Karl Barth wrote the above in a letter to his niece, which you can read on the Faith and Theology blog.
The creation story deals only with the becoming of all things, and therefore with the revelation of God, which is inaccessible to science as such. The theory of evolution deals with what has become, as it appears to human observation and research and as it invites human interpretation. Thus one’s attitude to the creation story and the theory of evolution can take the form of an either/or only if one shuts oneself off completely from faith in God’s revelation or from the mind (or opportunity) for scientific understanding.
– Karl Barth