FAUNA OF THE TROPICAL FOREST = 938 
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 unfitted 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 flight, 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. Galago) 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 allies, together with some other animals, 
show a character which gives as great security of 
grip, while 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 
