I read on Pharyngula about an evolutionary biologist who was planning to read the Origin of the Species for the first time. He had managed to get a Ph.D. without doing so. This did not surprise me.
I am a physicist, but I have never read Newton’s Principia. Of course the fact that it is written in Latin is an obstacle for me. From what I have heard of it, it would be quite hard for me to follow. Even though Newton invented calculus, he did many of the proofs geometrically. Newton got the physics right, but many people have figured out better ways of explaining it.
I find evolutionary biology interesting, especially evo-devo. So much has been learned about evolution since Darwin and I think I would want the much more complete view that is available now, than you would get from the Origin of the Species.