We visualize the evolutionary history of the group to have followed a course that began with the invasion of Central America by a _microcephala_ ancestral stock that differentiated into two populations in lower Central America--a _microcephala_-like frog on the Pacific lowlands and a _phlebodes_-like frog on the Caribbean lowlands. Differentiation could have been brought about by isolation by montaine or marine barriers. The population on the Pacific lowlands either was preadapted for subhumid conditions or became so adapted and dispersed northward onto the Pacific lowlands of northern Central America. Simultaneously the frogs on the Caribbean lowlands, which were adapted to humid environments, dispersed northward in the humid forested regions to southern México and crossed the Isthmus of Tehuantepec onto the Pacific slopes of Oaxaca and Guerrero northward to Jalisco. Subsequent development of arid conditions, possibly in the Pliocene, Pleistocene, or even as late as the Thermal Maximum in post-Wisconsin time, resulted in a restriction of the ranges in northern Central America, thereby isolating part of the _phlebodes_-stock on the Pacific slopes of México, where it adapted to drier conditions and evolved into _sartori_. The rest of the _phlebodes_-stock was restricted to the humid forests on the Caribbean lowlands of lower Central America. The increased aridity on the Pacific lowlands eliminated the _microcephala_-stock from southern Honduras and northwestern Nicaragua and in so doing left an isolated population on the lowlands of Chiapasand Guatemala, which differentiated into _robertmertensi_. The original stock on the Pacific lowlands of Panamá and southeastern Costa Rica became _microcephala_. If the _microcephala_-stock was, as we believe, better adapted for existence under subhumid conditions than was the _phlebodes_-stock, the development of subhumid conditions in much of the lowland region of northern Central America and southern México would have permitted the expansion of the range of _microcephala_ into the area now inhabited by _H. m. underwoodi_, while _phlebodes_ was being eliminated from this area by climatic conditions that were unsuited to its survival there. Perhaps the similarity in coloration of _H. m. underwoodi_ and _phlebodes_ is the result of convergence or possibly hybridization occurred at the time the former was expanding its range and the latter's range was being restricted. If hybridization did occur, the differences in mating call subsequently were enhanced, thereby providing a valid isolating mechanism in sympatric populations. _Hyla microcephala_ and _phlebodes_ range into northern South America. Probably both species entered South America in relatively recent times after they had differentiated from one another in Central America. LITERATURE CITED