ISOLATION 179 



were all kept apart so that intercrossing might be impossible. Then 

 we might, possibly, have a very large number of definite species, 

 but they would still belong all of them to the genus Rubus. 



All this applies to the position taken up by Romanes. He 

 failed to see that isolation alone had no cumulative power, 

 whereas Natural Selection as the generations pass, piles up, 

 specialises, makes complex, till at length a low organism becomes 

 a high one. The concession that I have made as to the powers 

 of isolation acting alone is no surrender, I believe, of an important 

 Darwinian position. It is merely a recognition that unimportant 

 variations, when there is no crossing with other varieties, may 

 remain constant or nearly so, not encouraged and not eliminated 

 by Natural Selection. 



There is another question connected with isolation which I 

 shall not discuss at length because I think there is no need to 

 reopen it: can physiological selection, as Romanes called it, i.e. 

 isolation due to differences in the reproductive cells, produce 

 new species, though unaccompanied by any beneficial variation ? 

 As far as I can see, the answer must be an emphatic No. If a 

 few members of a species, say twenty, are fertile only inter se 

 and not with others of the species, they will run a great risk of 

 leaving no offspring, for, being but a scattered handful among 

 a crowd, they will not be likely to pair together. But if some 

 peculiarity in the reproductive elements expresses itself in a 

 beneficial variation, then it is clear that a new species may be 

 founded. The belief that reproductive divergence alone can 

 make species must, I think, have its ultimate origin in the 

 common teleological fallacy. We wish to account for species, 

 and, therefore, we put Nature in our own position as if she 

 were anxious to divide up the undifferentiated herd and keep 

 them in different folds, so that specific differences may be 

 fostered and developed. 



Physiological selection must be rejected. But we cannot but 

 believe that variations that have survived have been protected 

 by isolation in some form or other. This view does not 

 dethrone Natural Selection ; it merely makes isolation one of 

 the necessary conditions, if Natural Selection is to multiply 



