It is admitted by most evolutionists
that mammals are descended from a marsupial form; and if so, the mammary
glands will have been at first developed within the marsupial sack. In the
case of the fish (Hippocampus) the eggs are hatched, and the young are
reared for a time, within a sack of this nature; and an American
naturalist, Mr. Lockwood, believes from what he has seen of the development
of the young, that they are nourished by a secretion from the cutaneous
glands of the sack. Now, with the early progenitors of mammals, almost
before they deserved to be thus designated, is it not at least possible
that the young might have been similarly nourished? And in this case, the
individuals which secreted a fluid, in some degree or manner the most
nutritious, so as to partake of the nature of milk, would in the long run
have reared a larger number of well-nourished offspring, than would the
individuals which secreted a poorer fluid; and thus the cutaneous glands,
which are the homologues of the mammary glands, would have been improved or
rendered more effective. It accords with the widely extended principle of
specialisation, that the glands over a certain space of the sack should
have become more highly developed than the remainder; and they would then
have formed a breast, but at first without a nipple, as we see in the
Ornithorhyncus, at the base of the mammalian series. Through what agency
the glands over a certain space became more highly specialised than the
others, I will not pretend to decide, whether in part through compensation
of growth, the effects of use, or of natural selection.
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