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It seems to me from the recent comments that there might be two
different camps in the "trees-down" group.  The first group appears to
support the developed of powered flight from some sort of glider, first
using gravity to assist flight and then developing the capability to lift body
mass actively against gravity.  Some others have been arguing that flight
developed to assist an arboreal theropod in jumping from one perch to a
higher one.  This latter scenario doesn't seem too much different, in my
opinion, from the cursorial theory.  Both involve the use of the forelimb
primarily or exclusively for propulsion upward through the air against
gravity, both would presumably use the forelimbs for stabilization and
guidance while moving through the air, and both would require precision
landings with the center of gravity directly above the landing point.  The
precision bipedal landing would be necessary for the rough terrain
situation, for catching a moving prey, for running and leaping onto a low
branch, or for leaping from branch to branch.  Perhaps the point-to-point
arboreal theory might explain the differences between the claws of
Archeopteryx and other similar theropods, with the former being more
adapted to grasping and perching.  The other arboreal group would
consist of the true gliders.  Gliders  using gravity-assisted flight seem to
have a more flexible "wing" membrane that involves all four limbs, and
frequently make landings that rely more on grasping ability than balance. 
Perhaps we should have "cursorial", "trees-down", and "trees-up"