690 Hans GADow, 
The caleulated and the observed mean temperatures differing 
oniy about 0,1 of a degree! Nevertheless the respective faunas and 
floras of these two places are by no means identical nor living 
under .the same conditions, because of the different pressure; further 
the Northern parts are droughty, the annual scanty rainfall being 
concentrated into a few irregular downpours, whilst the Southern 
part of the plateau has a regular wet season with considerably more 
rain; again the month of May is, in the North the time of excessive 
heat, when storms of dust rage, often for several days, whilst in the 
Southern part of the plateau such excesses. are much rarer. The 
Atlantic or Gulf side is hot and moist, the Pacific hot and much drier. 
It may therefore be doubted whether it is worth trying to draw 
conclusions from the altitudinal distribution of a Fauna. But Mexico 
is a favorable country for such studies thanks to its almost unique 
geographical conditions. For our purposes, and as a geographical 
entity, it ends as the Isthmus of Tehuantepec. The whole country 
is a broad-based southward extension of North America, a narrowing 
peninsula, the greater part of which is taken up by the obliquely 
slanting plateau, fringed, especially in the South and West by hich 
ranges of mountains, and sloping down sharply to East and West, 
leaving an Atlantic and a Pacific long strip of low Hotlands. The 
Southern portion is complicated by a coast range. The Hotlands 
are quite tropical in character at the Isthmus, 17° N. Whence they 
extend imperceptibly into Texas, and in the North West up to 
32° N assuming gradually subtropical features. 
Thus it comes to pass that every variety of climate exists in 
Mexico within short distances. Hot and moist, or hot and dry 
lowlands, to temperate and cold plateaux and snow capped moun- 
tains. And as Mexico lies well within the tropical belt, it is easy 
to get into and out of the various climates, easy not only for 
the components of the fauna and flora but also for the naturalist 
who wants to investigate them. 
Further, as I have shown in detail (The distribution of Mexican 
Amphibians and Reptiles, in: Proc. zool. Soc. London, 1905, p. 191—244) 
Mexico is the meeting ground of two of the fundamentally most 
distinct faunas and floras, the Nearctic and the Neotropical. The 
first has got a great lead Southwards by following the plateau 
which for more than one thousand miles so effectively counteracts 
the deereasing latitude by its steady rise. The host of Neotropicals 
in their northward spreading have surged against the wedge of this 
