Not all sounds made by animals serve as language, and we have only to turn to that extraordinary discovery of echo-location in bats to see a case in which the voice plays a strictly utilitarian role.
To get a full appreciation of what this means, we must turn first to some recent human inventions. Every one knows that if he shouts in the vicinity of a wall or a moutain side, an echo will come back. The further off this solid obstruction, the longer time will epalse for the return of the echo. A sound made by tapping on the hull of a ship will be reflected from the sea bottom, and by measuring the time inteval between the taps and the receipt of the echoes, the depth of the sea at that point can be calculated. So was born the echo-sounding apparatus, now in general use in ships. Every solid object will refect a sound, varying according to the size and nature of the object. A shoal of fish will do this. So it is a comparatively simple step form locating the sea bottom to locating a shoal of fish. With experience, and with improved apparatus, it is now possible not only to locate a shoal but to tell if is is herring, cod or other wellknown fish, by the pattern of its echo.
A few years ago, it was found that certain bats emit sqeaks and by receiving the echoes they could locate and steer clear of obstacles, or locating flying insects on which they feed on. This echo-location in bats is often compared with radar, the principle of which is similar.
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