"On the Origin of Species" proved unexpectedly popular, with the entire stock of 1,250 copies oversubscribed when it went on sale to booksellers on 22 November 1859. In the book, Darwin set out "one long argument" of detailed observations, inferences and consideration of anticipated objections. In making the case for common descent, he included evidence of homologies between humans and other mammals. Having outlined sexual selection, he hinted that it could explain differences between human races.</ref> He avoided explicit discussion of human origins, but implied the significance of his work with the sentence; "Light will be thrown on the origin of man and his history."</ref> His theory is simply stated in the introduction: