Distribution 



suffer retrogression in the structures which facilitated arboreal 

 habits. 



The distribution that we should expect from the physical 

 features of North America is exactly the one we find on a study 

 of its Salientia. The thermal barrier to their spreading gives 

 only a few species to southern Canada. Very few species are 

 reported from the Western plains and Rocky Mountain region; 

 among them are the burrowing Spadefoot Toads, which are al- 

 most peculiar to the region. The dry plateaus of the Southwest 

 yield mainly toads, some six species. The tree frogs are almost 

 wholly limited to southern and southeastern regions, with emi- 

 nent exceptions, such as Acris gryllus, the Cricket Frog, which 

 has the disks of the toes reduced, and may be found on the ground 

 either in forest regions or prairie districts. The riches of the 

 continent in genera and species of Salientia are found in the 

 moist forest-covered southeastern United States, in Florida, the 

 Gulf States, and eastern Texas. The northern part of the Pacific 

 Slope west of the Sierras also has some abundance of species. 



A comparative study of the distribution of living and ex- 

 tinct animals, together with a study of the geological history of 

 the world, has resulted in a division of the globe into faunal re- 

 gions, each of which has its subregions. The western hemisphere 

 has a tropical region (Neotropical) at the south, made up of South 

 America, Central America, the West Indies, and Mexico. A 

 boreal region at the extreme north consists of Canada from New- 

 foundland to Alaska, with a southern boundary corresponding 

 in general to the northern outline of the United States (the main 

 exceptions/to this correspondence are due to southerly extensions 

 along the mountain chains). The stretch of temperate America 

 between these two is the austral l region, which is very nearly 

 confined within the limits of the United States. 



The Austral region has been divided into life zones that 

 stretch across the continent south of the boreal zone from coast 

 to coast. Their boundaries are very irregular. Along the east 

 and west coasts the southern Austral zones penetrate far north- 

 ward, and in the high altitudes of the eastern and western 

 mountain districts the more northerly austral zones, and the 

 Boreal also, reach long arms southward. The eastern wooded 



1 The Austral and Boreal of Merriam (1892) correspond to the following: Nearctic of Slater 

 (1858); Medio-columhian and Aqulonian of Blanford (1890); Sonoran and Holarctic of Lydekker 

 (1896); Medio-columbian and Holarctic of Cope (1896). 



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