We view the Miocene _Smilisca_ as the prototype described in the preceding section, and suppose that it lived in the mesic tropical environment of the eastern part of the Central American Peninsula (in what is now Costa Rica and western Panama). Two stocks differentiated, probably in middle Miocene times; one of these, the ancestral stock of the _baudini_ group, was widespread on the Caribbean lowlands from the Nicaraguan Depression to the Bolivar Trough, and the other, the ancestral stock of the _sordida_ group, was restricted to the Pacific lowlands of the same region. In late Miocene time the ancestral stock of the _baudini_ group dispersed northwestward around the deep embayment in the Nicaraguan depression into upper Central America (in what is now Honduras and Guatemala) and thence into southern Mexico. Apparently differentiation took place on each side of the Nicaraguan Depression; the frogs to the south of the depression evolved into _S. phaeota_, whereas those to the north of the depression represented the stock from which _S. baudini_ and _cyanosticta_ arose. Prior to the uplift of the mountains in the late Miocene and the Pliocene the _baudini-cyanosticta_ stock probably was widespread in northwestern Central America. The elevation of the mountains resulted in notable climatic changes, principally the development of sub-humid environments on the Pacific lowlands. The frogs living on the Pacific lowlands became adapted to sub-humid conditions and developed into _S. baudini_. The stock on the Caribbean lowlands remained in mesic environments and evolved into _S. cyanosticta_. Possibly in the middle Miocene before the Talamanca Range in Costa Rica and western Panama was greatly uplifted, the ancestral stock of the _sordida_ group invaded the Caribbean lowlands of what is now Costa Rica. The subsequent elevation of the Talamanca Range in the Pliocene effectively isolated the ancestral stock of _S. sila_ on the Pacific lowlands from the _puma-sordida_ stock on the Caribbean lowlands. The former was subjected to the sub-humid conditions which developed on the Pacific lowlands when the Talamanca Range was uplifted. It adapted to the sub-humid environment by living along streams and evolving stream-adapted tadpoles. On the Caribbean side of the Talamanca Range the _puma-sordida_ stock inhabited mesic environments. The stock that evolved into _S. puma_ remained in the lowlands as a pond-breeding frog, whereas those frogs living on the slopes of the newly elevated mountains became adapted for their montane existence by developing stream-adapted tadpoles and thus differentiated into _S. sordida_.