246 NATURAL SCIENCE [October 



although there is much evidence to the contrary ; but, in either case, 

 isolation by sexual selection is necessary for the preservation of any 

 variations the males may possess. 



From this examination of the place of isolation in organic 

 evolution I conclude that species generally originate by the pre- 

 servation of individual variations by means of geographical or 

 physiological isolation, which may be brought about in many ways ; 

 and that, in most, but by no means all, cases they are improved and 

 developed by competition and natural selection ; and this competi- 

 tion they find only in large and well-populated areas. 



F. W. Hutton. 



