370 THE AGE OF MAMMALS 



such as grasses, are accompanied by the extinction of a large number of 

 the browsing Herbivora and by the rapid evolution of the grazing Her- 

 bivora, as was first thoroughly worked out in Kowalevsky's epoch-making 

 memoir of 1873. 



The influences of decreasing moisture are fivefold: (1) the character 

 of the food supply changes with diminution of the softer and more succu- 

 lent vegetation and increase of the harder and more resistant vegetation. 

 (2) There is an increase in the length and severity of the dry seasons of 

 the year. (3) Forest barriers are diminished or removed, and new com- 

 petitors enter the country. (4) There is a reduction of the water supply 

 and consequent elimination of the animals incapable of traveling long 

 distances for food and water. (5) The evolution of grazing quadrupeds is 

 favored, while that of browsing and forest-living quadrupeds is hindered. 

 In brief, prevailing or increasing droughts entirely disturb the balance of 

 nature; they compel migration; they expose quadrupeds to attack by car- 

 nivores by drawing them to restricted water pools; they favor quadrupeds 

 able to dispense with a daily supply of water. 



Facts of this kind enable us to understand the disappearance of the 

 browsing horses, of the browsing chalicotheres, of the browsing and grazing 

 rhinoceroses, incapable of traveling great distances, and of the browsing 

 camels, and the evolution at the same time of the wide-ranging long-limbed 

 types of grazing horses and grazing camels, which even in Pliocene times 

 were probably acquiring the power of dispensing with daily draughts of 

 water. 



Influence of droughts. — Darwin describes ^ the devastating effects of the 

 great drought in the pampas of South America between 1827 and 1830, 

 during which great numbers of birds, wild animals, cattle, and horses 

 perished from want of food and water. Cattle perished by thousands on the 

 muddy banks of the Parana River. Similarly Azara describes horses perish- 

 ing in large numbers in marshes, where driven in by thirst. In central 

 Africa to-day the influence of gradual decrease of moisture is clearly illus- 

 trated in the conditions observed by such writers as Gregory. ^ The drinking 

 places, or water pools, during long periods of drought become fewer in 

 number and more widely separated, and large animals driven to them by 

 thirst are more readily attacked and killed by Carnivora. Thirst, like 

 hunger, drives quadrupeds to take extreme risks, which they would abso- 

 lutely avoid during natural conditions of water supply. The pools become 

 separated by distances of thirty to forty miles, thus necessitating long 

 excursions to and from the various feeding places, during which quad- 

 rupeds are again exposed to attack. Finally, some of the pools dry up 

 entirely, and, as observed by Gregory {op. cit., p. 268): "Here and there 



' Darwin, C, Journal. . . . Voyage of H. M. S. Beagle around the World, 1845. New 

 ed., 1909, pp. 128-130. 



2 Gregory, J. W., The Great Rift Valley. 8vo, London, 1896. 



