The Charles Darwin Papers in Cambridge University Library hold nearly the entire existing collection of Darwin’s working scientific papers. Among these documents are Charles Darwin’s Evolution Manuscripts, his papers on the transmutation of species. Using these notebooks, annotations, and portfolios, Darwin wrote the nine of his fifteen books that set down, enlarged and defended the theory of evolution by natural selection. You can find the papers online at the Darwin Manuscripts Project.
The Darwin Correspondence Project also holds online resources for students, including a Darwin Timeline showing the key moments in Darwin's life and what was happening in Britain at the time, plus a series of audio clips and videos on Darwin's work. For example, the Face of Emotion series discusses Darwin’s work on expression in the context of current research in artificial intelligence, autism, and neuroscience. You can even try Darwin's Emotion Experiment for yourself here.