l rains, that portion of Mexico is without rain, and dry, and so
continues until the rainy belt returns in the following year. While the
belt is over Southern Mexico it is nearly all _precipitation_, and there
is little _evaporation_; while that belt is _absent_ it is all
_evaporation_, with little or no _rain_. Surely this is not consistent
with the prevailing belief of simple evaporation, ascent to a colder
stratum, commingling, and condensation, and rain. Southern Mexico at least
is not supplied by mere evaporation from its surface, and must therefore
form an exception to that belief, and to the Huttonian theory.

But we shall recur again to the peculiarity of distribution within the
tropics.

Turn now for a brief space to Northern Mexico, Southern New Mexico, and
Southern California. In Northern Mexico, Southern New Mexico, Utah, and
California, between the parallels of 28 deg. and 32 deg., and particularly west of
the mountain ranges, we find an almost rainless region, sterile and
worthless, resembling that which is found upon nearly the same parallels
of north latitude in Northern Africa, Egypt, Arabia, Beloochistan,
Afghanistan, and North-western India; and in corresponding latitudes south
of the Equator, in Peru, a portion of Southern Africa, and the northern
and middle portions of New Holland. Why Northern Mexico and the other
countries named are thus sterile and comparatively rainless, we shall see
hereafter, when we examine critically the machinery of distribution as it
operates within the tropics. It is the fact that it is thus sterile and
rainless to which we desire to call attention in this place.

Mr. Bartlett thus describes it:

"On leaving the head waters of the Concho, nature assumes a new
aspect. Here shrubs and trees disappear, except the thorny chaparral
of the deserts; the water-courses all cease, nor does any stream
intervene until the Rio Grande is reached, three hundred and fifty
miles distant, except the muddy Pecos, which, rising in the Rocky
Mountains, near Santa Fe, crosses the great desert plain west of the
Llano Estacado, or Staked Plain.

"From the Rio Grande to the waters of the Pacific, pursuing a
westerly course along the 32d parallel, near El Paso Del Norte, there
is no stream of a higher grade than a small creek. I know of none but
the San Pedro and the Santa Cruz--the latter but a rivulet, losing
itself in the sands nea