10. A lifespan is a billion heartbeats.
动物的寿命大约是十亿次心跳的时间。

Complex organisms die. Sad though it is in individual cases, it's a necessary part of the bigger picture; life pushes out the old to make way for the new. Remarkably, there exist simple scaling laws relating animal metabolism to body mass. Larger animals live longer; but they also metabolize slower, as manifested in slower heart rates. These effects cancel out, so that animals from shrews to blue whales have lifespans with just about equal number of heartbeats — about one and a half billion, if you simply must be precise. In that very real sense, all animal species experience "the same amount of time." At least, until we master #9 and become immortal.
复杂的有机体会死去。对个体来说这是个悲哀的事实，但在更高的层面上这是必须的：旧的总要为新的让路。值得注意的是，自然界存在着关于动物体型和新陈代谢的一套简单比例法则：体型更大的动物活得更久，但新陈代谢也更慢，表现出来就是心跳会更慢。这些效应相互抵消，结果就是从鼩鼱（一种外形似鼠的小型哺乳动物）到蓝鲸，各种动物的寿命都差不多是15亿次心跳的时间。从这个意义上来说，所有物种都经历了“相同的时间”——至少是在我们掌握了第9条并获得永生之前。