Much of the human interest in biology stems from the observation that we are surrounded by a great diversity of other organisms, and each culture has made up stories to explain where these creatures came from and how they are related. The genetic continuity of organisms and the evolutionary changes that occur in organisms over time are the fundamental themes throughout biology and this textbook. This textbook is constructed based on a spiral approach to learning, where general topics are presented first, and the author and the reader will "spiral back" to these topics, and build on the level of detail, at each turn.