346 POPULAR SCIENCE MONTHLY 



potamus possesses, as we should expect, four digits, arranged in pairs 

 of equal size. The wild swine, although attached to water and to boggy 

 ground, are nevertheless swift runners, and spend most of their time 

 on more solid footing. The outer digits are retained because they are 

 useful in keeping the foot from sinking in the mud, though they are 

 not functional a good share of the time. The greater strains brought 

 to bear upon the middle digits have resulted in their increased size 

 (by variation) and their monopoly of the carpal and tarsal joints, firm 

 articulation with which is no longer needed by the little-used outer 

 toes. That these changes were advantageous to the swine is shown 

 by the fact that related forms, less adaptive in this respect, have be- 

 come extinct. 



Of the deer family, the water chevrotain and its relatives are the 

 only forms possessing four complete digits. Again we have to do with 

 animals attached to swampy places, and the outer digits, though slender, 

 are retained intact because of the extra support they offer in traversing 

 boggy ground. The fusion of the metacarpal and metatarsal bones 

 of the middle digits, which characterizes the foot of other ruminants, 

 has evidently been prevented in the water deer by the spreading of 

 the toes. 



The red deer is one of the swiftest of runners and its usual habitat 

 is the wooded plain and upland. It, however, readily takes to the 

 water, as a large part of its food consists of aquatic plants. In roam- 

 ing the more solid floor of the forest only the middle digits support 

 the body. These have become relatively larger than those of swine, 

 and are further strengthened hy the union of the metacarpal and meta- 

 tarsal bones. The outer digits are perfectly useless in ordinary loco- 

 motion, but still perform two important functions: they serve to sup- 

 port the foot in yielding ground and give the deer a firm footing when 

 running rapidly, especially down-hill. Any one may observe that in 

 walking on fairly firm ground the foot of the deer leaves but two hoof- 

 prints, but that the foot of a running deer leaves four distinct marks. 

 The performance of these functions has caused the retention of the 

 lower portion of the outer toes. But as these digits no longer support 

 any part of the weight of the animal, no proximal articulation is 

 necessary and we find that the upper part of the metacarpal and meta- 

 tarsal bones has atrophied. 



Wild goats and sheep are mountain animals, feeding on rugged and 

 precipitous slopes where the footing is precarious. Both sheep and 

 goats are expert climbers and leapers, but in their ordinary habitat 

 only the middle digits are used for supporting the weight of the body. 

 The outer digits have, therefore, been reduced, but the hoofs and 

 rudiments of two phalanges have been retained, because these small 

 toes are used in climbing, and render the animals more sure-footed. 

 No such function is performed by the second and fifth digits of the 



