每天坚持背诵两篇
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.Everyone knows that if he shouts in the vicinity of a wall or a mountainside,an echo will come back.The further off the solid obstruction,the longer time will elapse 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 interval（intervals） between the taps and the receipt of echoes,the depth of the sea at that point will be calculated.So was born the echo-sounding apparatus,now in general use in ships.Every solid object will reflect a sound ,varying according to the size and the nature of the object.A shoal of fish will do this.So it is a comparatively simple step from 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 it is herring ,cod or other well-known fish,by the pattern of（its） echoes.
It has been found that certain bats emit squeaks and by receiving its echoes,（they can）locate and steer clear of the obstacles --and（or locate） flying insects on which they feed.This echo-location in bats is often compared with radar,the principle of which is similar.
