Turning now to the surface of the land, we may note that regions which are snow-covered in winter offer special facilities for rapid locomotion. In the northern part of North America, both in the tundra and the forest region, the aborigines used both snow-shoes and sledges. The Eskimos to the north use dogs to draw their heavy sledges, but to the south the Indians used a lighter form of sledge, which was dragged by women, and therefore represents a much more primitive form of transport. Even down to the present time the conditions in Arctic America are such that transport facilities are very much greater in winter than in summer, except in regions close to navigable rivers. Snow-sledges and snow-shoes of course occur also on the European side of the Atlantic to the north, but the domesticated reindeer replaces the dog in Lapland as the means of traction, though dogs are used in other parts of the tundra region of the old world. Apart from snow-covered regions deserts afford another example of areas in which the surface is frequently so uniform that friction is greatly reduced, and rapid movement is possible without specially prepared tracks. In the Sahara, for example, which is far from being the waste of sand which is popularly imagined, there are great areas of almost level surface, where "the soil is firm and elastic, strewn with gravel, and like a garden walk." In Algiers it is possible to drive in high dog-carts over the plateau region in any direction, regardless of roads, and in parts of the North American desert the same thing is true. It has been suggested, indeed, that wheeled vehicles were invented by races living near desert regions, and that the invention thus preceded the making of roads. In the general case, however, rapid movement is bound up with the existence of roads or tracks. In parts of North America, notably in the region south and east of the Great Lakes, where the rivers are generally unsuitable for navigation by very primitive forms of boats, the original Indian inhabitants mostly moved by means of "trails" through the woods. These trails were the lines of migration of the larger mammals, especially of the bison, and it is a remarkable fact that the roads made later by the white immigrants sometimes followed these old trails, which proved to be the most convenient routes. This suggests one method in which roads and paths might originate, but the Indian trails, like the African negroes' paths through the forest, were excessively narrow and inconvenient.