ME. Q. J. EOMANES ON PHTSIOLOGTCAL SELECTION. 397 



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 difiiculty 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 altliough natural selec- 

 tion alone could not induce sterility between allied types, yet 

 when this sterility is given by physiological 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, I believe that the 

 co-operation enables the two principles to effect very much more 

 in the way of species-making than either of them could effect if 

 working sepai'ately. 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, physio- 

 logical selection would be powerless to create any differences of 

 specific type, other than those of mutual sterility and trivial 

 details of structure, form, and colour — differences wholly without 

 meaning from a utilitarian point of view. But in their combi- 

 nation these two principles appear to me able to accomplish what 

 neither can accomplish alone — namely, a full and satisfactory 

 explanation of the origin of species. 



GrENERAL SUMMAET AND CONCLUSION. 



Seeing that the theory of natural selection is confessedly un- 

 able to explain the primary specific distinction of sterility, as 

 well as a large proportional number of the secondary specific dis- 

 tinctions ; seing also that, even as regards the remainder, it is 



