CHANGES OF PROPORTION 263 



and in the tropics, and the high table-lands of Africa alone 

 retain the grandeur of the Pliocene Epoch. 



The Definite Couuse of Chroiviatin Evolution in 



THE Origin of New Characters Partly 



Predetermined by Ancestry 



Some of the most universal laws as to the modes (p. 251) of 

 evolution emerge from the comparative study of the horses, 



127. Two Stalls i.\ lin, Jadlltiux or thi; Titaxutiieres. 



Transformation of the small hoofed quadruped Eotitanops {A) of the Eocene — a relatively 

 light-hmbcd, swift-moving, cursorial herbivore — into the gigantic Brontothcriitm (B) of 

 the Lower Oligocene — a ponderous, slow-moving, graviportal type, horned for offense 

 and defense. These titanotheres were remotely related to the existing rhinoceroses, 

 horses, and tapirs, but they became suddenly extinct on attaining this impressive 

 stage of evolution. They exemplify the increase of size characteristic of the evolution 

 of the greater number of the hoofed Herbivora. The time during which this trans- 

 formation occurred is estimated at 1,200,000 years — about one-third of the whole 

 Tertiary Epoch. 



the proboscidians, and the rhinoceroses, from areas so widely 

 separated geographically that there was no possibility of hy- 

 bridizing or of a mingling of strains. For example, during a 

 period estimated at not less than 500,000 years the horses of 

 France, Switzerland, and North America evolve in these widely 



