The text surveys evidence and principal patterns of biological evolution, beginning with geological and fossil records that indicate simpler forms preceded more complex ones. It discusses variation under domestication and traces transitions from single-celled organisms to multicellular life, then follows the diversification of worms, invertebrates, early vertebrates and fishes. Later sections examine the conquest of land, the emergence of reptiles, birds and mammals, and the anatomical and dental changes associated with these shifts. Throughout it uses fossil series, comparative anatomy, and morphological trends such as limb reduction and size change to reconstruct evolutionary lineages and consider mechanisms and tempo of evolutionary change.