The differences in the geographic distribution of mammal life between
North and South America and the relationships between our fauna and
that of the Old World are parts of the latest chapter of a wonderful
story running back through geologic ages. The former chapters are
recorded in the fossil beds of all the continents. While only a good
beginning has been made in deciphering these records, enough has been
done by the fascinating researches of Marsh, Cope, Osborn, Scott, and
others to prove that in all parts of the earth one fauna has succeeded
another in marvelous procession.
It has been shown also that these changes in animal life, accompanied
by equal changes in plant life, have been largely brought about
by variations in climate and by the uplifting and depressing of
continental land-masses above or below the sea. The potency of climatic
influence on animal life is so great that even a fauna of large mammals
will be practically destroyed over a great area by a long-continued
change of a comparatively few degrees (probably less than ten degrees
Fahrenheit) in the mean daily temperatures.
The distribution of both recent and fossil mammals shows conclusively
that numberless species have spread from their original homes across
land bridges to remote unoccupied regions, where they have become
isolated as the bridges disappeared beneath the waves of the sea.
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