IN ORGANIC EVOLUTION 88 



mainly in its static aspects, not as a necessary or 

 at least not a separate factor in evolution. 

 " Each species," he says, " has been produced 

 within one area and has migrated as far as it 

 could." This statement may be taken as the 

 central fact of our knowledge of geographical 

 distribution. The distribution of each species 

 covers the earth except in so far as it is unable to 

 reach distant parts through barriers, or as it has 

 been unable to maintain itself in regions which 

 it has reached — or as it has, through selection and 

 isolation, been changed in some part of its range 

 into a different species. In this case as else- 

 where selection and segregation must work to- 

 gether, the one producing adaptive divergence or 

 adaptive convergence, the other non-adaptive 

 divergence alone. 



Darwin quotes from Wallace that " every spe- 

 cies has come into existence coincident in space 

 and in time with a pre-existing closely alhed spe- 

 cies." This coincidence is attributed, by Darwin 

 and Wallace, to " descent with modification." 

 The language quoted is perhaps obscure, but the 

 meaning of Wallace is clearly a recognition of 

 the mutual relations of geminate species. 



Darwin further states : "I do not doubt that 

 isolation is of considerable importance in the for- 

 mation of new species." He goes on to say that : 

 " On the whole I am inclined to believe that large- 

 ness of area is of more importance, especially in 

 the production of species which will prove capa- 



