The possession by male mammals of functionally imperfect mammary organs
is, in some respects, especially curious. The Monotremata have the
proper milk-secreting glands with orifices, but no nipples; and as
these animals stand at the very base of the mammalian series, it is
probable that the progenitors of the class also had milk-secreting
glands, but no nipples. This conclusion is supported by what is known
of their manner of development; for Professor Turner informs me, on the
authority of Kolliker and Langer, that in the embryo the mammary glands
can be distinctly traced before the nipples are in the least visible;
and the development of successive parts in the individual generally
represents and accords with the development of successive beings in the
same line of descent. The Marsupials differ from the Monotremata by
possessing nipples; so that probably these organs were first acquired
by the Marsupials, after they had diverged from, and risen above, the
Monotremata, and were then transmitted to the placental mammals. (29.
Prof. Gegenbaur has shewn ( Jenaische Zeitschrift, Bd. vii. p. 212)
that two distinct types of nipples prevail throughout the several
mammalian orders, but that it is quite intelligible how both could have
been derived from the nipples of the Marsupials, and the latter from
those of the Monotremata. See, also, a memoir by Dr. Max Huss, on the
mammary glands, ibid. B. viii. p. 176.) No one will suppose that the
marsupials still remained androgynous, after they had approximately
acquired their present structure. How then are we to account for male
mammals possessing mammae? It is possible that they were first
developed in the females and then transferred to the males, but from
what follows this is hardly probable.
It may be suggested, as another view, that long after the progenitors
of the whole mammalian class had ceased to be androgynous, both sexes
yielded milk, and thus nourished their young; and in the case of the
Marsupials, that both sexes carried their young in marsupial sacks.
This will not appear altogether improbable, if we reflect that the
males of existing syngnathous fishes receive the eggs of the females in
their abdominal pouches, hatch them, and afterwards, as some believe,
nourish the young (30. Mr. Lockwood believes (as quoted in Quart.
Journal of Science, April 1868, p. 269), from what he has observed of
the development of Hippocampus, that the walls of the abdominal pouch
of the male in some way afford nourishment. On male fishes hatching the
ova in their mouths, see a very interesting paper by Prof. Wyman, in
180