Some animals that cannot see can still tell the difference between light and darkness, while others can react to chemicals, which is something like smelling and tasting. However, there may be a sense that we know nothing about, for otherwise it is very hard to explain how birds, animals and even insects and fish manage to travel long distances without losing their way.

Only man can tell his companions exactly what he wants, for he has the wonderful gift of speech; but many of the larger animals can show that they are hungry or frightened or pleased by making sounds that we have learned to recognize, such as a roar. Even man still uses animal-like sounds when he laughs or sighs.

Some animals can get further than these simple noises. For instance, a bird song is much more elaborate, for it tells other males to keep away, while at the same time inviting a female to come closer. Birds have other calls which warn of danger or help to keep the flock (鳥群) together. Of course, a parrot does not understand what it is saying when it imitates the human voice.

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（責任編輯：vstara）