398 MR. G. J. EOMANES ON PHYSIOLOGICAL SELECTION. 



difficult to see how natural selection alone could have evolved 

 them in the presence of free intercrossing ; seeing all this, it 

 becomes obvious that natural selection is not a theory of the 

 origin of species : it is a theory of the genesis of adaptive modifi- 

 cations, vsrhether these happen to be distinctive of species only, 

 or likewise of higher taxonomic divisions. Only, if species 

 were always distinguishable in points of utilitarian significance, 

 if natural selection were able fully to explain the fact of their 

 mutual sterility, and if it were a part of the theory to show 

 that in some way the mutual crossing of varieties is prevented ; 

 only under these circumstances could it be properly said that a 

 theory of the genesis of adaptive modifications is likewise a theory 

 of the origin of species. But, as matters stand, supplementary 

 theories are required. Of these the only ones hitherto suggested 

 are the theories of use and disuse, sexual selection, correlated 

 variability, prolonged exposure to similar conditions of life, and 

 prevention of intercrossing by geographical barriers, or by migra- 

 tion. The first three may here be neglected, as they do not touch 

 the subject-matter of the present paper. Prolonged exposure to 

 similar conditions of life has been shown inadequate to explain 

 the contrast between hybrids and mongrels in respect of fertility. 

 The prevention of intercrossing by geographical barriers and by 

 migration has been shown adequate to account for the frequent 

 appearance of non-adaptive specific characters. But the great 

 distinction of sterility between species is still left unexplained. 

 This it is that my theory of physiological selection seeks to 

 explain. And the theory consists merely in pointing to the fact that 

 wherever, among all the possible variations of the highly variable 

 reproductive system, there arises towards any parent form any 

 degree of sterility which does not extend to the varietal form, 

 there a new species must necessarily take its origin. For, even 

 though the varietal form continues to live on the same area as 

 its parent form, intercrossing is prevented by the primary 

 distinction of sterility, with the consequence of secondary distinc- 

 tions subsequently arising by way of independent variability — 

 just as happens when the barrier to intercrossing, instead of being 

 physiological, is geographical. 



It makes no essential difference to my theory whether the 

 causes of this particular variation on the part; of the reproductive 

 system are extrinsic or intrinsic ; nor does it make any difference 

 whether the variation first occurs in a high or in a low degree. 



