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 ground, 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 
