MB. G. J. EOMANES ON PHTSIOLOGHCAL SELECTION. 389 



their inliabitants ; but it does not follow that they are rich in 

 species if coutrasted with larger areas containing very many more 

 inhabitants. Therefore, the rules are that large areas turn out 

 an absolutely greater number of specific types than small areas ; 

 although, relatively to the number of individuals or amount of 

 population, the small areas turn out a larger number of species 

 than the large areas. 



Now, these two complementary rules admit of being explained 

 as Darwin explains them. Small and isolated areas are rich in 

 species relatively to the amount of population, because, as we 

 have before seen, this population has been permitted to develop 

 an independent history of its own, shielded from intercrossing 

 with parent, and from struggle with exotic forms. On the other 

 hand, large and continuous areas are favourable to the production 

 of numerous species, first, because they contain a large popu- 

 lation, so favouring the occurrence of numerous variations ; and, 

 secondly, because the large area furnishes a diversity of conditions 

 in its different parts, as to food, climate, altitude, and so forth. 



Such being the state of the facts, it is obvious that physiolo- 

 gical selection must have what may be termed a first-rate oppor- 

 tunity of assisting in the manufacture of species on large areas. 

 For, not only is it upon large and continuous areas that the an- 

 tagonistic effects of intercrossing are most pronounced (and, 

 therefore, that the influence of physiological selection must be 

 most useful in the work of • species-making) ; but here also the 

 large population, as well as the diversity in the external con- 

 ditions of life which the large area supplies to different parts of 

 that population, — both these circumstances cannot fail to furnish 

 physiological selection with a greater abundance of that particular 

 variation in the reproductive system on which its action depends. 

 For all these reasons, therefore, we might have expected, upon 

 my theory, that large and continuous areas should be good manu- 

 factories of species. 



Again, Mr. Darwin has shown that not only large areas, but 

 likewise " dominant" genera upon those areas, are rich in species. 

 By dominant genera he means genera 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 qualities ; that these would be transmitted to 

 the otherwise varying offspring ; and, therefore, that when these 



