It has been shown in the present volume that the offspring from the
union of two distinct individuals, especially if their progenitors have
been subjected to very different conditions, have an immense advantage
in height, weight, constitutional vigour and fertility over the
self-fertilised offspring from one of the same parents. And this fact is
amply sufficient to account for the development of the sexual elements,
that is, for the genesis of the two sexes.
It is a different question why the two sexes are sometimes combined in
the same individual and are sometimes separated. As with many of the
lowest plants and animals the conjugation of two individuals which are
either quite similar or in some degree different, is a common
phenomenon, it seems probable, as remarked in the last chapter, that the
sexes were primordially separate. The individual which receives the
contents of the other, may be called the female; and the other, which is
often smaller and more locomotive, may be called the male; though these
sexual names ought hardly to be applied as long as the whole contents of
the two forms are blended into one. The object gained by the two sexes
becoming united in the same hermaphrodite form probably is to allow of
occasional or frequent self-fertilisation, so as to ensure the
propagation of the species, more especially in the case of organisms
affixed for life to the same spot.
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