PEDIGREE OF MAMMALS. 



275 



of the aboriginal genus is repeated, and Phylogenesis is U. 

 unequivocally expressed in Ontogenesis. The Anchi- ' 

 therium is a three-toed horse, in which, however, the 

 middle toe has already undertaken the chief task. But 

 in the Hipparion the two side toes are entirely raised 

 from the ground, and by disuse are brought to the con- 

 dition of arrest which is completed in the horse. 



In the constitution of the molar teeth the tapirs have 

 remained most faithful to the ancestral type. The cir- 

 cumstance that the tapir has four toes in front, whereas 

 the Palseotheridae known to us, have three shows, how- 

 ever, that the genus Palaeotherium cannot have been 

 the ancestral stock of the tapirs. P'or the supposition 

 that the tapir acquired the fourth toe is contrary to all 

 experience respecting the formation of the extremities. 

 Rhinoceroses are also four-toed in front, and their close 

 kindred with the tapirs is testified by the structure of 

 their toes and a series of details in the skeleton. 



Hippopotami, Pigs. Tragulidse. Deer. Antelopes. Oxen, 



Anoplotheridse. 



An isolated branch of the Palaeotheridae seems to be 

 the fossil genus Macrauchenidae, which combines the 



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