There does not seem to be any great
difficulty in understanding how an organism, formed by the conjugation
of two individuals which represented the two incipient sexes, might have
given rise by budding first to a monoecious and then to an hermaphrodite
form; and in the case of animals even without budding to an
hermaphrodite form, for the bilateral structure of animals perhaps
indicates that they were aboriginally formed by the fusion of two
individuals.
It is a more difficult problem why some plants and apparently all the
higher animals, after becoming hermaphrodites, have since had their
sexes re-separated. This separation has been attributed by some
naturalists to the advantages which follow from a division of
physiological labour. The principle is intelligible when the same organ
has to perform at the same time diverse functions; but it is not obvious
why the male and female glands when placed in different parts of the
same compound or simple individual, should not perform their functions
equally well as when placed in two distinct individuals. In some
instances the sexes may have been re-separated for the sake of
preventing too frequent self-fertilisation; but this explanation does
not seem probable, as the same end might have been gained by other and
simpler means, for instance dichogamy. It may be that the production of
the male and female reproductive elements and the maturation of the
ovules was too great a strain and expenditure of vital force for a
single individual to withstand, if endowed with a highly complex
organisation; and that at the same time there was no need for all the
individuals to produce young, and consequently that no injury, on the
contrary, good resulted from half of them, or the males, failing to
produce offspring.
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