In the arid region, however, man was not provided with such convenient material. His shelter from the sun cost him much labour and he was obliged to transport his necessary wood long distances. Additions to the shade to make it more comfortable were therefore obtained by piling up stones or scraping together the mud after a rain, and these operations being repeated, a development of skill was the inevitable result; skill which eventually produced a wall all round the sun-shelter, with the beams of the latter resting upon them instead of upon posts. [Illustration: PRIMITIVE AMERIND LADDERS] [Illustration: A NAVAJO HOUSE] It seems, therefore, altogether probable that stone and mud house building originated in arid regions; but in a region treeless, like our great plains, the inevitable outcome in the line of a shelter was the portable tipi (teepee), because there bison hides were at hand for covering, but poles of the proper sort were difficult to secure and were carried along. In the forest, neither portable tents nor stone houses were necessary. It would only be when population was dense enough to destroy the game and timber, or when a people were forced to an arid region, that the stone house would develop. The Iroquois was a forest Amerind, and he built a house of wood that was excellent in construction and answered his purpose admirably. The Navajo occupying an arid region has been content with a rude shelter of boughs and branches or with boughs or poles covered with mud. They have never profited by the example of their Moki neighbours, and built substantial houses,—one reason, and the chief one, being that their habit of never occupying again any shelter where death has occurred has precluded it, for they do not care to bestow great labour on a structure that they may be called upon any time to abandon. There are then other causes besides ability, or inability, to build substantially that determine the character of the Amerind house. [Illustration: A SWEAT HOUSE]