FIG. 21-8 Postulated post-glacial 

 dispersal movements of two sub- 

 species of the chorus frog, 

 Pseudacris frlseriafa, In (a) the 

 climatic optimum, (b) early 

 xerothermic, (c) xerothermic 



(e) the present. The population 

 of frheriata left behind In New 

 Jersey has recently been recog- 

 nized as the subspecies halml 

 (Smith 1957). 



mountain-building was extensive in the Rocky Moun- 

 tains, Appalachians, Ozarks and Ouachitas, Cascades, 

 Sierra Nevada, and Coast Ranges. The epicontinen- 

 tal seas receded. The climate in the rain-shadows of 

 the mountain systems became increasingly arid, par- 

 ticularly in the Southwest, Great Basin, and on the 

 Great Plains. Concurrently, the climate became pro- 

 gressively cooler, a trend culminating in the severe 

 glaciation of the Pleistocene. 



As a result of these changes in physiography and 

 climate, the Neotropical-tertiary flora retreated to 

 the present tropics to constitute the tropical forest 

 and tropical savanna biomes of today. The Arcto- 

 tertiary forest withdrew southward and eastward to 

 form the temperate deciduous forest, coniferous 

 forest, and tundra biomes. Into the areas vacated by 

 these two floras the Madro-tertiary flora expanded 

 to form the woodland, chaparral, grassland, and 

 desert biomes. 



At maximum glaciation during the Pleistocene, 

 the tundra biome was greatly restricted in North 

 America and the coniferous forest biome was mostly 

 destroyed, except in refugia in Alaska, the northern 

 Appalachians, the northern Rocky Mountains, and 

 on the Pacific coast. Everywhere it extended to 



lower elevations in the southerly mountain areas. 

 The deciduous forest was modified by intrusion of 

 coniferous forest species, but was not otherwise 

 greatly disturbed, either in its extent or its composi- 

 tion. Because of heavy precipitation, grassland was 

 more widely distributed through the Great Basin and 

 the Southwest. 



With the retreat of the glacier in post-Pleistocene 

 times, the tundra and coniferous forest biomes re- 

 occupied most of northern North America. In the 

 northern states from Minnesota and Illinois east- 

 ward, pollen data indicate changes of climate from 

 cool-moist to warm-moist to warm-dry, then back to 

 the cooler, moister conditions of the present time. 

 Accompanying these climatic changes was a succes- 

 sion of vegetation from spruce-fir to pine to oak- 

 hemlock-beech to oak-hickory and the prairie penin- 

 sula, then back to oak-beech. 



Comparable changes in climate and vegetation 

 occurred in Europe throughout the Tertiary and 

 Quaternary eras. These changes in climate and veg- 

 etation had a profound effect both in Europe and 

 North America on the evolution and dispersal of 

 animals ; and, by so much, on the development of 

 present-day animal communities. 



292 Geographic distribution ot communities 



