The Descent of Man/Chapter II
Darwin begins by discussing how individual humans differ from each other in height, skull shape, teeth, artery courses, musculature, and internal organs. He also mentions that humans and monkeys alike differ in intelligence, and he touches on the work of Galton and the inheritance of genius.
He then discusses work that has been done to explain the causes for these differences within species. In the paragraphs on changed conditions, Darwin quotes an ancient Greek poem decrying the fact that human marriage arrangements were not made with the same concern for the offspring that farmers had for the breeding of their livestock. He can think of only two historic instances of selective human breeding, the Prussian grenadiers and the Spartans.
External conditions like climate and terrain had inconclusive results except for diet. He says that even though diet obviously plays some part in determining the height or weight of an individual, that is not the whole story.
The paragraphs on the use and disuse of body parts as a reason for differences in arm lengths, hand size, etc. include some ideas modern people would see as strange. He describes microcephalics and attributes the cause of this condition to arrests of development.
Another paragraph discusses cases of women with additional mammaries and double uteruses which Darwin attributes to reversion, inheriting traits from lower animals. He relates anecdotes of people with extra fingers and toes and mentions some sources, but finally concludes that this is not due to reversion. He then goes on to discuss various scientists who had noticed the similarity between human canine teeth and those of apes. He concludes that the connection is clear no matter how much some men might object even as they "retract...'snarling muscles'...so as to expose them [canines] ready for action, like a dog prepared to fight." Darwin cites cases published by other scientists showing humans with muscles similar to apes and concludes there is a "genetic connection" between the species (although he didnt use the word genetic because the term 'genetic' was coined by William Bateson in 1905.
Under variations Darwin discusses questions about whether one feature like arm muscles is affected by another feature like leg muscles. He calls this correlated variation and mentions other possible examples like sight related to hearing. He also mentions spontaneous variation.
The events that affect a population's rate of increase like space, food, and disease are also mentioned as reasons for differences between species. He proposes that increase in fertility may be inherited and discusses Malthus's description of the "checks" to the "savage" populations.
Again this chapter is heavily footnoted showing that Darwin was well-aware of the science of his day.