to explain the evolution of the species of Phyllomedusa (sensu lato) by assuming that they evolved from an advanced Hyla-like ancestor. Therefore, she placed those species having large, fully webbed hands and feet near the base of her phylogenetic scheme and hypothesized that evolutionary sequences involved stages of reduction and eventual loss of webbing, followed by the development of grasping toes. Such an evolutionary history is highly unlikely. The Agalychnis phyletic line has one kind of specialization for an arboreal existence. It is contrary to evolutionary theory that a specialized group would evolve into a generalized form and then evolve new kinds of specializations to meet the needs imposed by the same environmental conditions affecting the earlier specialized group. A more reasonable hypothesis is that the evolution of opposable digits took place in a phyletic line that had as its ancestral stock a frog with generalized hands and feet. If this assumption is correct, Phyllomedusa and Agalychnis represent different phyletic lines; each exhibits divergent modes of adaptation for arboreal habits, whereas Pachymedusa probably remains relatively 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 México. 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 México, 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.

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