There can be no genuine clarity in the understanding of man''s biol. nature until the role of the social factor in his development is understood. For rational systems of evolution it is necessary to look to those which consider progressive adaptation the driving force of the process. Man''s adaptation consists chiefly in developing his inventiveness. For any trait, its fixity or plasticity is genetically controlled and the direction of the evolutionary adaptation will depend on circumstances. First, the change is always detd. by the nature of the supply of mutational variability available. Secondly, the direction of the change is controlled by natural selection. Human social environments are notable for their complexity and also for the rapid changes to which immediate adjustment is demanded. Adjustment occurs chiefly in the psychical realm. Genetic fixation of behavioral traits would have been unfavorable for survival. The genetically controlled plasticity of mental traits is the most typical and unique human characteristic. The cultural evidence suggests that the essentially human organization of the mental capacities emerged early in human evolution. It is probable in view of the diff. environments of men that significant genetic diffs. in the mental capacities of the various ethnic groups of mankind exit.