VEGETATIOX OF BOX CANON. 1/ 



temperature. With what care, however, such general concki- 

 sions should be accepted is illustrated by the fact that the creosote 

 bush deports itself here, at an altitude of some 6,000 feet, pre- 

 cisely as it does in the neighborhood of Tucson, at an altitude 

 of 2,400 feet and upwards, occupying the talus slopes as its 

 special habitat, while Foiiquicria, Opiiiifia, and others are here, 

 at the higher altitude, with its consequent climatic differences, 

 quite as much at home as there, on the coarser rocks and more 

 rugged exposures, facts that emphasize the necessity of giving 

 due weight to edaphic as well as climatic factors in any attempt 

 to trace the pathway of these plants in their desert wanderings. 



How, now, about the third association of plants, that of coni- 

 fers and deciduous trees and shrubs growing in the bottom of 

 the canon ? Here we need a fact of history. Had they reached 

 the top of the Sacramento Mountain before the caiion was 

 formed? If so, they have advanced down the caiion as it has 

 finally come to offer conditions favorable for their growth, such 

 as do not exist on the bluffs and hills around. Concerning 

 this historical fact, however, there is at present little light. It 

 has been suggested that here, as in the eastern United States, 

 such mesophytes and their associates moved southward with the 

 increasing cold of the glacial epoch, and, here as there, subse- 

 C[uently moved northward, and also took to the mountains, where 

 so many of them are still living. The view has also been put 

 forward, that their presence here is the result of direct migra- 

 tion along the Rocky ^Mountains and the outlying elevations of 

 the southwest, which, if not continuous, served none the less as 

 good and sufficient stepping-stones for their progress. But more 

 than another day will have to be spent in the patient accumula- 

 tion of facts. Meantime, it would be hard to find a more fasci- 

 nating problem. 



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