1873.] 



Osteology of the Hyopotamidse. 



155 



this mode of reduction. The fourth digit does not even take the whole of 

 the unciform, and a part of this bone is still occupied by the useless 

 rudiment of the fifth digit ; the third has not extended over the whole 

 os magnum ; and the useless rudiment of the second digit occupies 

 its typical place on the trapezoid, touching the os magnum, and being 

 additionally supported by the trapezium. 



Following the second or adaptive mode of reduction, the middle digits 

 grow larger and thicker than in the first mode ; but whilst broadening 

 transversally they do not adhere to the ancestral pattern, but tend to 

 gain a better support on all the bones of the carpus and tarsus ; they de- 

 viate from the ancestral type, push the lateral digits (while these are yet 

 completely developed) to the side, and usurp their typical carpal and tarsal 

 bones for their (the middle digits) own use, thus gaining a better and 

 more complete support for the body. The lateral digits, deprived of 

 their typical carpal and tarsal bones, and taking henceforth no active part 

 in locomotion, tend to disappear ; and every millimetre that is lost by the 

 lateral digits is immediately taken possession of by the enlarged middle 

 ones ; so that even before the entire disappearance of the lateral digits the 

 two middle digits have usurped the whole of the distal surface of the carpus 

 and tarsus, the fourth digit has spread over the whole unciform (manus) 

 and cuboid (pes), and the third has taken possession of the trapezoid 

 (manus) and second cuneiform (pes). This once attained, the two middle 

 digits, being pressed from both sides by the carpal and tarsal bones, 

 begin to coalesce, forming the so-called cannon of the recent Euminantia, 

 or of the hind foot of Dicotyles. This mode of reduction I call the adaptive, 

 or reduction in which such modification keeps pace with inheritance. 



As an instance of this mode, I may cite the foot of Sus, Dicotyles, 

 Hyomoschus, Euminantia. Every anatomist will acknowledge that this 

 second mode of reduction is much more useful to the organism than the 

 first. 



If we inquire further what are the genera which follow the first or 

 inadaptive mode of reduction, we find that cdl extinct genera of Paridigitata 

 follow it, while all living * genera follow the second or adaptive mode of 

 reduction. 



This being the state of the case, the questions arise, Did they not be- 

 come extinct because of their incapacity to adapt themselves completely 

 to altered circumstances ? and did not the others survive because they 

 adapted themselves more fully to these circumstances ? I will try to 

 consider both cases in reference to the living and fossil Paridigitata. 



I said before that early in the Eocene period the group of Paridigitata 

 split dichotomously into two secondary groups, one with crescentic teeth, 

 the other with tubercular ; the first I have called the Selenodonta, the 

 second Bunodonta (or Suina). Now each of these secondary groups fol- 



* Or fossil forms which continue to live, or have left direct successors, as PalceochcBrm 

 and the Miocene Euminantia from Auvergne. ..... ,. 



