The laws regulating the geographical distribution of animals, and
their combination into distinct zooelogical provinces called faunae,
with definite limits, are very imperfectly understood as yet; but so
closely are all things linked together from the beginning that I am
convinced we shall never find the clew to their meaning till we carry
on our investigations in the past and the present simultaneously. The
same principle according to which animal and vegetable life is
distributed over the surface of the earth now, prevailed in the
earliest geological periods. The geological deposits of all times have
had their characteristic faunae under various zones, their zooelogical
provinces presenting special combinations of animal and vegetable life
over certain regions, and their representative types reproducing in
different countries, but under similar latitudes, the same groups with
specific differences.
Of course, the nearer we approach the beginning of organic life, the
less marked do we find the differences to be, and for a very obvious
reason. The inequalities of the earth's surface, her mountain-barriers
protecting whole continents from the Arctic winds, her open plains
exposing others to the full force of the polar blasts, her snug
valleys and her lofty heights, her tablelands and rolling prairies,
her river-systems and her dry deserts, her cold ocean-currents pouring
down from the high North on some of her shores, while warm ones from
tropical seas carry their softer influence to others,--in short, all
the contrasts in the external configuration of the globe, with the
physical conditions attendant upon them, are naturally accompanied by
a corresponding variety in animal and vegetable life.
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