What's the easiest way to tell species apart? Check their genitals. Researching private parts was long considered taboo, but scientists are now beginning to understand that the wild diversity of sex organs across species can tell us a lot about evolution. Schilthuizen invites readers to join him as he uncovers the ways the shapes and functions of genitalia have been molded by complex Darwinian struggles. He joyfully demonstrates that by learning about the private parts of animals the more humans can learn about their unique place in the diversity of life.