First, that the National Parks must be maintained in absolutely unimpaired form for the use of future generations as well as those of our own time; Second, that they are set apart for the use, observation, health, and pleasure of the people; and Third, that the national interest must dictate all decisions affecting public or private enterprise in the Parks.[339] Since then the program has made steady progress. Its purposes and policies have been enlarged and clarified. The acceptable qualities of parkhood were defined. Such an area must be supremely significant, having a national appeal whether scenic, archaeological, scientific, or historical. Only such features of natural architecture were to be included as would represent the highest accomplishment within its class. For example, Grand Canyon of the Colorado exemplifies the most extraordinary achievement of stream erosion, whereas Yellowstone is most unique in the realm of thermal activity.[340] The National Park represents the apex of the conservation program, wherein the principle of optimum use is the dominating force. The twin purposes of enjoyment plus conservation always remain uppermost. Commercialization beyond actual requirements is not to be tolerated, and to this end close supervision is maintained. Natural species of animals and plants were to abide in normal relationships, free from man’s interference, except under urgent circumstances such as were described in the discussion of wild life control. The primitive appearance of Yellowstone forests is distasteful to some people. It was a carping German traveler who said, “Look at your dead trees and burned stumps in the woods ... and your streams are full of driftwood. It is not cared for.”[341] In that sense Yellowstone is not a park but a wilderness full of the beauty of natural disorder. All things remain as nature leaves them. No man disturbs landslide, log jam, or wind-swept lodgepole avalanche.