atavism

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One striking example of an atavism is the human coccyx, or tailbone, which is a relic of the mammalian tail.
关于返祖的一个有力例证就是人类的尾骨，它是哺乳动物尾巴的遗留物。
Xiong Bite explains imperialism from sociological viewpoint, he thinks imperialism is the atavism in structure of a kind of society.
熊彼特从社会学的观点来解释帝国主义，他认为帝国主义是一种社会结构中的返祖现象。
If learning and gentility are signs of civilization, perhaps our almost-big brains are straining against their residual atavism, struggling to expand.
如果学习和温婉是文明的标志，那么也许可以说我们那不算太大的大的大脑正在超越落后的残余，努力扩展着自己。

What is causing the U.S. Ambassador, Robert Strauss, to bewail the "wild card" of the Red Army, and inducing our new Director of Central Intelligence to send solons to dictionaries with his predictions of "xenophobic atavism"?
CENTERFORSECURITYPOLICY: Gorby Invites the Deluge

the recurrence in a plant or animal of certain primitive characteristics that were present in an ancestor but have not occurred in intermediate generations