124 SUMMARY. 



ants 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 suc- 

 cess 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 modified offspring that superiority which 

 now makes them dominant in their own countries. Nat- 

 ural selection, as has just been remarked, leads to diverg- 

 ence of character and to much extinction of the less 

 improved and intermediate forms of life. On these prin- 

 ciples, 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 seen clustered round 

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

 endless cycles. If species had been independently created, 

 no explanation would have been possible of this kind of class- 

 ification; 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 



