In the second place, as the forms of the same trimorphic or
dimorphic heterostyled species are obviously identical in general structure,
with the exception of the reproductive organs, and as they are identical in
general constitution (for they live under precisely the same conditions), the
sterility of their illegitimate unions and that of their illegitimate offspring,
must depend exclusively on the nature of the sexual elements and on their
incompatibility for uniting in a particular manner. And as we have just seen
that distinct species when crossed resemble in a whole series of relations the
forms of the same species when illegitimately united, we are led to conclude
that the sterility of the former must likewise depend exclusively on the
incompatible nature of their sexual elements, and not on any general difference
in constitution or structure. We are, indeed, led to this same conclusion by the
impossibility of detecting any differences sufficient to account for certain
species crossing with the greatest ease, whilst other closely allied species
cannot be crossed, or can be crossed only with extreme difficulty. We are led to
this conclusion still more forcibly by considering the great difference which
often exists in the facility of crossing reciprocally the same two species; for
it is manifest in this case that the result must depend on the nature of the
sexual elements, the male element of the one species acting freely on the female
element of the other, but not so in a reversed direction.
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