104 NATURAL SELECTION J Chap IV. 



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 

 sxisting 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, 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 geolo- 

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

 From the first growth of the tree, many a limb and branch has 

 decayed and dropped off; and these fallen branches of various sizes 

 may represent those whole orders, families, and genera which have 

 now no living representatives, and which are known to us only in 

 a fossil state. As we here and there see a thin straggling branch 

 springing from a fork low down in a tree, and which by some 

 chance has been favoured and is still alive on its summit, so we 



