But occasionally a
variation may occur which has special advantageous characters of its own
(such as mimicking a protected species), and then this variation will
maintain itself by selection. In no less than three of my _polymorphic_
species of Papilio, one of the female forms mimics the _Polydorus_
group, which, like the _AEneas_ group in America, seems to have some
special protection. In two or three other cases one of the female forms
is confined to a restricted locality, to the conditions of which it is
probably specially adapted. In other cases one of the female forms
resembles the male, and perhaps receives a protection from the
abundance of the males, in the crowd of which it is passed over. I think
these considerations render the production of two or three forms of
female very conceivable. The physiological difficulty is to me greater,
of how each of two forms of female produces offspring like the other
female as well as like itself, but no intermediates?
If you "know varieties that will not blend or intermix, but produce
offspring quite like either parents," is not that the very physiological
test of a species which is wanting for the _complete proof_ of the
origin of species?
I have by no means given up the idea of writing my Travels, but I think
I shall be able to do it better for the delay, as I can introduce
chapters giving popular sketches of the subjects treated of in my
various papers.
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