her in a marsupial
pouch on her ventral surface, and here nourished till able to care
for themselves.

Pardon a moment's digression. The marsupials, except the opossum,
are confined to Australia, and the oviparous mammals, or monotremes,
to New Zealand. Formerly the marsupials, at least, ranged all over
Europe and Asia, for we have indisputable evidence in their fossil
remains. But they have survived only in this isolated area, and here
apparently only because their isolation preserved them from the
competition with higher forms. If the Australian continent had not
been thus early cut off from all the rest of the world, the only
trace of both these lower groups would have been the opossum in
America and certain peculiarities in the development of the egg in
higher mammals. This shows us how much weight should be assigned to
the formerly popular argument of the "missing links." The wonder is
not that so many links are missing, but that any of these primitive
forms have come down to us. For we see here another proof of the
fearful extermination of lower forms during the progress of life on
the globe. It seems as if the intermediate forms were less common
among these most recent animals than among the older types. This may
not be true, for it is not easy to compare the gap between two
mammals with that between two worms or insects, and mistakes are
very easily made. But it seems as if extermination had done its work
more ruthlessly among these highest forms than among their humbler
and lower ancestors. I would not lay much weight on such an opinion;
but, if true, it has a meaning and is worthy of study.

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 n