Yes, Nabokov at times willingly conceded that mimetic phenomena do afford protection, not only in his 1949 paper on Lycaeides Ms. Alexander cites. Even in the passage that is always quoted ("Gift" p.110) he writes that "the incredible artistic wit of mimetic disguise [...] was too refined for the mere [!] deceiving of accidental predators..." That is, he did not at all deny that mimicry was useful as a protective device. His argument was that there was a special quality to some mimetic phenomena that could not be explained by its usefulness and hence by natural selection ("the struggle for existence"). This additional quality he saw in a kind of aesthetic surplus: some cases of mimicry, he thought, are too refined for the discriminative powers of the predators--these cannot really appreciate the perfection of the thing. Now this is not philosophy of nature but sound science, for it is a testable hypothesis that can be either confirmed or falsified. It could not be done in a general way--each instance would have to be examined. To my knowledge there has not been a single instance so far proving that, yes, less perfect mimicry would afford just the same protection. Predation usually does not happen in an atmosphere of leisure where a predator can quietly contemplate and appreciate the degree of perfection in his potential prey. He has to decide quickly whether he wants to risk taking a pick at a lep that is in movement and only partly visible to him. So even a very imperfect mimic might enjoy some protection, benefiting from a moment of doubt as to its palatability. This, however, does not exclude that a more perfect mimic might enjoy more protection. It is not an all-or-nothing affair as Ms. Alexander seems to suggest. Still there actually might be cases of aesthetic surplus in mimicry, and as I argued in my 1999 paper they might be explainable by genetic drift (neutral evolution) or structural constraints (structural evolution). The important thing to realize is that neutral or structural evolution, if they can be shown to have produced a certain case of mimicry, would not have happened in place of natural selection but just supplemented it.