Darwin illustrated that natural selection sorts between the hereditary variations, for example, the weight of an organism, its height, and color of the coat and so on. The most current discussions of heredity focus on heredity traits like those that can be shown to be passed on genetically.
The discussions of heredity show confusions between the process accountable for the individual development and the processes accountable for the conduction of traits from one generation to the next.
The Darwin's understanding of heritability debilitated his arguments as he was not 100 % sure on what he actually is discussing about. However, if he could argue the truth and demonstrated them with facts, then it would be rational.