ON THE CAUSE OF THE ABSENCE OF TREES. 701 



from thirty-two to sixty inches, is sufficient to be depended upon for agricul- 

 tural purposes. 



Where the annual precipitation is below twenty, or perhaps twenty-four 

 inches, there can be no growth of forests, and this is the true cause of the absence 

 of trees on the great plains. But this does not prevent the existence in arid 

 regions of certain specialized types of arborescent vegetation. The sage brush 

 that covers the dreary wastes of the Rocky Mountain Region, the Laramie Plains, 

 the Bitter Creek Valley, and such vast areas of the West, while in its botanical 

 characters it is little more than an over-grown weed, is to all intents and purposes 

 a tree, and often attains a great age. The region it occupies is even more arid 

 than the great plains, yet no fires occur and no forests grow. In the nearly rain- 

 less areas of Arizona, southern Utah and New Mexico, and stretching eastward 

 into Texas, there occur a number of arborescent forms, the creosote bush ( Lar- 

 rea Mexicana), the mesquit i^Prosopis juliflord), various acacias and mimosas, and 

 one yucca (K brevifolid), together with the tree cactus [Cereus giganteus) . These 

 grow scattered at great distances from each other and rarely form thickets or 

 groves. Why no such characteristic species are found occupying the great plains 

 is not known, and it is probably a mere accident that none happens to exist, 

 adapted both to their temperatures and their arid condition. Did any such exist, 

 there seems no reason why it might not thrive as well as the sage brush farther 

 west or the mesquit of the south. 



The absence of forests or extensive tracts of timber land on those areas of 

 our western country where the rain-fall annually exceeds twenty-four inches, must, 

 as already remarked, be attributed to human agency in repeatedly burning over 

 these areas, whereby all forms of vegetation requiring more than one season to 

 mature their fruit are prevented from perpetuating their kind. The American 

 aborigines have inhabited this continent through a prolonged period of the past, 

 how long we need not here stop to inquire. Ages prior to European discovery 

 they roamed over the great plains, hunting the buffalo, and through the forest- 

 clad wildernesses to the eastward, hunting the deer, the elk, and the other large 

 animals that belonged to the fauna of the country. Doubtless they found it ad- 

 vantageous to locate themselves at points near the boundary line which separates 

 these great are as so well marked off by nature, where they could pursue the 

 antelope by going westward or the deer by going eastward. 



No human race has yet been found so low as to be ignorant of the art of 

 making fire, and all along this border of the forest region we may imagine the red 

 man's camp-fire to have glowed in periods too remote for profitable speculation. 

 It is wholly unnecessary to assume, as Prof. Meehan has done, that the grass of 

 these border wood-lands was fired intentionally or "with the object of retaining 

 food foj their wild animals." It is just possible that their knowledge that wild 

 animals would be attracted to the sweeter grasses of newly burned tracts, may 

 have led to the intentional burning of certain districts, but there is no need to 

 call in this partial explanation. No Indian would ever attempt to check the ravages 

 of fires accidentally allowed to escape, or prevent them from plunging into the 



