THREATENED ARIDITY ON THE PACIFIC SLOPE 161 



produced by excessive porosity of soil. Loose sand and gravel 

 or volcanic ashes are poor conservators of moisture, and part 

 with it readily, both through evaporation and percolation. Soils 

 of these sorts are not common, however, in these regions, where, 

 as a rule, moisture-retentive qualities are the predominant char- 

 acteristics. In the following discussion the question of soil 

 aridity is eliminated and the effects of climatic aridity alone 

 are considered. 



The variations of plant life which accompany the encroach- 

 ments of aridity are diverse and often very complex. Innumer- 

 able general modifications and adaptations occur, mostly tend- 

 ing toward a more or less successful resistance to the stress 

 imposed by drier climatic conditions. Local peculiarities, de- 

 pending on adjacent heights or depressions, specialize — that is 

 to say, they lessen or increase the general degree of aridity prev- 

 alent over any large area, thereby favoring minute adaptations 

 or gradual transitions to more extensive and pronounced modi- 

 fications. 



In the region west of the Rocky mountains, the forest as a unit 

 is the type of vegetation which, aside from the purely aquatic 

 element of the flora, suffers a more profound disturbance of its 

 equilibrium and is more quickly and thoroughly driven out by 

 the advancing aridity than any other. In the herbaceous and 

 to a lesser extent in the suffmtescent flora there is a gradual 

 evolution of new forms, or of entire groups of certain types, to 

 meet the changing environments. It is doubtless true that many 

 herbaceous and shrubby species have gone under in this strug- 

 gle within recent geologic times, while others are so rare and 

 scattered as to warrant the assumption that they, too. are rap- 

 idly approaching extinction; but, on the other hand, there are 

 many groups possessing the power of adaptation in a high de- 

 gree, and through the slow development of modifications or by 

 evolution of what we term species, they are enabled for a time 

 to withstand successfully very adverse conditions. 



It is different with the forest growth in this region. Over- 

 whelmingly composed of cone-bearing trees, representing com- 

 paratively few species, it has an extremely narrow margin for 

 the evolution of new forms or species. The fact stands out clear 

 and distinct that most of the types and species of the order of 

 conil'er;r west of the Rocky mountains possess the power of adap- 

 tation only in a very limited degree. Their outlying forms are 

 few and only vaguely definable. It is true that we can recog- 



