784 



THE AMERICAN NATURALIST 



[Vol. XL 



plant food, largely or wholly inadapted to the new or altered kinds 

 of food. This we shall show was probably the most potent factor 

 in the extinction of the Titanotheres, of the Chalicotheres, in fact 

 of all the quadrupeds with short-crowned molar teeth, adapted to 

 browsing habits. 



The correlation between an initial change of climate and the 

 consequent diminution of the softer kinds of vegetable food and 

 increase of the harder kinds, such as grasses, followed by the extinc- 

 tion of a very large number of Herbivora, was first thoroughly 

 worked out in an epoch-making memoir of Waldemar Kowalevsky 

 in 1873. 



Droughts and Numerical Diminuiion. — Darwin^ describes 

 the devastating effects of the great drought in the Pampas between 

 1827 and 1830 in which great numbers of birds, wild animals, 

 cattle, and horses, perished from want of food and water. The 

 cattle perished by thousands on the muddy banks of the Parana 

 River. Similarly Azara describes the horses perishing in large 

 numbers in the marshes. 



Increasingly prolonged summer droughts were characteristic of 

 the late Miocene and Pliocene of Europe, and we are beginning 

 to accumulate evidence that the same conditions prevailed in 

 North America. 



Influence of Droughts in Central Africa.— The influence of the 

 gradual dec-n^ase of moisture in a country is clearly illustrated 

 in the coiiditions which prevail in the African continent to-day, as 

 observe. 1 l)y such writers as Gregory,^ Foa, and Schillings. Thirst, 

 like luin^^er, first (h-ives quadrupeds to take extreme risks, which 

 they would al)solutely avoid during natural conditions. The 

 drinking-places or water-pools during long seasons of drought 

 become fewer in number and more widely separated, and large 

 animals driven to them by thirst are more readilv attacked and 

 killed by Carnivora. Thv pools become separati-.l by <h"stances 

 of thirty and forty miles, thus u.vcssirntiua- loun- cxcursious to aud 

 from the various feedin<r j»hices, in which the (luadruneds are 



• Darwin, Chas. Journal. . . Toija^ie of //. ^f. S. Hcagh anmnd the World,. 

 p. 128-130. 



^ Gregory. The Great Rift Valley. . . .8vo. London, 18r6 



