no tails; monkeys have tails, which are usually long; while baboons have
short tails, and their faces, instead of being round and with a man-like
expression as in apes and monkeys, are long and more dog-like. These
differences are, however, by no means constant, and it is often
difficult to tell whether an animal should be classed as an ape, a
monkey, or a baboon. The Gibraltar ape, for example, though it has no
tail, is really a monkey, because it has callosities, or hard pads of
bare skin on which it sits, and cheek pouches in which it can stow away
food; the latter character being always absent in the true apes, while
both are present in most monkeys and baboons. All these animals,
however, from the largest ape to the smallest monkey, have the same
number of teeth as we have, and they are arranged in a similar manner,
although the tusks or canine teeth of the males are often large, like
those of a dog.

The American monkeys, on the other hand, with the exception of the
marmosets, have four additional grinding teeth (one in each jaw on
either side), and none of them have callosities, or cheek pouches. They
never have prominent snouts like the baboons; their nostrils are placed
wide apart and open sideways on the face; the tail, though sometimes
short, is never quite absent; and the thumb bends the same way as the
fingers, is generally very short and weak, and is often quite wanting.
We thus see that these American monkeys differ in a great number of
characters from those of the Eastern hemisphere; and they have this
further peculiarity, that many of them have prehensile or grasping
tails, which are never found in the monkeys of any other country.
This curious organ serves the purpose of a fifth hand. It has so much
muscular power that the animal can hang by it easily with the tip curled
round a branch, while it can also be used to pick up small objects with
almost as much ease and exactness as an elephant's trunk. In those