How it came about that snakes manufactured（制造） poison is a mystery. Over the periods their saliva, a mild digestive（消化的） juice like our own, was converted（使转变） into a poison that defies（ 违抗） analysis even today. It was not forced upon them by the survival（ 幸存） competition. They could have caught and lived on prey（捕食） without using poison just as the thousands of non-poisonous snakes still do. Poison to a snake is merely（仅仅） a luxury. It enables（使能够） it to get its food with very little effort, no more effort than one bite.

  And why only snakes? Cats, for instance（实例）, would be greatly（大大地） helped. No running fights with large fierce（凶猛的） rats（老鼠） or tussles with grown rabbits, just a bite, and no more effort needed.