94 Bulletin American Museum of Natural History. [Vol. XXXVI, 



Where zonal boundaries are determined not only by altitude (= tem- 

 perature) but also by humidity, they are more clearly defined than when 

 attributable to temperature alone. As with faunas, suitability of haunt 

 or habitat here plays an important part. 



The upper limits of the Tropical Zone in the Cauca Valley, for example, 

 are arid and meet the lower borders of the Subtropical Zone at the lower 

 limit of condensation. In journeying from west to east one therefore passes 

 from the dripping, cloud-wrapped forests of the western slopes and crest 

 of the Western Andes to the arid, treeless eastern slope of the range. The 

 forest-inhabiting birds of the Subtropical Zone stop as suddenly as the 

 forest itself and they are succeeded by certain species of the arid Tropical 

 Zone which find a suitable haunt on these treeless slopes, whence they have 

 extended their range upward from the dry savannas of the Cauca Valley 

 below. 



Descending to and crossing this valley, we ascend the bare foothills of 

 the western slopes of the Central Andes only to reverse the experience, as at 

 the cloud line (about 6500 ft.) one leaves the arid Tropical Zone and enters 

 the forests of the Subtropical Zone. 



On both sides of the valley, however, an extremely interesting interdigita- 

 tion of zonal boundaries is observed as the arid Tropical Zone climbs up 

 the barren lidges or crests of the spurs of the foothills, while the forests of 

 the Subtropical Zone seem to flow down the drainage areas or arroyos be- 

 tween them. Under such conditions Tropical Zone species are found at 

 higher altitudes than Subtropical Zone species on the same mountain slopes, 

 and the importance of a personal knowledge of the local factors is obvious. 



The altitude to which the Tropical Zone ascends is determined, there^ 

 fore, primarily by temperature, but, as with faunal boundaries, humidity 

 may exert an important influence not only as it increases the temperature 

 but provides an environment better adapted to the wants of certain species 

 of the arid Tropical Zone than to those of the Subtropical Zone. 



Our experience at Buena Vista, in the Eastern Andes, made it apparent 

 that the altitude of the upper margin of the Tropical Zone may also, to 

 some extent, depend on the altitude of the range or ridge concerned. Here 

 a heavily forested spur rises from the Llanos, with only gallery forests, to 

 an altitude of 4500 feet. Its direct physical connections are therefore with 

 the Tropical Zone, and in spite of the favorable environment and the alti- 

 tude, very few Subtropical Zone species were secured. 



Accessibility may therefore be a factor in fixing zonal boundaries. It 

 is accessibility which chiefly distinguishes the Tropical Zone from zones 

 above it. Life may enter it wherever it comes in contact with areas lying 

 below an altitude of approximately 5000 feet; and a glance at an orographic 



