136 ■ FORREST SHREVE 



two mountains, and a still closer agreement in the manner in 

 which these factors control the vertical and local distribution of 

 vegetation. 



Between two regions as widely separated as Jamaica and 

 Arizona we would expect to find the very great dissimilarity of 

 flora and vegetation that is known to exist. Between the 

 physical controls of the two regions there are also great differ- 

 ences, sometimes even direct antitheses. In the dripping fog- 

 filled forests of Jamaica one would expect to find the local dis- 

 tribution of the vegetation dependent upon a very different set 

 of environmental conditions from those that control the sclero- 

 phyllous and semisucculent plants of the arid slopes of the 

 Arizona mountains. 



The difference of latitude from 19° N. to 32° N., and the 

 insular position of the Blue Mountains contrasted with the 

 continental position of the Santa Catalinas, lie at the bottom of 

 all else that may be said regarding their dissimilarities. The 

 tropical mountain gradient of temperature renders the summit 

 of the Blue Mountains cool, but very rarely and only locally 

 subject to frost, whereas the whole vertical extent of the Santa 

 Catalinas is exposed to frost and their upper altitudes to rela- 

 tively severe and prolonged low temperatures. The trade 

 winds of the Caribbean Sea carry a constant and enormous mass 

 of moisture-laden air to the slopes of the Blue Mountains, 

 making their windward side reek with fog and frequent rain, 

 and giving their leeward side a drier, but by no means dry cli- 

 mate. The winds which ascend the Santa Catalina Mountains 

 are both hot and extremely dry, usually exerting a very desiccat- 

 ing effect upon the vegetation of the lower half of thp mountain 

 and only occasionally, in the mid-summer or mid-winter rainy 

 seasons, gathering sufficient moisture to cause precipitation. 



The climatic conditions of the Blue Mountains are extremely 

 constant when contrasted with those of the Arizona Mountains. 

 The daily temperature curve of the former swings through six 

 to ten degrees, the curve of the latter through forty to fifty- 

 five degrees; and the annual amplitudes are quite as unlike. 

 The frequence and regularity of rain or fog, or at least of heavy 



