What was Darwin's basic argument in the Descent of Man?

Although Origin of Species (1859) is his best-known work, and he first lays out his theory of natural selection in that text, the influence of Darwin's The Descent of Man is perhaps even greater. In it, he explores the consequences of his ideas about evolution and natural selection in human development and society. It is these thoughts that become misinterpreted and warped into the doctrines of social Darwinism, although one can see how his ideas can be seen in this light.
Essentially he argues that civilized society, in seeking to protect everyone equally, heal the sick, and support the weaker members of the species, has curbed and even countered the basic evolutionary forces of "survival of the fittest." This idea comes to be used to justify elements of social Darwinism and eugenics, which involves encouraging the fertility of the fit and sterilization of the unfit.
He also looks at human cultures in evolutionary terms and places them into a hierarchy, from primitive to advanced. And yet this view is deeply skewed by his own cultural framework and biases.
In regards to human evolution and the development of the human species, he suggests that sexual selection is highly dependent on the female and creates a situation where men compete strenuously for females. In this respect he uses various examples of other species, like the Peacock, whose elaborate tail feathers really only serve the purpose of impressing the female peahen.