It is impossible to make more than the wildest guess at how many they kill, but they are hungry creatures, not content with only three meals a day.

它们一年中消灭了多少昆虫，我们简直无法猜测，它们是吃不饱的动物，不满意一日三餐。

It overran a whole continent.

整个澳洲兔子成灾。

Over the periods their saliva, a mild, digestive juice like our own, was converted into a poison that defies analysis even today.

蛇的唾液本来和我们人的消化液一样柔和，但经过漫长的时间，演变成了今天仍无法分析清楚的毒液。

it is very rarely that one comes across a dead body, except, of course, on the roads.

但是，除非在路上，很少有人遇到这些动物的尸体，

For our purposes, we are concerned only with one kind of cost----

就我们的目的而言，我们只关心一种费用-

and a period (which is the time it takes a wave crest to travel one wave length).

和周期（浪峰走过一个波长所需的时间）。