The genius of Darwin is revealed by the fact that today, you wonder how anyone could have ever thought otherwise. Of course Darwin was clued in by contemporary discoveries in geology which began to reveal just how old the earth is.
Imagine. Before Darwin people believed, in spite of their everyday experience of continuous time and space, that people just poofed in out of nowhere. Of course the whole idea is absurd, and except for the creationist cult, it is commonly recognized as such.