t, and most han mice.

    Eventually, tion, but to  nearly 150 millionyears for Megadynasty 3, to come to an abrupt end and make room forMegadynasty 4 and our own Age of Mammals.

    Eacransformations, as  on t paradoxically important motor of progress: extinction. It is a curiousfact t on Eart literal sense, a y billion is a commonly citedfigure, but t as ever tual total, 99.99percent of all species t o a first approximation,” asDavid Raup of ty of Co say, “all species are extinct.” For complexorganisms, t four million years—rougwhere we are now.

    Extinction is alims, of course, but it appears to be a good t. “ternative to extinction is stagnation,” says Ian tattersall of tural ory, “and stagnation is seldom a good thing in any realm.”

    (I se t ion as a natural, long-term process.

    Extinction broug by ter altogetory are invariably associated ic leaps afterive outburst of tinction of 440 million years ago cleared t of immobile filterfeeders and, someed conditions t favored darting fis aquatic reptiles.

    turn ion to send colonists onto dry land e Devonian period gave life anot  scatteredintervals tory. If most of ts   as t ainly  be here now.

    Eartinction episodes in its time—triassic, and Cretaceous, in t order—and many smaller ones. t about 80 to 85 percent ofspecies. triassic (210 million years ago) and Cretaceous (65 million years) eac 70 to 75 percent of species. But tinction of about 245million years ago, least 95 percent of animals kn