In Modern Philology for April and July, 1909, I attempted to construct a theory of literary development on the basis of genetic psychology. Literature was there defined to be a function of consciousness, a psychological and social product devised for the purpose of revealing the ethical and æsthetic values of human life. So far as the propagation and variation of literary forms are concerned emphasis was laid upon the fact that these processes take place in every case through the medium of a conscious personality. This human consciousness is endowed with certain powers or aptitudes by virtue of which it assimilates to itself the traditions, conventions, artistic forms, religious beliefs, ethical convictions, scientific ideas, etc., of the society in which it is born, and hands them down to the generation that follows. The physical and mental endowment which enables the individual thus to learn and to transmit his acquirements to others constitutes his physical or biological heredity. The process of learning imitatively from the models, patterns, or examples, of one's predecessors is called by the psychologists “imitative selection”; and the great body of traditions, conventions, forms, from which the individual must learn, and to which he must adjust himself, is termed his “social heredity.” Variation in literature may therefore be defined as the attempt more or less constant to adjust literature to the writer's social heredity—or, as Prof. Manly has shown in the case of the mediæval drama, it is the combination of elements or unit characters hitherto kept separate. The cardinal factor to be emphasized in any study of variation in literature is therefore the individual consciousness, its power of imitative selection, its dependence on social heredity for the materials with which it works, and its power of constructive imagination by which new combinations are produced. For example, I showed that Hauptman's naturalistic drama is to be explained as the combination of the traditional dramatic form with the evolutionary idea that man's destiny is controlled by heredity and environment and not by free will. Both these elements, the traditional dramatic form and the evolutionary idea, Hauptmann assimilated from his social heredity, and then by his power of æsthetic invention he united them in a new variation, the naturalistic drama.