ECOLOGICAL FACTORS 



415 



true deer ; spreading its broad hoofs it runs quickly over yielding crusts 

 and through deep snow in which a deer could not advance 50 m. 132 The 

 European elk and its ally the American moose have large and spread- 

 ing hoofs. 



In the arctic hare, the soles of the feet are unusually tough and 

 covered with spreading hair; Lepus americanus is called the "snow- 

 shoe rabbit" by hunters. 133 Like their prey, the predators are also 

 adapted to running over the snow: the Canada lynx has characteristi- 

 cally large feet; deer are hunted only at times of deep snow by the 

 puma. Aquatic birds are protected by their webs from sinking into 

 soft mud; the webbed toes of flamingos are chiefly used for this purpose. 



Fig. 115. — Digits of sand-inhabiting lizards, enlarged by fringes; a, foot 

 of Ptenopus gamdus, a South African gecko; b, Uma notata, an American 

 iguanid. After Cope. 



The various types of soil also differ from each other in their rela- 

 tion to heat. Rocky and gravelly soils on south and southwest slopes, 

 and especially alkaline soils, are heat accumulators. The southern 

 flora, for example, extends further to the north and up to the mountain 

 slopes on alkaline soil than on moist clay; certain plants, which are 

 found only on calcareous subsoil in north Germany and the British 

 Isles, are not so restricted in milder climates. 134 Rocks and hillsides 

 consequently form a favorable habitat for stenothermal, warmth- 

 limited animals. The rough calcareous cliffs like those of the upper 

 Rhine plateau below Basel and the vineyard-covered spurs of the 

 Vosges with their calcareous soil are foci of southern life. 135 



Some direct chemical effects of the nature of the substratum are 

 evident. Lime is a necessary part of the food of many animals, par- 

 ticularly of snails and mammals. Snails use lime for their shells, the 

 operculum, and the viscous mucus of the skin; mammals require lime 

 for their bones, and deer especially need it for the development of 

 the annual antlers of the male, which, in the red deer, weigh up to 



