and their occurrence in various vegetation types has been 
obtained from Miranda (1952-58) and supported by my 
field observations. Miranda’s classification will be fol- 
lowed throughout the discussion of the vegetation types. 
Chiapas today is characterized by considerable vegeta- 
tional variation controlled partially by topographic diver- 
sity, which also controls many of the climatic conditions. 
Chiapas is bounded on the north and south by coastal 
plains. On the north the coastal plain is a prolongation 
of the extensive Tabasco plain; on the Pacific side it is 
only 20-80 kilometers wide. The greatest heights (1500- 
4000 m.) occur in the Sierra Madre, located immediately 
to the north of, and rising parallel to, the Pacific coast. 
The Central Mesa of Chiapas averages 1200-2200 m., 
but with local higher peaks. Between the Tabasco coastal 
plain and the Central Mesa, an abrupt, highly eroded 
hilly area ranges between 1400 and 1800 m. ‘To the south 
of «he Central Mesa, and between it and the Sierra 
Madre, is the large Central Depression with the altitude 
varying from 42 to 800 m. 
The precipitation patterns are mainly controlled by 
the northeast trade winds and are modified by topo- 
graphic effects. There are two definite wet seasons alter- 
nating with dry seasons. Relatively violent storms occur 
as ‘‘Nortes,’’ and hurricanes are known to sweep in from 
the Gulf of Mexico and the Antilles. The highest annual 
precipitation recorded for Mexico (4-5 m.) occurs along 
the Chiapas and Tabasco coastal plain, and there are no 
truly arid conditions (less than 500 mm. annual precipi- 
tation) in Chiapas (Miranda, 1952). It is assumed, how- 
ever, that relatively drier conditions exist locally now 
than existed in the Oligo-Miocene. This is a result of 
rain shadow effects of currently higher ]and masses and 
latitudinal extension of tropical conditions much farther 
north in the past then than they are today. 
[ 267 ] 
