Unlike Darwin, Wallace began his career as a travelling naturalist already believing in evolution. Both he and Bates had read "Vestiges", a controversial work of popular science, published anonymously in 1844. This advocated an evolutionary origin for the solar system, the earth, and living things. He also read Malthus's "Principle of population", which he says "twenty years later gave me the long-sought clue to the effective agent in the evolution of organic species". He meant by that the struggle for existence, which led Wallace to "natural selection".