新概念英语85年上外美音版第四册 第25课：The
Snake
Lesson 25 The Snake蛇
How it came about that snakes manufactured poison is a mystery. Over the periods their saliva, a mild, digestive juice like our own, was converted into a poison that defies analysis even today. It was not forced upon them by the survival competition; they could have caught and lived on prey without using poison just as the thousands of non-poisonous snakes still do. Poison to a snake is merely a
luxury; it enables it to get its food with very little effort, no more effort than one bite. And why only snakes ? Cats, for instance, would be greatly helped; no running rights with large, fierce rats or tussles with grown rabbits just a bite and no more effort needed. In fact it would be an assistance to all the carnivorae--though it would be a two-edged weapon -When they fought each other. But, of the vertebrates,
unpredictable Nature selected only snakes (and one lizard). One wonders also why Nature, with some snakes concocted poison of such extreme potency.
In the conversion of saliva into poison one might suppose that a fixed process took place. It did not; some snakes
manufactured a poison different in every respect from that of others, as different as arsenic is from strychnine, and
having different effects. One poison acts on the nerves, the other on the blood.
The makers of the nerve poison include the mambas and the cobras and their venom is called neurotoxic. Vipers (adders) and rattlesnakes manufacture the blood poison, which is known as haemolytic. Both poisons are unpleasant, but by far the more unpleasant is the blood poison. It is said that the
nerve poison is the more primitive of the two, that the blood poison is , so to speak, a newer product from an improved formula. Be that as it may, the nerve poison does its
business with man far more quickly than the blood poison. This,however,means nothing. Snakes did not acquire their poison for use against man but for use against prey such as rats and mice, and the effects on these of viperine poison is almost immediate.
新概念英语85年上外美音版第四册 第25课：The Snake
