240 DR. H. GADOW ON MEXICAN [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 Pacificlowlands. 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, 
Difterent 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 25° C. In short, the Hot-land temperature 
averages from 25° to 28° C.=75° to 82° H.; 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 de- 
cidedly 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 wstwm, 
Phyllodactylus tuberculosus, Uta bicarinata, Zamenis mexicana. 
To another category belong those species which have a wide, 
but very scattered, discontinuous distribution, especially those 
