Evidences of Physiological Selection. 67 



Again, Darwin has shown that not only large 

 areas, but likewise "dominant" genera within those 

 areas, are rich in species. By dominant genera he 

 meant those which are represented by numerous 

 individuals, as compared with other genera inhabiting 

 the same area. This general rule he explains by the 

 consideration that the qualities which first led to the 

 form being dominant must have been useful ; that 

 these would be transmitted to the otherwise varying 

 offspring; and, therefore, that when these offspring 

 had varied sufficiently to become new species, they 

 would still enjoy their ancestral advantages in the 

 struggle for existence. And this, doubtless, is in part 

 a true explanation ; but I also think that the reason 

 why dominant genera are rich in species, is chiefly 

 because they everywhere present a great number of 

 individuals exposed to relatively great differences in 

 their conditions of life : or, in other words, that they 

 furnish the best raw material for the manufacture of 

 species by physiological selection, as explained in 

 the last paragraph. For, if the fact of dominant 

 genera being rich in species is to be explained only 

 by natural selection, it appears to me that the useful 

 qualities which have already led to the dominance 

 of the ancestral type ought rather to have proved 

 inimical to its splitting up into a number of sub- 

 ordinate types. If already so far ■ in harmony with 

 its environment ' as to have become for this reason 

 dominant, one would suppose that there is all the 

 more reason for its not undergoing change by the 

 process of natural selection. Or, at least, I do not 

 see why the fact of its being in an unusual degree 

 of harmony with its environment should in itself 



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