In his brief, superficial introduction to the world of mammals, George Laycock notes that there exist today nearly 4300 species, each of which can be placed into one of three groups: monotremes, marsupials, and placentals. He doesn't explain why the sixteen or so that he chooses to explore in somewhat greater depth happen to be limited to the placental group. Though he does warn us (mildly) of the possible extinction facing many mammals -- including man -- and man's responsibility for maintaining a healthy environment both for himself and his fellow mammals, Laycock's choice of topics does not seem to be limited by a concern for endangered species. But there is still much to interest the young reader in the lives of such common creatures as moles, shrews, cottontail rabbits and whitetail deer as well as those of more exotic animals such as muskoxen and caribou. Facts of their lives -- where they live, what they eat, when they mate, when and how many young are produced, and what dangers they face -- are presented in a readable, though sometimes too folksy style and in numerous black and white photographs, many taken by the author. A more comprehensive introduction can be found in Richard Carrington's The Mammals (Time-Life Nature Library -- Young Reader's Ed.).