Chap. IT.] SUMMARY. 161 



to divergence of character ; for the more organic beings 

 diverge in structure, habits, and constitution, by so 

 much the more can a large number be supported on the 

 area, — of which we see proof by looking to the inhabit- 

 ants of any small spot, and to the productions naturalised 

 in foreign lands. Therefore, during the modification of 

 the descendants of any one species, and during the 

 incessant struggle of all species to increase in numbers, 

 the more diversified the descendants become, the better 

 will be their chance of success in the battle for life. 

 Thus the small differences distinguishing varieties of the 

 same species, steadily tend to increase, till they equal 

 the greater differences between species of the same 

 genus, or even of distinct genera. 



We have seen that it is the common, the widely-diffused 

 and widely-ranging species, belonging to the larger 

 genera within each class, which vary most ; and these 

 tend to transmit to their modified offspring that superi- 

 ority which now makes them dominant in their own 

 countries. Natural selection, as has just been remarked, 

 leads to divergence of character and to much extinction 

 of the less improved and intermediate forms of life. 

 On these principles, the nature of the affinities, and the 

 generally well-defined distinctions between the innumer- 

 able organic beings in each class throughout the world, 

 may be explained. It is a truly wonderful fact — the 

 wonder of which we are apt to overlook from familiarity 

 — that all animals and all plants throughout all time 

 and space should be related to each other in groups, sub- 

 ordinate to groups, in the manner which we everywhere 

 behold — namely, varieties of the same species most 

 closely related, species of the same genus less closely and 

 unequally related, forming sections and sub-genera, 



