Limbs of Vertebrate Animals. 27 



distance above the level of the other fingers a part 

 which is clipped off, as being 'in the way' in sporting 

 dogs : that of the hyaena has the four fingers of the dog 

 without the rudimentary thumb, and thus the differences 

 met with in these several cases are brought about in 

 the same way, that is, by the disappearance, to a 

 greater or less degree, of the same part the thumb. 

 The thumb, too, is wanting in the fore-paw of the 

 Megatherium and of the Unau, or two-toed sloth, and 

 not this part only, but the 5th finger also, while in the 

 sloth the foot is still further simplified by the absence of 

 the 4th finger, so that the forefoot was three-fingered in 

 the extinct animal, and is two-fingered in its living 

 congener. The direct transition to the simple condition 

 met with in the parts corresponding to the hand or foot 

 in the horse, however, is not through the unguiculate 

 but through the ungulate members : and the instances 

 which have been cited are only brought forward for the 

 purpose of showing that the same rule is followed in 

 both cases. From the squat plantigrade foot of the 

 elephant with its five fingers, each encased in a separate 

 hoof, it is easy to pass to the semi-plantigrade foot of 

 the rhinoceros and hippopotamus, the one with four 

 fingers and the other with three, each one having its 

 own hoof ; and having arrived at this point there is no 

 difficulty in passing on, through the digitigrade forefoot 

 of the ox, with the tips of its two fingers capped in 

 horn so as to form the ' cloven-hoof/ to the digitigrade 

 forefoot of the horse with the end of its single fully- 

 developed finger enclosed in a single hoof. Moreover, 

 the passage from the four-fingered foot to the two, and 

 from this to the foot with one finger, is opened out 

 in other ways also. Thus : in the rein-deer the simple 

 cloven foot of the ox is made complex by the addition 



