1. Two opposing scenarios, the "arboreal" hypothesis and the "cursorial" hypothesis, have traditionally been put forward concerning the origins of bird flight.
关于鸟类飞行的起源，传统上提出了两种对立的假设：“地栖说”和“树栖说”。
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2. Two opposing scenarios, the "arboreal" hypothesis and the "cursorial" hypothesis, have traditionally been put forward concerning the origins of bird flight.
