In 1859, Charles Darwin's On the Origin of Species was published. In this book, he described his theory of evolution by natural selection. Both the work of Darwin and his contemporary, Alfred Russel Wallace —who was also working on the theory of evolution—were informed by the careful study of insects. In addition, Darwin collected many species of invertebrate during his time aboard ; many of the specimens collected were insects. Using these collections, he was able to study sexual dimorphism, geographic distribution of species, and mimicry; all of these concepts influenced Darwin's theory of evolution. Unfortunately, a firm popular belief in the immutability of species was a major hurdle in the acceptance of the theory.