240 DK. H. GADOAV ON BiEXicAN [June 6, 



and the gloom of the underwood. In a desert or semidesert 

 the amount and character of the scarce and precarious vegetation 

 remain practically stabile ; not so in the Pacific lowlands. During 

 the rainy season grows up a dense mass of herbaceous plants 

 covering the ground with a tangle of weeds, tall Salvias and 

 Composites, stinging herbs and spiny creepers ; all this disappears, 

 is burnt up, scattered during the dry season, and for months the 

 ground may be bare, whilst many of the trees are leafless. In 

 this Pacific type of Tierra Oaliente we have periodical extremes. 

 Different again is the moist Atlantic Tierra Caliente, and also 

 the ranges of mountain forests of the ^Southel•n and South-eastern 

 Tierra Temjolada. There are no extremes ; the very opposite to 

 arid tracts ; there is plenty of high and low vegetation all the year 

 i-ound. 



The important factor is not the tempeiuture, nor the altitude 

 as such, but the amount, or rather the distribution, of annual 

 moisture. Temperature : more than the northern half of the 

 Mexican plateau belongs to one of the hottest regions of the world, 

 the centre of heat being the State of Sonora. From May to July 

 the mean temperature for Sonora is 36° 0. = 96'8° F. ; for the 

 rest of the northern plateau 30° 0. = 86° F., which is more than the 

 summer average of South Mexico and Central America. But in 

 the winter the North averages 16° C. = 60-8° F., while the Tierra 

 Caliente enjoys 25° C. In short, the Hot-land temperature 

 averages from 25° to 28° C. = 75= to 82° E. ; the Northern plateau 

 from 60° to 96° F., with additional extremes from frost and snow 

 to unbearable broiling heat and drought. 



The overlapping, mentioned above, is mu,ch more generic than 

 specific. There are, indeed, very few species which , although having 

 a wide geographical range, are well established in stations of de- 

 cidedly veiy different physical aspect. For instance, species on the 

 higher mountains, or plateaux, and also in the Tierra Caliente : see 

 p. 231. But of all these only very few, e.g. Hylodes rhodopis, 

 Sceloporus scalaris, a Rattlesnake, and Trojndonotus ordinatus, 

 can, in their indifference to physical conditions, be compared with 

 the Puma, the Armadillo, Opossum, the Raven, and Turkey- 

 Buzzard. 



Some species, natives of the plateau, descend from it down 

 to the neighbouilng coast {Btifo simus, Hypsiglena torqiucta, 

 Zamenis grahavii) ; others ascend from the hot countries on 

 to the plateau, especially from the west by way of Guadalajara, 

 and thence to Guanajuato and further east, the means being the 

 alluvial plains spoken of before ; or the ascent can be traced 

 through the Balsas depression towards Iguala and Cuernavaca ; 

 another opportunity seems to lead from the east side to Zacual- 

 tipan in the State of Hidalgo. Such ascending species are £ufo 

 marinus, B. valliceps, Hyla miotympanum, Engystoma usttivi, 

 Phyllodactylus tuber culostis, Uta hicarinata, Zamenis mexicana. 



To another category belong those species which have a wide, 

 but very scattered, discontinuous distribution, especially those 



