br> In the United States and its territories alone, there are more than 40 national parks, varying considerably in landscape and mood. Each park was chosen for its unique features, representative of a particular region of the country. Everglades National Park, for example, at the southern tip of Florida, is a huge subtropical swamp, which provides refuge for such unusual reptiles as the alligator and for thousands of varieties of birds, including the graceful pink flamingo. In sharp contrast to the Everglades is the arid, dusty park of the West. Mesa Verde National Park in Colorado, for example, preserves the ruins of an Indian civilization which flourished in the twelfth and thirteenth centuries. Today, tourists can wander through the awesome cliff dwellings that the Indians carved out of the steep rock.