DISTRIBUTION OF PLANTS IN SOUTHERN MEXICO. 431 
being the plateau; the other, for instance, on the Sierra de Ajusco, which 
‘forms the southern border of the Valley of Mexico, finds its southern level 
in the plain of Cuernavaca, about 5000 feet, thence sloping further into the 
tropical Balsas basin. Similar conditions apply to Popocatepetl and to the 
Nevado de Colima. 
Consequently all these mountains possess a very mixed flora. First, 
northerners (or shall we call them long migrants?) ; secondly, endemie 
ascendants from a temperate or cool immediate base ; thirdly, southerners, 
or ascendants from the tropics; fourthly, the “ Andines.” 16 is difficult, 
perhaps impossible, to keep the main categories asunder, since the distinction 
between hot, temperate, cool and cold is arbitrary. Specialists in American 
plants will be able to do it better than the present writer, who has but little, 
secondhand, botanical knowledge. | 
Yet the attempt has been made to group the flora of Mexico into several 
main categories. | 
1. Northern plants”, with their presumable home in the temperate 
northern hemisphere. 
2. Endemies of Mexico, which again naturally fall into a cool to temperate 
and a tropical or * Southern " category. 
3. Southerners are of course also those which for the sake of shortness we 
refer to as Andines, leaving the question of their original home in 
abeyance ; for instance, Acwna (Rosacee), Tauschia ( Umbelliferze), 
Pernettyia (Ericacze), Chionolena (Composite). Those which occur 
on Mexiean high mountains do not, as a rule, extend further north, 
whilst they are found also in Central America and in South America, 
especially on the Andes. | 
I had no idea what the outcome would be of the necessary, almost endless, 
calculations. and plottings, and yet they assumed a shape apparently so 
reasonable that it can scarcely be accidental. Thanks mainly to Heilprin the 
flora from 10,000 feet upwards is fairly well known, and his lists proved of 
the greatest value. But thence downwards all available data are vague. 
Papers like that by Liebmann on Citlaltepetl (* Botanische Zeitung,’ 1844), 
and Kotschy’s * Survey of Mexican Plants," Sitzungsberiehte Wien. Akad. 
Wiss., Feb. 1852, viii. (partly suppressed 1), are very scarce; and unfortunately 
altitudinal distribution does not seem to have appealed much to the authors 
of the ‘Biologia Centrali-Americana,’ at least I have failed to find therein 
such general conclusions as I had, perhaps unreasonably, been looking for. 
The fourth of the volumes on Botany, more especially pp. 145-151 and 
282-315, by W. B. Hemsley, forms nevertheless the main supply of data for 
* For instance Pinus, Alies, Juniperus, Alnus, Quercus, Sambucus, Platanus, Ulmus, Cornus, 
Viscum, Veratrum, Pinguicula, Spirea, Thalictru n, Aquilegia, Delphinium, Pyrola. 
