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 fights 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 carnivores 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 saliva into why Nature， with respect from that of others， as other on the blood.

    In the conversion of saliva into poison， one might suppose that a fixed process took place. It did not； some snakes manufacture 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.

    saliva n.  唾液

    digestive adj. 助消化的

    defy v.  使不可能

    analysis n.  分析

    prey n.  被捕食的動物

    fierce adj. 兇猛的

    tussle n.  扭打

    carnivore n.  食肉動物

    vertebrate n.  脊椎動物

    lizard n.  蜥蜴

    concoct v.  調(diào)制

    potency n.  效力

    conversion n.  轉(zhuǎn)變

    arsenic n.  砒霜

    strychnine n.  馬錢子堿

    mamba n.  樹眼鏡蛇

    cobra n.  眼鏡蛇

    venom n.  毒液

    neurotoxic adj. 毒害神經(jīng)的

    viper n.  蝰蛇

    adder n.  蝮蛇

    rattlesnake n.  響尾蛇

    haemolytic adj. 溶血性的

    viperine adj. 毒蛇

    蛇是怎樣產(chǎn)生毒液的，這是一個謎。蛇的唾液本來和我們?nèi)说南�化液一樣柔和，但�?jīng)過漫長的時間，演變成了今天仍無法分析清楚的毒液。毒液不是生存競爭強加給它們的，它們也可以不用毒液捕捉動物而生存，就像今天成千上萬的無毒蛇那樣。毒液對毒蛇來說只不過是一種舒適生存的優(yōu)越手段，它使蛇不用費多大力氣就能捕獲到食物，輕咬一口即可。為什么只有蛇才有毒液呢？譬如說，如果貓有毒液，那對貓會大有幫助，它就不必再和又大又兇的老鼠邊跑邊博斗了，也不必再和大兔子扭斗了，只要咬一口，就不必再費大力氣。因此，任何食肉動物有了毒液，都能從中獲益。不過，當(dāng)它們相互撕打時，毒液就成了利弊參半的武，可以殺死對方，也可以被對方的毒液殺死。然而，在脊椎動物中，大自然神秘模測地只選擇了蛇（還有一種蜥蜴），人們弄不清楚大自然為什么在某些蛇的身上調(diào)制出如此高效的毒液來。

    人們可能認為，唾液轉(zhuǎn)變成毒液，其中有固定的程序。其實沒有。有些蛇產(chǎn)生的毒液也在各方面與另外一些毒蛇產(chǎn)生的毒液不同，就像砒霜不同于馬錢子堿一樣。不同毒蛇產(chǎn)生的毒液產(chǎn)生的效果不同，一種毒液作用于神經(jīng)，另一種毒液作用于血液。

    產(chǎn)生神經(jīng)毒液的蛇有一種非洲樹眼鏡蛇和眼鏡蛇，它們的毒液稱為神經(jīng)毒素。蝰蛇（蝮蛇）和響尾蛇產(chǎn)生血液毒素，稱為溶血性毒液。這兩種毒液都很可怕，但溶血性毒液尤其厲害。據(jù)說，神經(jīng)毒液在兩種毒液中是較為原始的一種，而溶血性毒液，打個比方說，是根據(jù)改良配方生產(chǎn)的一種較新的產(chǎn)品。不過，神經(jīng)毒辣液比溶血性毒液在人身上起作用快得多。但是，這沒有什么關(guān)系，因為蛇有毒液不是用來對付人的，而是對付它的獵物，諸如鼠類，毒液對這些獵物會立刻起作用。