266 Heredity and Environment 



The actual origin of new hereditary characters or mutations 

 is obscure. Practically all of the earlier workers and writers on 

 evolution found the principal causes of transmutation in the action 

 of extrinsic or environmental forces on the organism. As the 

 result of years of labor on this subject Darwin concluded that 

 "variability of every sort is due to changed conditions of life"; 

 but in the light of modern genetics such a statement is too sweep- 

 ing. 



It is well known that environmental changes produce many 

 kinds of modifications in organisms, and in general these modi- 

 fications 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 striking results of recent work is to show the small effect 

 of environmental changes on hereditary characters. Marked in- 

 dividual 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 chang- 

 ing environment 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. 99, 100). In most cases such mutations consist in the 

 dropping out of s,ome old character rather than in the addition of 

 a new one, but at least they represent modifications of hereditary 

 constitution and as such they furnish material for evolution. 

 Whence and how they appear we do not know, for like the king- 

 dom of heaven they come without observation. Their inf requency 

 amidst the multitude of fluctuations indicates the wonderful sta- 

 bility of racial types and teaches respect for Weismann's doctrine 

 of a germ plasm relatively stable, independent and continuous. 



This distinction between somatic and germinal variations, be- 

 tween those which concern only the individual and those which 

 are inherited and furnish material for evolution, marks the great- 

 est advance in the study of evolution since the work of Darwin. 

 And just as these germinal variations are the only ones of impor- 



