HEREDITY AND ENVIRONMENT IN DEVELOPMENT 441 



then bred to a white male and produced only black offspring, 

 presents conclusive evidence of the importance of heredity. Yet 

 much is due to environment. The interaction between these two 

 factors is so complex that it is impossible to sa}", for any character- 

 istic of the adult, how much is due to heredity and how much to 

 environment, although it is often possible to assign a preponderant 

 influence to one or the other. The greatest problem of Embryology 

 might even be said to be the determination of the relative impor- 

 tance of heredity and environment at each step in development. 

 This is not a new question, since it has been discussed for centuries; 

 and to-day the extent to which the environmental factor called 

 education can make amends for a defective heredity is of vital 

 importance to mankind. 



The possibilities of environmental influences are so readily 

 appreciable that men have often ascribed to them a major role and 

 have overlooked the more subtle factor of heredity in the develop- 

 ment of the individual. Scientists like Lamarck and his modern 

 followers have beheved that the environment, acting directly upon 

 the organism, can be the cause of evolution. To cite other exam- 

 ples, it has been supposed that such differences as those between 

 the sexes of higher animals may be the result of environmental 

 conditions, hke food and temperature, and that skin color in the 

 various races of mankind may have l)cen produced by the intensity 

 of the sunlight to which the individuals were subjected generation 

 after generation. Our democratic social institutions are based 

 largely upon the doctrine that "all men are created equal," by 

 which the importance of heredity' is virtually denied while that 

 of environment is magnified. The advance of biological knowledge 

 has shown the importance of heredity in the production of the indi- 

 vidual, and modern theories of the causes of evolution incline 

 toward factors that originate in the hereditary constitution rather 

 than in the en^'ironment. Yet it cannot be too strongly insisted 

 that the individual that develops from a fertilized egg is a product 

 of both of these factors. 



The environmental conditions that affect development may be 

 classified as the physical factors of mechanical relationships, heat, 

 light, gra\aty, density of medium, etc. ; and the chemical factors, 

 such as the oxygen, carbon dioxide, water, food, and such ele- 

 ments or compounds as may be necessary for normal development. 

 In the study of development, physical or chemical factors that 



