SELECTION: ORGANIC AND SOCIAL 211 



of a species reach sexual maturity at different 

 times of year ; habitudinal barriers, when a species 

 splits into two or more castes with different habits 

 of life ; physiological barriers, such as arise by some 

 variation in the reproductive organs ; and psycho- 

 logical barriers, which rest on profound antipathies. 

 The subject has been worked at a good deal since 

 Darwin's day, by Wagner, Gulick, Romanes, 

 Jordan, and others and Romanes went the length 

 of saying that Isolation was a sine qua non in the 

 origin of new species. The great difficulty is to 

 get a sufficient body of reliable facts. 



From many passages in Darwin's works it is 

 evident that he recognised that isolation, or segrega- 

 tion, is important in natural selection, just as it is 

 in artificial selection. " I do not doubt," he says, 

 " that isolation is of considerable importance in 

 the formation of new species/' But he did not 

 analyse the idea as some post-Darwinian workers 

 have done. 



When a species spreads, several contingents may 

 become isolated from one another, and, if different 

 variations spring up in the several contingents, 

 then the isolation will favour the origin of distinct 

 species. It works in two ways : (1) by preventing 

 intercrossing and its possibly levelling effects, and 

 (2) by involving close inbreeding, which develops 

 prepotency or stability of type. There is one 

 bird peculiar to Britain, namely, the red grouse, 

 but it is closely allied to the Scandinavian willow 

 grouse, and it seems impossible to doubt that 

 the literal isolation of Britain has allowed the 

 red grouse to diverge as a new species from the 

 willow grouse stock. 



There are said to be eighty species of the land- 



