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 be impossible." He asked Hooker how many pages would be available, but "If the Referees were to reject it as not strictly scientific I would, perhaps publish it as pamphet." He began his "abstract of Species book" on 20 July 1858, while on holiday at Sandown, and wrote parts of it from memory, while sending the manuscripts to his friends for checking. By early October, he began to "expect my abstract will run into a small volume, which will have to be published separately." Over the same period, he continued to collect information and write large fully detailed sections of