The Cat in the Hat use of the term "cat" to refer to jazz musicians. According to Nel, "Even as [Geisel] wrote books designed to challenge prejudice, he never fully shed the cultural assumptions he grew up with, and was likely unaware of the ways in which his visual imagination replicated the racial ideologies he consciously sought to reject." Geisel once called the fish "my version of Cotton Mather", the Puritan moralist who advised the prosecutors during the Salem witch trials. Betty Mensch and Alan Freeman support this view, writing, "Drawing on old Christian symbolism (the fish was an ancient sign of Christianity) Dr. Seuss
The Cat in the Hat for the series, including "The Cat in the Hat Comes Back" (1958), "Green Eggs and Ham" (1960), "Hop on Pop" (1963), and "Fox in Socks" (1965). He initially used word lists of limited vocabularies to create these books, as he had with "The Cat in the Hat", but moved away from the lists as he came to believe "that a child could learn any amount of words if fed them slowly and if the books were amply illustrated". Other authors also contributed notable books to the series, including "A Fly Went By" (1958), "Sam and the Firefly" (1958), "Go, Dog.