66 Darwin, and after Darwin. 



environment, will give natural selection an improved 

 chance of finding new points of departure for its 

 operation. On the other hand, large and continuous 

 areas are favourable to the production of numerous 

 species, first, because they contain a large population, 

 thus 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, attitude, &c., and thus so many 

 different opportunities for the occurrence of sundry 

 forms of homogamy. Now, it is obvious that of all 

 these sundry forms of homogamy, physiological 

 selection must have what may be termed a first-rate 

 opportunity of assisting in the manufacture of species 

 on large areas. For not only is it upon large and 

 continuous areas that the antagonistic 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 diversity in the external conditions of life, 

 which the large area supplies to different parts of 

 the extensive population, cannot fail to furnish physio- 

 1< gical selection with a greater abundance of that 

 particular variation in the reproductive system on 

 which its action depends. Again, and of still more 

 importance, on large areas there are a greater number 

 of species already differentiated from one another 

 as such ; thus a greater number of already sexually 

 differentiated forms are presented for further differen- 

 tiation at the hands of physiological selection. For 

 all these reasons, therefore, we might have expected, 

 upon the new theory, that large and continuous areas 

 would be good manufactories of species. 



