When Darwin received Alfred Russel Wallace's paper that described natural selection, Hooker and Charles Lyell arranged for a joint reading of papers by Darwin and Wallace to the Linnean Society on July 1, 1858. Since Darwin had nothing prepared, the reading included excerpts from his 1844 "Essay" and from a letter he had sent to Asa Gray in July 1857 outlining his theory on the origin of species. By that time, Darwin had begun writing his book "On the Origin of Species". The correspondence with Gray was thus a key piece of evidence in establishing Darwin's intellectual priority with respect to the theory of evolution by natural selection. Neither Darwin nor Wallace attended the meeting. The papers were published by the society as "On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection". By summer 1859 it was obvious to Gray and others working with Darwin that "On the Origin of Species" would be a ground-breaking book.