412 LAND ANIMALS 



history of the species, but is obviously associated also with climatic 

 factors. 



Such extensive seasonal migrations as are common among birds 

 also occur among some mammals. The migrations of the bats are most 

 nearly comparable to bird migrations. Many northern bats travel to 

 warmer winter habitats. 125 In northern America, the bats inhabiting 

 hollow trees migrate southward as far as the Bermudas to pass the 

 winter. 



Many ruminants undertake regular migrations at the beginning of 

 the cold season. Especially impressive were the migrations of the 

 American bison, whose enormous herds moved towards the south in 

 the fall, though not to the extent that they migrated from Canada to 

 the coastlands of the Gulf of Mexico; it is true that the bison were 

 migrating over the entire distance, but it was a parallel displacement 

 of the entire mass, of such a sort that the most northern herds did not 

 go beyond the Republican River, the northern source of the Kansas 

 (40° N. latitude), while those from that region moved further to the 

 south. Such migrations are also known among deer. The wapiti of 

 Yellowstone Park migrate to the high valley of Jackson's Hole in the 

 Rocky Mountains, where as many as 40,000 of them are found together 

 in the winter; 126 and caribou regularly migrate from the tundra into 

 the northern coniferous forests. The seals (Otaria) of the Pacific 

 migrate northwards to their breeding place, in the warm season; in the 

 autumn they return to the south. The great majority of non-migratory 

 birds and mammals of the high mountain ranges withdraw in autumn 

 into lower, protected valleys; even hibernators such as the marmots 

 make their winter quarters at lower altitudes. 



The alternation of rainy and dry seasons also occasions periodic 

 migrations. In Africa, great bat migrations have been observed at the 

 beginning of the dry period. Antelopes, zebras, and ostriches often 

 congregate into large herds, and abandon the hot arid steppe; during 

 the dry season, the elephants move into the mountain forests and 

 climb to high altitudes in the mountains of Abyssinia, on Mount 

 Kenia and Mount Kilimanjaro, or withdraw into the galleried forests 

 along the river courses. 



The substratum. — The nature of the substratum is of great im- 

 portance to many air-breathing animals, especially to those that must 

 live in burrows in the ground itself. Animals that move about upon 

 the ground also show effects of certain characteristics of the sub- 

 stratum and adaptations to its peculiarities. Animal species living in 

 the ground include burrowing forms such as earthworms, Enchy- 

 traeidae, and nematode worms, countless insect larvae or mature, 





