1892.] -*•*' * [Heilprin. 



feet below the line of the pines, and yet more below that of the last 

 junipers.* 



We observed the last specimens of this genus (Juiiiperus tetragona) 

 covering the bare rocks of Orizaba at some little distance beyond the 

 actual tree line. It is not always easy to determine just what are the 

 causes which operate towards establishing and regulating the succession 

 of special vegetal zones on mountain slopes any more than it is possible, 

 in our present knowledge, to explain the anomalies of succession on the 

 horizontal plain stretching towards the Pole. The law of parallelism in 

 horizontal and vertical succession, which Humboldt first formulated, and 

 which was founded on the perception of climatic influences almost alone, 

 while it touches the broader aspects of the problem, does not essentially 

 explain the detail ; nor can it be said that the modification of this law, 

 defined by an excess or decrease of solar illumination, the horizon- 

 tality or verticalitj^ of the solar rays, etc. (as elaborated by Wahlen- 

 berg, Grisebach and others), any more explains the special contra- 

 dictory features of this distribution. Preoccupation or first possession of 

 a region by a special group of plants has doubtless much to do with the 

 problem; it is an important factor towards determining supremacy, and 

 must, therefore, largely regulate the outcome from a competitive struggle 

 for existence. 



The oaks of the Mexican volcahoes occupy the lower pine belt, ranging 

 to about 10,000 feet. We obtained three species on Orizaba — Quercus 

 reticulata, Q. Orizabce, and a third form which we have not yet been able 

 to identify. Above 8000 feet they are comparatively rare and no longer 

 form forests, such as are to be met with in the lower region of 4000-6000 

 feet. In the more or less open dustcountry below the pines — i. e., below 

 where the pines appear on the western slope of Orizaba, about 9000 feet — 

 they are still fairly abundant, forming groves and copses, but once enter- 

 ing the pines they appear only as stragglers. The same condition prevails 

 on Popocatepetl and Ixtaccihuatl. Associated with the oaks are one or 

 more species of alder, Alnus JoruUensis and A. castanifolia ; we found 

 the former a tree of some 15-20 feet height, extending up to 13,000 feet or 

 more. The second form, which is now generally looked upon only as a 

 variety of A. JoruUensis, and which we found on Popocatepetl at an 

 elevation of about 11,000 feet, is a member of the flora of the Peruvian 

 Andes — one of the very few plants which are common to the two regions. 



As regards the non-arboreal vegetation of the Mexican summits, the 

 list of species given at the beginning of this paper sufficiently illustrates 



a very much lower altitude than where we actually observed it ; indeed, it appears that 

 Humboldt and Bouplaud met v/ith it not far from the forest of Chilpanzingo, south of 

 the Mexican plateau, at an elevation of barely more than 4000 feet. 



* It is true that Schiede mentions the tree as rising to the timber line on Orizaba 

 (Parlatore, in De Candolle's "Prodromus"), but I believe the statement to be erroneous. 

 Galeotli's observations, which accord almost exactly with my own, place its limit at 

 some 12,200 feet. Hemsley, on the other baud, reduces the elevation to 10,500 feet, a 

 figure which is 1500 feet too low. 



