436 DR. HANS GADOW ON ALTITUDE AND 
as barren, whilst the south and east appear luxuriant. Then there is the 
impressionist bias of the leading features to contend with. We are impressed, 
for instance, at Xometla (south-east side of Citlaltepetl, 8500 feet) by the 
forest composed of Pines, Oak, Arbutus, and Alder ; they are the dominant 
feature, truly characteristic of that level, but they do not make up the 
majority of the species, which being smaller, although much more numerous, 
are far less impressive, But when we gather during an hour's ramble at 
haphazard an armful of plants, even if with the unscientific intention of 
making a bouquet of flowers, then the true state of affairs soon becomes 
clear. The Northerners are comparatively easy to sort out, they being old 
friends, while the others are endemic Mexicans, and in such the country is 
extremely rich. 
They increase the lower down we go. At such a delightful level as 
Orizaba, Oaxaca, or Chilpancingo, i.e. between 4000 and 5000 feet elevation, 
whence high levels or a deep and wide valley can be reached by a few hours’ 
excursion, we are struck with another state of affairs. The Northerners are 
few and far between, no longer obtrusive, nearly every plant is new to us ; 
but the plants after an hour's climb higher up give a very different 
impression from those which we meet at the bottom of the valley. The 
latter kinds, in looks and behaviour, make up a tout ensemble which we 
recognise as typical of the Tierra caliente, even if we have but slight 
experience of the tropics. Indeed, it is surprising what a change a thousand 
feet up or down can imply, not everywhere, but at the right level, and such 
a level is that mysterious border-land between what the natives call the 
Tierra caliente and the Tierra templada. Not much happens when safely 
within the Tierra templada, whether we are at 4000, 5000, or 6000 feet, but 
the changes are truly surprising which meet the traveller who crosses the 
ranges of hills, themselves only a few thousand feet high, which run parallel 
with the coast of Guerrero. There, at the bottom of the valleys. for instance 
on the Rio Omitlan, we are in the unmitigated tropics with nothing whatever 
to remind us of Northern influence; but one thousand feet higher, on the 
ridge, we are amongst Oaks and Pines and the tropieal life is gone, although 
there, at 17? N., we are at a level of less than 2000 feet above the sea. 
We descend within an hour on the other side, to meet the same river 
winding its way through the full glory of the tropics. 
It is one of the great attractions, and a circumstance which makes Mexico 
so valuable for the study of altitudinal distribution, that there we can get so 
easily into and out of the tropics. This is the case, first because the country 
is so extremely varied in its structure, secondly because most of it lies just 
upon the border-line of the tropical belt of the world. 
