DISTRIBUTION OF PLANTS IN SOUTHERN MEXICO, 437 
But to return to our diagram. The area of the Southern species, and the 
long strip of the Northern species according to the plan adopted for the 
plotting out, enclose of necessity a large odd-shaped area which of course is 
nota vacuum. It represents the endemic temperate species of which we 
know how many extend into the alpine region, and of which many more 
grow down into the hot lands. It does not matter much whether we allow 
200 or 600 species for this base ; nor does it matter whether we increase the 
absolute total number from 2500 to 4000 and refer it either to the 3500 or 
to the 5000 feet level, The general shape of the diagram with its three 
areas will remain essential the same ; in other words, the broad principle 
expressed by it is correct, The shape of the diagram is odd, bulging out as 
it does at the 4500 feet level; but this very odd shape is another great point 
in its favour of being correct, since it graphically represents the respective 
areas of the hot, temperate, and cold regions of the country. А great portion 
of Mexico is high plateau with a mean elevation of perhaps 5000 feet, fringed 
east, south, and west by high Sierras, whilst the hot country, although formed 
by extensive coast-lands, is of comparatively small extent. But the southern 
portion of Mexico is much intersected by mountain-ranges and by deep 
depressions ; for instance, the large Balsas basin extends so far inland that 
it is exceedingly difficult to apportion the various areas, especially if we 
remember that much of the country is but imperfectly surveyed. 
The following calculations ean therefore be approximate only. Reckoning 
the country between 21? N. and the Isthmus to contain 185,000 square miles, 
I have apportioned 75,000 square miles to the Tierra caliente, 85,000 to the 
Tierra templada, and 25,000 square miles to the Tierra fria or to what lies 
above 7000 feet elevation. What lies above 10,000 feet can amount at most 
to a few hundred square miles. 
The quotient of the areal number of square miles, divided by the number 
of species occurring, I call the areal density of species. 
Thus we have :— 
Areal density, 
Area in Number of i, e., an average of 
square miles. species. — sq. miles to 
every species, 
Tierra caliente ...... 75,000 1600 41 
Tierra templada ... 85,000 2500 34 
Tierra fria ............ 25,000 1300 19 
The result is somewhat surprising : the areal density of species increases 
steadily from the tropical belt upwards! That of the tropics compared with 
