178 EVOLUTION 



riers, when a species splits into two or more 

 castes with different habits of life; jphysio- 

 IfiQdwl harriers, such as arise by some va- 

 riation in the reproductive organs; and 

 i^r^. which rest on profound 



antipathies. 



What probably happens is this: a suqresg- 

 a.Ty_r| adapfcye sfpenes spread 



_ 



the several (*Ymt,jngentfl become isolated 

 from onc-another; and> if Different. 

 spring W-in several or all r>f 



then. nt r her tiingff being equal, isolation 



do not doubt," Darwin said, "that isolation 

 is of considerable importance in the forma- 

 tion of new Species." "fa may ]bie pf j^prnp 

 importance in _ preventing intercrossing but 

 it is much more^ikel^lhaf^ifT^works by 

 bringing about jjose M> mb>reedmp[ 7 which c\&- 

 V^lops prepotenqy or stabiljf.y 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- 



