Curator and chairman of the Department of Invertebrates at the American Museum of Natural History, Eldredge has long been comfortable with the concept of mass extinction--but his concern for the future surfaces here as he examines just how easily Homo sapiens might help bring about the next great catastrophic wave. Admittedly, Eldredge says, mass extinctions have always been an essential step in the cycles of life's creation--but as participants in the current global ecosystem, we have a clear and justifiable stake in postponing the next debacle for as long as we can. In examining the circumstances, possible causes, and consequences associated with previous mass extinctions, Eldredge attempts to understand how clear the present danger really is and what is likely to happen if current abuses of the environment continue unabated. The examination is long and sometimes tedious-- no one but a paleontologist can eagerly turn the pages of a discussion of Early Triassic ammonites--but the author's conclusions are powerful enough to justify a little preliminary detail. Whether caused by the earth's collision with extraterrestrial bodies, a global drop in temperature, or a change in sea level (thanks in part, perhaps, to the continental drift), most extinction events come about through a change of the size and location of habitat. Is the human impact on the ecosphere sufficient to set off a major catastrophe? Citing agriculture and deforestation as major culprits, Eldredge gives an emphatic yes- -particularly when considered in concert with the even less controllable and predictable natural forces still at work. Any solution must begin with conserving ``as much [space] as possible'' in maintaining local ecosystems and with rescuing endangered species one by one, understanding that preserving each species is an effective way of underscoring our intent to rescue us all. Sober and sobering--Eldredge's facts imply a sick canary indeed.