The Descent of Man

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 trans-