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.
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