DISTRIBUTION OF PLANTS IN SOUTHERN MEXICO. 433 
balanced * by the 53 others, at 13,000 feet the Northerners have rapidly 
gained the preponderance, owing almost entirely to the rapid falling behind 
of the endemic ascendants, which are now reduced to the solitary Echeveria. 
This is very significant, as it seems to indicate that the native basal genera 
find it more difficult to adapt themselves to these arctic heights than do the 
long migrants, the Northerners, which were already prepared for such 
eventualities, to put the matter in homely language. But it is equally 
significant that the Andines, although always few in numbers, hold their own. 
Thus it comes to pass that above 14,000 feet the few Northern plants are 
balanced by the others, and that near the snow-line a solitary Northern 
Arenaria is confronted by the Andine Chionolena and by an Alchemilla. 
From 10,000 feet downwards species and genera increase rapidly. This is 
not so much a fancy level as it would appear from the round number in feet, 
or 3100 metres. It fairly well marks the average top level of the main 
cloud-belt with its resultant moisture. Various calculations, notably at 
9500 feet, at 8500 feet at our Xometla camp, and at 7000 feet, indicate that 
the increase is rather steady, and it seems that the absolute maximum otf 
species is reached somewhere between 4000 and 5000 feet, the level of Orizaba. 
It is therefore possible to construct a curve from the 4500 feet level with 2500 
species to the 10,000 feet level with 200 species. Further, it stands to reason, 
first, that the number of Southern species, which of course have their 
maximum in the tropics, decrease steadily from that base upwards ; secondly, 
that the Northerners decrease steadily to sea-level, where we are not far 
wrong in assuming that they represent less than 5 per cent. of the plants 
found in the true tropics. But it is very difficult to ascertain in which level 
lies the absolute maximum of the Northerners. It lies, of course, not as low 
as 4500 feet, that is quite obvious, not even at 5000 feet. On the other 
hand, it does not stand at 9000 feet, since thence downwards Northern species 
and genera show a rapid increase. The mean between the two limits is 7000 
feet, and this seems to agree fairly with actual conditions. It marks, more- 
over, the lower level of the cloud-belt, at least on the sides away from the 
plateau, with the average height of which it also coincides, and this is an 
important point. Lastly, broadly speaking, this level marks the change from 
the Oak to the Pine belt. 
All such calculations must be vague ; they are to a bewildering extent 
influenced by local conditions. On the northern side of Citlaltepetl, 
Popocatepetl, and Nevado de Colima the floral aspect is much more 
Northern than it is at the same level on the south slopes which are in direct 
communication with the tropics. The plateau side of Citlaltepetl strikes one 
* It is important to note that the proportion differs but slightly whether we consider all 
the highest Mexican mountains, or restrict ourselves to Citlaltepetl. 
