SELECTION 177 



AUXILIARY HYPOTHESIS OF ISOLATION. 

 We have already referred to the occurrence 

 of, particular species on particular islands in 



j and there are a 



great many similar cases which suggest that 

 isolation means^ soviet]] jpg .uj evolution. 

 The red grouse is peculiar to Scotland, but 

 it has doubtless been derived from the 

 closely-related stock of the Scandinavian 

 willow grouse. While the zoologist has lately 

 distinguished an Orkney vole and a St. Kilda 

 wren, every one. J^npws the Shetland, pony, 

 tfrp Highland pa.tt.lft. There are said to be 

 eighty species of the land-snail Cerion in the 

 Bahamas, and Gulick records over 200 

 species of the land-snail Achatinella in the 

 various valleys of the Sandwich Island Oahu. 

 Many evolutionists Wagner, Weismann, 

 Gulick, Romanes, Jordan, and others have 

 worked at %* jrjpa pf Tsnlatinn^s a directive 



Romanes maintained 



that it was a sine^qua nog in the origin of new 

 species. The term must not be thought of 

 in any narrow sense; it includes all the means 

 wiiich restrict the range_ of intercrossing 



as arise when a peninsula becomes an island; 

 ^temporal barriers, such as arise when the 

 members of a' species reach sexual maturity 

 at different times of year; habitudinal bar- 



