But we already see how it entails
extinction; and how largely extinction {128} has acted in the world's
history, geology plainly declares. Natural selection, also, leads to
divergence of character; for more living beings can be supported on the
same area the more they diverge in structure, habits, and constitution, of
which we see proof by looking to the inhabitants of any small spot or to
naturalised productions. Therefore during the modification of the
descendants of any one species, and during the incessant struggle of all
species to increase in numbers, the more diversified these descendants
become, the better will be their chance of succeeding in the battle for
life. Thus the small differences distinguishing varieties of the same
species, steadily tend to increase till they come to equal the greater
differences between species of the same genus, or even of distinct genera.

We have seen that it is the common, the widely-diffused, and widely-ranging
species, belonging to the larger genera, 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, I
believe, the nature of the affinities of all organic beings 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 group subordinate to
group, in the manner which we everywhere behold--namely, varieties of the
same species most closely related together, species of the same genus less
closely and unequally related together, forming sections and sub-genera,
species of distinct genera much less closely related, and genera related in
different degrees, forming {129} sub-families, families, orders,
sub-classes, and classes. The several subordinate groups in any class
cannot be ranked in a single file, but seem rather to be clustered round
points, and these round other points, and so on in almost endless cycles.
On the view that each species has been independently created, I can see no
explanation of this great fact in the classification of all organic beings;
but, to the best of my judgment, it is explained through inheritance and
the complex action of natural selection, en