sing the whole
constitution, structure, and habits of each creature, favouring the good
and rejecting the bad? I can see no limit to this power, in slowly and
beautifully adapting each form to the most complex relations of life.
The theory of natural selection, even if we look no further than
this, seems to be in the highest degree probable. I have already
recapitulated, as fairly as I could, the opposed difficulties and
objections: now let us turn to the special facts and arguments in favour
of the theory.

On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can
see why it is that no line of demarcation can be drawn between species,
commonly supposed to have been produced by special acts of creation,
and varieties which are acknowledged to have been produced by secondary
laws. On this same view we can understand how it is that in a 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 a larger
genera apparently have restricted ranges, and in their affinities they
are clustered in little groups round other species--in both respects
resembling varieties. These are strange relations on the view that each
species was independently created, but are intelligible if each existed
first as a variety.

As each species tends by its geometrical rate of reproduction to
increase inordinately in number; and as the modified descendants of
each species will be enabled to increase by as much as they become more
diversified in habits and structure, so as to be able 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 dif