230 DR. H. GADOW ON MEXICAN [June 6, 
in other words either Northerners, as natives of Old Sonovaland, 
dwellers of mountains and high plateaus, or Southerners, which 
were and are mostly tropical species. The temperate zone is in 
the present case rather no-man’s-land than the happy medium 
favourable to the majority. 
The configuration of the whole country lends every support to 
this result; broadly speaking, a high, mountainous plateau, 
abruptly falling off into tropical lowlands. 
The species which have such a considerable range of altitude 
that they occur in the cool, temperate and hot zones, are of further 
interest. The same kind which is bound to hibernate on the high 
mountains is active throughout the year in the moist and hot 
lands, and possibly there are some which also exstivate during 
prolonged drought. The species can be grouped as follows :— 
T. Undoubted Northerners, or originally at home in a cool 
climate, as indicated by their main distribution, or by that of allied 
species of the same genus. These have descended into the hot 
lands. 
Scaphiopus dugest. Tropidonotus melanogaster. 
Rana halecina. % ordinatus. 
> montezume. ss validus . 
Sceloporus scalaris. Coluber triaspis. 
5 microlepidotus. Crotalus horridus. 
Uia bicarinata. 
Gerrhonotus ceruleus. 
Cinosternum pennsylvamcum. 
IL. Essentially hot-country species which have ascended; and 
it is remarkable that most of these are not found on the plateau 
proper, although they ascend the surrounding mountains, up to an 
altitude equal to or surpassing that of the plateau. This fact 
seems to indicate that the respective species are still continuing 
their upward spreading, or that they have conquered these 
mountains comparatively recently. This fits well with the 
suggestion expressed on p. 244 that the Southern or tropical 
fauna of Mexico represents for the greater part the most recent 
immigrants. The Sierra Madre del Sur affords a good illustration. 
It is separated from the plateau by the depression of the basin of 
the Rio Balsas. Tropical species coming from the south can surge 
up to the Sierra, and they have ascended its higher mountains 
(e. g. those of Omilteme, Amula, Cerro de S. Felipe near Oaxaca), 
and the backbone itself is of no mean height; but then comes the 
descent into the hot basin, then again the ascent of the plateau. 
A tropical species, which has succeeded in acclimatising itself to 
life on the Sierra, will have to “undo” this hardening, become 
tropical again, and lastly once more ascend and accommodate itself 
to a cool climate. Of course all this can be done, but it takes 
time. The same applies to the fauna of the rather isolated Volcan 
and Nevado de Colima. The ranges of mountains which border 
