Why doesn’t Sam like Charles Darwin?

Sam says Darwin's book was hard to understand, and teases Hally for never finishing the book (which Hally has now hidden safely away in the theology section where no one looks). However, Hally passionately defends Darwin for revolutionizing science and showing human beings where we came from. Sam says that he does not believe Darwin's theory and Hally scoffs, calling Sam ignorant and bigoted. Sam suggests Abraham Lincoln instead, and Hally tells him to stop being so sentimental as Sam was never a slave.