Part II of the book begins with a chapter outlining the basic principles of sexual selection, followed by a detailed review of many different taxa of the kingdom Animalia which surveys various classes such as molluscs and crustaceans. The tenth and eleventh chapters are both devoted to insects, the latter specifically focusing on the order Lepidoptera, the butterflies and moths. The remainder of the book shifts to the vertebrates, beginning with cold blooded vertebrates (fishes, amphibians and reptiles) followed by four chapters on birds. Two chapters on mammals precede those on humans. Darwin explained sexual selection as a combination of "female choosiness" and "direct competition between males".