gestation, and not in Prototheria, they cannot be the only essential
source of the hormone which stimulates the development of the milk glands,
since the latter develop in Prototheria. Again it is difficult, it might
be said impossible, to believe that an accidental mutation gave rise to
corpora lutea the secretion of which caused uterine gestation and
ultimately the formation of the placenta. It seems more probable that the
retention of the originally yolked ova within the oviduct, however this
retention arose, was the essential cause of the formation of the placenta
and all the changes which the uterus undergoes in gestation. The
absorption of nutriment from the walls of the uterus, and the chemical and
mechanical stimulation of those walls, might well be the cause of the
diversion of nutrition from the ovary, leading gradually to the decline of
the process of secretion of yolk in the ova.

The conceptions and the mode of reasoning of the physiologist are very
different from those of the evolutionist. The former concludes from
certain experiments that a given organ of internal secretion has a certain
function. The corpora lutea, for example, according to one theory are
ductless glands, the function of whose secretion is to establish ova in
the uterus and promote their development. Another function suggested for
the secretion of the corpora lutea is to prevent further ovulation during
pregnancy. The evolutionist, on the other hand, asks what was the origin
of this corpora lutea, why should the ruptured ovarian follicles after the
escape of the ova in Mammals undergo a progressive development and persist
during the greater part of the whole of pregnancy? It seems obvious that
the corpora lutea in evolution were a consequence of intra-uterine
gestation, for they occur only in association with this condition, and it
is impossible to suppose that a mutation could arise accidentally by which
the ruptured follicles should produce a secretion which would cause the
fertilised ova to develop within the oviducts. The developing ovum within