402 HEREDITY AND ENVIRONMENT 



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 do not 

 know, for like the kingdom of heaven they 

 come without observation. Their infrequency 

 amidst the multitude of fluctuations indicates 

 the wonderful stability of racial types and 

 teaches respect for Weismann's doctrine of a 

 germ plasm relatively stable, independent and 

 continuous. 



This distinction between somatic and germi- 

 nal variations, between those which concern 

 only the individual and those which are in- 

 herited and furnish material for evolution, 

 marks the greatest advance in the study of 

 evolution since the work of Darwin. And just 

 as these germinal variations are the only ones 

 of importance in the process of evolution so 

 the question of their origin is the greatest evo- 

 lutionary problem of the present day. How 

 are such germinal variations produced? 



