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
Reactions to On the Origin of Species breeding that would eventually form part of "The Variation of Animals and Plants under Domestication". He wrote to Asa Gray and used the example of fantail pigeons to argue against Gray's belief "that variation has been led along certain beneficial lines", with the implication of Creationism rather than Natural Selection. Over the winter he organised a third edition of the "Origin", adding an introductory historical sketch. Asa Gray had published three supportive articles in the "Atlantic Monthly". Darwin persuaded Gray to publish them as a pamphlet, and was delighted when Gray came up with the title of "Natural Selection Not