his same {470}
view we can understand how it is that in each region where many species of
a genus have been produced, and where they now flourish, these same species
should present many varieties; for where the manufactory of species has
been active, we might expect, as a general rule, to find it still in
action; and this is the case if varieties be incipient species. Moreover,
the species of the larger genera, which afford the greater number of
varieties or incipient species, retain to a certain degree the character of
varieties; for they differ from each other by a less amount of difference
than do the species of smaller genera. The closely allied species also of
the larger genera apparently have restricted ranges, and in their
affinities they are clustered in little groups round other species--in
which respects they resemble varieties. These are strange relations on the
view of each species having been independently created, but are
intelligible if all species first existed as varieties.

As each species tends by its geometrical ratio of reproduction to increase
inordinately in number; and as the modified descendants of each species
will be enabled to increase by so much the more as they become diversified
in habits and structure, so as to be enabled to seize on many and widely
different places in the economy of nature, there will be a constant
tendency in natural selection to preserve the most divergent offspring of
any one species. Hence during a long-continued course of modification, the
slight differences, characteristic of varieties of the same species, tend
to be augmented into the greater differences characteristic of species of
the same genus. New and improved varieties will inevitably supplant and
exterminate the older, less improved and intermediate varieties; and thus
species are rendered to a large extent defined and distinct objects.
Dominant species belonging to the {471} larger groups tend to give birth to
new and dominant forms; so that each large group tends to become still
larger, and at the same time more divergent in character. But as all groups
cannot thus succeed in increasing in size, for the world would not hold
them, the more dominant groups beat the less dominant. This tendency in the
large groups to go on increasing in size and diverging in character,
together with the almost inevitable contingency of much extinction,
explains the arrangement of all the forms of life, in groups subordinate to