So, there's nothing like the internet for links. Several months back, I had no sooner finished reading the piece about CSL's getting his name on a stained glass window in Westminster Abbey's Poets' Corner, than I noticed link in the sidebar to another story about some Lewis letters going to a Belfast library.
This piqued my interest, because while evolution is of little concern to CSL, it was of central concern to Owen Barfield's views on the evolution of consciousness. Barfield believed evolution was real, but that it had been guided, and that the key event in human history was the emergence of self-consciousness from a previous state of what might be called shared consciousness. And, since Lewis often adopts and repeats arguments and ideas originating in his fellow Inklings, I was curious whether any Barfieldian ideas would appear in the guise of Lewisian phrasing.
Apparently not, now that I've had a chance to read all five of the ten [actually eleven] surviving letters from CSL to Acworth printed in COLLECTED LETTERS,* and also an article based on the discussion of the correspondence, "C. S. Lewis on Creation and Evolution: The Acworth Letters, 1944-1960" by Gary B. Ferngren and Ronald L. Numbers** :
Against this, it's only fair to cite the rebuttal to Gerngren & Numbers' article by Lary Gilman, "The Shift That Wasn't: C. S. Lewis and Bernard Acworth" , which persuasively argues that the shift in Lewis's position that Acworth's son and Ferngren & Numbers see is unsupported by the evidence:
The simple truth seems to be that Lewis had no problem in believing in both Xianity and evolution, like most Christians. This is best demonstrated by his little rumination, in THE PROBLEM OF PAIN I think it was, about how there must have been a point at which someone crossed the line between 'pre-human/almost-human' on the one side and fully 'human' on the other, with names like 'Adam' and 'Eve' being a handy shorthand for the first individuals to cross that almost imperceptable line.
All of which, it must be stressed, wd have been anathama to Acworth, who was far wackier than the Belfast story or Frengren & Numbers' piece suggests. Now that I've had a chance to get a copy of his most famous book, THIS PROGRESS: THE TRAGEDY OF EVOLUTION , I've discovered that Acworth was so weird that his ideas deserve a post in themselves.
--to be continued
current reading: THIS PROGRESS by Bernard Acworth 
*For a full listing, and brief synopsis, of the whole correspondence, letter by letter, see the following link:
The ones published in letters are those of
Dec 9 '44 (Vol II.632-633) [Belfast #2]
Sept 13 '51 (III.138) [Belfast #5]
Oct 4 '51 (III.149-141) [Belfast #6]
Sept 18 '59 (III.1087-1088) [Belfast #10]
March 5 '60 III.1137-1138 [Belfast #11]
In addition, F & N quote from CSL's letters of Sept '44 [Belfast #1], June 14 '50 [Belfast #4], and Dec 16 '53 [Belfast #8] (the last of these about Piltdown Man), so only three letters (#3, #7, #9, dating to Feb '46, Sept '52, and Dec '54, respectively) are omitted from both sources.
Finally, Lewis mentions Acworth in passing in a letter to Arthur Greeves (Sept 20 1952; III.226) and discusses Acworth's conspiracy-theories dismissively in a letter to Warfield Firor (Dec 20 1951; III.150).
**Numbers is himself the author of a well-regarded book THE CREATIONISTS, a history of the Creationist movement, which I have not read.