568 EOCENE AND OLIGOCENE PERIODS 



of forest and marsh. Forests perhaps helped to preserve a section 

 of the evolving order in its more primitive form. 



Back of these influences lay the physical conditions that pro- 

 moted them. In western America, where the evolution is best 

 known, the lakes and rivers were undergoing changes. As they 

 shrank or shifted, they left behind them borders of grassy or sedgy 

 ground which, on fuller drainage, may have become prairies. Such 

 changes were suited to the evolution of herbivorous prairie life, and 

 this in turn must have invited predaceous animals. If these con- 

 siderations are valid, the prime factors in the evolution of the un- 

 gulates were (i) 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 modifica- 

 tion 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 intel- 

 ligence 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 defence. Nevertheless 

 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. 

 477), 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 singularly small for such ponderous bodies. In 

 them, brute mass and low brain-power seem to have reached their 

 climax among mammals. 



Divergence of ungulates into odd- and even-toed. Early in 

 the Eocene, hoofed animals began to diverge into v odd-toed (peris- 

 sodactyls) 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 the lateral 



