The anterior and
posterior limbs in all the higher vertebrate classes are plainly
homologous. So it is with the wonderfully complex jaws and legs of
crustaceans. It is familiar to almost every one, that in a flower the
relative position of the sepals, petals, stamens, and pistils, as well as
their intimate structure, are intelligible on the view that they consist of
metamorphosed leaves, arranged in a spire. In monstrous plants, we often
get direct evidence of the possibility of one organ being transformed into
another; and we can actually see, during the early or embryonic stages of
development in flowers, as well as in crustaceans and many other animals,
that organs, which when mature become extremely different are at first
exactly alike.
How inexplicable are the cases of serial homologies on the ordinary view of
creation! Why should the brain be enclosed in a box composed of such
numerous and such extraordinarily shaped pieces of bone apparently
representing vertebrae? As Owen has remarked, the benefit derived from the
yielding of the separate pieces in the act of parturition by mammals, will
by no means explain the same construction in the skulls of birds and
reptiles. Why should similar bones have been created to form the wing and
the leg of a bat, used as they are for such totally different purposes,
namely flying and walking? Why should one crustacean, which has an
extremely complex mouth formed of many parts, consequently always have
fewer legs; or conversely, those with many legs have simpler mouths? Why
should the sepals, petals, stamens, and pistils, in each flower, though
fitted for such distinct purposes, be all constructed on the same pattern?
On the theory of natural selection, we can, to a certain extent, answer
these questions.
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