han to-day. Then the Glacial Period spread an
ice-sheet from the North Pole to approximately the fiftieth parallel,
forced back life to the lower latitudes, and confined the bio-sphere to
the smaller land-masses of the southern hemisphere and a girdle north of
the equator. The sum total of life on the globe was greatly reduced at
the height of glaciation, and since the retreat of the ice has probably
never regained the abundance of the Middle Tertiary; so that our period
is probably one of relative impoverishment and faulty adjustment both
of life to life and of life to physical environment.[292] The continent of
North America contained a small vital area during the Later Cretaceous
Period, when a notable encroachment of the sea submerged the Atlantic
coastal plain, large sections of the Pacific coast, the Great Plains,
Texas and the adjacent Gulf plain up the Mississippi Valley to the mouth
of the Ohio.[293]

The task of estimating the area supporting terrestrial life which the
earth presented at any given time is an important one, not only because
the amount of life depends upon this area, but because every increase of
available area tends to multiply conditions favorable to variation.
Darwin shows that largeness of area, more than anything else, affords
the best conditions for rapid and improved variation through natural
selection; because a large area supports a larger number of individuals
in whom chance variations, advantageous in the struggle for existence,
appear oftener than in a small group. This position is maintained also
by the most recent evolutionists.[294]

On purely geographical grounds, also, a large area stimulates
differentiation by presenting a greater diversity of natural conditions,
each of which tends to produce its appropriate species or variety.[295]
Consider the different environments found in a vast and varied continent
like Eurasia, which extends from the equator far beyond the Arctic
Circle, as compared with a small land-mass like Australia, relatively
monotonous in its geographic conditions; and observe how much farther
evolution has progressed in the one than in the other, in point of
animal forms, races and civilization. If we hold with Moritz Wagner and
others that isolation in naturally defined regions, alternating with
periods of migration, offers the necessary condition for the rapid
evolution of type forms, and thus go farther than Darwin, who regards
isolation merely as a fortunat