230 DR. II. GADOW ON MEXICAN [June 6, 



in other words either ISTortherners, as natives of Old Sonoraland, 

 dwellers of mountains and high plateaus, or Southei-ners, which 

 were and are mostly tropical species. The temperate zone is in 

 the present case rather no-man's-land than the happy medium 

 favourable to the majority. 



The configuration of the whole country lends every support to 

 this result ; broadly sjDeaking, a high, mountainous plateau, 

 abruptly falling off into tropical lowlands. 



The species which have such a considerable range of altitude 

 that they occur in the cool, temperate and hot zones, are of further 

 interest. The same kind which is bound to hibernate on the high 

 mountains is active throughout the year in the moist and hot 

 lands, and possibly there are some which also sestivate during 

 prolonged drought. The species can be grouped as follows : — 



I. Undoubted Northerners, or oi-iginally at home in a cool 

 climate, as indicated by their main distribution, or by that of allied 

 species of the same genus. These have descended into the hot 

 lands. 



Scaphio2ncs dicgesi. Tropidonotios melcmiogaster. 



liana halecina. ,, ordinatus. ■> 



,, montezumm. ,, validus, 



Sceloporus scalaris. \ Coluber triaspis. 



,, microlepidotios. I Grotalus horridus. 



Uta hicarinata. Oinosteroiitm pennsylvanicum. 



Gerrhonotus cmrideus. 



II. Essentially hot-country species which have ascended ; and 

 it is remarkable that most of these are not found on the plateau 

 proper, although they ascend the sui-rounding mountains, up to an 

 altitude equal to or surpassing that of the plateau. This fact 

 seems to indicate that the respective species are still continuing 

 their upward spreading, or that they have conquered these 

 mountains comparatively recently. This fits well with the 

 suggestion expressed on p. 244 that the Southern or tropical 

 fauna of Mexico represents for the greater part the most recent 

 immigrants. The Sierra Madre del Sur affords a good illustration. 

 It is separated from the plateau by the depression of the basin of 

 the Rio Balsas. Tropical species coming from the south can surge 

 up to the Sierra, and they have ascended its higher mountains 

 (e. g. those of Omilteme, Amula, Cerro de S. Felipe near Oaxaca), 

 and the backbone itself is of no mean height ; but then comes the 

 descent into the hot basin, then again the ascent of the plateau. 

 A tropical species, which has succeeded in acclimatising itself to 

 life on the Sierra, will have to " undo " this hardening, become 

 tropical again, and lastly once more ascend and accommodate itself 

 to a cool climate. Of course all this can be done, but it takes 

 time. The same applies to the fauna of the rather isolated Yolcan 

 and Nevado de Colima. The ranges of mountains which boi'der 



