From The Inklings of Oxford: C. S. Lewis, J.R.R. Tolkien, and Their Friends (Harry Lee Poe, Grand Rapids: Zondervan, 2009), page 39: In the 1930s "Owen Barfield said that he himself believed in evolution but never changed at all. On the other hand, he said that Jack Lewis did not believe in evolution and was constantly changing."
How or whether evolution fits into a Christian worldview continues to be a raging debate (at this writing Googling "evolution creationism debate" yields nearly 3 million hits), especially in the United States, with groups such as Answers in Genesis and the Institute for Creation Research seeking to reconcile scientific findings with the six days of Creation recorded in Genesis. Apparently Lewis's position on the issue changed over the years (see quote above). However, when writing Mere Christianity Lewis clearly came down on the side of those who believe evolution can be reconciled with the Bible's account of Creation, recorded in Genesis 1-3. In "The New Men" Lewis gives his views on what the next step in evolution might be.