Scientifically, of course, they are not at all necessary or universal
features of the female sex, but are peculiar to the mammalian class of
Vertebrates in which they have been evolved. Milk glands, then, are
somatic sex-characters common to a whole class, instead of being
restricted to a family like the antlers in Cervidae. There is not the
slightest trace or rudiment of them in other classes of Vertebrates, such
as Birds or Reptiles. They are not actually sexual in their nature, since
their function is to supply food for the young, not to play a part in the
relations of the sexes. What is sexual about them is--firstly, that they
are normally fully developed only in the female, rudimentary in the male;
secondly, that their periodical development and functional activity
depends on the changes which take place in the ovary and uterus. Many
investigators have endeavoured to discover the nature of the nexus between
the latter organs and the milk glands.

That this nexus is of the nature of a hormone is generally agreed, and may
be regarded as having been proved in 1874 when Goltz and Ewald [Footnote:
_Pfluegers Archiv,_ ix., 1874.] removed the whole of the lumbo-sacral
portion of the spinal cord of a bitch and found that the mammae in the
animal developed and enlarged in the usual way during pregnancy and
secreted milk normally after parturition. Ribbert [Footnote: _Fortschritte
der Medicin,_ Bd. 7.] in 1898 transplanted a milk gland of a guinea-pig to
the neighbourhood of the ear, and found that its development and function
during pregnancy and at parturition were unaffected. The effective
stimulus, therefore, is not conveyed through the nervous system, but must
be a chemical stimulus passing through the vascular system.

Physiologists, however, are not equally in agreement concerning the source
of the hormone which regulates lactation. Starling and Miss Lane-Claypon
concluded from their experiments on rabbits that the hormone originated in