438 DR. HANS GADOW ON ALTITUDE AND 
that of the temperate zone is almost exactly like 2 to 3 ; tropics compared 
with the cool zone like 1 to 21. If the temperate zone be taken as the 
normal condition, then the hot zone with its 75,000 square miles should have 
2200 species instead of the assumed 1600, and the cool zone with 25,000 
square miles should possess only 730 instead оЁ 1300 species—i. e., the cool 
zone has twice as many as expected ; and this is the ease because most of 
the 1300 cool zone species have got up there so easily from the temperate 
base. 
It would be ridiculous to conclude from the above Table that the Tierra fria 
is more favourable for plants, as regards number of species, than the Tierra 
ealiente. ' 
It seems a paradox that the areal density should increase from the tropical 
belt upwards. But the underlying principle, or solution, appears clear if we 
say that of 10,000 feet elevation— with 
200 species, the areal density would be enormous, perhaps averaging one 
remember that on a very small ared 
species for every square mile, since in such plaees the species, although few, 
are crowded as if they were on refuge islands. If the area of the Tierra fria 
were three times as large as it is, equal to that of the tropieal belt, there 
would probably be still not more than 1300 species, 7. e. with an areal density 
of 19 x 3=57, i. e. much smaller than that of the tropical belt. 
Dut the faet remains that the Tierra templada of Southern Mexico is richer 
than the tropical belt, probably for the following reasons. The upper limit 
of the Tierra caliente is generally assumed to lie at 3000 feet elevation ; 
undoubtedly this is not enough on the moist Atlantic side, but rather too 
much on the much drier Pacific slope. The belt of the Tierra templada, in 
this paper from 3000 to 7000 feet, comprises 33 per cent. more in vertical 
extent, and it comprises, moreover, in its larger area the most varied features, 
more varied and juxtaposed than exist in the other regions, from sweltering 
tropieal rain-forests to vast arid tablelands. All the mountain-ranges of 
middle height lie in the Tierra templada, with their mysterious eloud-belt (not 
to be confounded with the cloud-belt which reaches from 7000 to 10,000 feet 
elevation on the giant mountains). Innumerable brooks exist in the Tierra 
templada before they combine into larger and therefore less varied rivers : 
there are deep depressions in the Central Plateau with a semi-tropical climate, 
and also high wind-swept plateaus exposed to excessive heat in the summer 
days and to frost in the winter. 
In short, the great variety of environmental conditions is responsible for 
the richness of the flora of this intermediate belt, the most delightful and 
prettiest of all Mexico. 
It is therefore all the more interesting that this obviously reasonable 
