That the intra-uterine gestation, or its cessation, were not originally
necessary to determine the functional periodicity of the milk glands is
proved by their presence in the Monotremes, which are oviparous. It is
evident from the conditions in these mammals that both hair and milk
glands were evolved before the placenta.

It may also be pointed out here that, according to the evidence of
Steinach, in the milk glands at least among somatic sexual characters
there is no difference between the male and female in the heredity of the
organs. The zygote therefore, whether the sex of it is determined as male
or female, has the same factor for the development of milk glands. On the
chromosome theory as formulated by Morgan this factor must be in the
somatic chromosomes and not in the sex-chromosomes, and must be present in
every zygote. All the cells of the body, assuming that somatic segregation
does not occur, must possess the same chromosomes as the zygote from which
it developed, and whether the sex chromosomes are _XX_ or _XY_ or _X_,
there must be at any rate one chromosome bearing the factor for milk
glands. The functional development of these depends normally, according
to the evidence hitherto discovered, on the presence or absence of
hormones from the ovary or from the uterus.

If we attribute, as in my opinion we must, the primary origin of the milk
glands in evolution to the mechanical stimulus of sucking, we may attempt
to reconstruct the stages of the evolution of the present relation of the
glands to the other organs and processes of reproduction. In the earliest
stage represented by the Monotremata or Prototheria, there was no
intra-uterine development. We must suppose that in the beginning the
sucking stimulus caused both growth and secretion, for at first there was
nothing but sebaceous or sweat glands, and although a mutation might be