DISTRIBUTIONAL PATTERNS OF VERTEBRATES 453 



southern Texas (Blair, 1952). Peromyscus gossypinus of the coastal 

 l^lain and P. leucopiis overlap in a generally narrow zone along the 

 border of the coastal plain and along the deciduous forest border in 

 eastern Texas and Oklahoma (Osgood. 1909; McCarley, 1954). The 

 present distribution of this species pair can be explained as a result 

 of the westward and northward spread of the coastal-plain-adapted 

 gossypinus from a refuge in Florida and the northward and eastward 

 spread of leucopus from a Mexican refuge, where it would have been 

 adapted to less mesic upland forests. The remarkable call "races" 

 of the gray treefrog {Hyla versicolor) described by Blair (1958) 

 imply the splitting of this species into three populations along north- 

 south axes, with subsequent spread to bring about the present 

 relationships. 



Other examples of forest-restricted isolates, discussed by Martin 

 and Harrell (1957), include: (1) the red-bellied snake {Storeria 

 occipitomaculata) , which is widely distributed in the eastern forest 

 with relicts in the central grasslands and in Mexico, (2) the yellow- 

 lipped snakes, which comprise Rhadinea flavilata to the east on the 

 coastal plain and a closely related species, R. laureata, disjunct in 

 Mexico, and (3) the barred owl (Strix varia), which has a Mexican- 

 Central American disjunct. 



The largest group providing evidence of past or present disjunc- 

 tion into eastern and western populations is the one in which the 

 eastern population inhabits forests and the western population is 

 adapted to, or tolerant of, grasslands. The members of some of 

 these species pairs meet or approach at or near the forest boundary 

 in eastern Texas and Oklahoma. Two hylid frogs, Pseitdacris nigrila 

 and P. clarki, which are interfertile in the laboratory, overlap nar- 

 rowly along this boundary, where they show a complex set of isola- 

 tion mechanisms (Lindsay, 1958). Two narrow-mouth frogs, 

 Microhyla carolinensis and M. olivacea, overlap narrowly in this 

 same area, where they hybridize to a limited extent and where 

 their isolation mechanisms are apparently being reinforced (Blair, 

 1955). Two populations of toads referred to by some (as by A. P. 

 Blair, 1941) as an eastern species, Bufofowleri, and a western species, 

 B. woodhousei, meet and freely interbreed, secondarily, in this same 

 area (Meacham, 1958). The eastern pine snake {Pituophis melano- 

 leucus) and the western bullsnake (P. catenifer) either approach 

 range or interbreed in southeastern Texas (Smith and Kennedy, 



