There is a post over at Uncommon Descent that goes through Darwin's metaphysical presuppositions that led to his theory. For a full treatment of this theme, read Cornelius Hunter's book, Darwin's God.
It shows how an unbiblical view of God, man, creation, and evil that arose in 17th-19th century Europe led Darwin to think that, since this view isn't reflected in nature, the only alternative, naturalistic common descent, must be true. Thank you natural theology for creating Darwinism! (sarcasm) Yick!