The next level of mammal life
The next level of mammal life, the highest level that it attains
in Australia (apart from recent invasions), is the Marsupial. The
pouched animals (kangaroo, wallaby, etc.) are the princes of
pre-human life in Australia, and represent the highest point that
life had reached when that continent was cut off from the rest of
the world. A few words on the real significance of the pouch,
from which they derive their name, will suffice to explain their
position in the story of evolution.
Among the reptiles the task of the mother ends, as a rule, with
the laying of the egg. One or two modern reptiles hatch the eggs,
or show some concern for them, but the characteristic of the
reptile is to discharge its eggs upon the warm earth and trouble
no further about its young. It is a reminiscence of the warm
primitive earth. The bird and mammal, born of the cooling of the
earth, exhibit the beginning of that link between mother and
offspring which will prove so important an element in the higher
and later life of the globe. The bird assists the development of
the eggs with the heat of her own body, and feeds the young. The
mammal develops the young within the body, and then feeds them at
the breast.
But there is a gradual advance in this process. The Duckbill lays
its eggs just like the reptile, but provides a warm nest for them
at the bottom of its burrow. The Anteater develops a temporary
pouch in its body, when it lays an egg, and hatches the egg in
it. The Marsupial retains the egg in its womb until the young is
advanced in development, then transfers the young to the pouch,
and forces milk into its mouth from its breasts. 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. It is more difficult
to identify Monotreme remains, but the fact that Monotremes have
survived to this day in Australia, and the resemblance of some of
the Mesozoic teeth to those found for a time in the young
Duckbill justify us in assuming that a part of the Mesozoic
mammals correspond to the modern Monotremes. Not single specimen
of any higher, or placental, mammal has yet been found in the
whole Mesozoic Era.
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