uthwest 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 the climate of the home of the Mayas in the days
of their prime was much more nearly like that which now prevails where
civilization rises highest.

From first to last the civilization of America has been bound up with
its physical environment. It matters little whether we are dealing with
the red race, the black, or the white. Nor does it matter whether we
deal with one part of the continent or another. Wherever we turn we can
trace the influence of mountains and plains, of rocks and metals from
which tools are made, of water and its finny inhabitants, of the beasts
of the chase from the hare to the buffalo, of domestic animals, of the
native forests, grass-lands, and deserts, and, last but not least, of
temperature, moisture, and wind in their direct effects upon the human
body. At one stage of human development the possibilities of agriculture
may be the dominant factor in man's life in early America. At another,
domestic animals may be more important, and at still another, iron or
waterways or some other factor may be predominant. It is the part of
the later history of the American Continent to trace the effect of these
various factors and to chronicle the influence t