Puzzled by the geographical distribution of wildlife and fossils he collected on the voyage, Darwin began detailed investigations, and in 1838 conceived his theory of natural selection. Although he discussed his ideas with several naturalists, he needed time for extensive research and his geological work had priority. He was writing up his theory in 1858 when Alfred Russel Wallace sent him an essay that described the same idea, prompting immediate joint publication of both of their theories. Darwin's work established evolutionary descent with modification as the dominant scientific explanation of diversification in nature. In 1871 he examined human evolution and sexual selection in "The Descent of Man, and Selection in Relation to Sex", followed by "The Expression of the Emotions in Man and Animals" (1872). His research on plants was published in a series of books, and in his final book, "The Formation of Vegetable Mould, through the Actions of Worms" (1881), he examined earthworms and their effect on soil.