In higher, true, placental mammals the period of pregnancy is much
longer, and the young are born in a far higher stage of development,
or rather, growth. The stage of growth at which the young are born
differs markedly in different groups. A new-born kitten is a much
feebler, less developed being than a new-born calf. An embryonic
appendage, the allantois, used in reptiles and birds for
respiration, has here been turned to another purpose. It lays itself
against the walls of the uterus, uterine projections interlock with
those which it puts forth, and the blood of the mother circulates
through a host of capillaries separated from those of the blood
system of the embryo only by the thinnest membrane. This is the
placenta, developed, in part from the allantois of the embryo, in
part from the uterus of the mother. It is not a new organ, but an
old one turned to better and fuller use. In these closely
associated systems of blood-vessels, nutriment and oxygen diffuse
from the blood of the mother into that of the embryo, and thus rapid
growth is assured. The importance and far-reaching effect of this
new modification in the old reproductive system cannot be
over-estimated. The internal intra-uterine development of the young,
and the mammalian habit of suckling them, far more than any other
factors, have made man what he is. Some explanation must be sought
for such a fact.

We have already seen that any animal devotes to reproduction the
balance between income and expenditure of nutriment. Now, the
digestive system is here well developed, and the income is large.
But we have already noticed that, as animals grow larger, the ratio
between the digestive surface and the mass to be supported grows
continually smaller. On account of size alone the mammal has but a