Man is the only animal that laughs．What is the history of this 'happy laughter', as someone once【67】it?
We are not【68】of theories to explain the mystery; researchers have been seeking a【69】explanation of laughter．One writer put【70】the theory that its function is to frighten others or to humiliate them．Another took the opposite【71】that we laugh in order not to cry．A psychologist offered the explanation that laughter【72】as a remedy to painful experiences and it serves to protect a person from【73】the psychologist called 'the many minor pains to which man is exposed'．In the seenth century a writer set forth the theory that we laugh when we【74】ourselves with others and find ourselves【75】to others; in effect, we laugh at the【76】of others.
Almost every theory has been【77】with either the structure or the function of laughter,【78】relatively few have been devoted【79】the question of its origin．I propose to offer a theory which has not previously been set forth,【80】only those animals capable of speech are capable of laughter; and therefore man, being the only animal that【81】, is the only animal that can laugh.
Those who have【82】chimpanzees(黑猩猩) closely feel confident that chimpanzees occasionally【83】behavior．that looks like a primitive human laughter．This【84】, however, has been observed only in a humanlike way; whether or not it is laughter is doubtful; the【85】fact that under any condition an ape is capable of【86】behavior．is no more than passing interest—it only indicates that early man might have the basic laughter.
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