EVOLUTION IN NATURE 149 



environmental conditions, and according to our 

 expressed interpretation of the organism the ap- 

 pearance of all characters must, in some degree, 

 be governed by the environment. The mere fact 

 that conditions which permit the existence of the 

 organism normally permit the development of all 

 hereditary characters does not contradict this point, 

 hence we are brought very close to a union of the 

 idea of environmental modification of living things 

 and the idea of mutation.^ 



Several years ago I analyzed the *' acquired 

 characters" of the literature and pointed out that 

 all significant cases involved the response of a he 

 reditary factor as well as the occurrence of an 

 environmental stimulus.^ The distinction between 

 slight variations in the expression of a character 

 as a given heritage responds to fluctuations of 

 environment and the appearance of a mutation, or 

 its failure to appear, according to environmental 

 conditions, seems very tenuous. It is evident that 

 many discussions of this point in evolution fail to 

 strike at the heart of the problem. 



With these matters in mind it is profitable to 

 consider the ways in which evolution, as we see its 

 results in the natural world, may have taken place. 

 Thus we may apply the facts drawn from experi- 



' Cunningham, J. T., Modern Biology, Ch. V, 1928, criticizes this interpre- 

 tation of the environmental factors in normal development as expressed 

 by Goodrich. 



4 Am. Nat., Vol. LXI, pp. 251-265, 1927. 



