26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



there are well-marked arid and humid divisions ; the Upper Tropical, 

 or Subtropical Zone, and the Temperate Zone. 



An exhaustive ecological treatment of the animals and plants of 

 the region should recognize aquatic, littoral or riparian, and other 

 associations which, except in a few such instances as those of Chiro- 

 nectcs panamensis, Rheomys raptor, Hydrochccras isthmius, and 

 Trichechus manatus have comparatively slight significance with 

 reference to mammals alone. 



As in the neighboring regions, the life zones are the expression 

 of the influence on organisms of various factors, or varying combi- 

 nations of factors, of which temperature and moisture, more or less 

 intimately associated, and light, are of prime importance. 



The approximate boundaries between zones on different slopes 

 vary in conformity with many of the same modifying conditions as 

 elsewhere ; the humidity of a given area is clearly determined by the 

 height of mountains in combination with the direction of prevailing 

 winds. 



The zone lists include all of the mammals known from the region, 

 except certain widely ranging species whose distribution have no 

 obvious zone significance A species or subspecies may occur regu- 

 larly in two or more life zones, but is usually assignable to one in 

 which it reaches its maximum abundance. Here, as elsewhere, some 

 of the mammals exhibit a tendency to become differentiated in accord- 

 ance with rather local environmental conditions ; thus, a species 

 characterized by dark colors in the humid belt, may be represented 

 by a paler counterpart in more arid territory. The lists of birds are 

 made up mainly of the more characteristic species, and together 

 with the short lists of plants tend to corroborate deductions which 

 might be based on the mammals alone. 



LOWER TROPICAL ZONE 



The Lower Tropical Zone, an area of high temperature, includes 

 by far the greater part of the Isthmian land surface from the Atlantic 

 and Pacific shore lines across at low elevations from sea to sea and 

 to about 3,000 to 3,500 feet in average altitude along the slopes of the 

 higher mountains. As might be expected, owing to its greater 

 extent, the majority of the animals and plants of the general region 

 are assignable to this zone, and many species, especially of bats, 

 have extended their ranges into all its parts. The zone is, however, 

 divisible into humid and arid divisions, which are denominated the 

 Humid Lower Tropical Zone, and the Arid Lower Tropical Zone, 



