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.
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