On the principle of successive
variations not always supervening at an early age, and being inherited at a
corresponding not early period of life, we clearly see why the embryos of
mammals, birds, reptiles, and fishes should be so closely similar, and so
unlike the adult forms. We may cease marvelling at the embryo of an
air-breathing mammal or bird having branchial slits and arteries running in
loops, like those of a fish which has to breathe the air dissolved in water
by the aid of well-developed branchiae.
Disuse, aided sometimes by natural selection, will often have reduced
organs when rendered useless under changed habits or conditions of life;
and we can understand on this view the meaning of rudimentary organs. But
disuse and selection will generally act on each creature, when it has come
to maturity and has to play its full part in the struggle for existence,
and will thus have little power on an organ during early life; hence the
organ will not be reduced or rendered rudimentary at this early age. The
calf, for instance, has inherited teeth, which never cut through the gums
of the upper jaw, from an early progenitor having well-developed teeth; and
we may believe, that the teeth in the mature animal were formerly reduced
by disuse owing to the tongue and palate, or lips, having become
excellently fitted through natural selection to browse without their aid;
whereas in the calf, the teeth have been left unaffected, and on the
principle of inheritance at corresponding ages have been inherited from a
remote period to the present day.
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