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.
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