SELECTION 177 



AUXILIARY HYPOTHESIS OF ISOLATION. 

 We have already referred to the occurrence 

 of particular species on particular islands in 

 the Galapagos Archipelago, and there are a 

 great many similar cases which suggest that 

 isolation means something in 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 knows the Shetland pony, 

 the Highland cattle. 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 the idea of Isolation, as a directive 

 factor in evolution ; and Romanes maintained 

 that it was a sine qua non in the origin of new 

 species. The term must not be thought of 

 in any narrow sense; it includes all the means 

 which restrict the range of intercrossing 

 within a species: geographical barriers, such 

 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- 



