CHAP. IV.] SUMMARY. 97 



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 inhabitants of any small spot, and to the produc- 

 tions naturalised in foreign lands. Therefore, during the modifi- 

 cation 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 dis- 

 tinguishing 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 modi- 

 fied offspring that superiority 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 innumerable 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, subordinate 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, 

 species of distinct genera much less closely related, and genera 

 related in different degrees, forming sub-families, families, orders, 

 sub-classes and classes. The several subordinate groups in any 

 class cannot be ranked in a single file, but seem clustered round 

 points, and these round other points, and so on in almost endless 

 cycles. If species had been independently created, no explana- 

 tion would have been possible of this kind of classification ; but it 

 is explained through inheritance and the complex action of 

 natural selection, entailing extinction and divergence of character, 

 as we have seen illustrated in the diagram. 



The affinities of all the beings of the same class have sometimes 

 been represented by a great tree. I believe this simile largely 

 speaks the truth. The green and budding twigs may represent 

 existing species ; and those produced during former years may 

 represent the long succession of extinct species. At each period 

 of growth all the growing twigs have tried to branch out on all 

 sides, and to overtop and kill the surrounding twigs and branches, 

 in the same manner as species and groups of species have at all 

 times overmastered other species in the great battle for life. The 



