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Show 1 9 0 5 .] AMPHIBIANS AND REPTILES. 2 4 1 which, like most Gerrhonotus, are now restricted to the higher mountains. Lastly, a considerable number of Southern species ascend from the hot lowlands high up onto mountains which rise isolated, or which fringe the plateau. Of course it is difficult, perhaps premature, to generalise in this respect, and sharp lines cannot be drawn between these categories. Not the least cause is the vagueness or doubtful nature of many of the reported localities. For instance, Cope had various correspondents in Mexico, and some of the alleged localities are quite impossible. Peters had a good correspondent resident in Puebla City, but the specimens which now figure as " Puebla " came from anywhere in that State, which has the most perplexing, intricate boundaries, and contains altitudes from 3500 to less than 100 metres ! " Vera Cruz " is another snare to the unwary. Others have bought specimens, even collections, in Mexico City. I myself found in a shop at Orizaba several large glass vessels full of well-preserved snakes for sale, but I left them alone since nobody knew where they came from. Sumichrast lived for many years in Tehuantepec and he travelled widely, all over the Isthmus and beyond. The town is situated on a plain, about 100 feet above the not distant sea ; within a few hours' ride are mountains, covered with pines, well above the Tierra Caliente, as typical of which every specimen labelled " Tehuantepec " is put down. I shall not, at least in this paper, go into the detail of the generic overlapping, a very important question. Suffice it to say, that in many cases the species of a genus are so distributed that some are decidedly northern, living on the plateau, typical inhabitants of the Tierra Fria; another species lives in the adjoining Tierra Templada, more often on the western than on the eastern slopes and descending more or less far into the lowlands ; while a third kind is confined to the typical tropical Tierra Caliente. Such cases are clearly illustrative of the evolution of species due to the prevailing physical conditions, especially when none of these species has a wide geographical range. Are we justified in calling a certain species ancient because it has a wide continuous range ? For instance, Tropidonotus orcli-natus, Crotalus terrificus. It is rather doubtful, because these creatures are so indifferent to climatic conditions. With more light we consider those as ancient which have to be very particular about their terrain, and which are now scattered, without the least chance of communication-as, for instance, Thorius, Chirotes, Eeloderma, and other slow, or digging, creatures. 3. Northern and Southern Immigration. In the following table the Mexican Amphibia and Reptiles are divided into a Northern or Nearctic and a Southern or Neotropical mass according to their presumable ancestral home or centre of P roc. Z ool. Soc.-1905, V o l . II. No. XVI. 16 |
