Keith Burgess-Jackson has an interesting post on the phrase "in virtue of". Which is correct: "in virtue of" or "by virtue of"? According to Bryan A. Garner, A Dictionary of Modern American Usage, the latter; the former, he says is an archaism. Burgess-Jackson was puzzled because, as he says, philosophers use the phrase "in virtue of" all the time. I was puzzled, too, because while I hear and read "in virtue of" quite often, it has been a long time since I can remember having heard the phrase "by virtue of." This piqued my interest, so I ran a Google search.
"In virtue of" came up with approximately 112,000 hits. Virtually all of the hits in the first thirty pages were from three fields: philosophy, theology, and law, in that order, with law only yielding a handful. The overwhelming majority were philosophical; very few of the uses of the phrase were casual.
"By virtue of" came up with approximately 920,000 hits. The first thirty pages were far more diverse than was the case with "in virtue of", with many casual uses of the phrase; but a clear majority of the hits were from the field of law.
So the results seem to be this. "In virtue of" is not an archaism, but it is clear that it survives primarily in philosophy. The theological remnants appear to be largely due to older translations of the Bible; and "in virtue of" is used in law as a rare synonym of "by virtue of". "In virtue of" is clearly the preferred phrase in philosophy, just as "by virtue of" is clearly the preferred phrase in law. Neither phrase is particularly common in ordinary speech, but there is a clear preference for "by virtue of."
It is worth noting that the on-line OED gives no suggestion whatsoever of "in virtue of" being an archaism; and this is also true of the Merriam-Webster on-line dictionary .
It's curious that I never noticed how philosophy-bound "in virtue of" is. This morning I thought I spoke English; now I find I speak Philosophy....