Here is more about C. S. Lewis' thinking on creation and evolution from my book, Mere Theology....
As we have already seen, Lewis believed that certain parts of evolutionary theory might be correct, and in the strictly scientific theory of evolution he saw no conflict with the Bible. However, Lewis strongly held that evolutionism, the belief that life on earth is getting better and better, was a myth. And by myth, in this instance, he means a picture of reality which results from imagination. Lewis considers this myth to be a wonderful story, but not one which is true to reality. He points out that an illegitimate transition is often made from the Darwinian theory in biology to the modern myth of evolutionism, developmentalism, or progress in general. Lewis documents how the myth arose earlier than Darwin’s theory, in advance of all evidence. He notes two great works that embody an idea of a universe where the “higher” always supersedes the “lower.” One is Keats’ poem Hyperion, and the other is Wagner’s Ring cycle. Both works of art, Lewis emphasizes, are earlier than the Origin of Species. The idea that the myth is a result of Darwin’s biology is unhistorical. On the contrary, Lewis contends, the attraction of Darwin’s theory of evolution was that it gave to a pre-existing myth of evolutionism the scientific reassurances it required.
Perhaps one reason why Lewis does not have any religious difficulty with accepting the biological concept of evolution is because he believes that creation is taking place at every moment, not just at one point millions of years ago. The reason Lewis views creation in this way is because of his understanding of God being outside of time. He explains that there is no question of God, at one point in time, adapting the material history of the universe to free acts that human beings perform at a later point. To God, all the physical events and all the human acts of time are present in an eternal “Now.” The liberation of finite wills and the creation of the whole material history of the universe is, to God, a single act. God did not create the universe long ago; rather, He creates the universe every minute.
Along with this idea, Lewis has no problem accepting the idea that man is in the process of evolution. Though he would prefer to say that man is in the process of being created, since the latter terminology implies a personal God who is involved in the whole process.
Lewis takes this idea of man being in the process of evolution and he uses it in a unique way: to suggest that the next step in man’s evolution has already happened, and the next step is that of men, who are merely creatures of God, becoming sons of God.
The Value of Lewis’ Approach
Whether the biological theory of evolution is right or wrong was irrelevant to Lewis. If it was found to be wrong or right, either way, it would have had no effect on Lewis’ Christianity. What Lewis wanted to do as an apologist was to show that there was no final conflict between true science and the Bible. The value of Lewis’ theistic evolutionary stance, whether one agrees with it or not, was that it allowed Lewis to focus on what he considered to be more important issues and to lead his readers more immediately to consider what he thought to be more important religious questions.