The real reason
for this is that the Marsupial falls far short of the higher
mammals in the structure of the womb, and cannot fully develop
its young therein. It has no placenta, or arrangement by which
the blood-vessels of the mother are brought into connection with
the blood-vessels of the foetus, in order to supply it with food
until it is fully developed. The Marsupial, in fact, only rises
above the reptile in hatching the egg within its own body, and
then suckling the young at the breast.
These primitive mammals help us to reconstruct the mammal life of
the Mesozoic Epoch. The bones that we have are variously
described in geological manuals as the remains of Monotremes,
Marsupials, and Insectivores. Many of them, if not most, were no
doubt insect-eating animals, but there is no ground for supposing
that what are technically known as Insectivores (moles and
shrews) existed in the Mesozoic. On the other hand, the lower jaw
of the Marsupial is characterised by a peculiar hooklike process,
and this is commonly found in Mesozoic jaws. This circumstance,
and the witness of Australia, permit us, perhaps, to regard the
Jurassic mammals as predominantly marsupial.
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