356 THE BOOK OF THE ANIMAL KINGDOM 



The interpretation of these facts is not difficult. The Eohippus 

 is the descendant of a five-toed ancestor, the progeny of which had 

 no use for all its toes. That ancestor may have been the five-toed 

 Eocene Phenacodus, which some claim to have been the original 

 parent of all hoofed animals : but the claim is disputed. According 

 to the laws of natural development, the used limb becomes stronger, 

 the disused ones atrophy. So in the history of the Horse. Out of 

 the five toes of the Horse's original ancestor, the Eohippus had lost 

 two from its hind feet, and one was disappearing from the front 

 feet. The middle toe of each foot was being used most extensively, 

 and becoming stronger and bigger. This does not mean that these 

 missing toes suddenly disappeared : the atrophy was a very gradual 

 process, and must have been due to the operation of natural selec- 

 tion through very many generations. The five-toed originals must 

 have been handicapped in the struggle for existence; the progeny 

 that used fewer toes became swifter and were favoured in that 

 respect. The slower ancestors perished, while the swifter progeny 

 bred and produced young with tendency to use fewer toes. The 

 toes that were used remained in the Eohippus, those that had not 

 been used for many generations had disappeared. Some day 

 remains may be found of intermediate developments of this gradual 

 process. The Orohippus represents another stage in the evolution 

 of the Horse the entire loss of the fifth digit and an increase in 

 size. The Mesohippus exhibits a stronger growth of the central 

 toe due to much usage; it had lost completely two toes from each 

 limb, and other two were showing signs of disuse, and the promise 

 of decay. In the larger Protohippus the process was greatly 

 accentuated; the central toe was still increasing in power and use- 

 fulness; the two smaller digits no longer touched the ground and 

 had fallen into desuetude. That same desuetude doomed them to 

 atrophy, the promise of which is fulfilled in our noble, swift, useful 

 and intelligent one-toed Horse. 



This developmental story is rendered clearer by knowledge of 

 some elementary facts concerning the Horse. The hoof of this 

 animal is really a big nail of a highly developed middle digit. 

 Quadrupeds usually have two distinct bones, known as the ulna and 

 radius, in their fore limbs. An examination of the fore limb of a 

 Horse would seem, at first glance, to show but one bone where two 

 are naturally expected. A closer scrutiny, however, discloses the 

 fact that two bones have been blended into one. The knee of the 



