OCR Text |
Show 230 DR. H. GADOW ON MEXICAN [June 6, in other words either Northerners, as natives of Old Sonoraland, dwellers of mountains and high plateaus, or Southerners, 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 speaking, 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 aestivate during prolonged drought. The species can be grouped as follows :- I. Undoubted Northerners, or originally 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. Scaphiopus dugesi. liana halecina. ,, montezmiue. Sceloporus scalaris. ,, rnicrolepidotus. Uta bicarinata. Gerrhonotus cceruleus. II. Essentially liot-country species which have ascended; and it is remarkable that most of these are not found on the plateau proper, although they ascend the surrounding 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 011 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 Volcan and Nevado de Colima. The ranges of mountains which border Tropidonotus melanogaster. ,, ordinatus. ,, validus. Coluber triaspis. Crotalus horridus. Cinosternum pennsylvanicum. |