The Evolution of Cooperation insects, where the workers – which are sterile, and therefore incapable of passing on their genes – benefit the queen, who is essentially passing on copies of "their" genes. This is further elaborated in the "selfish gene" theory of Richard Dawkins, that the unit of evolution is not the individual organism, but the gene. (As stated by Wilson: "the organism is only DNA's way of making more DNA.") However, kinship selection works only where the individuals involved are closely related; it fails to explain the presence of altruism and cooperation between unrelated individuals, particularly across species. In a 1971 paper
On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection Darwin apparently mistook Wallace's meaning, writing "nothing very new... Uses my simile of tree, [but] it seems all creation with him". However, he spelt out the details of Natural Selection to Lyell, who found the idea hard to accept but urged Darwin to publish to establish priority. On 14 May 1856 Darwin began what became his draft for a book titled "Natural Selection". Wallace collected specimens and corresponded with Darwin from Borneo. In December 1857, he wrote to ask if Darwin's book would delve into human origins, to which Darwin responded that "I think I shall avoid the whole subject,