The Descent of Man/Chapter VI
Darwin lists the similarities between man and lower animals which argue against special creation. He also dismisses the idea expressed by some naturalists of his day that the natural world be divided into human, animal, and plant kingdoms. He criticizes the classification systems that use only one or two points of comparison, even if brains are compared.
Darwin then briefly explains the classes and orders of vertebrates in general and gives examples of similarities between dinosaurs and birds, reptiles and mammals, and animal embryos across the spectrum. He reiterates that there are animals still living like the echidna that link classes of animals together. He mentions new information on the lancelet embryo which helped scientists to classify it as a worm and points to that class, ascidians, as the likely group where the vertebrates began.
Finally Darwin also mentions the rudimentary female sex organs found in males which point to a hemaphroditic ancestor, probably belonging to a class lower than mammals. Again in a footnote Darwin gives credit for this idea to another scientist, a comparative anatomist. He also lists four scientists who have written on hermaphroditism in lower animals. As further evidence Darwin discusses the development of the nipple from marsupials and retained by both sexes in the higher mammals. He further points to male animals that take care of the eggs and young and cases of male lactation.
In the final paragraph, once again Darwin traces the evolution of vertebrates from ancient marine worms to man. He adds that humans should not be ashamed of their humble origins because even the lowest animals are marvelous in their structure.