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 sentence [Light will be thrown on the origin of man and his history] I show that I believe man is in same predicament with other animals. Many modern writers have seen this sentence as Darwin’s only reference to humans in the book; Janet Browne describes it as his only discussion there of human origins, while noting that the book makes other references to humanity. Some other statements in the book are quietly effective at pointing out the implication that humans are simply another species, evolving through the same processes and principles affecting other organisms. For example, in Chapter III: "Struggle