According to your correspondent John Schmid (November AD2000), Pope Leo XIII, in his 1880 encyclical Arcanum, merely gives "a description of creation as in the Bible", without thereby implying anything either for or against the theory of human evolution.
Such a reading of Leo XIII's paraphrase of Gen 2:7 and Gen 2:21-22 fails to take into account its literary and historical context. The Pope is insisting here that this Biblical "description" records real historical facts about human origins, and so stands in opposition to the novel theories of persons he describes as "revilers" (vituperatores) of the Christian faith (i.e., Darwin, Huxley, and their followers). Here is the entire Arcanum passage in question (with emphasis added in key expressions):
"What is the true origin of marriage? That, Venerable Brethren, is a matter of common knowledge. For although the revilers of the Christian faith shrink from acknowledging the Church's permanent doctrine on this matter, and persist in their long-standing efforts to erase the history of all nations and all ages, they have nonetheless been unable to extinguish, or even to weaken, the strength and fight of the truth. We call to mind facts well-known to all and doubtful to no-one: after He formed man from the slime of the earth on the sixth day of creation, and breathed into his face the breath of life, God willed to give him a female companion, whom He drew forth wondrously from the man's side as he slept. In bringing this about, God, in His supreme Providence, willed that this spousal couple should be the natural origin of all men."
The idea of Adam's being "formed from dust/slime" (Gen 17) can perhaps be given a figurative, but still basically historical, interpretation that would be compatible with evolution. But the notion of being "formed from a sleeping man's side" is not nearly so malleable: if Gen 2:21-22 is true as history, then woman cannot have evolved. But Leo XIII is clearly insisting that Eve was truly and historically formed from the sleeping adult male (vir).
He even uses the adverb mirabiliter ("wondrously" or "miraculously") to emphasise that her body came about by a supernatural, not a natural, process. This "permanent doctrine" was then confirmed in 1909 by the Pontifical Biblical Commission. Partly, no doubt, because the truth of the New Testament is also at stake here (cf 1 Cor 11:8, 12), the PBC insisted the formation of the first woman from the first man is a truth "touching the foundations of the Christian religion."
FR BRIAN HARRISON OS
Ponce, Puerto Rico