l8 NELSON 



tropical peninsula of Yucatan, continues on to the Isthmus 

 of Panama. The Arid Tropical areas are characterized by low- 

 scrubby forests ; the Humid Tropical by greater luxuriance of 

 vegetation. The rainfall on the mountain slopes facing the sea 

 on both coasts is much heavier than on the adjacent lowlands, 

 and is distributed more generally throughout the year, produc- 

 ing a heavier forest growth. In eastern Mexico this causes a 

 northward extension of the Humid Tropical area, in the form of a 

 long narrow tongue along the mountains, which reaches eastern 

 San Luis Potosi. On the west coast the uniformity of the Arid 

 Tropical area is broken by humid tropical islands at intervals 

 along the mountains northward nearly to the border of Guer- 

 rero, and by belts of heavy forest along streams flowing through 

 the plains. 



Above the Tropical zones in Mexico the higher mountain 

 slopes extend through all the succeeding life zones to extreme 

 timberline, and in Central America to altitudes varying from 

 6000-14,000 feet. Differences of altitude are always accom- 

 panied by climatic changes, but in addition local causes also 

 often exert a marked influence. Thus the proximity of high 

 mountains, or the trend of a range or spur in relation to the 

 course of the prevailing wind, frequently has a direct effect 

 upon the rainfall of the surrounding district. In Vera Cruz 

 the north and northeast slopes of mountains are by far the most 

 humid owing to the fact that during the dry winter months 

 ' northers ' prevail, bringing fogs and rain. A striking example 

 of the climatic differences that may exist within a limited area 

 is afforded by the slopes of Mt. Orizaba, in western Vera Cruz. 

 The top of this mountain rises over 18,000 feet above sea level 

 and is perpetually covered with snow ; its southeastern base 

 reaches the hot coast plain, and at an altitude of 800 feet the 

 foothills are covered with a humid tropical forest so dense in 

 places that the dew drops all day from the undergrowth ; while on 

 the opposite side, at an altitude of 8000 feet, lie the arid treeless 

 tablelands. 



HISTORY AND NOMENCLATURE. 



The first tree squirrel of tropical North America to receive 

 a distinctive scientific name was Scitirtis aureoQ-aster, described 



