232 GEOLOGY. 



modification (a branch of the primitive Condylarthra in particular); 

 (2) a plant group susceptible of becoming advantageous food for the 

 new type, notably the grasses and subordinately the fodder-furnishing 

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

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

 ing in prairies whose open field and hard turf invited the development 

 of foot and limb modification in the interest of the greatest speed. The 

 era of simple bulk and heavy armor had largely passed, and an era of 

 agility, dexterity, and of light but effective weapons had begun. No 

 small factor in this progress was the increase in intelligence disclosed by 

 the larger brains. Intelligence henceforth proved an advantageous sub- 

 stitute for mass and mere brute strength. Corresponding with the 

 lighter and more agile structure there was the development of smaller, 

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

 theless continued to be a factor of importance, and some species in 

 almost every suborder grew in bulk until they reached and passed the 

 point of mass-advantage, and thereafter declined. 



Side branches that became extinct. — In the course of the early evolu- 

 tion some notable forms appeared, and a little later, became extinct. 

 Of these the Amblypoda (blunt feet) took precedence for a time. They 

 were a rather low type with diminutive, smooth brains, heavy bodies, 

 stocky limbs ending in stumpy five-toed feet, with a partly digitate 

 habit. They reached elephantine size; indeed they were much such 

 a development of massiveness and clumsiness on the mammalian stem, 

 as the dinosaurs had been on the reptilian stem, but the times did not 

 equally favor their dominance and perpetuity. The most prominent 

 offshoot from the Amblypoda in the Lower Eocene was the Coryphodon 

 (Fig. 428). Near the middle of the period (Bridger epoch) the remark- 

 able Dinoceras (terrible horn) appeared, followed later in the epoch by 

 Tinoceras, with which the line of the Amblypoda seems to have become 

 extinct. The Dinocerata (Fig. 429) were 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, an extravagant attempt at armature on both upper and 

 nether sides, but with meager results, if the short history of their endur- 

 ance is a true index. Their brains were smooth and singularly small 

 for such ponderous bodies. All mammalian brains of the time were 

 diminutive and simple, compared with later forms (see Fig. 430), but 



