Descent of Man
For Barfield, of course, there was no "descent of man" as Darwin (or for that matter Teilhard de Chardin) conceived of it: though accepting of the idea of evolution, Barfield drew upon the use of the metaphor as understood in a tradition stretching back to Plato and beyond.1

Consider Burgeon's discussion of the descent in Worlds Apart:

"But in the past," Burgeon objected, "there still remained some memory of pre-existence. And when that was gone, there still remained the tradition. Why is it that all the myths of men's origin, without exception, tell of a descent--a descent from some former state of happy intercourse with the gods into present exclusion from it, from a paradisal state of light and joy into an earthy one of sorrow and darkness? I defy you to find a single myth that portrays an ascent from unconsciousness to consciousness, from ape to man, from matter to spirit. Why not, if it was the fact? Why should those who were still near to the great change know nothing of it, report nothing of it?
    I will tell you why. It is because the story of a linear ascent is itself the product of the last stage of total exile from even the memory of the light--the last darkness in which there is nothing left to do, but play the guessing-game with the help of the language-game. I should have thought the analogy was plain to high heaven! And if there has indeed occurred this imprisonment, this descent into the darkness of the material world--and what else is the history of Western civilization?--why may the West not work its way towards an awakening?--an awakening, if you like, to what it has always been in the spirit. . . ." (WA 87)
If humanity did not "descend" in the Darwinian sense, the question still remains: where did we come from? Barfield's answer (offered here by Sanderson in Worlds Apart) is heretical to the core but a central pillar of his thinking nonetheless:
I say [man] was there, in his unconscious, from the beginning. And I say it is just that beginning to which those paradise-myths. . . , found all over the earth, point back; . . . they are a dim recollection in tradition of the state of affairs that obtained before his more conscious life developed. . . . Why do none of the myths anywhere symbolize [the] ascent of man from animals . . .? (159-60)
See in particular "Man, Thought, and Nature" (RCA 223-240) and "Philology and the Incarnation" (RM 228-36).
1Drawing on the work of one of the great scholars of Romanticism, Barfield supplies (in "The Rediscovery of Allegory II") an historical perspective on the descent metaphor:
    In his Natural Supernaturalism, M. H. Abrams lays his finger on the difference between [Neo-Platonic thought & evolution] by remarking that we have transformed Plotinus's emanation into evolution. That is to say, we have ceased to conceive of ascent or 'progress' as representing in any sense a 'return' to the world of archetypes behind the symbols, to the home where with our authentic humanity we belong. . . . But the crucial issue between ascent only and ascent-cum-descent is, I fancy one that will have to be faced before long. . . . And I should say that the necessity of facing that issue will bring into focus an even more crucial one. For the concept of a descent of man presupposes the validity of that overall view of the relation between mind and matter-anathema today to intellectual establishments of both Left and Right-which the Scholastic philosophers classified in their succinct way as the doctrine of universalia ante rem. (RM 109)