CONTROL OF HEREDITY: EUGENICS 389 



isms, and in general these modifications are the 

 more profound the earlier they occur in onto- 

 geny ; it is known that slight alterations of the 

 germ cells may produce great modifications of 

 adult structure, and yet one of the most strik- 

 ing results of recent work is to show the small 

 effect of environmental changes on racial char- 

 acters. Marked individual modifications may 

 be produced which do not become racial. 

 Usually not one of thousands of variations 

 which occur has any evolutionary value. 

 These fluctuations come with changing en- 

 vironment and with changing environment 

 they disappear. Very rarely a sudden 

 variation or mutation arises which is per- 

 petuated by heredity and which forms the 

 basis of a new race (Figs. 94, 95). In most 

 cases such mutations consist in the dropping 

 out of some old character rather than in the 

 addition of a new one, but at least they repre- 

 sent modifications of hereditary constitution 

 and as such they furnish material for evolu- 

 tion. Whence and how they appear we rarely 

 know, for like the kingdom of heaven they 

 come without observation. Their infrequencv 



