280 Heredity and Environment 



3. Causes of Mutations. The causes of new hereditary char- 

 acters, or rather of mutations in genes, are 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 mod- 

 ern genetics such a statement is too sweeping. 



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 ontog- 

 eny; 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 of the germplasm 

 arises which forms the basis of a new race (Figs. 99, 100, 101). 

 In most cases such mutations manifest themselves in the failure 

 of some old character to develop 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 infrequency 

 amidst the multitude of fluctuations indicates the wonderful sta- 

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

 of a germplasm 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- 



