PLEISTOCENE ECOLOGY AND BIOGEOGRAPHY 393 



cations of climatic change are found in plant-animal distribution in 

 fragmented and isolated habitats such as the Cloud Forest. Griscom 

 (1932, 1950) noted rather remarkable uniformity from northern 

 South America to Mexico in bird life of the Subtropical Life Zone, 

 which includes Cloud Forest. This habitat generally appears on 

 windward slopes between 3,000 and 7,000 feet elevation. To account 

 for the faunal uniformity Griscom (1932) postulated Pleistocene 

 continuity of the Subtropical Zone, the result of its depression to 

 sea level. It displaced the lowland tropical fauna which withdrew 

 southward. 



Stuart (1951, p. 32) noted that the present range of montane 

 lizards and other temperate animals on either side of the Isthmus of 

 Tehuan tepee (e.g., Barisia and Sceloporus malachiticiis) indicated a 

 past cool corridor across the arid lowlands. But Stuart questioned 

 the displacement of lowland Tropical Rainforest, which Griscom 

 (1950, p. 358) located far south of its present limit in the period of 

 extreme Pleistocene glaciation. "... a descent to sea level of a sub- 

 tropical zone would have brought about either widespread exter- 

 mination of the tropical fauna or acclimatization of that fauna to 

 subtropical conditions. . . . The evidence, therefore, points to the 

 presence of a [lowland] tropical environment in northern Central 

 America even at the height of Pleistocene glaciation" (Stuart, 1951, 

 p. 29). It seems we must have the argument both ways, altitudinal 

 depression of subtropical conditions to achieve some continuity of 

 Cloud Forest through Middle America from Mexico to Colombia, 

 but with persistence as far north as Mexico of Arid-Tropical scrub 

 and Tropical Rainforest. Actually, 3,000 feet may be too rigid a lower 

 altitudinal limit for marginal populations of Cloud Forest species. 

 Under extremely humid conditions subtropical animals may descend 

 to 2,000 or 1,000 feet (Wetmore, 1943, p. 223). 



The nature of Cloud Forest vegetation, avifauna, and biogeog- 

 raphy in tropical America is under study by B. E. Harrell (1951). 

 Marshall's exemplary analysis (1957) of Mexican oak-pine wood land 

 also illustrates the biogeographic advantage of studying environ- 

 mental rather than political units. 



A brief summary of environmental changes in western North 

 America and Central America during the late Pleistocene would 

 include the following points: (1) direct correlation between glacial 

 conditions in the Cordilleras and the growth of pluvial lakes in the 



