With a brief and simple consideration of the genetic and hormonal mechanisms regulating growth, the emphasis is on the how of growing and aging. Each species and individual grows and matures according to its own inherited schedule, modified by environmental factors of food supply, habitat, climate and relationship to other forms of life. Children who love to collect facts will be rewarded by the infinite and fascinating variety of adaptation reported here. They can follow the life cycle of a kangaroo or a fungus, the rate at which species of wild animals have become extinct, the pattern of tooth development in man, and learn how to tell the age of a tree, a whale or a puffin. The discussion of how populations grow considers the deleterious effects of civilization on other forms of life and the final chapter, ""The Secret of Long Life,"" names environmental poisons and nervous tension as villains, emphasizing the possibility of remaking the world in the interest of good health and long life. Consistently clear and instructive.