70 



from near the base of this branch, but have diverged greatlj^ 

 Like the Sirenia (with which they have no direct or close 

 relationship) they are modified to suit an aquatic existence, 

 and in some respects — especially in the condition of the 

 limbs and the almost total absence of hair— they exhibit 

 considerable degeneration. 



The primitive Ungulata were continued upwards into the 

 ancestors of the Artiodactyla and Perissodactyla, the two 

 groups into which the existing Ungulata fall, and of 

 numerous extinct groups which unite the Artiodactyla and 

 Perissodactyla, or diverge from them to varying extents. The 

 ancestral Rodentia, Hyracoidea and Proboscidea, and many 

 extinct groups known by their fossil remains, arose from the 

 series of primitive Ungulates.* 



The Rodentia are the most important of these divergent 

 branches. They have undergone very considerable evolution 

 since their origin, and are now a well-defined group of large 

 size, and containing a number of distinct families. But a 

 passage can be traced through the extinct forms Mesotherium 

 and Toxodon from undoubted Rodents to typical Ungulates. 

 Similarly the Proboscidea, although now a very isolated 

 group, may be joined to the Ungulata by the extinct 

 Proboscidea, the Dinotheria, and the Dinocerata. The 

 primitive Hyracoidea probably arose from the Ungulate stem, 

 close to the point of origin of the Proboscidea, but diverged 

 in a different direction, and the group as it is now known 

 (Hyrax) occupies a very isolated position. Thus from the 

 primitive Ungulates, which arose from the generalised 

 Eutherian, ancestral lines of descent diverged towards such 

 very different groups as the Cetacea, the Hyracoidea, the 

 various series of living and fossil Ungulata, the Proboscidea 

 and the Rodentia ; but the relations of these different 



* See Flower, article " Mammalia" in Encijcl. Brit., 9th edition, v. xv, 

 p. 372. 



