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 vegetation. 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 entirely 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 quadrupeds as frequent the ice, as well as occasionally true land-animals, might often be carried from place to place in this way. But the uprooted 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
