454 \V. F. BLAIR 



1951). The pigmy rattlesnakes comprise an eastern, forest species, 

 Sistruriis miliarius, a grasslands species, 5. catenates, and a disjunct 

 species, 5. raviis, in eastern Mexico (Smith and Taylor, 1945). The 

 eastern box turtle {Terrapene Carolina) and western box turtle (7". 

 ornata) overlap narrowly along the forest border. Two harvest mice, 

 the eastern Reithrodontomys humulis and the western R. montanus, 

 approach one another but apparently do not meet along the forest 

 border. The ranges of two packrats, the eastern Neotoma floridana 

 and the western N. micropus, Interdigitate in the broad forest-grass- 

 land ecotone. Two skinks, the eastern Eumeccs anthracmus and the 

 western E. septentionalis, overlap in eastern Oklahoma and Kansas. 

 An eastern newt, Diemictylus viridescens, ranges west to the edge of 

 the forest and is separated by a grassland gap from the related 

 species D. meridionalis of southern Texas and northeastern Mexico. 

 The group of species pairs discussed above interpret as having 

 reached their present distributional relationships through post- 

 Wisconsin spread to the margin of their respective environments, 

 where they have attained contact or near contact with their sibling 

 species. Another sizable group of species pairs shows a quite different 

 pattern in that the Mississippi Embayment, deep within the 

 Austroriparian forest, is involved in their separation. The species 

 pairs in this group have mostly remained widely disjunct. Two pairs 

 appear to be limited by soil types. An eastern pocket gopher, Geomys 

 pinetis, occurs on sandy soil of the coastal plain to the east of the 

 embayment, and a western species, G. bursarius, occurs on sands to 

 the west of it. The alluvial soils of the embayment appear to be the 

 ecological factor separating the present ranges of these allopatric 

 species. Two species of spadefoots (Scaphiopus) have essentially 

 similar distributions. The eastern 5. holbrooki and western S. hurteri 

 have been shown to be interfertile in the laboratory (Wasserman, 

 1956). A broader hiatus, of forested land, separates two chorus 

 frogs, Pseudacris streckeri and P. ornata (Fig. 8). P. streckeri occurs 

 west of the forest border, which imposes a limit to its eastward 

 distribution. P. ornata ranges west on the coastal plain to the Mis- 

 sissippi Embayment. There is little differentiation between the two 

 in mating call or morphology. Hybrids between them have been 

 produced, but one attempted backcross of a male hybrid to streckeri 

 failed (Mecham, 1957). Two species of Rana have a rather similar 

 distribution: The eastern gopher frog {R. capito) ranges west on the 



