242 DARWINISM TO-DAY. 



on the "geological study of the migration of marine inverte- 

 brates," Smith 10 has pointed out the general principles and 

 conditions upon which the interpretation of the geographic 

 distribution of the marine invertebrates of the earlier geo- 

 logic ages must be based. 



In closing this consideration of the status of geographic 



isolation as a factor in species-forming, I should not omit to 



Isolation not call attention to the fact, which should be obvi- 



an all-sufficient ous enO ugrh to anv reader, that isolation in itself 



agent of species- 

 forming, cannot be the basic and all-sufficient cause for 



the production of specific differentiation, any more than any 

 selective factor can. The prerequisite in both cases is the 

 occurrence of variation. What are the variations, and how 

 are they produced : these are the fundamental questions in 

 species-forming. Isolation is a tremendously favouring con- 

 dition but not a primary cause of species-forming. It tends 

 to help along, to hurry up species disintegration, not to- 

 initiate it. It is a biological catalytic agent. 



In this present connection the pertinent question is, is the 

 influence of geographical isolation in the cumulation of 

 variation or intra-specific differentiation due to its com- 

 pelling in-and-in breeding and hence the fostering and cumu- 

 lation of fortuitous Darwinian variations occurring in the 

 comparatively few individuals of the isolated group, or is 

 there a spur to variation, or even actual production of it along 

 determinate lines, in the new environmental conditions com- 

 mon to all the isolated group but inevitably different from 

 the conditions to which the parent type is exposed ? Is there 

 a gradual accumulation of differences between the split-off 

 group and the parent group due to environmental influence 

 plus in-and-in breeding? If we could reply yes to this 

 question, we should have a sufficient explanation of 

 how the isolated group splits rapidly away in many 

 small, and in a few large, characteristics from the parent 

 stock. 



