THE EOCENE PERIOD 789 



known, the lakes and rivers were undergoing changes. If these 

 followed the methods of to-day, they left behind them, as they 

 shrank or shifted, borders of grassy or sedgy ground which, on 

 fuller drainage, often became prairie. Such changes were suited 

 to the evolution of herbivorous prairie life, and this in turn must 

 have invited its appropriate contingent of predaceous animals. 

 If these considerations are valid, the prime factors in the evolution 

 of the ungulates were (1) an undifferentiated plastic animal group 

 susceptible of modification; (2) a plant group (grasses and fodder- 

 furnishing angiosperms) affording appropriate food for the new 

 type; and (3) the shrinkage and shifting of lakes, marshes, and 

 lodgment plains, and the drying up of the plains of the continent, 

 resulting in prairies whose hard turf favored the development of 

 foot and limb modification in the interest of speed. 



The era of bulk and heavy armor, such as had been possessed 

 by the reptiles, had passed, and an era of agility and dexterity had 

 begun. No small factor in this progress was the increase in in- 

 telligence indicated by the larger brains. The lighter and more 

 agile frame was accompanied by the development of smaller, 

 simpler, but more effective weapons of attack and defense. Never- 

 theless size continued to be important, and some species in almost 

 every sub-order reached and passed the limit of bulk-advantage, 

 and then declined. 



In the course of the early evolution strange forms appeared, 

 and soon became extinct. Among them were the Dinocerata (Fig. 

 530) , grotesque monsters whose skulls were armed with three pairs 

 of protuberances, perhaps horn cores, and a pair of enormous 

 canine teeth or tusks projecting below (at least in the male), and 

 an extravagant attempt at armature on both upper and nether 

 sides. Their brains were smooth and singularly small for such 

 ponderous bodies. In them, brute mass and low brain-power 

 seem to have reached their climax among mammals. 



The divergence of ungulates into odd- and even-toed. Early 

 in the Eocene, the hoofed animals began to diverge into odd- 

 toed (perissodactyls) and even-toed (artiodactyls) types. In the 

 former, the main line of support is in the axis of the middle toe ; in 

 the latter, between the third and fourth toes. In the course of time 



