T MHE course of development of the herpetofauna of the present day-that is to say, the herpetogeny-of Mexico and Guatemala is rather obscurely indicated. It is a remarkable fact that from the entire area mentioned no fossil amphibians and very few fossil reptiles are known, other than Pleistocene remains generally conspecific with living forms. Accordingly all discussion of herpetogeny in the prescribed area must be based upon less direct indications: (1) phylogenies of groups sufficiently well known and studied to be reasonably useful; (2) fossil remains in adjacent areas; (3) paleophysiography, especially as indicated by Schuchert (1935); (4) herpetogenies of limited areas which have been well studied, as for example Sonora (Bogert and Oliver, 1945); (5) modern physiography and herpesian1 life areas; and (6) implications of studies on other animals and on plants. Since the classes of reptiles and amphibians are much more ancient than those of mammals or birds, the impression is easily obtained that modern groups are equally ancient and, thus, have had a history of migration much different from that of more recent classes. On the contrary, modern groups of both reptiles and amphibians apparently flowered in the Cenozoic, as did mammals and birds, despite the fact that scattered remains are known from the Mesozoic. Thus it is not unreasonable if, as appears the case, the gross features of herpetogeny are closely correlated with those of mammalogeny and aviogeny. Known herpesian phylogenies establish securely the fact that the Mexican and Guatemalan plateaux are populated preponderantly by species. In groups thus represented it is remarkable that the living primitive forms are rarely, if ever, generalized in all respects. They are specialized in certain features, often rather highly. Radiating away from the Mexican and Guatemalan centers are forms representing various lines of specializations and stabilizations. Naturally phylogenetic trees thus almost invariably have their bases in the plateau area, and the branches directed away from that area. Phylogenies of intrageneric groups indicate the great importance at the species and/or subspecies level of the Miocene-Pliocene portal at the Isthmus of Tehuan-