As
quadrupeds are divided into two nearly equal bodies of different sexes, so here
we have two bodies, approximately equal in number, differing in their sexual
powers and related to each other like males and females. There are many
hermaphrodite animals which cannot fertilise themselves, but most unite with
another hermaphrodite. So it is with numerous plants; for the pollen is often
mature and shed, or is mechanically protruded, before the flower's own stigma is
ready; and such flowers absolutely require the presence of another hermaphrodite
for sexual union. But with the cowslip and various other species of Primula
there is this wide difference, that one individual, though it can fertilise
itself imperfectly, must unite with another individual for full fertility; it
cannot, however, unite with any other individual in the same manner as an
hermaphrodite plant can unite with any other one of the same species; or as one
snail or earth-worm can unite with any other hermaphrodite individual. On the
contrary, an individual belonging to one form of the cowslip in order to be
perfectly fertile must unite with one of the other form, just as a male
quadruped must and can unite only with the female.
I have spoken of the legitimate unions as being fully fertile; and I am fully
justified in doing so, for flowers artificially fertilised in this manner
yielded rather more seeds than plants naturally fertilised in a state of nature.
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