Fortunately, however, ancient men made tools of stone, especially flint,

然而， 幸运的是，远古人用石头制作了工具，特别是用燧石，

The bank must obey its customer's instructions, and not those of anyone else.

银行必须遵照储户的嘱托办事，不能听从其他人的指令。

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.

为什么只有蛇才有毒液呢？譬如说，如果猫有毒液，那对猫会大有帮助，它就不必再和又大又凶的老鼠边跑边博斗了，也不必再和大兔子扭斗了，只要咬一口，就不必再费大力气。

shape in only two dimensions.

形态只有两种尺度。

It is a world of red sandy deserts (hence its name----the Red Planet),

红色行星），

and see that nothing particularly alarming is happening to its companions.

并且看到自己的同伴身上没有发生让自己担惊受怕的事。