One of the things Darwin never understood was the mechanism responsible for the inheritance of the traits upon which natural selection would work. Of course, he knew the fact of inheritance, and even realized at some point that it must have a digital basis (else the end result would be a blending of traits upon which natural selection could not possibly work).
It would be Gregor Mendel who would first come to understand and document this process when he noticed that organisms are endowed with discrete units of inheritance (that can even skip generations). Once he laid the foundation, a whole new science of Genetics would be born, and the scientists who took the baton would run for it like their life depended on it. Here is an extremely brief and equally impressive history:
Anyone want to swap genes? :)