No. 543] PROBLEMS OF EVOLUTION 



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of regulation which causes a return to the type, when 

 once this has been departed from. 



In several instances recent investigators have found, 

 or have thought they have found, experimental evidence of 

 the inheritance of characters acquired through environ- 

 mental changes. But these evidences are by no means 

 conclusive. In a few cases it is known that the effects 

 of changed environment last through two or three genera- 

 tions and then disappear. In such cases racial, or speci- 

 fic, regulation is slow ; in most cases this regulation takes 

 place in the first generation after the environmental 

 change disappears. Perhaps in this lingering effect of 

 a changed environment we have the first indication of the 

 appearance and fixation of a new character. Here, un- 

 doubtedly, much work of value remains to be done. 



Very rarely a sudden variation, or mutation, arises 

 which is perpetuated by heredity and which forms the 

 basis of a new race. In most cases which have been care- 

 fully studied such mutations consist in the dropping out 

 of some old character rather than in the addition of a new 

 one, but at least they represent modifications of the hered- 

 itary characters, and as such they furnish material for 

 evolution. Whence and how they appear we do not know, 

 for like the kingdom of heaven, they come without obser- 

 vation. Their infrequency, amidst the multitude of 

 somatic variations, indicates the wonderful stability of 

 racial types and teaches respect for Weismann's doctrine 

 of a germplasm, relatively stable, independent and con- 

 tinuous. 



This distinction between somatic and germinal vann^ 

 tions, between those which concern only the individual 

 and those which are inherited 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 evolutionary problem of the present day. 

 How are such germinal variations produced ? Do they 

 occur as the result of extrinsic or of intrinsic causes! By 



