have admitted the existence of varieties, which they think sufficiently
distinct to be worthy of record in systematic works. No one can draw
any clear distinction between individual differences and slight
varieties; or between more plainly marked varieties and sub-species,
and species. Let it be observed how naturalists differ in the rank
which they assign to the many representative forms in Europe and North
America.
If then we have under nature variability and a powerful agent always
ready to act and select, why should we doubt that variations in any way
useful to beings, under their excessively complex relations of life,
would be preserved, accumulated, and inherited? Why, if man can by
patience select variations most useful to himself, should nature fail
in selecting variations useful, under changing conditions of life, to
her living products? What limit can be put to this power, acting during
long ages and rigidly scrutinising 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 looked no further than this, seems to me to be in
itself 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 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 they are clustered
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