FAUNA OF THE TROPICAL FOREST 93 



luxuriant undergrowth, and by great density, we find 

 not a few arboreal forms which never voluntarily put 

 foot to the ground. This is true of the South American 

 sloths, whose feet are quite imfitted for terrestrial pro- 

 gression, also of some of the monkeys, of forms like the 

 flying lemur, and so forth. Where the forest is less 

 dense than usual, such purely arboreal forms must at 

 times find it difficult to pass from one tree to another, 

 and it is notable that very many different kinds of 

 arboreal animals have some form of parachute, giving 

 them partial powers of fiight, or at least of taking long 

 leaps. Thus we have flying frogs (Rhacophorus) ; a fly- 

 ing lizard {Draco volans) ; flying phalangers ; African 

 flying squirrels, differing in several respects from the 

 forms found in temperate climates; the so-called 

 flying lemur, &c. The true lemurs do not possess 

 parachutes, but some forms (e.g. Oalago) have the 

 ankle greatly elongated, which gives them a very frog- 

 like appearance, and enables them to leap from one 

 tree to another. 



As regards the structure of the feet, two conditions 

 occur in the more highly specialized forms. Some- 

 times, as in the sloths of South America, the fingers 

 and toes are converted into hooks, incapable of being 

 separated from one another, and having as their sole 

 function the suspending of the body from the branches 

 of the trees. This condition is suggested also in some 

 of the thumbless monkeys, but in general the monkeys 

 and their aUies, together with some other animals, 

 show a character which gives as great security of 

 grip, whUe conducing to greater freedom of movement 

 and greater agility. This is the modification of one 

 digit on both hands and feet, which is so inserted that 

 it can be opposed to each of the other digits, producing 



