Lewis was an evolutionist. Lewis was also a human creature, and fallible.
"Lewis was an evolutionist..." ~ Jedi Master Pikachu
There are various theories of evolution. C.S. Lewis would have pretty much agreed with Pope John Paul's opinion on the subject. bttt
See #15 for details:
http://www.freerepublic.com/focus/news/1657102/posts?page=15#15