1 72 Royal Society : — 



touching the os lnagnurri, 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 pat- 

 tern, but tend to gain a better support on all the bones of the carpus 

 and tarsus ; they deviate 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 E-uminantia, or of the 

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

 reduction in which such modification Tceeps pace with inheritance. 



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

 cotyles, Hi/omoschus, Euminantia. Every anatomist will acknow- 

 ledge 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 all extinct genera 

 of Paridigitata follow it, while all living * genera follow the second 

 or adaptive mode of reduction. 



Early Eocene Paridigitata. 



,0, 



Tubercular- toothed Crescentic-toothed 

 Paridigitata. Paridigitata. 



This being the state of the case, the questions arise, Did they 

 not become 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 circum- 

 stances ? I will try to consider both cases in reference to the living 

 and fossil Paridigitata. 



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

 Palteockoerus and the Miocene Rurninantia from Auvergne. 



