蛇类为何进化出毒液，至今仍是一个谜。
How it came about that snakes manufactured poison is a mystery.
经过漫长的地质年代，蛇类的唾液——一种与人类唾液相似的温和消化液——演变成了一种至今连化学分析都无法解释的毒素。
Over the periods their saliva, a mild, digestive juice like our own, was converted into a poison that defies analysis even today.
蛇毒并非是生存竞争强加给它们的；即便不使用毒液，蛇类本来也可以捕捉猎物为生，正如当今数以千计的无毒蛇仍在做的那样。
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.
对蛇而言，毒液不过是一种奢侈品；它使蛇能够以极小的代价获取食物——所需的努力不过是咬一口而已。
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.
那么，为何偏偏只有蛇进化出了毒液？
And why only snakes?
比如猫，就会从中大为受益；不必再与大型凶猛的老鼠长时间搏斗，也不必与成年兔子扭打——只需咬一口，毫不费力。
Cats, for instance, would be greatly helped; no running fights with large, fierce rats or tussles with grown rabbits--just a bite and no more effort needed.
事实上，毒液对所有肉食动物都将是一种助力——尽管当肉食动物相互争斗时，它就会变成一把双刃剑。
In fact, it would be an assistance to all carnivores though it would be a two-edged weapon when they fought each other.
然而，在所有脊椎动物中，难以预测的大自然偏偏只选择了蛇（以及一种蜥蜴）。
But, of the vertebrates unpredictable Nature selected only snakes (and one lizard).
人们还不禁追问：大自然为何要让某些蛇制造出如此极端强烈的毒素？
One wonders also why Nature, with some snakes concocted poison of such extreme potency.
在唾液转化为毒液的过程中，人们或许会以为存在一个固定的演化程序。然而事实并非如此；某些蛇类制造出的毒素在各方面都与其他蛇类的毒素截然不同，其差异之大，如同砒霜与马钱子碱之间的差距，且两者产生的效果也完全不同。
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.
一种毒素作用于神经，另一种作用于血液。
One poison acts on the nerves, the other on the blood.
神经毒素的"制造者"包括曼巴蛇和眼镜蛇，它们的毒液被称为神经毒素（neurotoxic）。
The makers of the nerve poison include the mambas and the cobras and their venom is called neurotoxic.
蝰蛇（蝮蛇）和响尾蛇制造血液毒素，这种毒素被称为溶血毒素（haemolytic）。
Vipers (adders) and rattlesnakes manufacture the blood poison, which is known as haemolytic.
两种毒素都令人不快，但血液毒素（溶血毒素）要比神经毒素不愉快得多。
Both poisons are unpleasant, but by far the more unpleasant is the blood poison.
据说神经毒素是两者中更为原始的，而血液毒素——可以这么说——是从改进配方中产生的更新版产品。
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.
不管怎样，神经毒素对人类发挥作用的速度，远比血液毒素快得多。
Be that as it may, the nerve poison does its business with man far more quickly than the blood poison.
然而，这（神经毒素速效这一事实）并不意味着什么（即并不能说明神经毒素更危险）。
This, however, means nothing.
蛇类获得毒液，并不是为了对付人类，而是为了对付老鼠和田鼠等猎物；而蝰蛇毒液对这些猎物的作用几乎是立竿见影的。
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.