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since they inhabit almost all the tropical regions but do not range more than about 10° beyond the southern and 12° beyond the northern tropic, while the great bulk of the species are found only within an equatorial belt about 30° wide. But as these animals are almost exclusively fruit-eaters, their distribution depends as much on vegetation as on temperature; and this is strikingly shown by the fact that the Semnopithecus schistaceus inhabits the Himalayan mountains to a height of 11,000 feet, where it has been seen leaping among fir-trees loaded with snow-wreaths ! Some northern animals are bounded by the isothermal of 32°. Such are the polar bear and the walrus, which cannot live in a state of nature far beyond the limits of the frozen ocean; but as they live in confinement in temperate countries, their range is probably limited by other conditions than temperature.
We must not therefore be too hasty in concluding, that animals which we now see confined to a very hot or a very cold climate are incapable of living in any other. The tiger was once considered a purely tropical animal, but it inhabits permanently the cold plains of Manchuria and the Amoor, a country of an almost arctic winter climate. Few animals seem to us more truly inhabitants of hot countries than the elephants and rhinoceroses ; yet in Post-tertiary times they roamed over the whole of the northern continents to within the arctic circle; and we know that the climate was then as cold as it is now, from their entire bodies being preserved in ice. Some change must recently have occurred either in the climate, soil, or vegetation of Northern Asia which led to the extinction of these forerunners of existing tropical species; and we must always bear in mind that similar changes may have acted upon other species which we now find restricted within narrow limits, but which may once have roamed over a wide and varied territory.
Valleys and Rivers as Barriers to Mammals. To animals which thrive best in dry and hilly regions, a broad level and marshy valley must often prove an effectual barrier. The difference of vegetation and of insect life, together with an unhealthy atmosphere, no doubt often checks migration if it is attempted. Thus
many animals are restricted to the slopes of the Himalayas or to the mountains of Central India, the flat valley of the Ganges forming a limit to their range. In other cases, however, it is the river rather than the valley which is the barrier. In the great Amazonian plains many species of monkeys, birds, and even insects are found up to the river banks on one side but do not cross to the other. Thus in the lower part of the Rio Negro two monkeys, the Jacchus bicolor and the Brachiurus couxiou, are found on the north bank of the river but never on the south, where a red-whiskered Pithecia is alone found. Higher up Ateles paniscus extends to the north bank of the river while Lagothrix humboldtii comes down to the south bank; the former being a native of Guiana, the latter of Ecuador. The range of the birds of the genus Psophia or trumpeters, is also limited by the rivers Amazon, Madeira, Rio Negro and some others; so that in these cases we are able to define the limits of distribution with an unusual degree of accuracy, and there is little doubt the same barriers also limit a large number of other species.
Arms of the Sea as Barriers to Mammals. Very few mammals can swim over any considerable extent of sea, although many can swim well for short distances. The jaguar traverses the widest streams in South America, and the bear and bison cross the Mississippi ; and there can be no doubt that they could swim over equal widths of salt water, and if accidentally carried out to sea might sometimes succeed in reaching islands many miles distant. Contrary to the common notion pigs can swim remarkably well. Sir Charles Lyell tells us in his “Principles of Geology” that during the floods in Scotland in 1829, some pigs only six months old that were carried out to sea, swam five miles and got on shore again. He also states, on the authority of the late Edward Forbes, that a pig jumped overboard to escape from a terrier in the Grecian Archipelago, and swam safely to shore many miles distant. These facts render it probable that wild pigs, from their greater strength and activity, might under favourable circumstances cross arms of the sea twenty or thirty miles wide; and there are facts in the distribution of this tribe of animals which seem to indicate that they have sometimes done so. Deer
take boldly to the water and can swim considerable distances, but we have no evidence to show how long they could live at sea or how many miles they could traverse. Squirrels, rats, and lemmings often migrate from northern countries in bands of thousands and hundreds of thousands, and pass over rivers, lakes and even arms of the sea, but they generally perish in the saltwater. Admitting, however, the powers of most mammals to swim considerable distances, we have no reason to believe that any of them could traverse without help straits of upwards of twenty miles in width, while in most cases a channel of half that distance would prove an effectual barrier.
Ice-floes and Driftwood as Aiding the Dispersal of Mammals.In the arctic regions icebergs originate in glaciers which descend into the sea, and often bear masses of gravel, earth, and even some vegetation on their surfaces; and extensive level icefields break away and float southwards. These might often carry with them such arctic quadrupeds as frequent the ice, or even on rare occasions true land-animals, which might sometimes be stranded on distant continents or islands. But a more effectual because a more wide-spread agent, is to be found in the uprooted trees and rafts of driftwood often floated down great rivers and carried out to sea. Such rafts or islands are sometimes seen drifting a hundred miles from the mouth of the Ganges with living trees erect upon them; and the Amazon, the Orinoco, Mississippi, Congo, and most great rivers produce similar rafts. Spix and Martius declare that they saw at different times on the Amazon, monkeys, tiger-cats, and squirrels, being thus carried down the stream. On the Parana, pumas, squirrels, and many other quadrupeds have been seen on these rafts; and Admiral W. H. Smyth informed Sir C. Lyell that among the Philippine islands after a hurricane, he met with floating masses of wood with trees growing upon them, so that they were at first mistaken for islands till it was found that they were rapidly drifting along. Here therefore, we have ample means for carrying all the smaller and especially the arboreal mammals out to sea; and although in most cases they would perish there, yet in some favourable instances strong winds or
unusual tidal currents might carry them safely to shores perhaps several hundred miles from their native country. The fact of green trees so often having been seen erect on these rafts is most important; for they would act as a sail by which the raft might be propelled in one direction for several days in succession, and thus at last reach a shore to which a current alone would never have carried it.
There are two groups of mammals which have quite exceptional means of dispersal—the bats which fly, and the cetacea, seals, &c., which swim. The former are capable of traversing considerable spaces of sea, since two North American species either regularly or occasionally visit the Bermudas, a distance of 600 miles from the mainland. The oceanic mammals (whales and porpoises) seem to have no barrier but temperature; the polar species being unable to cross the equator, while the tropical forms are equally unfitted for the cold polar waters. The shorefeeding manatees, however, can only live where they find food; and a long expanse of rocky coast would probably be as complete a barrier to them as a few hundred miles of open ocean. The amphibious seals and walruses seem many of them to be capable of making long sea journeys, some of the species being found on islands a thousand miles apart, but none of the arctic are identical with the antartic species.
The otters with one exception are freshwater animals, and we have no reason to believe they could or would traverse any great distances of salt water. In fact, they would be less liable to dispersal across arms of the sea than purely terrestrial species, since their powers of swimming would enable them to regain the shore if accidentally carried out to sea by a sudden flood.
Means of Dispersal of Birds.-It would seem at first sight that no barriers could limit the range of birds, and that they ought to be the most ubiquitous of living things, and little fitted therefore to throw any light on the laws or causes of the geographical distribution of animals. This, however, is far from being the case; many groups of birds are almost as strictly limited by barriers as the mammalia; and from their larger numbers and the avidity with which they have been collected, they furnish
materials of the greatest value for our present study. The different groups of birds offer remarkable contrasts in the extent of their range, some being the most cosmopolite of the higher animals, while others are absolutely confined to single spots on the earth's surface. The petrels (Procellariidoe) and the gulls (Laridæ are among the greatest wanderers; but most of the species are confined to one or other of the great oceans, or to the arctic or antarctic seas, a few only being found with scarcely any variation over almost the whole globe. The sandpipers and plovers wander along the shores as far as do the petrels over the ocean. Great numbers of them breed in the arctic regions and migrate as far as India and Australia, or down to Chili and Brazil; the species of the old and new worlds, however, being generally distinct. In striking contrast to these wide ranges we find many of the smaller perching birds, with some of the parrots and pigeons, confined to small islands of a few square miles in extent, or to single valleys or mountains on the mainland.
Dispersal of Birds by Winds.—Those groups of birds which possess no powers of flight, such as the ostrich, cassowary, and apteryx, are in exactly the same position as mammalia as regards their means of dispersal, or are perhaps even inferior to them; since, although they are able to cross rivers by swimming, it is doubtful if they could remain so long in the water as most land quadrupeds. A very large number of short-winged birds, such as toucans, pittas, and wrens, are perhaps worse off; for they can fly very few miles at a time, and on falling into the water would soon be drowned. It is only the strong-flying species that can venture to cross any great width of sea; and even these rarely do so unless compelled by necessity to migrate in search of food, or to a more genial climate. Small and weak birds are, however, often carried accidentally across great widths of ocean by violent gales. This is well exemplified by the large numbers of stragglers from North America, which annually reach the Bermudas. No less than sixty-nine species of American birds have occurred in Europe, most of them in Britain and Heligoland. They consist chiefly of migratory birds which in autumn