178 EVOLUTION 



riers, when a species splits into two or more 

 castes with different habits of life; physio- 

 logical barriers, such as arise by some va- 

 riation in the reproductive organs; and 

 psychological barriers, which rest on profound 

 antipathies. 



What probably happens is this: a success- 

 fully vigorous and adaptive species spreads; 

 the several contingents become isolated 

 from one another; and, if different variations 

 spring up in several or all of the contingents, 

 then, other things being equal, isolation 

 will favour the origin of distinct species. "I 

 do not doubt," Darwin said, "that isolation 

 is of considerable [importance in the forma- 

 tion of new species." It may be of some 

 importance in preventing intercrossing, but 

 it is much more likely that it works by 

 bringing about close inbreeding, which de- 

 velops prepotency or stability of type. 



In the human world, the manifold range 

 of individuality presented by regions favour- 

 able to family, village, and clan isolations, 

 such as Scotland or Norway, Greece or Pales- 

 tine, thus becomes intelligible. Again, in 

 that deterioration of the cities of the plain, 

 which is so frequent throughout history, so 

 evident to-day, we may increasingly fear an 

 organic factor underlying the obvious social 

 ones that of reversion through intercross- 



