It is no valid objection to this
belief that the structure of some flowers, and the dichogamous condition
of many others, suffice to prevent the pollen reaching the stigma of the
same flower; for we should remember that with most species many flowers
expand at the same time, and that pollen from the same plant is equally
injurious or nearly so as that from the same flower. Nevertheless, the
belief that self-sterility is a quality which has been gradually
acquired for the special purpose of preventing self-fertilisation must,
I believe, be rejected. In the first place, there is no close
correspondence in degree between the sterility of the parent-plants when
self-fertilised, and the extent to which their offspring suffer in
vigour by this process; and some such correspondence might have been
expected if self-sterility had been acquired on account of the injury
caused by self-fertilisation. The fact of individuals of the same
parentage differing greatly in their degree of self-sterility is
likewise opposed to such a belief; unless, indeed, we suppose that
certain individuals have been rendered self-sterile to favour
intercrossing, whilst other individuals have been rendered self-fertile
to ensure the propagation of the species. The fact of self-sterile
individuals appearing only occasionally, as in the case of Lobelia, does
not countenance this latter view. But the strongest argument against the
belief that self-sterility has been acquired to prevent
self-fertilisation, is the immediate and powerful effect of changed
conditions in either causing or in removing self-sterility.
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