Charles Darwin didn't know how inheritance works. The first glimpse of inheritance accepted today was published by Gregor Mendel shortly after Darwin published his famous Origin of the Species. A copy of Mendel's Experiments on Plant Hybridization was found in Darwin's library after his death with the pages left uncut - suggesting that he never bothered reading it.
From the timeline shown below, one can see that Mendel's work was published after Origins was published - thus the material wasn't yet available to Darwin.
Darwin perpetuated some of the inheritance principles of his predecessor Jean-Baptiste Lamark including acquired characteristics - the idea that an organism can pass on characteristics that it acquired during its lifetime to its offspring.
The Lamarkian view does two things of note. First, as an aside, it paints a picture of species quickly evolving useful adaptations - thus lending false credibility to Darwin's main point. Second, and more importantly, the idea of acquired characteristics gave a view of inheritance that reinforced the activities of social constructionists like Marx and Stalin who prefer that society could completely construct the individual without genetic constraints.
It is noteworthy that Karl Marx read Origin and commented in a letter that "Darwin's book is very important and it suits me well that it supports the class struggle in history from the point of view of natural science". It can be argued that it was the erroneous Lamarkian elements in Origin that appealed to Marx.
It is also noteworthy that the Soviet Union in the Stalin era rejected Mendelian inheritance in favor of Lysenko's view of use-inheritance which is a neo-Lamarkian theory. Here we see a prime example of a political movement choosing a scientific theory based on how it suits its purposes.
Social Constructionists continue to favor theories of human nature that dispense with genetic constraints on the ability for society to adjust how people behave. Stephen Pinker's The Blank Slate: The Modern Denial of Human Nature chronicles this development.
A great context for Mendelian Inheritance is offered by Paul Ekman in his introduction of Oxford Press 3rd Edition of The Expression of the Emotions in Man and Animals by Charles Darwin.
