162 NATURAL SELECTION". [Chap. IT. 



species of distinct genera much, less closely related, and 

 genera related in different degrees, forming snb -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 classifica- 

 tion ; 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 

 flourished when the tree was a mere bush, onlv two or 

 three, now grown into great branches, yet survive and 

 bear the other branches ; so with the species winch lived 

 during long-past geological periods, very few have left 



