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Show 240 DH. IT. GADOW ON MEXICAN [ J u n e 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 Caliente we have periodical extremes. Different again is the moist Atlantic Tierra Caliente, and also the ranges of mountain forests of the Southern and South-eastern Tierra Templada. There are no extremes ; the very opposite to arid tracts ; there is plenty of high and low vegetation all the year round. The important factor is not the temperature, 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° C. = 96'8° F .; for the rest of the northern plateau 30° C. = 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 2.5° 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 much more generic than specific. There are, indeed, very few species which, although having a wide geographical range, are well established in stations of decidedly very 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 Tropidonotus 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 neighbouring coast (Bufo simus, Hypsiglena torquata, Zamenis grahami); 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 Bufo marinus, B. valliceps, Hyla miotympanum, Engystoma ustum, Phyllodactylus luberculosus, Uta bicarinata, Zamenis mexicana. To another category belong those species which have a wide, but very scattered, discontinuous distribution, especially those |