THE FATE OF THE HIND-LIMBS 51 



climbing hand over hand and testing every new hold 

 that it obtains before finally trusting its weight to it. 

 When it has reached the limit of its ascent, it commonly 

 turns round, and, hanging by its feet, eats its meal or 

 performs its toilet head downwards. It is from such a 

 position that it descends, and its descent is carried out 

 in exactly the same manner as its ascent, but naturally 

 in a reversed position it crawls and climbs down head 

 forwards. All this is very easily watched in Nycticebus, 

 because its actions are so deliberately leisured and orderly, 

 but the same head forward descent is typical of all the 

 Lemurs I have had the opportunity of watching. 



The New-World monkeys do the same thing, but in 

 them the use of the prehensile tails of some species rather 

 complicates the process of climbing down head foremost. 

 Now, it is an observation easily made wherever a higher 

 Old-World monkey is to be seen, that although it climbs 

 up a tree, it walks down again hind end foremost. Most 

 monkeys come down a tree just as a man does, bearing 

 the weight of the body by the suspending hand grasp 

 and by the supporting foothold. As a man descends 

 a ladder, so a higher monkey descends a tree. We may 

 sum up this process by saying that the lower Primates 

 climp up trees and climb down again, but the higher 

 Primates climb up and then walk down. 



Now the difference shown in these two simple cases is 

 in reality a very great one. The arboreal habit conferred 

 its benefits by emancipating the fore-limb from the duties 

 of support and progression, and, by differentiating its 

 functions from that of the hind-limb, it saved the animal 

 from becoming quadrupedal. In differentiating the 

 functions of the two sets of limbs, the animal gains a 

 great deal. Some animals, one might almost say, have 

 gone too far in adapting themselves to the arboreal habit. 

 An animal, saved by the arboreal habit from becoming 

 quadrupedal, does not gain the maximum of the benefits 

 derivable from its new mode of life, if it is saved from 



