Darwin was hard at work on his "big book" on Natural Selection, when on 18 June 1858 he received a parcel from Wallace, who stayed on the Maluku Islands (Ternate and Gilolo). It enclosed twenty pages describing an evolutionary mechanism, a response to Darwin's recent encouragement, with a request to send it on to Lyell if Darwin thought it worthwhile. The mechanism was similar to Darwin's own theory. Darwin wrote to Lyell that "your words have come true with a vengeance, ... forestalled" and he would "of course, at once write and offer to send [it] to any journal" that Wallace chose, adding that "all my originality, whatever it may amount to, will be smashed". Lyell and Hooker agreed that a joint publication putting together Wallace's pages with extracts from Darwin's 1844 Essay and his 1857 letter to Gray should be presented at the Linnean Society, and on 1 July 1858, the papers entitled On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection, by Wallace and Darwin respectively, were read out but drew little reaction. While Darwin considered Wallace's idea to be identical to his concept of natural selection, historians have pointed out differences. Darwin described natural selection as being analogous to the artificial selection practised by animal breeders, and emphasised competition between individuals; Wallace drew no comparison to selective breeding, and focused on ecological pressures that kept different varieties adapted to local conditions. Some historians have suggested that Wallace was actually discussing group selection rather than selection acting on individual variation.

Which book was Darwin working on when he received a package from Wallace?