动物发出的声音并不是都能当作语言，因此我们只好求助于蝙蝠回声定位的这种非凡的发现，探究一下语音在何种情况下起着绝对有用的作用。

Not all sounds made by animals serve as language, and we have only to turn to that extraordinary discovery of echolocation in bats to see a case in which the voice plays a strictly utilitarian role.

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