THE TROPICAL FOREST 95 



of the limbs is necessary, we note that these are not 

 tied, down to the body by skin, as in most quadrupeds. 

 A comparison of such animals as dog or cat with 

 monkey or man will make this point clear. Again, as 

 the arms are of very great importance in arboreal 

 animals, we find that they are proportionately very 

 long. In the sloth, in the monkeys and lemurs, this is 

 very noticeable. With the loss of other arboreal charac- 

 ters man has also lost his long arms, these being pro- 

 portionately very much shorter than in the apes. 



Another common feature of arboreal mammals is 

 the presence of collar-bones, which tend to be lost in 

 quadrupeds. This gives such mammals a charac- 

 teristically broad chest, easily seen on comparing the 

 position of the fore-limbs in, e. g. a sheep, and in a 

 monkey. It is the presence of the clavicle which per- 

 mits of the upward movement of the arm, so necessary 

 in climbing, a movement impossible to pure quadru- 

 peds like antelopes and deer. Again, in quadrupeds, 

 and notably in the swift ungulates, the two bones of 

 the forearm and the corresponding bones of the fore- 

 leg tend to fuse, in order to give a rigid support to the 

 body ; but in most arboreal animals they are separate, 

 giving much greater freedom of movement to wrist 

 and ankle, but preventing the animals from acquiring, 

 any great speed on the groimd, even when their fingers 

 and toes make it possible for them to attempt to run. 



Finally, arboreal forms have often only one or two 

 young at a birth, and the mammae are frequently 

 pectoral in position, for only thus can the young be 

 carried without loss of balance by the mother. 



To this account of the chief peculiarities of arboreal 

 animals we may add a few words upon the tropical 

 forest considered as a source of food. In the tropical 



