From Metamorphoses of Science Fiction, p. 29:
The lowest form of analogic modeling is that in which an extrapolation backwards is in fact a crude analogy to the past of the Earth, from geological through biological to ethnological and historical. The worlds more or less openly modeled on the Carboniferous Age, on tribal prehistory, on barbaric and feudal empires — in fact modeled on handbooks of geology and anthropology, on Spengler’s Decline of the West and Dumas père’s Three Musketeers — are unfortunately abundant in the foothills of SF. Some of this may be useful adolescent leisure reading, which one should not begrudge; however, the uneasy coexistence of such worlds with a superscience, which is supposed to provide an SF alibi, largely or wholly destroys the story’s cognitive credibility. The E.R. Burroughs-to-Asimov space opera, cropping up in almost all U.S. writers right down to Samuel Delany, belongs to the uneasy territory between inferior SF and non-SF…