n structure, habits and constitution,
by so much the more can a large number be supported on the area, of
which we see proof by looking to the inhabitants of any small spot, and
to the productions naturalised in foreign lands. 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 the descendants become, the better will be their chance
of success in the battle for life. Thus the small differences
distinguishing varieties of the same species, steadily tend to increase,
till they 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 within each 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