On the Origin of Species for Existence" Darwin includes "slow-breeding man" among other examples of Malthusian population growth. In his discussions on morphology, Darwin compares and comments on bone structures that are homologous between humans and other mammals. Darwin's early notebooks discussed how non-adaptive characteristics could be selected when animals or humans chose mates, with races of humans differing over ideas of beauty. In his 1856 notes responding to Robert Knox's "The Races of Man: A Fragment", he called this effect sexual selection. He added notes on sexual selection to his "big book on species", and in mid-1857 he added a section heading; "Theory applied
On the Origin of Species it made more widely available; the price was halved to 7s 6d by printing in a smaller font. It includes a glossary compiled by W.S. Dallas. Book sales increased from 60 to 250 per month. In the United States, botanist Asa Gray, an American colleague of Darwin, negotiated with a Boston publisher for publication of an authorised American version, but learnt that two New York publishing firms were already planning to exploit the absence of international copyright to print "Origin". Darwin was delighted by the popularity of the book, and asked Gray to keep any profits. Gray managed to negotiate