1905.] AMPHIBIANS AXD REPTILES. 241 



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 Avhich rise isolated, or 

 which fringe the plateau. 



Of coui'se 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 corre- 

 spondents 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 ! " Yera 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-presei'ved 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 eveiy siDecimen 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 ad- 

 joining 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 tyj>ical tropical Tierra Caliente. 

 Such cases are clearly illustrative of the evohition 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 ordi- 

 7iatits, Crotalus terrificus. It is rather doubtful, because these 

 creatiu^es are so indifl:erent to climatic conditions. With more 

 right we consider those as ancient which have to be very par- 

 ticular about their terrain, and which are now scattered, without 

 the least chance of communication — as, for instance, Thoriv.s, 

 Chirotes, Heloderma, and other slow, or digging, creatures. 



3. Northern and riouthern Immigration. 



In the following table the Mexican Amphibia and Eeptiles are 

 divided into a Northern or Nearctic and a Southern or Neotropical 

 mass, according to their jjresumable ancestral home or centre of 



Proc. Zool. Soc— 1905. Yol. II. No. XYI. 16 



