American Museum of Natural History paleontologist Eldredge is the colleague and coauthor (with Stephen Jay Gould) of the punctuated equilibria theory of evolution. This theory maintains that species generally remain stable over time. Rapid changes can and do occur at intervals, however, leading to declines and extinctions on the one hand, and, on the other, to the emergence of new forms or the rapid spread (""adaptive radiation"") of a previously existing species to fill new ecological niches. In this scholarly volume, Eldredge presents evidence for his point of view, now more often referred to in terms of ""pulses"" than the more grandiose p.e. To build his case, Eldredge takes the reader on a trip in time, beginning with the Precambrian epoch and the earliest life forms yet discovered: microfossil bacteria dated at 3.5 billions years ago. Not much happened for the next billion years except for the appearance of stromatolites: blue-green algal forms visible to the naked eye. Multicellular forms of life came into being with the Cambrian epoch (Cambria was the Roman name for Wales, the terrain mapped by Darwin's geology mentor, Adam Sedgwick). Eldredge is very much at home in Cambrian times because he is an expert on trilobites--primitive arthropods that have left their mark worldwide. So there is much poring over the varieties and stabilities of these and other marine forms (clams, brachiopods, worms). Indeed, the text space that Eldredge devotes to the various flora and fauna appears to vary directly with their duration on earth. By that token even the dinosaurs get relatively short shrift and man, hardly a mention. But this serves to underscore Eldredge's point: Evolution has been conservative; nature doesn't abandon a good thing, and many a species persists across epochs. What then is the instrument of change? The author posits extinctions: events that cause widespread loss of species and thereby create opportunities for others. Eldredge holds no brief for the out-of-space theories on why the dinosaurs crashed (they were already well on their way out, he contends). And yes, there are still gaps in the record that render his theory moot. But his well-marshalled arguments, his scholarly command of the facts, and his knack of tossing off witty lines and metaphors while deep in the heart of Devonian or Silurian strata may well win him readers.