162 NATURAL SELECTION. [Chap. IT. 



genera, species of distinct genera much less closely re- 

 lated, 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 end- 

 less 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 

 classification of all extinct and living species in groups 

 subordinate to groups. Of the many twigs which flour- 

 ished 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 lived 

 during long-past geological periods, very few have left 



