_Ptychohyla schmidtorum_ is thought to resemble more closely the parental stock of the genus than does any other species of _Ptychohyla_ now extant. This parental stock is discussed above in the account of the generic relationships. _Ptychohyla schmidtorum_ has a red eye, white lateral stripe, frontoparietal fontanelle, funnel-shaped mouth in tadpoles, and lacks nuptial spines; in all of these characters it resembles members of the _Hyla uranochroa_ group. Probably during times of glaciation during the Pleistocene, when climates in Mexico and Central America were depressed, the _Ptychohyla_ stock was more widespread than it is now. Subsequent elevation of climatic zones during interglacial periods would have isolated populations as they are today in regions of cloud forests. Thus, through geographic isolation populations could have differentiated and evolved into the present species. Climatic fluctuation in the Pleistocene must have been of sufficient magnitude to permit the spread of cool, moist forests containing _Ptychohyla_ across the Isthmus of Tehuantepec into the mountains of Oaxaca. Because of its small nuptial spines, small triangular vomers, coloration, and absence of a rostral keel, _Ptychohyla euthysanota_, more than any of the other species in the _P. euthysanota_ group, resembles _P. schmidtorum_. At the present time _P. euthysanota_ and _P. schmidtorum_ are sympatric. As I have mentioned previously, ecological segregation and interspecific competition probably is highly developed in the tadpoles of _Ptychohyla_. If this ecological segregation resulted from intraspecific competition in a stock of _Ptychohyla_, possibly _P. euthysanota_ and _P. schmidtorum_ differentiated sympatrically in this way. Specific identity is maintained, at least in part, by different breeding calls in males. _Ptychohyla spinipollex_ and _P. leonhardschultzei_ seem to be more closely related to one another than either is to _P. euthysanota_. Probably a stock of _P. euthysanota_ was isolated on the Atlantic slopes of northern Central America from _P. euthysanota_ on the southern slopes. The frogs on the Atlantic slopes differentiated and spread into the mountains of Oaxaca, where through isolation by the barrier of the Isthmus of Tehuantepec they developed into _P. leonhardschultzei_, while the stock on the northern slopes of Central America evolved into _P. spinipollex_. Subsequent to the differentiation of _P. leonhardschultzei_ and _P. spinipollex_ from _P. euthysanota_ and during a time of cooler more equable climate than exists now, _P. euthysanota_ and _P. schmidtorum_ invaded the Central Highlands of Chiapas. Subsequent climatic changes isolated populations of each in the Central Highlands, where _P. euthysanota macrotympanum_ and _P. schmidtorum chamulae_ evolved. _Ptychohyla ignicolor_ probably represents stock of _P. schmidtorum_ that crossed the Isthmus of Tehuantepec and became isolated in Oaxaca on the western side of the isthmus.