306 FORREST SHREVE 



Lonicera arizonica Rehd. 

 Parthenocissus vitacea (Knerr.) Hitchk. 

 Ptelea angustifolia Benth. 

 Vaccinium scopariu?n Leiberg 



Each of these lists will undoubtedly be greatly extended by 

 further exploration. To the last of them, however, there at- 

 taches an importance which has no relation to its length. Each 

 of the species listed as local in the Santa Catalina range is found 

 in only a small portion of a particular area over which the physi- 

 cal conditions are identical, and over which the species charac- 

 teristic of that area are found to recur constantly. Each of these 

 seven species is common in the Pinaleno Mountains and found 

 throughout its particular altitude and habitat. It is to be ex- 

 pected that each of them will gradually spread in the Santa 

 Catalinas until all of them are likewise coextensive with a par- 

 ticular set of limiting conditions. We are here brought to face 

 with the entire problem of the manner in which the isolated desert 

 mountains received their plant population. This problem is a 

 large and difficult one, on which the evidence is at present ex- 

 tremely meagre. There seems to be no evidence, however, in 

 favor of such a radical change of climate within the life of these 

 mountains as to have brought about a continuous lowland vege- 

 tation like that which is now confined to the mountain summits. 

 In other words we can not think of the forest flora as having been 

 derived by the retreat up the mountains of a forest which for- 

 merly occupied the desert valleys. The floristic evidence is against 

 such a view, and is such as to indicate that the isolated mountain 

 forests have secured their plant populations by some means of 

 dispersal across the arid areas which lie between them and the 

 mountain masses to the north-east and the south-east. 



The means by which such dispersals took place were undoubt- 

 edly numerous, and the rate at which they operated was probably 

 very slow. The limited area of the mountain tops has in itself 

 been an important factor in making this process a slow one. In the 

 Pinaleno Mountains there are 35 square miles above 8000 feet 

 in elevation, in the Chiricahua Mountains 25 square miles and 

 in the Santa Catalinas only 5 square miles. In the last named 



