PLEISTOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 507 



his conclusions as follows: that the most rational way of explaining the 

 extinction in Alaska is the gradual change from more temperate conditions 

 which reduced and finally destroyed the forest vegetation, thus reducing 

 the food supply and reducing the fauna to those forms capable of adapting 

 themselves to the recent tundra vegetation. 



Extermination of horses. — Among all the problems of Pleistocene extinc- 

 tion presented in America, that of the horses is certainly one of the most 

 difficult. These animals are far superior to cattle in their adaptability to 

 changed conditions of life and in resourcefulness during severe winter sea- 

 sons. They were extraordinarily numerous in North America at the begin- 

 ning of the Pleistocene; at the close it appears that they were entirely 

 extinct. Similar extinction occurred both in North and South America 

 in Pleistocene times. It is consequently impossible to connect this phenom- 

 enon directly with the Ice Age. In Pleistocene times there was a ready 

 escape to the high plateaux of Mexico, which must have presented all the 

 most favorable conditions for equine life, of climate, soil, and food. The nu- 

 merous and highly specialized horses of Mexico shared in this extinction. It 

 has consequently been suggested by the writer and by others that the horses 

 may have been swept out of existence by some epidemic disease or diseases. 

 These diseases are carried by flies and are favored by moist conditions 

 occurring chiefly during or immediately after heavy rainfalls, though in 

 sporadic cases they may occur at other seasons of the year; such moist 

 conditions occurred periodically in the Great Basin of Oregon and Nevada 

 and in the valley of Mexico. The disease known in India as 'surra' has a 

 widespread geographic distribution. In Africa there is a similar malady, 

 'nagana,' or tse-tse fly disease. In Algeria, France, and Spain the horse 

 and the ass are both liable to the attacks of a trypanosome (T. equiperdum) . 

 In South America the nial de caderas affects horses, asses, cattle, and certain 

 other animals, and is attributed to a trypanosome; it is distinctively a wet 

 weather disease, almost completely disappearing in the dry seasons. The 

 tse-tse fly of Africa renders thousands of square miles uninhabitable by 

 horses. The rapid rate at which such diseases may travel is illustrated by 

 the spread of the rinderpest, which traversed the whole length of Africa in 

 fifteen years. 



This theory of an epidemic among the American horses during the wet 

 weather periods of Glacial times receives some support from the discovery 

 by Cockerell in the Miocene insect fauna of Florissant, Colorado, of two 

 species of tse-tse fly (Glossina) very similar to the African types. The 

 application to the Pleistocene is that a moist or rainy period extending 

 over the Southern States and down into Mexico during Pleistocene times 

 would have favored the distribution of some flies or other parasite-bearing 

 insects, such as ticks, and have resulted in the extinction of the horses. 



Influence of increased rain supply. — Dry or moderately dry conditions, 

 if not too extreme, are generally more favorable to quadrupeds than moist 



