No. 614] FACTOR MUTATIONS IN EVOLUTION 1 1 9 



example, Harrison lias discovered that certain species of 

 moths, which are natives of different continents, but which 

 resemble each other so closely morphologically as to be 

 sometimes indistinguishable, exhibit extreme physiolog- 

 ical differences. These physiological divergencies were 

 indicated by the failure of hybridization between these 

 species to produce offspring which were viable, or, if 

 viable, which were fertile. Harrison concludes that geo- 

 graphical differences play a very important part in the 

 production and accentuation of such physiological diver- 

 gencies. 



The role of the environment in the production of factor 

 mutations is still an unsolved problem. As Caullery 

 points out, the role of external factors in the production 

 of mutations is no longer very clearly or directly appar- 

 ent. It even appears that factor mutations occur in "all 

 directions" quite independently of those elements of the 

 environmental complex which are outside the organism. 

 This does not mean that factor mutations are not caused. 

 Like any other natural event, they must be dependent upon 

 or conditioned by certain antecedent events, and, a priori, 

 there is no reason why such antecedent events should 

 not occur outside the organism. In other words, it is 

 reasonable to suppose that specific elements of the ex- 

 ternal environment might induce permanent alterations 

 in genetic factors. But, as yet, such a specific relation 

 between the external environment and factor mutations 

 can not be said to have been determined beyond reason- 

 able doubt. On the other hand, migration, isolation and 

 geographical differences along with other elements of the 

 environment play an important role in the selection of mu- 

 tations, and must, therefore, be recognized as of funda- 

 mental importance in organic evolution. It is conceiv- 

 able, indeed, that, given the occurrence of factor mutations, 

 the continuous impingement of some definite element in 

 the environmental complex during long periods of time 

 might condition a definite orthogenetic trend in phylogeny, 

 as in the evolution of the elephant and the horse. But 



