575 240 [October 
575.3 
iti 
The Place of Isolation in Organic Evolution 
LTHOUGH 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 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? 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. 
