Living in the humid forests of the foothills are salamanders of the genus _Lineatriton_, frogs of the _spatulatus_ group of _Eleutherodactylus_, _Anotheca coronata_, _Hyla miotympanum_, and _Phyllomedusa moreleti_. All of these occur in the foothills of the Sierra Madre Oriental in eastern Mexico and in Los Tuxtlas. _Lineatriton_, _Hyla miotympanum_, and the _spatulatus_ group of _Eleutherodactylus_ do not occur in the foothills of the Guatemalan-Chiapan Highlands; those amphibians reach the end of their ranges at the isthmus. _Phyllomedusa moreleti_ and _Anotheca coronata_ are found in the northern foothills of the Guatemalan-Chiapan Highlands, and _Phyllomedusa moreleti_ is found in the foothills on the Pacific slopes of the Chiapan Highlands. Although the above analysis is not so detailed as that of the lowland inhabitants, it does show that all of the genera and species of amphibians known to inhabit the foothills and highlands adjacent to the isthmus, only two species of amphibians cross the isthmus from one highland mass to the other. Thus, it is evident that the Isthmus of Tehuantepec presents a great barrier to dispersal of these groups of amphibians. ESTABLISHMENT OF PRESENT PATTERNS OF DISTRIBUTION From the foregoing analysis of geographical and ecological distribution in the Isthmus of Tehuantepec we may strive for an interpretation of the events that led to the establishment of patterns of distribution displayed not only by the amphibians, but other terrestrial vertebrates as well. The thesis that I am proposing below is based on the premise that in southern Mexico and northern Central America climatic fluctuation during the Pleistocene was of sufficient magnitude to cause vegetational shifts, both vertically and latitudinally, resulting in the establishment of alternating continuous and discontinuous lowland and highland environments, although this climatic fluctuation was not so great as to eliminate tropical lowland environments from the region. I feel that the present patterns of distribution of the amphibians in the Isthmus of Tehuantepec may be explained on this premise. Many authors dealing with the herpetofauna of Middle America have followed Schuchert's (1935) suggestion of a seaway in the isthmus during the Cenozoic. Thus, Burt (1931), Duellman (1956, 1958a), Gloyd (1940), Oliver (1948), Smith and Laufe (1946), and Stuart (1941) employed the presence of a seaway to explain distribution and speciation in various genera. Durham, Arellano, and Peck (1952), Olson and McGrew (1941), and Stirton (1954) have provided geological evidence that there probably was no Cenozoic seaway in the Isthmus of Tehuantepec. Even if there were a seaway in the Pliocene or Miocene (the dating of this possible seaway is open to question), its presence is not necessary to explain the present patterns of distribution in the isthmus.