458 LAND ANIMALS 



by the mass appearance of individual species, that are also distributed 

 in the neighboring regions. 39 Only where large steppe areas are in the 

 vicinity does migration of typical steppe animals occur from the larger 

 into the smaller. 



In general, the fauna of the arid, open lands shows about the fol- 

 lowing composition : 



Aquatic animals are the scarcer, the more temporary the bodies of 

 water. Forms with a short developmental period and resistant, quies- 

 cent stage are the most common; hence the large number of phyllopod 

 species in the steppe. Many more species of these crustaceans are 

 known from Australia with its extensive arid regions than from well- 

 investigated Europe which is one-fourth larger; almost one-half of all 

 the known phyllopod species (88 out of 184) come from Australia and 

 Africa together. 40 Not uncommonly, a number of genera occur side by 

 side in the same pool, e.g., Limnadopsis, Limnetis and Estheria; 41 the 

 smaller the species, the more numerous they are, for their develop- 

 mental period is so much shorter. Aquatic insects and insect larvae are 

 found with them, such as water beetles, water bugs, and especially gnat 

 and mosquito larvae. 



Of the hygric animals, only a few are able to subsist in the arid, 

 open country; they appear in decreasing number of species as the 

 environment varies from grassland to desert. The snail fauna of the 

 steppe is impoverished, for only species can occur that can endure a 

 long quiescence during drought, sometimes more than a year. Within 

 the same species, this ability may vary according to the locality; Helix 

 lactea in the Sahara may awaken from an aestivation of several years ; 

 individuals of the same species from Madeira are dead after a like num- 

 ber of months. 42 The llanos of Venezuela have a relatively rich snail 

 fauna; 43 in the North American prairies, however, they are limited 

 mainly to the tree-grown banks of the rivers; 44 the pampas is almost 

 lacking in mollusks, 45 and in central Australia snails are very uncom- 

 mon. All these snails have bright-colored, whitish, rather thick shells; 

 the stronger the shell, the greater is their resistance to the drought. 

 Desert individuals of widely distributed species of snails, such as Helix 

 pomatia, are much thicker shelled than typical specimens and tend to 

 lose their color bands. 46 



Amphibians are very uncommon in arid, open lands. Many of them 

 are dependent upon water for breeding, and because of the short rainy 

 period their development must be unusually accelerated. At the very 

 first spring rain, they come forth and begin to spawn; growth and 

 metamorphosis are rapid, and the young frogs conceal themselves, like 

 the old ones, in the mud during the dry season. All the amphibians of 



