The distribution of both the recent and the fossil mammals plainly indicates that num berless species have passed from their origi nal home across land bridges to other con tinents where they are now isolated by the dis appearance of the bridges beneath the sea. In many instances the species has no surviving representative in the region of its origin. For ages Asia appears to have been a vast and fecund nursery for the production of mammal types which passed thence to other parts of the earth. Most of the mammals now inhabiting temperate and Arctic North America were evidently derived from northern Asia by way of a land bridge which united the continents in the region about Bering Strait. By this route came the ancestors of our moose (Alces), elk (Certnis), caribou (Rangifer), mountain sheep (Ovis), mountain goat (Oreamnos), prong horned antelope (Antilocapra), musk ox (Ovibos), wolves (Canis), foxes (Vulpes), bears (Ursus), wolverine (Gulo), marten (Martes), otter (Lutra), Arctic hares (Lepus), woodchucks (Marmots), field mice (Microtus), lemmings (Lemmus and Dicrostonyx), and others, nearly all of which still have close rela tives living in northern Asia and Europe.
Similar proof of former land connection be tween now distant areas is afforded by the dis tribution of other species of living mammals. This is well illustrated by the distribution of the marsupials, an extremely ancient group which, as shown by fossils, was once widely distributed in northern continents. They are represented to-day only by the opossums of South and Middle America and by the kanga roos and numerous other marsupials which in form and habits resemble our mice, moles and carnivores, in Australia and neighboring islands. Equally good indication of former land connec tions is given by the existence of tapirs in tropi cal America and in southeastern Asia; also by the octodont rodents now peculiar to tropical America and Africa as well as by the monkeys of the Old and New Worlds. Land bridges are believed to have once united South America, the Antarctic continent, and Australia on one side, and to have joined Brazil and West Africa on the other.
The comparative length of time during which certain land areas have been separated is at times indicated in a marked way by the degree of differentiation between the related mammals of the regions. The geologically recent union of northern North America and northern Asia is clearly indicated by the close likeness between the living mammals of the two regions. The converse is shown by the monkeys of South and Middle America which, having a close external resemblance to Old World species, have also certain fundamental structural char acters not known to occur in any living or ex tinct Old World forms. This indicates that the New World animals branched from the parent stock at a remote period and have developed free from any subsequent interchange between the old and new continents. It is interesting to note that the man-like monkeys, including the gorilla, chimpanzee and orang, are limited to Africa, southern Asia and the Islands off its southeastern coast. The long isolation of Australia and the surrounding islands is indi cated by the presence of a mammal fauna made up mainly of marsupials, an ancient group of animals once widely spread but now ex tinct elsewhere except for the American opossums. Aside from the bats Australia has representatives of only a few groups of modern mammals, mainly the mice and rats of the genera Mus and Rattus of the rodents, and the dingo or wild dog (Canis), be longing to the carnivores, which may have been introduced by man. Madagascar is an other example of an area separated from other lands since a remote period, its most char acteristic mammals representing types which exist nowhere else to-day, but of which fossil remains occur in adjacent parts of Africa. The recent great abundance and variety of large mammals in North America, both of species and individuals, is in strong contrast to the situation in South America. In the north we have the bison, musk ox, mountain goat, prong horned antelope, mountain sheep, elk or wapiti, several well-marked species of deer, caribou, and an unequalled variety of bears, including four types, the polar, the black, the grizzly and the huge brown bears of Alaska. To offset
these, South America outside the tropics has only two or three deer, the guanaco and vicuna (from which have come the domesticated llamas and alpacas) and the spectacled bear. The fauna of the tropical parts of South and Central America has many distinctive types of smaller mammals, including the prehensile-tailed monkeys, marmosets, coati-mundis, peccaries, agoutis, prehensile-tailed porcupines, sloths, armadillos and anteaters. Both the scarcity of large mammals in South America and the num ber of surviving peculiar types evidence the long isolation of that continent from contact with other lands on either side. Certain groups of mammals are typical of cool regions, such as the moose, caribou, musk oxen, marmots and lem mings of northern parts of the Old and New Worlds. Others like the monkeys are typical of the tropics. Some mammals appear to be indif ferent to climate conditions as in the case of the mountain lion (Fells concolor and its geo graphic races), which ranges from southern Canada to Patagonia, and appears to he equally at home among the sun-scorched desert ranges of the southwestern United States, the cool slopes of high mountains and the gloomy depths of the vast tropical forests of Central and South America, amid almost constant rains. A simi lar hardiness is shown by the tiger of the Old World which has representatives from the hot lowlands of southern India to the valley of the Amur River in eastern Siberia, where Arctic temperatures prevail in winter. The distribu tion of certain species serve as unmistakable records of former climatic conditions, as in the case of the field mice (Microtus) which occupy high isolated mountain tops in southern Mexico and Guatemala, separated from their nearest northern relatives by intervening tropical low lands. It is evident that they spread southward while a suitably cold climate prevailed on the lowlands, and with the change of climate were forced up and stranded on the elevated islands they now occupy. This discontinuous distri bution on mountain ranges is as significant as is the existence of closely related types on is lands and on distinct continental lands, although they may be separated by a broad and deep ex panse of the sea. Isolation on islands, and to a lesser extent on high mountains, where com petition from invading species may be mainly or entirely absent tends to preserve the representa tives of ancient forms long after they have van ished from other parts of their former range. Both Australia and Madagascar are extreme ex amples of the effect of isolation in bringing down to recent times representatives of ancient faunas which have become elsewhere extinct. The relation of the distribution of living mam mals to their near kin of the geologically not distant past is well shown by the discovery on the open plains of North America of fossil re mains of several types of antelope now abun dant on the plains of Africa. Camels, once oc curring from the coast of Florida to the Pacific, now exist in the feral state only in central Asia. They have living relatives, however, in the guanaco and vicuña of South America. Members of the elephant tribe, once common in most parts of the world, exist now only in Africa and southern Asia. Fossil remains abundantly prove that the horse developed on the great plains of North America where it existed in extraordinary variety of size and form. At the time of the discovery of America the horse had completely vanished from its confines, al though representatives still survived in the wilds of Asia and Africa. The fact that the horses brought by the Spaniards to America found the plains on both continents perfectly adapted to their needs, and going wild increased enormously, renders still more inexplicable the cause of the disappearance of the original stock from these haunts. In the past many mammals of extraordinary size existed in various parts of the world which are now represented by smaller species. Among these may be men tioned the giant deer of the Irish peat bogs, the huge extinct marsupials of Australia, the gigan tic armadillos of South America and the great wolves and lions of southern California.