575 240 [October 



575.3 



II 



The Place of Isolation in Organic Evolution 



ALTHOUGH most writers on evolution mention the subject of 

 isolation, very few attach much importance to it, Professor 

 Cope even considering it as a function of natural selection, 1 which 

 is putting the cart before the horse. This neglect of isolation is 

 probably due to the term ' selection ' having been used in such a 

 variety of ways and having been made to include almost every 

 process in evolution, even the origination of variations. But such 

 indiscriminate use of a word which has a very definite meaning is 

 objectionable, for it confuses in our minds several totally different 

 things. To me it seems self-evident that all the known factors of 

 organic evolution should be arranged under two heads : (1) the 

 origin of variations capable of being transmitted by amphimixis or 

 by environment, or by use and disuse, or by any other means ; and 

 (2) the preservation of variations by isolation or segregation, as it 

 has also been called. Possibly ' internal tendency,' ' kinetogenesis,' 

 or ' action of the environment ' may be other causes which tend to 

 preserve variations, but they have not yet been clearly established as 

 such, while there is strong evidence in favour of isolation being the 

 chief, if not the only, cause of the preservation of variations. The 

 subject of this paper is to point out the important part which 

 isolation must play in evolution. 



Professor Y. Delboeuf has shown 2 that if in any species a 

 number of individuals, bearing a ratio not infinitely small to the 

 entire number of births, are in every generation born with a 

 particular variation, which is neither beneficial nor injurious, and if 

 it is not counteracted by reversion, then the proportion of the 

 new variation to the original form will increase until it approaches 

 infinitely near to equality. Now the effect of the isolation of a few 

 individuals is to largely increase the ratio of any new variation 

 which may appear among them to the total number of births, and 

 thus to largely increase the chances of its preservation. On the 

 other hand, every variation which arises in a few individuals, and 

 which is subject to the free intercrossing of a large number of other 

 individuals, will tend to disappear. Intercrossing is probably 

 favourable to the production of variations, although it is unfavour- 



1 "Primary Factors of Organic Evolution," p. 386. 



2 Quoted in Murphy's " Habit and Intelligence," p. 241. 



