The remains of early mammals
The remains of these interesting early mammals, restricted, as
they generally are, to jaws and teeth and a few other bones that
cannot in themselves be too confidently distinguished from those
of certain reptiles, may seem insufficient to enable us to form a
picture of their living forms. In this, however, we receive a
singular and fortunate assistance. Some of them are found living
in nature to-day, and their distinctly reptilian features would,
even if no fossil remains were in existence, convince us of the
evolution of the mammals.
The southern continent on which we suppose the mammals to have
originated had its eastern termination in Australia. New Zealand
seems to have been detached early in the Mesozoic, and was never
reached by the mammals. Tasmania was still part of the Australian
continent. To this extreme east of the southern continent the
early mammals spread, and then, during either the Jurassic or the
Cretaceous, the sea completed its inroad, and severed Australia
permanently from the rest of the earth. The obvious result of
this was to shelter the primitive life of Australia from invasion
by higher types, especially from the great carnivorous mammals
which would presently develop. Australia became, in other words,
a "protected area," in which primitive types of life were
preserved from destruction, and were at the same time sheltered
from those stimulating agencies which compelled the rest of the
world to advance. "Advance Australia" is the fitting motto of the
present human inhabitants of that promising country; but the
standard of progress has been set up in a land which had remained
during millions of years the Chinese Empire of the living world.
Australia is a fragment of the Middle Ages of the earth, a
province fenced round by nature at least three million years ago
and preserving, amongst its many invaluable types of life,
representatives of that primitive mammal population which we are
seeking to understand.
It is now well known that the Duckbill or Platypus
(Ornithorhyncus) and the Spiny Anteater (Echidna) of Australia
and Tasmania--with one representative of the latter in New
Guinea, which seems to have been still connected--are
semi-reptilian survivors of the first animals to suckle their
young. Like the reptiles they lay tough-coated eggs and have a
single outlet for the excreta, and they have a reptilian
arrangement of the bones of the shoulder-girdle; like the
mammals, they have a coat of hair and a four-chambered heart, and
they suckle the young. Even in their mammalian features they are,
as the careful research of Australian zoologists has shown, of a
transitional type. They are warm-blooded, but their temperature
is much lower than that of other mammals, and varies appreciably
with the temperature of their surroundings.* Their apparatus for
suckling the young is primitive. There are no teats, and the milk
is forced by the mother through simple channels upon the breast,
from which it is licked by the young. The Anteater develops her
eggs in a pouch. They illustrate a very early stage in the
development of a mammal from a reptile; and one is almost tempted
to see in their timorous burrowing habits a reminiscence of the
impotence of the early mammals after their premature appearance
in the Triassic.
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