388 HEREDITY AND ENVIRONMENT 



factors are comparable to the changes which 

 take place in certain atoms, for example ra- 

 dium, by which the element itself is changed in 

 an irreversible manner. Evolution depends 

 upon the appearance of new characters; the 

 discoveries of Mendel show us how to follow 

 old characters through many combinations and 

 through many generations, but they do not 

 show us how new characters arise. These dis- 

 coveries have given us an invaluable method 

 of sorting and combining hereditary qualities, 

 but Mendelian inheritance as such does not 

 furnish the materials for evolution. 



The actual origin of new hereditary charac- 

 ters or mutations is obscure. Practically all 

 of the earlier workers and writers on evolution 

 have found the principal causes of transmuta- 

 tion 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- 



