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Show 280 DR. H. GADOW ON EVOLUTION [Mar. 20, Oaxaca at 5400, whilst they are swarming at 5200 feet level. Near Chilpancingo they do not go beyond 4500 feet. I suspect that they are stopped by those changes which on so many mountains coincide with the usual lowest level of the clouds. Although frequently found in ravines and on the spurs of mountain-ranges, they avoid the mountains themselves, and above all they are averse to crossing a system of cut-up ridges even of moderate height. Some species, C. deppei, immutabilis, and guttatus, are natives of the Tierra caliente, which they do not leave, so that any continuous rise beyond 3000 feet is to them an absolute barrier. When, by the way, Cope mentions C. deppei from Guadalajara, this certainly cannot refer to the plain of 5000 feet upon which this town lies, but to the deep depression of the neighbouring Rio de Santiago, 2000 and more feet lower! Southern, tropical species do not ascend far; but northerners, or let us say highlanders, extend their range frequently into the lower, tropical climes, and thereby they undergo considerable changes. It was stated that these lizards are very plastic. There are some species which average only 50 mm., while others reach a nose to vent length of 140 m m. I have selected only a few characters, chiefly the supraoculars, the composition of the collar, the rows of scales of the humerus, Text-fig. 61. Lepidosis of the front of the left forearm of Cnemidophorus mexicanus. A, B. Two specimens from Cuevnavaca. C. From Balsas No. 2. D. Cuer-navaca No. 8. E-I. Diagrammatic; I, an arrangement occurring in Cuernavaca No. 6 and Rincon No. 2. |