This book grew from a series of lectures on vertebrate natural history. The topics have been developed over a period of nearly 30 years, and today scarcely resemble the original subject matter. The progress is primarily technical. Some concepts provide a synthetic framework for viewing much modern research, but many of these concepts either date from Darwin or have developed from obser- vations of later students. Animal science courses follow a sequential pattern in which there are three discrete levels of undergraduate instruction. Initially, students study subject mat- ter contained in such courses as biology and general zoology. These courses intro- duce students to animal phylogeny, basic plans of morphology and certain phys- iological aspects; incidental to these subjects the student acquires a broad zoological vocabulary. At the other end of the academic spectrum are courses that emphasize synthe- sis and theory: evolution, zoogeography, behavior and ecology are important courses whose role is to explore the relationships of various aspects of the physical and biological world. In these courses theory and analysis prevail. They are not, however, essentially "subject matter" courses with distinct bodies of knowledge.