Lesson 20 Snake poison_亿城英语教学网

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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 rabbitsjust
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.

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