That introduction, or a revision thereof, continued to be printed in subsequent Everyman Library editions by Dent right through the 1951 edition. However, in 1956, the next Everyman edition came out with a new introduction. (This might plausibly have been considered by the publisher to be something of a centennial edition - since it was the last planned edition prior to the actual centenary - but who knows.)
At any rate, the new introduction was by W.R. Thompson, and while I haven't been able to confirm it first-hand, I think Thompson's essay, not Keith's earlier one, is the better candidate for the origin of that quotation. In fact, I think there's a very decent chance that Thompson's introduction actually contains those words or something close to them. That suspicion is buttressed by the fact that, according to the Cambridge Darwin bibliography, in 1967 some anti-evolution outfit reprinted Thompson's 1956 Origin of Species preface in its entirety as a separate work.
