Evidences of Physiological Selection. 99 



theories of natural and of physiological selection. For 

 which purpose I will quote the final paragraph of my 

 original paper. 



So much, then, for the resemblances and the differences 

 between the two theories. It only remains to add that the two 

 are complementary. I have already shown some of the respects 

 in which the newer theory comes to the assistance of the older, 

 and this in the places where the older has stood most in need of 

 assistance. In particular, I have shown that segregation of the 

 fit entirely relieves survival of the fittest from the difficulty under 

 which it has hitherto laboured of explaining why it is that sterility 

 is so constantly found between species, while so rarely found 

 between varieties which differ from one another even more than 

 many species ; why so many features of specific distinction are 

 useless to the species presenting them ; and why it is that 

 incipient varieties are not obliterated by intercrossing with parent 

 forms. Again, we have seen that physiological selection, by 

 preventing such intercrossing, enables natural selection to 

 promote diversity of character, and thus to evolve species in 

 ramifying branches instead of in linear series— a work which I 

 cannot see how natural selection could possibly perform unless 

 thus aided by physiological selection. Moreover, we have seen 

 that although natural selection alone could not induce sterility 

 between allied types, yet when this sterility is given by physio- 

 logical selection, the forms which present it would be favoured in 

 the struggle for existence ; and thus again the two principles are 

 found playing, as it were, into each other's hands. And here, as 

 elsewhere, 1 believe that the co-operation enables the two prin- 

 ciples to effect very much more in the way of species-making 

 than either of them could effect if working separately. On the 

 one hand, without the assistance of physiological selection, 

 natural selection would, I believe, be all but overcome by the 

 adverse influences of free intercrossing— influences all the more 

 potent under the very conditions which are required for the 

 multiplication of species by divergence of character. On the 

 other hand, without natural selection, physiological selection 

 would be powerless to create any differences of specific type, 

 other than those of mutual sterility and trivial details of structure, 



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