supposed to have produced larger glands, no mutation could explain the
influence of hormones on the growth and function of such glands. Then
heredity of the effect of stimulus took place to some slight degree, and
this would occur, according to my theory, only in the presence of the
hormone from the ovary in the same condition as that in which the
modification was first caused. This would be of course after ovulation,
and after hatching of the eggs. In the next stage, if we adopt the modern
view that Marsupials are descended from Placental Mammals, the eggs would
be retained for increasing periods in the uteri, and would be born in a
well-developed condition, since lactation would demand active sucking
effort on the part of the young. The early Placentalia would inherit from
the Monotreme-like ancestors the development of the milk glands after
ovulation, although no sucking was taking place while the young were
inside the uterus. It seems probable that the relation between parturition
and actual milk secretion originated with the sucking stimulus of the
young after birth.

There is good evidence that the secretion of milk may continue almost
indefinitely under the stimulus of sucking or milking. Neither
menstruation nor gestation put an end to it. Cows may continue to give
milk until the next parturition, and if castrated during lactation will
continue to yield milk for years. Women also may continue to produce milk
as long as the child is allowed to suck, and this has been in some cases
two or three years or even more. Moreover, lactation may be induced by the
repeated act of sucking without any gestation. This has happened in mares,
virgin bitches, mules, virgin women, and in one woman lactation continued
uninterruptedly for forty-seven years, to her eighty-first year, long
after the ovary had ceased to be functional. Lactation has also been
induced in male animals, _e.g._ in a bull, a male goat, male sheep, and in
men. [Footnote: Knott, 'Abnormal Lactation,' _American Medicine_, vol. ii