522 EVOLUTION OF DEER. 



The Pachyderms form a large group, in which extinct fossil 

 genera have marked resemblances to living species. Thus the Rhino- 

 ceros was preceded by Acerotherium, Palceotherium, and Paloplo- 

 therium, arid differences are found in the form of the skull and the 

 dentition. Acerotherium is essentially a Rhinoceros without horns, 

 and consequently with smaller nasal bones. Fossil species of Rhino- 

 ceros show all intermediate conditions of this region of the skull. 

 The American species of Acerotherium have three pairs of incisor 

 teeth, as in Palceotherium ; and this number is preserved in the 

 extinct Rhinoceros sivalemis of India, though in other species the 

 incisors become lost. Tapirs are traced back by the American genus 

 Hyrachyus to Lophiodon in the Eocene. Pigs of the genus Sits 

 appear in the Middle Miocene. Sus Lockarti is nearly allied to Hyo- 

 therium, and this genus is closely allied to Palceochcerus, which 

 resembles the peccary of South America. Palceochwrus passes to 

 Choeropotamus, and this genus is closely related to Dichobune of the 

 Headon beds. Pachyderms are the prevalent type in the older 

 Tertiaries of Europe. 



The oldest known Ruminants are Xiphodon, Dichodon, and Am- 

 phim.eryx. Most of the older American Ruminants possess, like 

 Xiphodon, some characters of Pachyderms. This is exemplified by 

 the French genera Gelocus and Dremotherium. All the chief ruminant 

 types are found in the Upper Miocene. The older genera, such as 

 Oreodon, are allied to Anoplotherium, and without horns. The first 

 antelopes have the horns very small ; and horns appear to have de- 

 veloped gradually, becoming relatively large in Antelope recticornis 

 of the Lower Pliocene. The antlers of Ruminants also developed 

 gradually in geological time, just as they grow in complexity during 

 life in existing deer. In the Middle Miocene the genus Dicroceras 

 has antlers with two prongs. In the Upper Miocene deer first appear 

 in which the antler has three prongs ; and it is only in Pliocene and 

 Post-Tertiary deposits that antlers attain their full complexity, though 

 since the extinction of types like Cervus Sedgwicld of the Forest Bed, 

 and the Cervus megaceros, no greater development of antler has 

 occurred. Since the older fossil antlers are always attached to the 

 skull, they have been regarded as permanent appendages in those 

 types. When they began to become deciduous, at first only the upper 

 portion of the antler was shed, long pedestals remaining attached to 

 the frontal bones. The portion shed gradually increased in length. 



The earlier Ruminants, Oreodon, Dichodon, and Xiphodon, have 

 canines and incisors in the upper jaw like Pachyderms. It is, how- 

 ever, impossible to discover from study of the teeth what the Pachy- 

 derm type was from which the Ruminants were evolved. In the 

 digits of the limbs existing genera show types in which a transition 

 may be traced from those with four metapodial bones to those in 

 which there is but one ; and similarly, by tracing the ancestral forms 

 of specialised living genera, a like series of modifications is met with, 

 until in the older genera the metapodial bones are all separated. The 

 steps by which this evolution was brought about are, according to 



