The disease develops gradually and imperceptibly.

该疾病难以察觉地逐渐发展着。

《柯林斯英汉双解大词典》

Slowly and almost imperceptibly the road started to climb upwards, looping its way in languid curves around the forested hills.

道路缓慢而几乎不可察觉地开始向上攀升，在林木环绕的山丘上曲折地蜿蜒前进。

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Allopatric speciation can also be brought by the imperceptibly slow but colossal movements of the tectonic plates that make up Earth's surface.

构成地球表面的那些构造板块极其缓慢但剧烈的运动，也可能导致异地物种形成。

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"The heat causes the wires to expand, imperceptibly to the naked eye, " Donovan said.

WSJ: Commuters warned: Summer heat may slow Metro-North

He might have added that it made progress in a normal way: almost imperceptibly.

ECONOMIST: IMF/World Bank: Business as usual? | The

History is littered with once-dominant institutions that were imperceptibly hollowed out and then suddenly collapsed.

ECONOMIST: Political parties: Lonely at the top | The