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THE AMERICAN NATURALIST [Vol. XL 



as possible; the darker the animal the safer he appears to be from 

 the attack of flies." In this connection we recall the dark color 

 of the true Bovinse. In South America the mal de caderas affects 

 horses, asses, cattle, hogs, and certain other animals, and is attrib- 

 uted to the protozoan known as Trypanosoma equinum. It is 

 distinctively a wet- weather disease, almost completely disappearing 

 in dry seasons. Asses, swine, and water hogs are said to be 

 affected, and horses are never known to recover. It is chronic 

 in course, lasting from two to five months in horses, and from six 

 to twelve in asses and mules. See Voges * for fuller details. 



Immunity and Adaptation. — Existing conditions among the 

 large quadrupeds of Africa are especially important because of 

 the increasing conviction that North American conditions in the 

 Oligocene, ]\Iiocene, and Pliocene are most closely paralleled 

 in the great upland region of modern Africa, the central life belt 

 as distinguished from the coast belt. 



From these recent discoveries' it appears that immunity from 

 disease is one of the most im])()rtant tVatnnvs of animal adaptation 



been our of tlu- potent caiiM's of diminntion and extinction. T. 



adaptive processes. He states, as his personal opinion, however, 

 that certain of these plienotneiia could not be explained as due to 



large number of instances of acquired immunity eamiot be directly 

 explained as adaptive phenomena. 



Variations in Immunity. — There are in Africa diseases fatal 

 to both wild and domesticated animals, otliers fatal to domesti- 

 cated animals to which wild animals are imnume. Some to which 

 all successive generations succumb; others to which inununitv 

 is acquired in the second generation or amonu' 'natives.' Still 

 more remarkable is the fact that both wild and domesticated 



