In this small book Professor Castle, who was a pioneer and is a recognized authority in the field of mammalian genetics, has summarized what is known about the inheritance of characters in laboratory and domestic mammals. Especially interesting is a table of comparative gene mutations found in the rabbit, house mouse, Norway rat, black rat, field mouse and guinea pig. His discussions of sex differentiation and sex determination are brief but they are good summaries of what is known about these topics. In his chapter on maternal inheritance he takes to task those who would attribute all inheritance to the genes. There is only an occasional reference to the genetics of human characters. The book is highly recommended to those who do not have the time to read the detailed papers in this important field. To those working in the field it is a welcome summary and synthesis.