NATURAL SELECTION 235 
class, 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 lived during long-past 
geological periods, very few have left living and modified descendants. 
