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THE AMERICAN NATURALIST [Vol. XLIII 



and southwestern North America, moreover, are better 

 known than are the animals. "Whatever may be the 

 cause, not only is there a resemblance between families 

 and genera in the two areas; the general similarity of 

 the landscape and especially the occurrence in both of a 

 profusion of cactuses strikes the casual observer at once. 

 Many specimens of northern plants recur in identical 

 forms in those distant regions of South America. 



Professor Asa Gray 31 and Sir Joseph Hooker long ago 

 directed attention to this very remarkable phenomenon. 



More recently, Professor Engler 32 and Professor Bray 

 have emphasized this affinity and speculated on the prob- 

 lems connected with it. The flora of the Rocky Moun- 

 tains including the Sierra Nevada Mountains above the 

 transition zone, and the mountains far to the south of 

 them, though separated from one another by a stretch 



Anemone, Geranium, Spira?a, Geum, Rubus, Saxifraga, 

 Vaceinium, Gentiana, Hieracium and others. The 

 greater number of forms occurring in the southern con- 

 tinent are endemic, pointing to long-continued isolation. 



alpine region, reappear in the extra -tropical Andes 

 towards the southern extremity of South America, being'. 



mountains as well as the tropical Andes. Among these 

 Professor Bray mentions particularly Gentiana pros- 

 variety maqrUunir,,. Dnihn i„r,nm. AlnfxTiim* ulpinns, 

 Saxifraga cespitosa, Polemonium microemthum and 

 Collomia gracilis. 



The lower Sonoran elements of plant life are likewise 

 represented in the extreme south. Oxytheca dendroidea, 

 Chorizanthe commissuralis and Lastarricea chilensis, for 

 example, do not occur in the vast regions that separate 



