I will now sum up very briefly the chief conclusions which seem to follow from
the observations given in this volume. Cleistogamic flowers afford, as just
stated, an abundant supply of seeds with little expenditure; and we can hardly
doubt that they have had their structure modified and degraded for this special
purpose; perfect flowers being still almost always produced so as to allow of
occasional cross-fertilisation. Hermaphrodite plants have often been rendered
monoecious, dioecious or polygamous; but as the separation of the sexes would
have been injurious, had not pollen been already transported habitually by
insects or by the wind from flower to flower, we may assume that the process of
separation did not commence and was not completed for the sake of the advantages
to be gained from cross-fertilisation. The sole motive for the separation of the
sexes which occurs to me, is that the production of a great number of seeds
might become superfluous to a plant under changed conditions of life; and it
might then be highly beneficial to it that the same flower or the same
individual should not have its vital powers taxed, under the struggle for life
to which all organisms are subjected, by producing both pollen and seeds. With
respect to the plants belonging to the gyno-dioecious sub-class, or those which
co-exist as hermaphrodites and females, it has been proved that they yield a
much larger supply of seed than they would have done if they had all remained
hermaphrodites; and we may feel sure from the large number of seeds produced by
many plants that such production is often necessary or advantageous.
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