1 6 THE PLANT WORLD. 



to analysis, and their provisional interpretation, in this stage of 

 our study, is quite as scientific a process and quite as necessary 

 as the collection of further data, indispensable as this ma}- be. 



In the first place, then, the desert plants on the right side of 

 the caiion have, without doubt, merely extended their range, in 

 comparatively recent time, with the development of the canon, 

 and had it not been formed in such a way as to offer exposures 

 favorable for them and generally unfavorable for plants of the 

 pinon zone, they would not have reached the height they have 

 already attained. We may, therefore, speak without hesitation 

 of an upward advance of Covillca, Yucca, Dasylirioii, Fouquieria, 

 etc., which has been going on step by step with the growth of 

 the canon, and is apparently still in j^rogress. 



In the second place, the dead pihons scattered amongst the 

 agaves and other desert plants mark a lower limit, determined 

 by a mininunn water supply, beyond which the continued exist- 

 ence of this species is precarious, and a period of unusual drouth 

 results in its extinction. If, at any point, the slope, exposure, 

 etc., result in lowering the available water supply below this 

 minimum, the lower limit of the species is thereby shifted upward. 

 Thus we have on the right side of the canon at the present time 

 an upward movement of plants belonging to two different zones, 

 an advance on the part of Covillca and its desert allies, and a 

 retreat of the pihon. 



It should be remarked in passing that the deportment of indi- 

 vidual species belonging to these different associations is highly 

 instructive and suggests a rational basis for the delimitation of 

 " zones " which they occupy. It is significant that while the 

 pihon has locally failed to maintain itself, the two junipers, its 

 characteristic associates, have lived and done well through the 

 drouth to which it has succumbed. It would be quite impossible, 

 therefore, to indicate with exactness the limits of this zone in 

 terms of physical measurements, although it is occupied by 

 as well marked an association of plants as one is likely to meet 

 with anywhere. As far as present evidence goes, water supply 

 is the chief factor determining the lower limit of the pihon zone, 

 while its upper limit is almost certainly determined largely by 



