16 HOESE AND MAN. 



Now we can find a clue to the modification of the 

 foot, or hand, whichever we like to call it. The long 

 straight bone which reaches from the wrist to the 

 pastern (' cannon,' or ' shank bone,' as it is sometimes 

 called) is formed of the third and fourth metacarpals 

 fused together. But what has become of the rest of 

 the hand ? Where is the thumb ? What has been 

 done with the second (index) and little (fifth) 

 fingers ? We can answer these questions by geology 

 combined with comparative anatomy. 



In every instance where the phalanges become 

 rudimentary, the thumb is the first to be withdrawn. 

 In the dog and cat, the thumb is familiar to all as 

 the ' dew-claw.' Now, in the horse tribe the thumb 

 was withdrawn even before the time of the anchi- 

 therium, leaving only three toes (or fingers) capable 

 of resting on the ground — i.e. in the middle were the 

 third and fourth toes fused into one, and on either 

 side the index and little finger. 



In the hipparion these external toes are partially 

 rudimentary, so that though they still possess hoofs, 

 they do not rest on the ground, and are as useless for 

 walking purposes as those of the deer. In the true 

 horse, as we now know the animal, these external 

 toes have become perfectly rudimentary. There is 

 no external trace of them, but when the skin is re- 

 moved from the horse's shank two skewer-like bones 



