524 EVOLUTION OF THE CAMEL. 



is distinguished chiefly by the teeth from the true horses which occur 

 fossil in the most recent beds. 1 This series renders it likely that 

 closely allied genera may descend by parallel lines from a distant 

 parent stock. 



What Professors Huxley and Marsh have done for the evolution 

 of the Perissodactyla by tracing the history of the horse, Professor 

 Cope has attempted for the Artiodactyla in the history of camels. 

 The oldest American representative of the group is Poebrotherium 

 of the Lower Miocene. This genus has four incisor teeth, and 

 has the metapodial bones distinct throughout life. In Protolabis 

 the upper incisor teeth also remain well marked. This genus is 

 associated in the Upper Miocene with Procamelus. in which there is 

 first a marked retardation in development of the first incisor which 

 leads to its atrophy ; and secondly, an acceleration of ossification, by 

 which a canon bone is formed by the blending of the metapodial bones, 

 though they long remain separate. A further step in the change is 

 seen in Pliauchenia, in which the number of premolar teeth is reduced 

 from four to three, and the first and second incisors are lost. In 

 Camelus the premolars are further reduced to two, and in Auclienia to 

 one , though even this type shows its affinity with Poebrotherium by 

 retaining a separate condition of the elements of the canon bone during 

 a portion of foetal life. Procamelus was about the size of a llama, 

 with the head and muzzle more elongated, limbs more slender, and 

 neck shorter. 2 The giraffes have similarly been traced by Lydekker 

 back to the Sivatheres, in which a shortening of the neck is associated 

 with the development of palmate horns. 



Thus both Artiodactyla and Perissodactyla are traced back to 

 simpler types of ungulata, and Gaudry urges that they result from 

 the necessity that the foot, when applied to the ground, should sup- 

 port the pressure of the body evenly. In the Perissodactyla the 

 fibula rests directly against the tibia, instead of articulating with the 

 calcaneum, as in the Artiodactyla. This arises from the external digit 

 becoming rather more slender than the internal digit, and leads to the 

 articulation of the heel-bone or calcaneum with the cuboid-bone 

 becoming smaller, and this causes an attenuation of the fourth digit ; 

 so that the whole weight of the animal is thrown on the astragalus, 

 which is carried by the naviculare, which is itself carried by the third 

 cuneiform bone, which supports the third or middle digit of the foot. 

 Hence it would follow that the astragalus of the odd-hoofed mammals, 

 which are flattened in foot, is modified from the type of the even-hoofed 

 mammals, and this conclusion is supported by the other tarsal bones. 

 Therefore, though the modern classification into Artiodactyla and 

 Perissodactyla is valuable as an evidence of evolution, the classifica- 

 tion becomes less marked as ungulates are traced back in geological 

 time. 



North and South America have yielded several extinct orders of 

 Mammals defined by Owen, Marsh, and Cope. 



1 Mareh, Amer. Jour. Sci., vol. xvii. p. 497. 



8 Rep. U. S. Geog. Surveys West of looth Merid., part ii. vol. iv., 1877. 



