612 HISTORICAL GEOLOGY 



adequate since the climate of the continent south of the ice sheets 

 was not unfavorable, and at the present time horses on the western 

 plains survive a temperature of many degrees below zero, without 

 shelter or any food other than that which they can obtain for 

 themselves, being able, in fact, to withstand conditions fatal to 

 cattle and sheep. The suggestion that the extinction was due to 

 some epidemic receives some support from the discovery of two 

 species of tsetse fly in the Miocene deposits of Colorado, similar to 

 the African types which, in that country, render thousands of square 

 miles uninhabitable by horses. Epidemics such as that carried by 

 the tsetse fly, the tick, and other insects are most prevalent in wet 

 seasons. The moist conditions which are believed to have prevailed 

 in North America during glacial times would favor the spread of 

 such a disease over, perhaps, the whole of the New World and might 

 readily wipe out of existence the entire race of horses. 



REFERENCES FOR HORSES 



Beddard, F. E., — Mammalia. 



Flower, Sir W. H., — The Horse. 



International Encyclopedia, — Horse. 



Lankester, E. R., — Extinct Animals, pp. 132-142. 



Lucas, F. A., — Animals of the Past, pp. 159-175. 



Lull, R. S., — The Evolution of the Horse: Am. Jour. Sci., Vol. 23, 1907, pp. 161-182. 



Matthew, W. D., — The Evolution of the Horse : Am. Museum Jour., Vol. 3, No. 1, 1903. 



Osborn, H. F., — The Evolution of the Horse in America: Century Mag., Vol. 69, 1905, 



PP. 3-17- 

 Scott, W. B., — A History of Land Mammals in the Western Hemisphere, pp. 291-308. 



Elephants. — The massive body and legs, the long tusks, and 

 flexible trunk of the elephant combine to make it one of the strangest 

 of animals and, from external appearance alone, one which might 

 seem least likely to be descended from the generalized mammals of 

 the early Eocene. 



The earliest known fossil elephants (Moeritherium) (Fig. 548) 

 have been found in Upper Eocene deposits of Egypt. They were 

 about three and one half feet high and, even at this time, were of 

 stocky build, although they would hardly be recognized as belonging 

 to the elephant family were it not for later forms which became 

 more and more elephantlike. The structure of the skull shows 

 thai a flexible upper lip, the beginning of a trunk, was present in 

 life. The teeth had already been reduced to 36, and one pair of 



