Variation and Natural Selection. 99 



permitted. Whence it inevitably follows, as a matter of 

 simple logic, that where divergence has occurred, inter- 

 crossing and interblending must in some way have been 

 lessened or prevented. 



Thus a new factor is introduced, that of isolation, or 

 segregation. And there is no questioning the fact that it is 

 of great importance.* Its importance can, indeed, only be 

 denied by denying the swamping effects of intercrossing, 

 and such denial implies the tacit assumption that inter- 

 breeding and interblending are held in check by some form 

 of segregation. The isolation explicitly denied is implicitly 

 assumed. 



There are several ways in which isolation, or segregation, 

 may be effected. Isolation by geographical barriers is the 

 most obvious. A stretch of water, a mountain ridge, a 

 strip of desert land, may completely, or to a large extent, 

 prevent any intercrossing between members of a species on 

 either side of the barrier. The animals which inhabit the 

 several islands of the Galapagos Archipelago are closely 

 allied, but each island has its particular species or well- 

 marked varieties. Intercrossing between the several 

 varieties on the different islands is prevented, and diver- 

 gence is thus rendered possible and proceeds unchecked. 

 It is said that in the Zuyder Zee a new variety of herrings, 

 the fry of which are very small compared with open-sea 

 herrings, is being developed. And the salmon introduced 

 into Tasmania seem to be developing a fresh variety 

 with spots on the dorsal fin and a tinge of yellow on the 

 adipose fin. In the wooded valleys of the Sandwich Islands 

 there are allied but distinct species of land-shells. The 

 valleys that are nearest each other furnish the most nearly 

 related forms, and the degree of divergence is roughly 

 measured by the number of miles by which they are 

 separated. Here there is little or no intercrossing between 



* Its importance in artificial selection was emphasized by Darwin : " The 

 prevention of free crossing, and the intentional matching of individual animals, 

 are the corner-stones of the breeder's art " (" Animals and Plants under 

 Domestication," ii. 62). 



