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”。

Two opposing scenarios, the "arboreal" hypothesis and the "cursorial" hypothesis, have traditionally been put forward concerning the origins of bird flight.

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因此，你可能会想，现代陆龟可能从早期的陆栖时代就一直呆在陆地上，就像大多数哺乳动物，他们中的一些返回海洋之后所做的那样。

You might think, therefore, that modem land tortoises have probably stayed on land ever since those early terrestrial times, as most mammals did after a few of them went back to the sea.

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但是其它因素也会带来影响：栖地的流失、污染、疾病和入侵的物种。

But other factors have also had an impact: habitat loss, pollution, disease and invasive species.

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