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great age of the Vikings, the Norsemen. As toward its close it grow
less dry, the wet began to rob the Vikings of their livelihood and their
lands as surely as the drought had robbed the steppe dwellers of
theirs; and they poured forth in a burst of migration which took them
across the Atlantic, and eventually, in the guise of Normans, as far
as Sicily.


In the New World too the climatic changes were similar and had
the same general effects, notably upon the story of the remarkable
Maya civilization of Yucatan. The huge monuments of the Mayas
are now buried in dense tropical jungle, which no primitive people
could hope to keep at bay. After the first flourishing period of the
Mayas, civilization retreated for centuries from Yucatan, but recolon-
ized its northern part for a short time about A.D, rooo. The two
flourishing periods of Maya history correspond with what we have
called cold, wet periods. But these were wet only in regions at a
certain distance from the poles. During these times, the storm tracks
shifted further toward the equator; and accordingly the dry belts
between temperate and tropical were shifted equatorward too. To-
day, Yucatan lies just south of where the northern dry zone passes
over into the tropical. When the temperate rainy zone shifted south,
the margin of the dry zone also was forced southward over Yucatan,
the forest melted, and the Mayas could build an empire there.
In the temperate zones, after the short wet period of the eleventh
century, there followed a series of minor and drier fluctuations. There
was one cold spell in the thirteenth century. There was another in
the first half of the seventeenth, in which the tradition of the <<! old-
fashioned" severe winter probably takes its origin (though doubtless
perpetuated by the common failing of age to decry the present in
favour of the past). Since then there has not been any great change.
True, there have been shiftings of sea currents, such as that which
brought the herrings to the Baltic, or that which sent the cod away
from the coast of Brittany; but there have been no marked move-
ments of the storm belt.
This long string of conclusions is drawn from the most diverse
sources—from the deposits in northern peat bogs, from the old shore
lines of the Caspian, from the salt lakes of Central Asia, from the now
waterless cities, such as Palmyra, that once lay on great trade routes,
from legend and historical record. But they find a wonderful corro-
boration within the trunks of the big trees of the western United
States. Rain is the limiting factor of the tree's summer growth, and