TERTIARY ANIMALS. 539 



Tertiary animals is decidedly smaller relatively than that of their liv- 

 ing congeners. Marsh has, moreover, traced a gradual increase in the 

 relative size of the brain from the earliest Eocene to the present time. 

 The brain of the Coryplwdon, Lower Eocene, is not only extremely 

 small in }3roportioii to the size of the animal, but the higher portion of 

 the brain — the cerebral lobes — is very small in proportion to the cere- 

 bellum. The brain of the Middle Eocene Dinoceras is only about one 

 eighth the size of that of a living Ehinoceros of equal bulk. The brain 

 of the Miocene Brontothere is larger than that of the Eocene Dino- 

 ceras, but much smaller than that of the Pliocene Mastodon of nearly 

 the same size. Through the whole line of ancestry of the horse the 

 gradually-increasing size of the brain may be traced step by step. As 

 already seen, page 491, the same was true of early birds arid reptiles. 

 There has been a gradual increase in brain-power, and therefore in 

 nerve and muscular energy, in all classes. 



2. Genesis of Existing Orders. — We have seen that the main branches 

 of the Mammalian class if traced backward, approach one another very 

 closely in the Early Tertiary ; and if we could trace them still further 

 back, they would unite in the Cretaceous in a common stem or primal 

 mammal. This was doubtless a plantigrade, five-toed, bunodont, omniv- 

 orous animal. From this common stem the Carnivores and the Ungu- 

 lates — to take only the two most widely-contrasted types — diverged more 

 and more in all these characters — the one becoming more and more 

 adapted to flesh-eating, the other to herb-eating ; the one for seizing, 

 the other for escaping — until the present extreme types were attained. 



3. Genesis of Existing Families. — Xot only did these two main 

 branches separate more and more, but each of them branched again to 

 form existing families. To illustrate this we take the order of Ungu- 

 lates as the best known. 



Cuvier divided all Ungulates into two orders, viz., Pachyderms and 

 Ruminants. The Pachyderms are a heterogeneous order, but the Ku- 

 minants have been regarded as one of the most distinct of all mamma- 

 lian orders. Their horns in pairs, their hoofs in pairs, absence of upper 

 front-teeth, complex stomachs, and the habit of rumination, differenti- 

 ated them widely from all other animals. But Prof. Owen showed that 

 this distinction, so clear in zoology, was untenable in paleontology. 

 He found, in studying extinct Ungulates, that another distinction, viz., 

 foot-structure, was more fundamental and persistent. He therefore 

 divided all Ungulates into Perissodactyls (odd-toed) and Artiodadyls 

 (even-toed). A Perissodactyl may have five toes, as in the Corypho- 

 don and the Elephant ; or three toes, as in the Palseothere, the Ehi- 

 noceros, and the Tapir ; or one toe, as in the Horse. The Artiodactyls 

 always have their toes in pairs : there may be only two toes, as in 

 Anoplothere and in Ruminants ; or four, as in the Hog and the Hippo- 



