Evidences of Physiological Selection. 69 



argument tells strongly (though unconsciously) in 

 favour of physiological selection. For the larger a 

 genus, or the greater the number of its species, the 

 greater must be the opportunity for the occurrence 

 of that particular kind of variation on which the 

 principle of physiological selection depends. The 

 species of a genus may be regarded as so many 

 varieties which have already been separated from one 

 another physiologically ; 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 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 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 as a rule, and in 

 the first instance, determined by changes in the repro- 

 ductive system, 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 I can see, we 

 are not able to understand on the theory of natural 

 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 



