Unfortunately for the therapsids, their cousins the diapsids were also productively evolving, in their case into dinosaurs (among other things), which gradually proved too much for the therapsids. Unable to compete head to head with these aggressive new creatures, the therapsids by and large vanished from the record. A very few, however, evolved into small, furry, burrowing beings that bided their time for a very long while as little mammals. The biggest of them grew no larger than a house cat, and most were no bigger than mice. Eventually, this would prove their salvation, but they would have to wait nearly 150 million years for Megadynasty 3, the Age of Dinosaurs, to come to an abrupt end and make room for Megadynasty 4 and our own Age of Mammals.
兽孔目爬行动物的运气不佳，它们的表亲双孔亚纲在进化过程中也繁殖力很强，有的进化成了恐龙。兽孔目爬行动物渐渐被证明不是恐龙的对手。它们无力与这种凶猛的新动物展开势均力敢的竞争，总的来说从记录中消失了。然而，少量进化成了毛茸茸的穴居小动物，在很长时间里作为小型哺乳动物存在，等待合适时机的到来。其中最大的也长不到家猫的大小，大多数的个儿不超过老鼠。最后，这将证明是一条活路。但是，它们还得等待将近，l5000万年，等着第三大王朝即恐龙时代突然告一段落，为第四大王朝和我们自己的哺乳动物时代让路。

Each of these massive transformations, as well as many smaller ones between and since, was dependent on that paradoxically important motor of progress: extinction. It is a curious fact that on Earth species death is, in the most literal sense, a way of life. No one knows how many species of organisms have existed since life began. Thirty billion is a commonly cited figure, but the number has been put as high as 4,000 billion. Whatever the actual total, 99.99 percent of all species that have ever lived are no longer with us. "To a first approximation," as David Raup of the University of Chicago likes to say, "all species are extinct."
每一次大规模的转化，以及其间和其后的许多较小规模的转化，都取决于那个说来矛盾的重要原动力：灭绝。在地球上，说句实在话，物种死亡是一种生活方式。这是个很有意思的事实。谁也不清楚自生命起步以来究竟存在过多少种生物。一般引用的数字是300亿种，但是有人估计那个数字高达4万亿种。不论其总数是多少，99，9%存在过的物种已经不再和我们在一起。“基本的估计是，”芝加哥大学的戴维·劳普喜欢说，“所有的物种都已灭绝。”