414 LAND ANIMALS 



the sand of the dunes. 129 The consistency of the soil is a determining 

 factor for the soil-inhabiting animals. 



Certain non-burrowing animals are also directly dependent on the 

 nature of the subsoil. Birds such as the barn swallow, that nest in the 

 houses of man, are inhabitants of rocky regions in nature. The baboons 

 are usually found in rocky regions. The hyraces live in rocky areas of 

 southwest Asia and Africa and are fitted for climbing by suction pads 

 on their soles. Many hoofed animals, such as chamois, mountain goats, 

 and sheep, are animals of rocky regions ; other hoofed animals (horses, 

 wild cattle, antelopes) require at least hard firm ground that affords 

 a good foothold for running. The speed of the runner increases with 

 the decrease of the hoof surface on the ground; and relatively small, 

 delicate hoofs appear to be an adaptation in this direction. These 

 horny hoofs are adapted to great wear and are continually growing; 

 where the animals are kept on too soft ground, like the horse on the 

 boggy ground of the Falkland Islands, the hoofs become elongated be- 

 cause of lack of wear. All two-legged jumpers among the mammals 

 (with the exception of the tree-dwelling Tardus and the arboreal 

 kangaroos of New Guinea) are limited to such ground. In Europe, 

 Asia, Africa, and America these are rodents;* in Australia they are 

 kangaroos. Among the birds, the large Ratitae (ostrich, rhea, and 

 emu) are inhabitants of firm ground. Adaptation to running is at- 

 tained in these by the reduction in number of toes to three or even 

 two. Many invertebrates are also limited to rocky regions; 130 rock 

 snails such as Patula rupestris and many Clausiliae are examples. 

 The mountain butterfly, Parnassius mnemosyne, is found in waste 

 places covered with large boulders ; it avoids meadows. 



Looser, yielding subsoil, like sand, hinders the rapid movement of 

 heavy animals. For this reason, it is important that their soles be 

 widened. Deep sand is difficult terrain even for the camels with their 

 very broad soles and long legs. 131 The white gazelle (Gazella loderi) 

 and the antelope (Addax nasomacidatus) in the drifting sand district 

 of the inner Sahara have broad hoofs, differing thus from their rela- 

 tives. Small mammals and birds, however, are less inconvenienced by 

 loose sand. Even so, sand-inhabiting mammals, birds, and lizards often 

 have fringed toes, which give them traction on the sand. 



Deep snow and marshy ground also offer difficulties for larger 

 animals. Mammals inhabiting such regions also possess a broad foot. 

 Tapirs, hippopotami, and pigs are true hoofed animals of the swamps. 

 The reindeer is much better adapted to such yielding ground than the 



* Dipus, Alactaga, Pedetes, Jaculus. 





