NATURAL SELECTION 235 



clas?, which vary most; and these tend to transmit to their modified 

 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 explanation 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 limbs divided into great branches, and 

 these into lesser and lesser branches, were themselves once, when the 

 tree was young, budding twigs; and this connection of the former and 

 present buds by ramifying branches may well represent the classifica- 

 tion of all extinct and living species in groups subordinate to groups. 

 Of the many twigs which flourished when the tree was a mere bush, 

 only two or three, now grown into great branches, yet survive and bear 

 the other branches; so with the species which Hved during long-past 

 geological periods, very few have left living and modified descendants. 



