McLoughlin, who has brought adult readers up to date on dinosaur theory (Archosauria) and the origin of mammals (Synapsida), now traces for young readers the whole tree of life, from the first self-replicating organic molecule to humans, who are pruning back the tree at an alarming rate. (""Some five hundred thousand species of plants and animals may become extinct during the last fifty years of the twentieth century alone, ending the Cenozoic era forever."") As each branch arises and develops, McLoughlin intersperses the text with a double-page, freee-form picture of the tree so far: First, on a mostly blank double page, there is only the one branch, of protozoa, growing from its prokaryote roots. Next, sponges have appeared and a diverging branch of planula has split off into coelenterates in one direction and mollusks and annelids in another; and so it goes until the tree is completed, an illuminating visual metaphor and a useful memory review, with all the forms drawn in outline in the outlined branches. The sequence itself provides the book's structure, and relevant concepts are introduced in process, via concrete examples. Very early there is an unusually clear and thorough explanation of the place of mutation and the process of evolution in general, and on the way we see the repeated and ongoing processes of adoptive radiation, diversification as a response to competition, coevolution of predator and prey, the complicated interaction of form, function, and environmental pressure (exemplified in one case in the development of the gizzard as an indirect response to two-legged posture), and parallel evolution to fill parallel econiches. (The book begins with a look at kangaroos and cattle in Australia--two animals who have developed ""the same clever mode of digestion"" though they are ""not at all close relatives."" An even more striking example is the marsupial wolf.) The early pages, which introduce many of the principles and mechanisms, are clear, well ordered, and creatively presented; still, they require some attention from young readers who may not be accustomed to such respect. Those who respond will be amply rewarded, and amply aided by McLoughlin's many graceful, integral drawings.