nchus has a bill like a duck's, webbed feet, and a great many
quaint anatomical peculiarities which closely ally it to the birds and
reptiles. Both, in fact, are early arrested stages in the development of
mammals from the old common vertebrate ancestor; and they could only
have struggled on to our own day in a continent free from the severe
competition of the higher types which have since been evolved in Europe
and Asia. Even in Australia itself the ornithorhynchus and echidna have
had to put up perforce with the lower places in the hierarchy of nature.
The first is a burrowing and aquatic creature, specialised in a thousand
minute ways for his amphibious life and queer subterranean habits; the
second is a spiny hedgehog-like nocturnal prowler, who buries himself in
the earth during the day, and lives by night on insects which he licks
up greedily with his long ribbon-like tongue. Apart from the
specialisations brought about by their necessary adaptation to a
particular niche in the economy of life, these two quaint and very
ancient animals probably preserve for us in their general structure the
features of an extremely early descendant of the common ancestor from
whom mammals, birds, and reptiles alike are originally derived.

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