ossils 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 de