The ordinary Australian pouched mammals belong to far less ancient types
than ornithorhynchus and echidna, but they too are very old in
structure, though they have undergone an extraordinary separate
evolution to fit them for the most diverse positions in life. Almost
every main form of higher mammal (except the biggest ones) has, as it
were, its analogue or representative among the marsupial fauna of the
Australasian region fitted to fill the same niche in nature. For
instance, in the blue gum forests of New South Wales a small animal
inhabits the trees, in form and aspect exactly like a flying squirrel.
Nobody who was not a structural and anatomical naturalist would ever for
a moment dream of doubting its close affinity to the flying squirrels of
the American woodlands. It has just the same general outline, just the
same bushy tail, just the same rough arrangement of colours, and just
the same expanded parachute-like membrane stretching between the fore
and hind limbs. Why should this be so? Clearly because both animals have
independently adapted themselves to the same mode of life under the same
general circumstances. Natural selection, acting upon unlike original
types, but in like conditions, has produced in the end very similar
results in both cases. Still, when we come to examine the more intimate
underlying structure of the two animals, a profound fundamental
difference at once exhibits itself. The one is distinctly a true
squirrel, a rodent of the rodents, externally adapted to an arboreal
existence; the other is equally a true phalanger, a marsupial of the
marsupials, which has independently undergone on his own account very
much the same adaptation, for very much the same reasons.
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