I
have also noticed an extraordinary amount of difference in the length of the
pistil in cultivated varieties of Primula veris and vulgaris.
As most plants are at least occasionally cross-fertilised by the aid of insects,
we may assume that this was the case with our supposed varying plant; but that
it would have been beneficial to it to have been more regularly cross-
fertilised. We should bear in mind how important an advantage it has been proved
to be to many plants, though in different degrees and ways, to be cross-
fertilised. It might well happen that our supposed species did not vary in
function in the right manner, so as to become either dichogamous or completely
self-sterile, or in structure so as to ensure cross-fertilisation. If it had
thus varied, it would never have been rendered heterostyled, as this state would
then have been superfluous. But the parent-species of our several existing
heterostyled plants may have been, and probably were (judging from their present
constitution) in some degree self-sterile; and this would have made regular
cross-fertilisation still more desirable.
Now let us take a highly varying species with most or all of the anthers
exserted in some individuals, and in others seated low down in the corolla; with
the stigma also varying in position in like manner. Insects which visited such
flowers would have different parts of their bodies dusted with pollen, and it
would be a mere chance whether this were left on the stigma of the next flower
which was visited.
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