464 W. F. BLAIR 



hypothesized by previous workers, is considered to be the southward 

 shifting of cHmatic belts during the Glacial stages of the Pleistocene 

 and the resultant enforced withdrawal of the warmth-adapted biota 

 into separate refuges in Florida and Mexico. 



The evidence from fossil pollens and from plant macrofossils 

 indicates that at times in the Pleistocene northern species of trees 

 such as spruce, hemlock, fir, larch, and arbor vitae extended onto 

 the Gulf and southern Atlantic coastal plains. Boreal mammals and 

 a few fishes of northern affinity are known from Pleistocene deposits 

 far south of their present distributions and as far south as southern 

 Nuevo Leon in Mexico. Both the plant and animal fossils are indica- 

 tive, then, of major ecological changes in the southern United 

 States in the Pleistocene. 



Evidence from various sources indicates that the southern grass- 

 lands have not acted as a continuous barrier to the exchange of 

 forest biotas between the eastern United States and the Mexican 

 highlands since their origins in the late Miocene and early Pliocene. 

 The Pleistocene mammalian fauna of Florida includes various groups 

 of South American origin, some of them forest types, that must have 

 crossed the area of the present grasslands barrier, as their enti-y into 

 North America would necessarily have followed the emergence of the 

 Central American land bridge in the late Pliocene. Various Pleisto- 

 cene fossils from the area of the present grasslands are indicative of 

 greater moisture and of forests at times in this area in the Pleisto- 

 cene. These include such indicators of extreme departure from 

 present ecological conditions in the area as Neofiber in the Texas 

 panhandle and Tapirus in trans-Pecos Texas. An impressive number 

 of present day relicts of forest-adapted species in the grasslands 

 also argues against past continuity of grassland in the area. 



Present distributional patterns of coastal plain vertebrates indi- 

 cate many east-west and very few north-south disjunctions. Some 

 of these involve eastern, forest-adapted and western, grasslands- 

 adapted populations, with the forest-grasslands boundary important 

 in their present distributional relationships. Other patterns involve 

 the Mississippi Embayment as a distributional boundary, and in 

 this group the hiatus between eastern and western populations may 

 include either forest or grassland, or both. Still other patterns in- 

 volve eastern and western forest-adapted types, some of which have 

 their western populations as relicts in the Mexican highlands. 



