40U HEREDITY AND ENVIRONMENT 



The actual origin of new hereditary charac- 

 ters 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 sweeping. 



It is well known that environmental changes 

 produce many kinds of modifications in organ- 

 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 hereditary 

 characters. 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 



