little changed from the basic phyllomedusine stock. On the basis of modern distribution and areas of diversification alone (no fossils are known), it is evident that _Phyllomedusa_ underwent its adaptive radiation in South America, _Agalychnis_ evolved in Central America, and _Pachymedusa_ ended up in western Mexico. If we follow the Matthewsian concepts of the American herpetofauna outlined by Dunn (1931) and modified by Schmidt (1943) and Stuart (1950), _Pachymedusa_ represents a "hanging-relict" of a group that moved southward. According to Savage's (1966) interpretation of the origins and history of the American herpetofauna, _Agalychnis_ and _Pachymedusa_ are members of the Mesoamerican fauna, and _Phyllomedusa_ is part of the Neotropical fauna. Perhaps the phyllomedusines arose in South America; from there a primitive stock spread northward and survived as _Pachymedusa_ in Mexico, whereas the stock in Central America and South America evolved into _Agalychnis_ and _Phyllomedusa_, respectively. Evidently the primitive phyllomedusines evolved the habit of arboreal egg deposition and a walking gait; the latter is best developed in the small, highly specialized species of _Phyllomedusa_ (Lutz, 1966). Probably the other divergent arboreal adaptations resulted from environmental stresses and competition. The generalized _Pachymedusa_ inhabits relatively dry areas characterized by low forest. Throughout its range it coexists with no more than five other arboreal hylids. The species of _Agalychnis_ live in rain forests and humid montane forests. In any given area one species of _Agalychnis_ occurs sympatrically with no more than a dozen other arboreal hylids. With few exceptions the species of _Agalychnis_ are more arboreal in their habits than are other hylids. The species of _Phyllomedusa_ live in the same kinds of habitats as do those of _Agalychnis_, but throughout the ranges of most of the species of _Phyllomedusa_ the diversity of arboreal hylids is much greater than in Central America. In the upper Amazon Basin as many as 35 hylids occur sympatrically. Many groups of _Hyla_ in this area (for example, the _Hyla boans_ and _Hyla marmorata_ groups) are equally as arboreal in their habits as are the species of _Agalychnis_ in Central America. Conceivably, competition within this array of tree frogs resulted in selection for modification of the extremities, thereby bringing about a different mode of climbing in _Phyllomedusa_. The walking gait already present in phyllomedusines provided a source for further modification, which resulted in the development of opposable digits and the associated lemuroid manner of climbing.