NATURAL SELECTION 1511 



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 can not 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 indepen- 

 dently created, no explanation would have been pos- 

 sible of this kind of classification; but it is explained 

 through inheritance and the complex action of natu- 

 ral selection, entailing extinction and divergence of 

 character. 



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 spe- 

 cies; 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 classi- 

 fication of all extinct and living species in groups sub- 

 ordinate to groups. Of the many twigs which flour- 



