The Descent of Man/Chapter VII
In the beginning of this chapter Darwin says he intends to ask what are the origins of the races and the value of the differences among races as to classification. After noting that non-Caucasian people from different cultures were dissimilar in many ways, he then lists the reasons why naturalists might classify them as different species. These reasons include differences in habitat, physical features, intelligence, emotional makeup, and even body lice.
Darwin then argues against these reasons. He points out that individuals differ from others within their group or culture, and unlike lower animals, they are able to interbreed with other groups of people. Furthermore,individual differences are outweighed by the many general similarities among all races. The characteristics of two ancient stocks of pig could be seen in modern pigs, but this could not be said for man which argues against more than one ancestral line according to Darwin. He describes the similarities of culture, nonverbal language and mental processes among the races which Darwin prefers to call "subspecies" if such a differentiation must be made at all.
In discussing the reasons for the extinction of ancient peoples, Darwin points to competition and the struggle for existence. He mentions cases where contact with "civilized" nations has led to the deaths of native peoples and explores the possible reasons for the mortality increase. Darwin argues against most reasons offered by others and states the main cause is sterility brought on by changes in living conditions most of which happened as a result of contacts with Europeans. He adds that interbreeding appears to increase fertility in at least some cases. He also surmises that the changes in living conditions could have been caused by contact with other native peoples too.
In the last paragraphs Darwin discusses the opposing views on the reasons for different shades of human pigmentation and the relationship of pigmentation to yellow fever immunity. The chapter ends with a note on the comparison of the brains in humans and apes. Darwin reiterates that there are fewer differences between the brains of a chimpanzee and a human than between a chimp and a lemur. He also quotes one of his critics on this point and offers further arguments as counterpoint.