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This is one of two evolutionist theories regarding how reptiles—a terrestrial life form—began
to fly. According to the arboreal theory, the ancestors of birds were tree-dwelling
reptiles that gradually developed wings by leaping from branch to branch. (The other
view is the cursorial theory, which maintains that birds took to the air directly from the
ground.) But this first theory is utterly imaginary, and has no scientific evidence
to support it.
John Ostrom, who first proposed the cursorial theory, admits that the proponents of both
hypotheses can do nothing more than speculate. He wrote that his theory of "cursorial predator"
was in fact speculative—but the arboreal theory was also speculative.
In addition, none of the transitional forms (See Transitional form) that should have existed
on Earth in ages past has ever been discovered