From Larry Febo:
I`m not sure exactly how feathers developed,but actinofibrils arranged in a
pattern similar to flight feather rachis is very suggestive. Whatever the
path, a changeover from a fully functional membrane to feathers would have
to be gradual, and would require a "reason" for it to happen. I believe the
reason might have been the invasion of a new, and colder enviornment.
Perhaps a more upland enviornment, and natural selection acting on this
primitive wing to cut down on exposed area in a colder enviornment. Contour
feathers, once developed, could have extended to long flight feathers as the
membrane, and wing finger gradually (over generations) shrunk, until this
wing finger was itself no longer necessary.
The passage includes the idea that this transition must have been "gradual". I do not think that is necessarily so. A saltation is also a possibility.