170 



Royal Society: — 



Diagram of a Typical Foot in Ungulata Paridigitata. 

 Manus. Pes. 



Beginning from this typical structure of the ruanus and pes, 

 which was probably exhibited by the progenitors of the Paridi- 

 gitata, we may follow its gradual reduction along both lines of 

 descent in the crescent-toothed (Selenodont) and tubercular-toothed 

 (Bunodont) Paridigitata. Both lines present a series of parallel 

 modifications, and the parallelism is often carried to the minutest 

 details. The only difference is, that along the crescent-toothed 

 line (Selenodonta) the reduction is proceeding at a much quicker 

 rate than along the tubercular-toothed (Bunodonta). The reason of 

 this may consist perhaps in the commencing faculty of rumination 

 in the former group, which faculty gave it an immense advantage 

 over the latter. For the comparative anatomist this slow rate of 

 change in the Suina is exceedingly welcome, as it brings the modi- 

 fication of the Suilline foot to our own time, and allows us to dis- 

 cover all the intermediate stages of modification, which, being 

 passed over very rapidly, and in ancient periods, by the crescent- 

 toothed group, have left none or but few traces of their existence. 



By the reduction of the foot in Paridigitata, I simply mean that 

 the function of locomotion which has been performed primitively 

 by all the four (or five) digits begins to be carried on chiefly by the 

 middle two, the lateral digits undergoing a gradual decrease. This, 

 as I have said before, seems to be of great advantage to the or- 

 ganism, and is manifested by all descending lines of Ungulata. 



In trying to ascertain the exact method of this reduction and its 

 final results in recent and fossil genera, we come to very interesting 

 facts that have not been duly noticed before, and which furnish us 

 with the explanation of the presence of so many very reduced forms 

 even in old Eocene and Miocene deposits. In both groups, the 

 crescent-toothed (Selenodonta) and the tubercular-toothed Paridi- 

 gitata (Bunodonta), we meet with a twofold mode of reduction of 

 the manus and pes — a simple or inadaptive, and an elaborate or 

 adaptive mode. 



Following the first or inadaptive mode of reduction, the foot, 

 whilst losing its lateral digits, acquires no better adaptation to 

 altered conditions of locomotion and support of the body than 

 that which is derived from the mere thickening of the remaining 



