oacan--Lakes Cuitzeo and Patzcuaro. The
remarkable groups of lakes in the Valley of Mexico, around which the
Aztec civilisation flourished, comprise six salt-water and one (that of
Chalco) fresh-water lake. The two maps given in these pages, of the
disposition of these lakes at the time of the Conquest[26] and at the
present day, respectively, show how remarkably their waters have shrunk
during the intervening centuries. Indeed, this may have followed a
certain drying-up process which seems to have been going on throughout
the whole Andine region of the Americas, and which is evidenced by
retiring snow-caps in Peru, and the receding of Lake Titicaca.

[Footnote 26: See page 76.]

The climate and temperature of Mexico follow certain marked zones,
depending upon elevation, as already indicated in the opening chapter.
Both on the Atlantic and Pacific slopes these zones are
encountered--the _tierra caliente_ up to 3,000 feet elevation above
sea-level; the _tierra templada_ to 5,000 or 6,000; and the _tierra
fria_ above that altitude. On the tropical lowlands the heat of the
torrid zone is experienced, but is not necessarily oppressive, although
the European or American traveller who prefers a less enervating
climate hastens to exchange that region for the more bracing air of the
uplands. The night breezes, however, compensate largely for the heat of
the sun, and render bearable, and indeed agreeable, the Vera Cruz
littoral and the Yucatan peninsula, by the lowered temperature they
afford. The rains also, which have their season from June to November,
do much to refresh the atmosphere. Indeed, the year is divided mainly
by the matter of rainfall into a wet and dry period, the summer and
winter of other countries being unknown; or, rather, one might say,
that the daytime is the summer and the night-time the winter, so marked
are the diurnal changes of temperature.

In the _tierra caliente_ the mean temperature varies from 77 degrees to
80 degrees F., but often rises to 100 degrees, and in some of the
hottest coast regions to 105 degrees F. In the _tierra templada_ the
mean is from 62 degrees to 70 degrees F., and this is the climatic
region which the Mexicans love to term "perpetual spring." In point of
fact, it is a zone not unworthy of the designation, being equable,
healthy, and with a beautiful and varied _flora_. It is to be
recollected that the greater part of the area of the country lies in
this temperate zone, alth