To begin at the beginning, it is certain that monkeys have a first-class
sensory equipment, especially as regards sight, hearing, and touch. The
axes of the two eyes are directed forwards as in man, and a large
section of the field of vision is common to both eyes. In other words,
monkeys have a more complete stereoscopic vision than the rest of the
mammals enjoy. They look more and smell less. They can distinguish
different colours, apart from different degrees of brightness in the
coloured objects. They are quick to discriminate differences in the
shapes of things, e.g. boxes similar in size but different in shape, for
if the prize is always put in a box of the same shape they soon learn
(by association) to select the profitable one. They learn to
discriminate cards with short words or with signs printed on them,
coming down when the "Yes" card is shown, remaining on their perch when
the card says "No." Bred to a forest life where alertness is a
life-or-death quality, they are quick to respond to a sudden movement or
to pick out some new feature in their surroundings. And what is true of
vision holds also for hearing.