58 FORREST SHREVE 



of the same nature whether we alKide to them as ''biotic" 

 factors, ''mechanical factors" or what not. These two classes 

 of factors are further distinguishable in the fact that the direct 

 ones are chiefly constant in their operation, while the indirect 

 ones are intermittent or discontinuous in all cases. 



In the isolated desert mountains of southern Arizona the great 

 bulk of the species are physically controlled in their distribution, 

 as is abundantly shown by the universality of each of these 

 species throughout a given altitudinal range or a given set of 

 habitats, and by the definiteness with which it is limited by rather 

 sharply drawn lines. When one of these mountains is compared 

 with another the flora is found to be closely similar but not identi- 

 cal. Certain species range over the western Cordillera of Mexico 

 or over the central Rocky Mountains and have reached certain 

 of the isolated desert mountains without having reached all of 

 them. At least a few cases are known in which a given species 

 is common throughout several mountains and is known from 

 only one or two restricted localities in another mountain, al- 

 though an immediately contiguous area of favorable environ- 

 ment awaits it. In view of the universality of the distribution 

 of most of the mountain flora these cases appear to be the open- 

 ing invasions by which new components are being added to the 

 flora. They are in a way analagous to the novitiate species of 

 larger areas, and their presence in no way vitiates the evidence 

 for the physical control of the distribution of the major portion 

 of the mountain flora. 



Wherever I have had an opportunity to study vegetation with 

 any knowledge of the physical conditions attending it, there 

 has been patent evidence of the control which these conditions 

 exerted over the local and general distribution of the common 

 components of the vegetation. This control is more sharply 

 manifest in the desert mountains of southern Arizona than in 

 other localities which I have seen, because of the great differ- 

 ences of climate encountered within short vertical distances. 

 The force of the physical controls is almost equally patent, 

 however, along the greatest east and west diameter of Texas, 

 where the changes of elevation are small and differences of soil 



