"excepting only yosemite, hetch hetchy is the most attractive and wonderful valley within the bounds of the great yosemite national park and the best of all the camp grounds. people are now flocking to it in ever-increasing numbers for health and recreation of body and mind. though the walls are less sublime in height than those of yosemite, its groves, gardens, and broad, spacious meadows are more beautiful and picturesque. . last year in october i visited the valley with mr. william keith, the artist. he wandered about from view to view, enchanted, made thirty-eight sketches, and enthusiastically declared the in varied picturesque beauty hetch hetchy greatly surpassed yosemite. it is one of god's best gifts, and ought to be faithfully guarded."
john muir, century magazine, 1909

john muir's thought above expresses what major change in americans' views of the natural environment?