The mammals of the Mesozoic had been small and
primitive animals, rarely larger than a rat, and never rising
above the marsupial stage in organisation. They not only
continued to exist, and give rise to their modern representatives
(the opossum, etc.) during the Tertiary Era, but they shared the
general prosperity. In Australia, where they were protected from
the higher carnivorous mammals, they gave rise to huge
elephant-like wombats (Diprotodon), with skulls two or three feet
in length. Over the earth generally, however, they were
superseded by the placental mammals, which suddenly break into
the geological record in the early Tertiary, and spread with
great vigour and rapidity over the four continents.
Were they a progressive offshoot from the Mesozoic Marsupials, or
Monotremes, or do they represent a separate stock from the
primitive half-reptile and half-mammal family? The point is
disputed; nor does the scantiness of the record permit us to tell
the place of their origin. The placental structure would be so
great an advantage in a cold and unfavourable environment that
some writers look to the northern land, connecting Europe and
America, for their development.
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