JOURNEYINGS AND DISPERSAL OF ANIMALS. 581 



rivers, and finds its way through the densest jungles. Other groups 

 are much more limited in their wanderings, as witness the monkeys, 

 lemurs, and apes, animals so strictly adapted to an arboreal life that 

 they cannot roam far beyond forest limits. Equally essential to the 

 existence of others is the desert or open country. The range of many 

 mammals appears to be limited by climate, or by its resulting vege- 

 tation. Thus the Quadrumana are chiefly found within an equatorial 

 belt of 30 wide, but these animals live almost exclusively on fruit, 

 which is the abundant product of the tropics. The polar bear and 

 walrus, which, in a natural state, are limited by the frozen ocean, in 

 confinement may live in temperate regions ; the tiger, once regarded as 

 a purely tropical animal, now has his permanent home in Mantchooria, 

 a country of almost arctic climate ; and, in post-Tertiary times, the 

 elephant and rhinoceros roamed over the northern continents, even 

 to regions beyond the arctic circle. Hence it does not follow that 

 animals, which we now see inhabiting extremely warm or extremely 

 cold climates, may not, under changed conditions, thrive equally well 

 elsewhere. 



Valleys and rivers often prove effectual barriers to mammals. 

 Thus, in the plains along the Amazon, many species of insects, birds, 

 and monkeys, are found extending to the river -bank on one side, 

 which do not cross to the other. And on the northern bank of the 

 Rio Negro there are found two monkeys, the Brachiurus couxion and 

 the Jacchus bicolor, which are never seen on its southern bank. Many 

 mammals can swim well for short distances, but none over any great 

 extent of sea. It is not unusual for the bear and bison to swim across 

 the Mississippi, and from Lyell's " Principles of Geology " we learn 

 that in 1829, during the floods in Scotland, pigs six months old, which 

 were carried to sea, swam five miles back to shore ; and it seems en- 

 tirely probable that wild-pigs, from their greater activity and power 

 of endurance, might cross arms of the sea twenty or thirty miles 

 wide, and facts in the distribution of these animals lead us to infer 

 that they have sometimes done so. Lemmings, rats, and squirrels, 

 often migrate in enormous bands, but they generally perish in the 

 sea-water. And, admitting that many mammals have power to swim 

 considerable distances, it remains true that a channel ten or twenty 

 miles wide would, in most cases, prove an effectual barrier to them. 

 The bats, provided as they are with wings, and the Cetacea, which 

 swim, -have exceptional powers of dispersal. In the arctic regions 

 glaciers give rise to icebergs; these descend to the sea, often carrying 

 with them masses of earth and some vegetation. Such arctic quad- 

 rupeds as frequent the ice, as well as occasionally true land-animals, 

 might often be carried from place to place in this way. But the up- 

 rooted trees and rafts of drift-wood which float down large rivers and 

 out to sea, are more effectual agents in the dispersal of animals. Such 

 islands or rafts are sometimes seen drifting hundreds of miles from 



