temperature, and the steady humidity are as hostile to
civilization as are the cold of the far north and the dryness of
the desert.

The only explanation of this anomaly seems to be that in the past
the climatic zones of the world have at certain periods been
shifted farther toward the equator than they are at present.
Practically all the geographers of America now believe that
within the past two or three thousand years climatic pulsations
have taken place whereby places like the dry Southwest have
alternately experienced centuries of greater moisture than at
present and centuries as dry as today or even drier. During the
moist centuries greater storminess prevailed, so that the climate
was apparently better not only for agriculture but for human
energy. At such times the standard of living was higher than now
not only in the Southwest but in the Gulf States and in Mexico.
In periods when the deserts of the southwestern United States
were wet, the Maya region of Yucatan and Guatemala appears to
have been relatively dry. Then the dry belt which now extends
from northern Mexico to the northern tip of Yucatan apparently
shifted southward. Such conditions would cause the forests of
Yucatan and Guatemala to become much less dense than at present.
This comparative deforestation would make agriculture easily
possible where today it is out of the question. At the same time
the relatively dry climate and the clearing away of the
vegetation would to a large degree eliminate the malarial fevers
and other diseases which are now such a terrible scourge in wet
tropical countries. Then, too, the storms which at the present
time give such variability to the climate of the United States
would follow more southerly courses. In its stimulating qualities