CENOZOIC TIME — TERTIARY. 929 



ters began to disappear. The teeth had the typical number, 44, reduced ; their 

 structure made more complex ; and their characters varied otherwise through 

 use and adaptations to different purposes. 



The feet had the number of digits reduced in most Ungulates, but not in 

 the Coryphodon line, or in the Carnivores, or the Quadrumana, or rarely in the 

 Insectivores or Rodents. Moreover, the feet lost the plantigrade tread in 

 the Herbivores, and Carnivores, but not in the Quadrumana, Insectivores, 

 or Rodents. 



In most of the larger species the regularity in the carpal and tarsal series 

 of the feet gave way to the oblique or alternating position o£ the bones 

 required for firmness in running. 



Some of the causes favoring change. — The development of so great a 

 diversity of Eocene Vertebrate structures is the more remarkable in view of 

 the absence of all evidence as to any great physical or meteorological dis- 

 turbance to require new adaptations. No change of climate is indicated 

 beyond what might have occasioned a feeble amount of migration. No evi- 

 dence of disquiet in the earth's crust has been noted, excepting that relating 

 to the imperceptible geosynclinal movements over the areas of the Eocene 

 lakes attending the slow deposition of sediments. 



The only sources of disquiet that can be appealed to as causes of bio- 

 logical change, are biological sources proceeding from the appetites or needs 

 or impulses of the animals. Of these appetites the dominant one, the most 

 imperative, the only daily recurring one, was the demand for food. As 

 nearly half of the Mammals lived on animal food, there was perpetual strife 

 between the stronger flesh-eaters and the weaker, and between all flesh-eaters 

 and other species. It would naturally have driven the weak kinds to holes, 

 or somewhere out of reach of their enemies, where poor food, darkness, and 

 other privations, would have been unfavorable to high progress. The strife, 

 moreover, as writers on the derivation of species have illustrated, would 

 have promoted fleetness, cunning, devices for protection, and have favored 

 those changes in the Mammalian structures that would better fit or accom- 

 modate the species to the new demands. 



The evolution of the Horse through the necessity of running to escape 

 from enemies has often been set forth as an example of the effects, under 

 certain conditions, of such a cause. An animal of primitive Ungulate type, 

 having the third or middle toe the longest of the five, raising itself on its 

 toes for greater speed in running, and forcing itself forward naturally by its 

 longer toe, had this toe, as Eocene and Miocene time passed, with the bone 

 of the foot above it (the metatarsal and the metacarpal) enlarged and 

 elongated, while the less-used toes either side dwindled till too short to reach 

 the ground ; and finally, through these and other concurrent changes, there 

 was evolved, a long-legged one-toed animal — the Horse. It became tall and 

 long-legged, not only by elongating growth in certain bones, but also through 

 the functional appropriation by the leg of all of the foot excepting the 

 terminal hoofed joint. 



DANA'S MANUAL — 59 



