It is, however,
incredible that so peculiar a form of mutual infertility should have been
specially acquired unless it were highly beneficial to the species; and although
it may be beneficial to an individual plant to be sterile with its own pollen,
cross-fertilisation being thus ensured, how can it be any advantage to a plant
to be sterile with half its brethren, that is, with all the individuals
belonging to the same form? Moreover, if the sterility of the unions between
plants of the same form had been a special acquirement, we might have expected
that the long-styled form fertilised by the long-styled would have been sterile
in the same degree as the short-styled fertilised by the short-styled; but this
is hardly ever the case. On the contrary, there is sometimes the widest
difference in this respect, as between the two illegitimate unions of Pulmonaria
angustifolia and of Hottonia palustris.
It is a more probable view that the male and female organs in two sets of
individuals have been by some means specially adapted for reciprocal action; and
that the sterility between the individuals of the same set or form is an
incidental and purposeless result. The meaning of the term "incidental" may be
illustrated by the greater or less difficulty in grafting or budding together
two plants belonging to distinct species; for as this capacity is quite
immaterial to the welfare of either, it cannot have been specially acquired, and
must be the incidental result of differences in their vegetative systems.
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