THE THEORY OF ORGANIC EVOLUTION. 439 



nished, as we have seen, by constitutional or germinal 

 variations. What these may amount to depends in 

 the long run on the potentialities resident in living 

 matter, especially of reacting to external influences, 

 and this forces us finally back to the institution of 

 the order of nature which at some level or other the 

 evolutionist takes for granted. In organic evolution, 

 variation supplies the materials; heredity (or the 

 relation of genetic continuity between successive 

 generations) is one of the conditions; natural selec- 

 tion or elimination is one of the directive factors. 

 But there may be others, and one has been indicated 

 in what is called the theory of isolation. 



A formidable objection to the Darwinian theory, 

 first clearly stated by Professor Fleeming Jenkin, 

 and familiar to everyone who has thought out the 

 matter, is that variations of small amount and sparse 

 occurrence would tend to be swamped out by inter- 

 crossing. In artificial selection, the breeder takes 

 measures to prevent this by pairing similar or suit- 

 able forms together ; but what in nature corresponds 

 to the breeder ? 



Various suggestions have been made in answer to 

 this question. Thus Professor Weismann says, 

 " The necessary variations from which transforma- 

 tions arise must in all cases be exhibited over and 

 over again by many individuals," but there is still 

 a lack of concrete evidence to bear this out. We do 

 not mean to deny it, but before we lean heavily upon 

 it we should like to be able to furnish numerous ex- 

 amples of many similar variations occurring at once 

 within the same group. 



The favourite answer of recent years is that 

 worked out by the late Dr. Romanes, Mr. Gulick, 

 and others — ^the theory of isolation. They point to 



