LAWS CONTROLLING HUMAN SOCIAL HEREDITY 335 



ural selection is tending in the human race toward 

 the enhancement of the ethical nature of man. 



Final Conteast of Organic and Social Heredity 



Organic evolution has produced for man his body 

 and brain with mental powers in which the amount 

 of fixed inheritance is slight while the plasticity is 

 great. Natural selection acting upon man has pre- 

 served those races in which the social instinct is best 

 developed, together with such other instincts as lead 

 toward a willingness to sacrifice self-interests in 

 some degree. These latter factors have been slowly 

 developing during his history. In distinction from 

 the evolution of animals, among which the tendency 

 has been for fixed adjustments of the nervous system 

 to be formed and inherited (instincts), mankind has 

 apparently developed less and less fixity in nervous 

 structure and become more plastic. With mankind 

 natural selection has largely resulted in the elimina- 

 tion of fixed instincts, leaving a race whose nervous 

 system is very complex and whose possibilities of 

 combination are extremely great, but a nervous 

 system that is not preformed at birth and is therefore 

 capable of an almost unlimited amount of subsequent 

 molding by the action of the environment upon it. 

 Up to this point we are dealing with factors which 

 are organic in nature and transmitted by germinal 

 heredity. 



Inasmuch as this brain is not preformed, and its 

 machinery is not already adjusted into its intricate 

 relations by prenatal influences, it is possible that the 

 structure which arises upon this plastic substratum 

 may be immensely varied. Upon this plastic nature 

 the force of social heredity has engrafted the struc- 



