DISTRIBUTIONAL PATTERNS OF VERTEBRATES 461 



had occurred. Their existence does not controvert the evidence 

 presented above of extensive east-west fragmentation of ranges 

 among warmth-adapted coastal plain vertebrates in the Pleistocene. 



Instances of north-south fragmentation of ranges involving 

 coastal plain species, exclusive of the urodeles, are contrastingly 

 scarce. The southern rough green snake {Opheodrys aestivus) and 

 northern smooth green snake (0. vernalis) constitute one example. 

 However, relicts of the northern species in central and southeastern 

 Texas (Davis, 1949) indicate a very different past distribution and 

 suggest the possibility that the present distribution is derived from 

 an east-west disjunction comparable to that postulated for the 

 Peromysciis leucopus group. In the common water snake {Matrix 

 sipedon) a freshwater ecotype shows secondary interbreeding with a 

 salt-marsh ecotype around the Gulf Coast. Pettus (1956j hypothe- 

 sized continuous distribution of the salt-marsh type in the Pleisto- 

 cene and a southern refuge or refuges for the freshwater type. Hyla 

 andersoni of the New Jersey pine barrens is an apparent relict of a 

 formerly more northern extension of H. cifierea on the xA.tlantic 

 coastal plain (see Blair, in press). 



The Pseiidacris nigrita group, with the most complex distribution 

 pattern of any group of North American anurans, does show evi- 

 dence of north-south speciation in the eastern United States. A 

 small group of relicts of grasslands-adapted mammals in Mexico 

 and on the Texas coast (Blair, 1954) is consistent with the thesis of 

 southward displacement of cold climates and of the Arcto-Tertiary 

 forest. In the east there are a few such relicts of groups that occur 

 today in the northern United States. The spotted turtle {Clemmys 

 guttata) occurs as a relict in northern Florida. A northern species of 

 frog, Rana sylvatica, has left relict populations in northwestern 

 Arkansas and in the Flint Hills of Kansas (Smith, 1950). 



The distributional patterns of warmth-adapted vertebrates on the 

 coastal plain as discussed above are overwhelmingly indicative of 

 east-west fragmentation of ranges as the initial agent of geographic 

 speciation in this fauna. The variations in distributional patterns 

 shown by allopatric populations undoubtedly reflect differences in 

 ability to reoccupy territory after the initial disjunction, differences 

 in the time that has elapsed since the initial separation, and other 

 more subtle factors. It might be argued that the east-west speciation 

 simply reflects adaptation to forest and grassland environments. 



