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Incipient wings

        Now here's a question about flight evolution- how do  aerofoils
become large enough to make a substantial difference in prey pursuit/
predator evasion on or from the ground?  
        In other words, the old "what good is part of a wing?" I think the
gliders pretty well establish that this pathway is easy enough to achieve
for treetop leapers. Every additional square inch is probably going to
slow falling rates and slightly extend leaping range, even at the crucial
phase when nothing even remotely resembling an aerofoil exists. I submit
that treetop leapers can gain from going from 0% of a wing to 5% of a wing
(say, slightly extended insulatory feathers on the forearm and tail,
something like the sakis), likely every additional milimiter of a wing
(selective pressure on Darwin's finches acts on something like milimeter
or tenth-milimiter beak length differences).
        My question isn't whether or not it's feasible once you've got
there. Chickens establish that you can go from 100% to  50%. But do they
establish that you can go the other way around- 0% to 50% of a wing?