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It is almost always due to some very special circumstances that traces of land animals survive, as by falling into inaccessible caves, or into an ice crevasse, like the Siberian mammoths, when the whole animal is sometimes preserved, as in a refrigerator.
几乎总是由于某些特殊的条件，陆地动物的遗体才被存下来，如掉进难以到达的洞穴，或者像西伯利亚长毛象那样掉进冰窟中，有时整个动物像被放在冰箱里一样被保存下来。
as by falling into inaccessible caves, or into an ice crevasse：as表示“按照”“依照”“像”，引导方式状语从句， 修饰 survive，从句置于主句之后。
like the Siberian mammoths作方式状语修饰动词 falling into，表示像西伯利亚长毛象那样。
when...refrigerator是When引导的定语从句修饰mammonths