Common Descent
Charles Darwin strongly advocates for the theory that all human races are descended from a common ancestor. This argumentation seems to be important enough to attribute a whole chapter of his book to the dispute between the two sides in the scientific discussion. However, at the same time he argues that some human races are inferior to others, not fully following through with his own argumentation. It is nonsensical for the evolutionary theory that he sees most indigenous cultures as underdeveloped races that will be either wiped out or absorbed into the genetic pool of the more civilized, i.e. European, people.
Eugenics
While Darwin discusses this concept without irony, his intentions were of a purely evolutionary kind. The results of his argumentation, combined with the mind set of his contemporaries, resulted in horrifying theories on the treatment of mental illness or disability. After his death it also resulted in genocide, such as the racial ideologies of the Third Reich. It is ironic that Darwin's original idea was that his findings prove that the most civilized races have developed a moral code that should prevent such atrocities.
Female Choice
Darwin's original theories have often been used to argue that women's mental and physical capacities make them inferior to men. This argumentation stays the same in this book, even though Darwin clearly describes the power of female choice in sexual selection. Most species that display some form of different physical features between the sexes have a strong focus on female choice, i.e. the male fights for the female's attention. This argument, with all we know about the human physiology, is also true for humans. However, this point of view is not present in Darwin's second book.
Human Abilities
One of the main reasons Darwin decided to write this book. Ironically, the height of human mental capacities has been used to disprove the evolutionary theory. People argued that the difference between animal mental abilities and those of humans were so vast that they could not be explained with gradual change. Darwin, again ironically, used indigenous cultures and animals to describe the gradual change to the civilized human. He put the mental capacities of animals at the bottom, however still displaying the same type of ability. He then argued that indigenous cultures were the missing link between the low abilities of for example apes and the high abilities of the British man.
The Evolution of Defence
One of the most striking ironic developments after the publication of The Descent of Man, and Selection in Relation to Sex was the over usage of the theory of sexual selection. Each trait that could not be explained by the normal theory of evolution was labelled as sexual selection. However, this completely ignored the evolution of defense mechanisms in animals, which used similar features than sexual traits. For example, the wide neck of a cobra was thought to be used for sexual purposes, but today we know that it is used as a deterrent for predators.
