LIFE IN TERTIARY TIMES 297 



a form probably originating in the Condylarthra, related to 

 Pantolesies, and which still possesses, besides a complete 

 dentition, five digits on the fore feet and four on the hind feet ; 

 the pollex is already very small and the four persisting digits 

 are all equal in size. Oreodon was closely related to the 

 ancestors of the Camel, whose first representatives are 

 Leptotragidus of the Eocene of North America, which still 

 had four digits on its fore limbs, and lateral meta- 

 tarsals without toes on the hind. The Pabrotherium of 

 the Oligocene of America had only two toes and two 

 rudimentary metacarpals in the fore limbs. The meta- 

 carpals and the metatarsals are united into a single 

 cannon-hone in Protolabis and Procamelus of the Miocene. 

 This same union has come about in another and quite 

 independent series of Ruminants. In these initial forms there 

 are four complete digits on each foot, only two of which touch 

 the ground ; in Dorcatherium and Hypertragidus, both of which 

 are Miocene, there is no union between the metacarpals. 

 Union is present, however, in Gelocus, also Miocene, so far as 

 the metatarsals are concerned, and also in Hycemoschus, still 

 living in Western Africa. There is union in both in Tragidus 

 of the Pliocene. In all other Ruminants the metacarpals 

 and metatarsals are respectively united to form a cannon-bone. 

 In Cervidae and Bovidae there are still two lateral toes, but 

 the metacarpals and metatarsals are more or less incomplete 

 and reduced often to a simple splint. In the Ovidae there is 

 no longer any trace of lateral toes, and they are absent also in 

 Giraffes, Sivatherium, Samotherium of the Miocene in Samos, 

 Helladotherium and the Okapis, although these forms are 

 less highly evolved than the Cervidae so far as the horns are 

 concerned. Here the feet are completely consolidated and no 

 longer comprise any useless parts. If the animal had not in 

 the beginning immobilized its metacarpals and metatarsals — 

 or practically done so — by a deliberate act which became 

 habitual, their union, itself a proof of such immobilization, 

 could not have taken place. The part played by the animal 

 in the modification of its own organism is thus clearly 

 apparent. 



The Perissodactyla present digital reductions parallel to 

 those of the Artiodactyla. They had a common ancestor in 

 the five-toed Phcnacodus, with seriated carpals and tarsals. 



