Known for developing the concept of Müllerian mimicry, whereby poisonous species with a common predator display similar warning signals, the naturalist Johann Friedrich Theodor (Fritz) Müller (1821-97) spent most of his working life in Brazil. Having emigrated from Germany, owing partly to his radical atheism, he became a strong early supporter of Darwinism. Drawing on his studies of crustaceans, he originally published this work in German as Für Darwin (1864), and sent the great naturalist a copy. Müller became a regular correspondent, and Darwin supported the translation of Müller's work, firstly for his personal use and also in the published 1869 version that is reissued here, rendered into English by the naturalist William Sweetland Dallas (1824-90), with several updates by Müller. Using case studies of crustaceans to provide evidence for certain aspects of Darwinian theory, Müller draws up evolutionary classifications of the various species examined.