Observations of heredity preceded Darwin by thousands of years, this was not a theory of Darwin's he merely made use of existing ideas.
http://www.mala.bc.ca/~johnstoi/darwin/sect5.htm
"Observations of heredity preceded Darwin by thousands of years"
It's called "standing on the shoulders of giants". It's how siciense works. Nothing Darwin said had not been said before, Darwin was just the one that manged to put the pieces together and present it in a package that "sold".
That does not change the fact that his theory predicted many things that have been proved. That does not mean that he was 100% on the ball, no one ever is. Details are still hotly debated but the overall theory holds.