各个物种的灭绝原因都不相同，而且找不到任何遗传特性来区别灭绝物种与存活物种。
The extinctions differ from species to species, and we don't find any common genetic characteristic to distinguish extinct from surviving species.
长度可达18米（60英尺）的北太平洋东部露脊鲸，数目远在50以下，低于世界自然保护联盟作为一个可能存活物种的起码数量。
Numbering well under 50, the eastern North Pacific right whale --which can reach up to 18 metres (60 feet) in length -- falls below theIUCN's threshold of likely viability as a species.
长度可达18米（60英尺）的北太平洋东部露脊鲸，数目远在50以下，低于世界自然保护联盟作为一个可能存活物种的起码数量。
Numbering well under 50, the eastern North Pacific right whale --which can reach up to 18 metres (60 feet) in length -- falls below theIUCN's threshold of likely viability as a species.
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