214 NATURAL HISTORY OF 



these are the natural directing lines of nations exploring the 

 way to unknown regions ; and the necessity of facilitating 

 progression is the cause why all tribes, however rude, are 

 acquainted with some mode of conveyance by water. Other 

 roads were early indicated, by local necessities, differing from 

 the subsequent caravan routes, which took directions from and 

 to points already known to be most favorable for trafficking 

 with distant nations, who had objects of barter to exchange, 

 and, therefore, on both sides, had an interest in the speediest 

 and safest passage. From the well-known proceedings of sub- 

 sequent ages, it is clear that outcasts and scouts, then hunter 

 families, would naturally be the first adventurers, and tribes 

 would follow onwards only as far as immediate necessity or 

 convenience might dictate ; pushing further when more was 

 known of the world before them, and pressure from new 

 colonists urged them from behind. Starting through the 

 gorges of the great river outlets to the plains, and following 

 their course, or ranging along the flanks of mountain chains, 

 to turn deserts, or escape the necessity of attempting elevated 

 ridges, or interminable swamps, which were, or might be, im- 

 passable ; while, at the same time, water, game, and wild fruits 

 would be most abundant. 



Deserts and plains are never so absolutely impassable as to 

 prevent ulterior progress. Water is found in some localities, 

 and occasionally verdure ; and these oases are soon marked 

 by the wanderer, who then guides his family or moving tribe 

 along them, till they reach a better region. Impediments of 

 this kind are, therefore, incentives to progress, and generally 

 much less obstacles than morasses and dense forests; for it is 

 by the river courses alone that these last are penetrated. 



In the progressive colonization some leading tribe would find 

 a natural obstacle to retard or prevent its further migration ; 

 halting on the spot, other clans would come up ; and where no 

 forests near the sea, nor a great stream, would favor the struc- 

 ture of rafts or canoes, intercourse occurring, more or les9 



