If you are interested in the life and scientific contribution of Charles Darwin, you may want to check out the exhibit, Darwin, A Reluctant Revolutionary at the St Louis Science Center. It was a light on substance but that might have been because of the way that the curator displayed the items. Specifically, the exhibit was in an overly spacious gallery with lots of unused space and walking room in some spots between displays. This is great if there is a crowd but nothing more than a distraction zone when there isn't. Overall, it was a nice little exhibit, just don't expect the level of curation that you'd find at an art or history museum.
The exhibit Darwin, A Reluctant Revolutionary is open through August 1st at the St. Louis Science Center.