Perhaps Darwin’s only major failure was that he didn't understand inheritance correctly. He proposed his own theory of inheritance, called Pangenesis, but it was based on scant and unreliable evidence and has long been rejected. In his publications he often talks about the inheritance of acquired characteristics, or evolution by ‘use and disuse’ – the famous example being that blacksmiths (who have big muscles) would pass on their bulk to their children.
Part of Darwin’s genius however was to realise that not understanding inheritance wasn’t a problem for his theory of natural selection. He had proof from his work on domesticated animals and plants, and from his reading and communication with other scientists, that characteristics can be, indeed are, inherited. This was enough to build his theory. In On the Origin of Species Darwin leaves inheritance largely as a ‘black box’.
Had things gone slightly differently Darwin could have heard the answer, for only two years after Darwin’s death Weismann published his correct “Germ-Plasm” theory, and during Darwin’s own lifetime Gregor Mendel had published the results of his experiments on crossing peas which began the field of genetics. Combining Darwin’s theories of evolution with Mendel’s genetics and Weismann’s Germ-Plasm theory was the most important breakthrough in biology since Darwin published The Origin. The discovery of DNA and it’s structure and method of replication was the next great update, opening doors to whole new fields of research and a whole new world of evidence for evolution.