Aridity and Evolution. 



229 



cacti in tropical rain-forests and on the high northern plains of 

 Nevada, Idaho and Montana. If plants of wider latitudinal 

 distribution are taken into consideration it may be seen that with 

 an extension of polar climate, the extermination of a species in 

 the higher part of its range would be coincident with additions 

 to the eligible area on the southward. If the land area was limit- 

 ed, or if mountain barriers intervened, such dissemination would 

 of course be impracticable and the forms involved would soon 

 perish. 



The unfavorable influence of increasing moisture upon the 

 xerophytic forms of a region would also include effects of an 

 indirect character. Soil temperature and moisture relations 

 would undergo great alterations, humus would increase and other 

 changes would ensue, entailing conditions which the specialized 

 structures would be unfitted to meet. Furthermore, succulents 

 and spinose forms being advanced types, their retrogressive 

 evolution to conform to moist conditions would be a process re- 

 sulting in enormous loss of species. Some spinose types repre- 

 senting the lesser specialization would seem to offer the best 

 morphological possibilities for such a change. 



Perhaps the most important of all the altered conditions 

 brought about by increasing moisture, however, would be the 

 total transformation of the competitive struggle for existence. 

 Animals would no longer play the predominating role as in arid 

 areas. The number of individuals representing the constituent 

 species of a flora would be multiplied a hundred fold, perhaps a 

 thousand fold, and once more the amount of food material 

 offered to animals would decrease their total importance in 

 selection, while the intensest crowding between roots and be- 

 tween shoots would once more be resumed and horizontal differ- 

 entiations of associations such as that in forests, would ensue. 



The element of the flora which would respond most readily 

 to ameliorated aridity would, of course, be the hygrophytic an- 

 nuals and perennials, which had survived the period of desicca- 

 tion in their refuge of the rainy seasons, and in the moist areas 

 along streamways and on elevated peaks. These would quickly 

 occupy the greater part of the surfaces available for plants to 

 the great intensification of the inter-vegetal struggle for exist- 

 ence As these hygrophytes survived in the moist situations and 



