INTRODUCTION. 11 



When one of these mountains is compared with another the floras 

 are found to be closely similar but not identical. 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 moun- 

 tain, although it is there surrounded by an area possessing a favorable 

 environment for it. In view of the universality of the distribution of 

 most of the mountain species, these cases appear to be the opening 

 invasions by which new components are being added to the flora. 

 They are in a way analogous 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. 



In determining the limits of the types of vegetation which are shown 

 in plate 1, no consideration whatever has been given to the ranges of 

 individual species of plants. It is true, nevertheless, that each of these 

 great vegetations possesses many species, particularly among its 

 dominant forms, which are roughly confined to the area occupied by 

 the vegetation itself. Picea sitchensis, for example, is nearly coincident 

 in distribution with the Northwestern Hygrophytic Evergreen Forest; 

 Pinus twda extends very little beyond the range of the Southeastern 

 Mesophytic Evergreen Forest, and Bulbilis dactyloides extends over 

 nearly the same area as the Grassland and the Grassland-Deciduous 

 Forest Transition. Other forms are nearly coincident with groups of 

 areas, as is the case with Covillea tridentata, which is found in the 

 California Microphyll Desert, the Arizona Succulent Desert, and the 

 Texas Succulent Desert. Since the limits of groups of dominant 

 plants have been unconsciously and necessarily taken into account 

 in delimiting the vegetational areas, it is to be expected that there 

 should be many cases in which the physical conditions concomitant 

 with the distribution of a given type of vegetation and those concomi- 

 tant with the distribution of some of its dominant species should prove 

 to be nearly identical. 



We regard the view that vegetation is determined in its distribution 

 by complexes of physical conditions, as estabhshed beyond all cavil 

 by the work of a large number of men, if indeed it is not almost axio- 

 matic. That the distribution of individual species is also controlled 

 by physical conditions is equally well demonstrated, so long as we con- 

 fine our attention to the common forms which are important elements 

 of the vegetation. There are doubtless many of the novitiate and 

 rehct species of plants which find the physical conditions of the present 

 time a barrier to their spread, but such cases have not yet been demon- 

 strated. We have, therefore, confined our consideration of individual 

 species to forms which are extremely abundant, except in a few cases 

 which will be noted. 



