172 



MEANING OF THE SYSTEM. 



especially interesting to trace the recent mammalian fauna back 

 through the pleistocene forms to the forms of the oldest tertiary 

 period. It is possible to trace the ancestry of a number of mam- 

 malian species. Riitimeyer was the first to undertake to trace out the 

 ancestral line of the Unyulata, and especially of the Rumina/ntia, 

 so as to obtain a palaeontological developmental history, and succeeded 

 in obtaining results, by means of detailed geological and anatomical 

 (deciduous teeth) comparison, which leave no room to doubt that 

 whole series of species of existing mammalia are collaterally or 

 directly related with each other and with fossil species. Riitimeyer's 

 investigations have received corroboration in their essential points 

 from the recent comprehensive works of W. Kowalevski, and have 

 resulted in the establishment of a natural classification of the ungulate 

 animals founded on phylogeny. 



n 



FIG. 117. Bones of the feet of the different genera of the Equida (after Marsh). , Foot of 

 Orohipptis (Eocene), b, Foot of AnchUlic-rlum (Lower Miocene), c, Foot of Hipparion 

 (Pleiocene). (', Foot of the recent genus Equits. 



In addition to these works we have the recent researches of 

 Marsh, who has completed to an extraordinary degree our knowledge 

 of the genealogy of the genus Eqw.s, by numerous discoveries 

 (fig. 117) in America (Wyoming, Green River, White River]. The 

 eocene OroMppus, in which the small posterior toes were present as 

 well as the three principal toes which rested on the ground, was 

 succeeded in the Lower Miocene formation by AncMtherium with 

 three hoofs ; and the latter was followed by the Hipparion of the 

 Pleiocene formations ; and this is the ancestral form of the existing 

 genus Equus. 



The origin of most orders of Mammalia, such as Rodentia, Cheirop- 

 tera, Probosddea, Cetacea, etc., cannot be clearly traced out, but 

 for certain orders, as the Prosimice, Carnivora, Unrjulata, and Ro- 



