16 VEGETATION IN CENTRAL AMERICA. 



greater dryness of the atmosphere and the lengthening of the dry 

 season, conditions which are evidently intensified by the exposure of 

 the earth to the sun. Denudation allows the rain water to run off at 

 once, and thus reduces the amount of moisture available for evapora- 

 tion into the atmosphere. At the same time the relative humidity of 

 the air is decreased through the greater radiation of heat from the 

 exposed, sun-baked earth. The earlier and later rains are diminished 

 and the dry season is lengthened until only the drought-resistant 

 types of vegetation are able to survive. Even if there were no change 

 at all in the amount of rainfall, changes in the distribution of the 

 rainfall would have serious effects on the vegetation. 



On large continental land masses where there is an extensive circu- 

 lation of the atmosphere exposures of the bare earth may appear to 

 have no very definite effect upon the climate, but in Central America 

 the effects of local conditions are often readily appreciable. The 

 regular alternation of land and sea breezes within a few hours does 

 not compel any general circulation of the atmosphere, but only slight 

 local displacements. Precipitation follows relatively small fluctua- 

 tions of temperature and is the more readily and obviously influenced 

 by local conditions. A column of heated air rising from a denuded 

 district can often be seen to absorb the clouds which drift over from a 

 neighboring forested area. A striking example of this phenomenon a 

 can be seen in the spring months from the crest of the mountain 

 which overlooks the valley of Salama, on the road to Kabinal. (See 

 PI. VII.) 



PREVIOUS OCCUPATIONS SHOWN BY ARCHAEOLOGICAL REMAINS. 



Many localities which are now occupied by apparently virgin forests 

 are shown by archaeological remains to be regions of reforestation. 

 Thus in the Senahu-Cahabon district of Alta Vera Paz relics of two 

 or three very different types of primitive civilizations indicate that 

 as many ancient populations have occupied successively the same 

 areas which are now being cleared anew by the coffee planters as 

 though for the first time. 



It does not yet appear that any considerable region of forest has 

 been explored in Central America without finding similar evidence 

 that the present forests are not truly virgin growth. Even in ex- 

 tremely humid and insalubrious lowlands of the Atlantic slope of 

 Costa Rica many relics of ancient civilizations have been uncovered 

 in clearing away the heavy tropical forests to make banana planta- 



a A similar effect is reported by Darwin from Brazil (see Voyage of the 

 Beagle, Chap. II), but is described as occurring near sunset, whereas it may be 

 seen in Guatemala during several hours of the day. The clouds are not collected 

 about elevated summits, but float higher in the air. 

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