the limits of variation which nature permits, and within those limits of variation there is a large amount of freedom of choice, both to individuals and to groups. Human societies, therefore, may be conceivably free to take one of several paths of development at any particular point. But in the long run they must conform to the ultimate conditions of survival; and this probably means that the goal of their evolution is largely fixed for them. Human groups are free only in the sense that they may go either backward or forward on the path which the conditions of survival mark out for them. They are free to progress or to perish. But social evolution in any case, in the sense of social change either toward higher or toward lower social adaptation, is a necessity that cannot be escaped. Sociology and all social science is, therefore, a study not of what human groups would like to do, but of what they must do in order to survive, that is, how they can control their environment by utilizing the laws which govern universal evolution. From this brief and most elementary consideration of the bearings of evolutionary theory upon social problems it is evident that evolution, in the sense of what we know about the development of life and society in the past, must be the guidepost of the sociologist. Human social evolution, we repeat, rests upon and is conditioned by biological evolution at every point. There is, therefore, scarcely any sanity in sociology without the biological point of view. SELECT REFERENCES _For brief reading:_ FAIRBANKS, _Introduction to Sociology,_ Chaps. XIV.-XV. JORDAN, _Foot-Notes to Evolution,_ Chaps. I.-III. ELY, _Evolution of Industrial Society._ Part II, Chaps. I.-III. _For more extended reading:_ DARWIN, _Descent of Man._ FISKE, _Outlines of Cosmic Philosophy._ WALLACE, _Darwinism._ _On the religious aspects of evolution:_ DRUMMOND, _Ascent of Man._ FISKE, _The Destiny of Man._ FISKE, _Through Nature to God._ CHAPTER III THE FUNCTION OF THE FAMILY IN SOCIAL ORGANIZATION