In certain species of whales there is a tendency to
the formation of irregular little points of horn on the palate; and it
seems to be quite within the scope of natural selection to preserve all
favourable variations, until the points were converted, first into
lamellated knobs or teeth, like those on the beak of a goose--then into
short lamellae, like those of the domestic ducks--and then into lamellae,
as perfect as those of the shoveller-duck--and finally into the gigantic
plates of baleen, as in the mouth of the Greenland whale. In the family of
the ducks, the lamellae are first used as teeth, then partly as teeth and
partly as a sifting apparatus, and at last almost exclusively for this
latter purpose.
With such structures as the above lamellae of horn or whalebone, habit or
use can have done little or nothing, as far as we can judge, towards their
development. On the other hand, the transportal of the lower eye of a
flat-fish to the upper side of the head, and the formation of a prehensile
tail, may be attributed almost wholly to continued use, together with
inheritance. With respect to the mammae of the higher animals, the most
probable conjecture is that primordially the cutaneous glands over the
whole surface of a marsupial sack secreted a nutritious fluid; and that
these glands were improved in function through natural selection, and
concentrated into a confined area, in which case they would have formed a
mamma.
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