254 Heredity and Environment 



The part played by these different factors of development may 

 be graphically illustrated by the accompanying diagram (Fig. 88), 

 in which the base line represents heredity and the other lines rep- 

 resent the extrinsic factors of environment and education. For 

 each individual heredity is a constant factor but environment and 

 training are variables. With a given heredity the characteristics 

 of the developed organism may vary enormously depending upon 

 the extrinsic factors. Hereditary possibilities are not changed by 

 accidents of environment but development is so changed. After 

 the fertilization of the egg the hereditary potencies of every organ- 

 ism are unalterably fixed but the extrinsic factors remain variable 

 and may be controlled. 



All of our social and ethical institutions such as government, 

 education and religion deal only with extrinsic factors of develop- 

 ment and of life. Nevertheless there is no evidence that such 

 extrinsic influences ever modify heredity, no evidence that the 

 effects of good environment or good training ever change the ger- 

 minal constitution. The influences of environment and education 

 affect only the development of the individual and not the consti- 

 tution of the race, and consequently such influences are temporary 

 in effect and must be repeated generation after generation. 



Social versus Germinal Inheritance But though the effects of 

 environment and training are not inherited, the environment and 

 training and experience of former generations are handed down 

 to later generations through custom, tradition, history. We do 

 not inherit through the germ cells the effects on our ancestors of 

 their training and environment, but we do inherit, in the property 

 sense of that word, their environment, customs, institutions. In 

 short the experiences and accomplishments of past generations 

 are not inherited through the germ cells but are inherited through 

 society. In this sense "we are the heirs of all the ages." 



Man alone of all animals can profit largely by the experiences 

 of others and especially by the experiences of former genera- 

 tions. In the human species only are successive generations born 



