Snake River Valley of Idaho, who used this device in their treeless land to cross the streams, when the water was too cold for swimming.[537] Still cruder rafts of reeds, without approach to boat form, were the sole vehicles of navigation among the backward Indians of San Francisco Bay, and were the prevailing craft among the coast Indians farther south and about the Gulf of Lower California.[538] Trees abounded; but these remnant tribes of low intelligence, probably recent arrivals on the coast from the interior, equipped only with instruments of bone and stone, found the difficulty of working with wood prohibitive.

The second step in the elaboration of water conveyance was made when mere flotation was succeeded by various devices to secure displacement.  The evolution is obvious.  The primitive raftsman of the Mesopotamian rivers wove his willow boughs and osiers into a large, round basket form, covered it with closely sewn skins to render it water-tight, and in it floated with his merchandise down the swift current from Armenia to Babylon.  These were the boats which Herodotus saw on the Euphrates,[539] and which survive to-day.[540] According to Pliny, the ancient Britons used a similar craft, framed of wicker-work and covered with hide, in which they crossed the English and Irish channels to visit their kinsfolk on the opposite shores.  This skin boat or coracle or currach still survives on the rivers of Wales and the west coast of Ireland, where it is used by the fishermen and considered the safest craft for stormy weather.[541] It recalls the “bull-skin boat” used in pioneer days on the rivers of our western plains, and the skiffs serving as passenger ferries to-day on the rivers of eastern Tibet.[542] It reappears among the Arikara Indians of the upper Missouri,[543] and the South American tribes of the Gran Chaco.[544] The first wooden boat was made of a tree trunk, hollowed out either by fire or axe.  The wide geographical distribution of the dug-out and its survival in isolated regions of highly civilized lands point it out as one of those necessary and obvious inventions that must have been made independently in various parts of the world.

[Sidenote:  Relation of the river to marine navigation.]

The quieter water of rivers and lakes offered the most favorable conditions for the feeble beginnings of navigation, but the step from inland to marine navigation was not always taken.  The Egyptians, who had well-constructed river and marine boats, resigned their maritime commerce to Phoenicians and Greeks, probably, as has been shown, because the silted channels and swamps of the outer Nile delta held them at arm’s length from the sea.  Similarly the equatorial lakes of Central Africa have proved fair schools of navigation, where the art has passed the initial stages of development.  The kingdom of Uganda on Victoria Nyanza, at the time of Stanley’s visit, could muster a war fleet of 325 boats, a hundred of them