KINETOGENESIS. 355 



rium (Fig. 1 1 1 . ) A similar successive reduction is to be 

 seen in the lines of the Perrisodactyla, as we pass from 

 the smaller and lighter to the heavier and more bulky 

 types. Such series are those which commence in the 

 Lophiodontida?, and terminate in the Menodontidae on 

 the one hand, and the rhinoceroses on the other. The 

 elephants display the end of a similar line, which com- 

 mences in the Condylarthra. In all of these bulky 

 mammals the weight in progression is borne on the 

 extremities of the metapodial bones, and the phalanges 

 take but little share in it. They are turned forwards 

 and are nearly useless. Their great reduction in di- 

 mensions in these forms appears to me to have fol- 

 lowed disuse, and this is then the probable cause of it. 



c. Atrophy of the Ulna and Fibula. 



Successive atrophy of the ulna and fibula has been 

 already referred to (p. 135). This is coextensive with 

 reduction of the number of the digits in the ungulate 

 Mammalia, and with the development of the digital 

 patagium in the bats. This is in broad contrast to the 

 subequal development of the ulna and radius in the 

 Cetacea, where the fore limb functions as the blade of 

 an oar. The cause of the reduction of the two ele- 

 ments in the Ungulata is the restriction of the func- 

 tions of the fore and hind limb to the radius and tibia 

 respectively. The distal extremities of the ulna and 

 fibula are supported by the external bones of the carpal 

 and tarsal series respectively. The reduction of the 

 external digit deprives the external bones in question 

 of their share in the support of the general weight, and 

 consequently relieves them of the impact which passes 

 through the longer median digits which remain. The 

 median digits, on the other hand, support the radius 



