310 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



Darwin's impressions of tropical scenery, or rather of tropical 

 vegetation, were probably the same as those of others when, for the 

 first time, they gazed on the splendid vegetation of the humid Tropics ; 

 that is, those parts where abundant and frequent rains produce a 

 perennial verdure, for the contrast in the vegetation of the humid 

 with that of the arid Tropics is even greater than between the former 

 and that of the more humid parts of the Temperate Zone. 



The dominant feature of the humid Tropics is the forest. This 

 covers the whole country, except where cleared by man, with a con- 

 tinuous heavy mantle, only the extreme summits of the higher moun- 

 tains being treeless; and in Costa Rica the "timber-line" is not the 

 effect of altitude but of eruptions of hot ashes, cinders, or gases from 

 the volcanic vents. Although continuous in extent, the forest pre- 

 sents great differences in its appearance and composition according 

 to altitude, the amount and continuity of rainfall, slope exposure, and 

 character of soil. According to these variations the forest may be 

 roughly divided into the following main types: 



I. The perennially evergreen forests of the Atlantic or Caribbean 

 slope, with the following subdivisions: (a) The forests of the coastal 

 plain; (5) the forests of the foothills and lower mountain slopes; 

 (c) the forests of the cloud zone. 



II. The only partly evergreen forest of the semiarid Pacific slope. 

 Of whatever type, however, all present one feature in common, 



though in varying degree, namely, extreme exuberance of growth and 

 variety of composition. 



The Atlantic or Caribbean coastal-plain forest I am least familiar 

 with, having seen it only in passing through by train, except that 

 portion which, in a more or less modified form, overlaps or merges 

 into the mountain forest among the foothills; consequently, the fol- 

 lowing observations apply to the other subdivisions only. It may be 

 remarked, however, that the coastal-plain forest represents the acme 

 of luxuriance in tropical vegetation, maximum development in size 

 of the trees, and of variety of palms and succulent plants with large 

 and conspicuous foliage. 



The forests which cover the foothills and greater part of the slopes 

 of the mountains are, in general appearance, more monotonous and 

 somber than those of the coastal plain. Seen from the outside or at 

 a little distance, this forest does not seem very different in appear- 

 ance from our northern primitive hardwood forests. The general 

 color is perhaps a darker or less vivid green and there is something in 

 the contour of the tree tops which seems unfamiliar to the observer 

 from the North; they are relatively broader, flatter and more com- 

 pact, and when a bare stem can be seen it seems disproportionately 

 slender for the broad crown which it supports. For much the 



