America's system of national parks, a model for similar set-ups around the world, was based on the idea of limiting human presence to passing visits, rather than permanent habitation.

美国的国家公园就是分布于世界各地类似的系统中的典范，其理念就是在保护区内人类只是游客，而不能成为永久居民。

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White Sands National Monument is part of America's National Parks System.

VOA: special.2010.01.04

In the US, more than 270 million people visit the national parks system each year.

BBC: Birth and spread of the world��s national parks