forests composed of uniformly large trees corresponding to our
oaks, maples, and beeches will not thrive unless the ground is
wet most of the time. Of course there may be no rain for a few
weeks, but there must be no long and regularly recurrent periods
of drought. Smaller trees and such species as the cocoanut palm
are much less exacting and will flourish even if there is a dry
period of several months. Still smaller, bushy species will
thrive even when the rainfall lasts only two or three months.
Hence where the rainy season lasts most of the year, rain forest
prevails; where the rainy and dry seasons do not differ greatly
in length, tropical jungle is the dominant growth; and where the
rainy season is short and the dry season long, the jungle
degenerates into scrub or bush.

The relation of scrub, jungle, and rain forest is well
illustrated in Yucatan, where the ancient Mayas reared their
stately temples. On the northern coast the annual rainfall is
only ten or fifteen inches and is concentrated largely in our
summer months. There the country is covered with scrubby bushes
six to ten feet high. These are beautifully green during the
rainy season from June to October, but later in the year lose
almost all their leaves. The landscape would be much like that of
a thick, bushy pasture in the United States at the same season,
were it not that in the late winter and early spring some of the
bushes bear brilliant red, yellow, or white flowers. As one goes
inland from the north coast of Yucatan the rainfall increases.
The bushes become taller and denser, trees twenty feet high
become numerous, and many rise thirty or forty feet or even
higher. This is the jungle. Its smaller portions suggest a second
growth of timber in the deciduous forests of the United States