Well, a couple of things to understand about Lewis: first off, he made no pretense of expertise outside of his area of study. If I remember his writings aright, he always allowed for the possibility of gradual development, though he never made any of his thinking predicate on that being true or false. He might have admitted the possibility of gradual development of species (and, you must admit, it is a possibility, though current genetic science can't explain how it would happen), but in at least one place he pointed out that even in his day biological science was starting to draw away from the Darwinist position. He also wrote a very good essay somewhere where he pointed out that the acceptance of Darwinism took place on the basis of philosophical enthusiasm rather than proper scientific examination - to use modern jargon, it wasn't well "peer-reviewed" - and that in itself made the theory suspect and weak.
Another point is that he used the term "myth" in a totally different way than most moderns. Usually in our culture "myth" is effectively translated "elaborate fairy tale". The way Lewis, Tolkien, and the rest of the Inklings used it, it meant "a story that contained more truth than it stated." A good example of how he explored this is his classic
Till We Have Faces. Thus he could (and, I believe, did) refer to "The Incarnation Myth" in such a way that didn't deny a bit of its historicity (in which he firmly believed), but reinforced the fact that there was a LOT MORE going on during the Incarnation than even eyewitnesses could grasp.
Thus you needn't fret when Lewis uses a term like "Creation Myth". He believed in creation, all right - he was just pointing out that there was a lot more to the story than a simple reading would deliver.
On that point, I highly recommend John Walton's superb
The Lost World of Genesis One. Fantastic work.
