Why was Darwin's theory of evolution right?

At was right, at the time, because it was a plausible, parsimonious model consistent with everything that was known at the time. However, it is no longer entirely right. We've learned many things since Darwin's time, and made significant alterations to the model. Evolutionary theory today includes genetics, elaborate mathematical moDELLing of population dynamics, developmental Biology, and so forth, and so on. Evolutionary theory today is still partly based on Darwin's work, but is mostly a synthesis of more recent findings and understanding of biology.