EXTENSION OF EVOLUTIONARY THEORIES 325 



However, we saw at an early stage that the theory 

 of selection will not explain everything in the present 

 condition of science. It could not satisfactorily 

 explain the various characters that are peculiar to 

 one sex. The auxiliary principle, sexual selection, 

 which we called to our aid, was so unsatisfactory that 

 we sought to reduce it again to natural selection. We 

 also failed to explain many of the rudimentary organs 

 because we had to reject, on the strength of 

 Weismann's objections, the Lamarckian principle 

 that might have shed light on them. On the other 

 hand, when we came to the question why there are 

 sharply distinct species, we discovered an agency 

 that really solved the problem — amphimixis. 



However, there seemed to be other difficulties in 

 the way of amphimixis. The origin of one species 

 from another — in other words, the divergence of 

 species — seemed to be inconsistent with it. How 

 can a new property, even when it only arises amongst 

 one section of a species, maintain itself and lead to 

 the founding of a new species, if the differently 

 constituted individuals are always crossing with the 

 modified ones ? The new feature ought to be 

 absorbed by the majority in this general crossing. 



But we have had frequent occasion to remind the 

 reader of a power that can prevent crossing, and so 

 maintain the incipient specific character. This agency 

 is isolation. We will now deal with the importance 

 of isolation in the formation of species, to which 

 attention was first drawn by Moritz Wagner. 



The most common effect of isolation is that a 



