274 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



of the lost lateral digits is constantly observed, and when these 

 disappear it is to be finally replaced by the rudiments of the ad- 

 joining toes in i^rocess of similar reduction. The bones of the sec- 

 ond row of the tarsus which are in connection with the toes are 

 not reduced so rapidly as the toes themselves; hence, the bones of 

 the toes, in order to maintain the fit of the i^arts, increase in 

 width, and consequently in strength. As is well known, in the 

 horse the single toe is as stout as several united toes of lower 

 forms, and the two toes of the ruminants have their basal seg- 

 ments (metatarsals) united into a stout solid mass, the cannon- 

 bone. At the same time several of the small bones of the sec- 

 ond tarsal row become coossified, so that we have, in the rumi- 

 nants and horse, the greatest consolidation of structure, connected 

 in the former with the most elegant mechanism. It is scarcely 

 necessary to add that, in the various cases of coossification and 

 consolidation described, the foetus displays the original elements 

 separated. 



In the fore limb the same successional reduction in the number 

 of toes may be traced as I have described in the hind foot ; but, 

 as the bones of the palm differ from those of the sole, the succes- 

 sional modification of these is also characteristic. The bones of 

 the second row of the carpus are four in number, but as the toes 

 are reduced, in the lines of the hoofed animals, the inner (tra- 

 pezium) is soon dropped, and the second (trapezoides) becomes 

 united with the third (magnum). In the carnivorous order, the 

 trapezoides is always separate, but the inner pair of bones of the 

 first row (scaphoid and the lunar) become consolidated into a single 

 mass, although their original distinctness is easily determined by 

 examination of the foetus. 



The two bones of the leg which articulate with the foot and 

 hand, exhibit a succession of changes of relation in progress to- 

 ward the more specialized types. In the Corypliodon and Umta- 

 therium of the Eocene, each of these bones has considerable share 

 in the articulation ; but as we rise in the series, the surface of at- 

 tachment of the lesser bones, the fibula in the foot, and the ulna 

 in the hand, becomes successively smaller, until in the ruminants 

 the fibula is almost obliterated, its distal end remaining as a small 

 tuberosity coossified to the side of the end of the tibia. In the 

 same manner the articular end of the ulna in the fore leg is suc- 

 cessively reduced, until this bone also becomes a thin strip coossi- 

 fied to the lower side of the radius, with no distinct termination. 



