MB. a. J. BOMANES ON PHTSIOLOOICAL SELECTION. 391 



small, so that iu this respect the species of the larger genera 

 resemble varieties more than do the species of the smaller genera." 

 Therefore the argument, while undoubtedly a very forcible one 

 in favour of the fact of evolution, appears to me scarcely consistent 

 with the theory of natmral selection. On the other hand, the 

 argument tells strongly (though unconsciously) in favour of 

 physiological selection. For the larger a genus, or the greater 

 the number of species it contains, the greater must be the oppor- 

 tunity aiforded for the occurrence of that particular kind of 

 variationon which the principle of physiological selection depends. 

 All the species of a genus may be regarded as so many varieties 

 which have already been separated from one another physio- 

 logically; therefore each of them may now constitute a new 

 starting-point for a further and similar separation — particularly 

 as, in virtue of their previous segregation, many of them are now 

 exposed to different conditions of life. Thus, it seems to me, we 

 can well understand why it is that genera already rich in species 

 tend to grow still richer ; while such is not the case in so great 

 a degree with genera that are poor in species. Moreover, we can 

 well understand that, multiplication of species being in the first 

 instance determined by changes in the reproductive system 

 alone, wherever a large number of new species are being turned 

 out, the secondary differences between them should be " often 

 exceedingly small" — a general correlation which, so far as 1 can 

 see, we are not able to understand on the theory of natura 

 selection. 



The two subsidiary facts, that very closely allied species have 

 restricted ranges, and that dominant species are rich in varieties, 

 both seem to tell more in favour of physiological than of natural 

 selection. For "very closely allied species" is but another name 

 for species which scarcely differ from one another at all except in 

 their reproductive systems ; and, therefore, the more restricted 

 their ranges, the more certainly would they have become fused by 

 intercrossiug with one another, had it not been for the barrier of 

 sterility imposed by the primary distinction. Or rather, I should 

 say, had it not been for the original occurrence of this barrier, 

 these now closely-allied species would never have become species. 

 Again, that dominant species should be rich in varieties is what 

 might have been expected ; for the greater the number of indi- 

 viduals in a species, the greater is the chance of variations taking 

 place in all parts of the organic type, and particularly in the 



