A little historical perspective is required to appreciate the import of the Museum of Modern Art's stimulating new exhibition "Groundswell: Constructing the Contemporary Landscape." A century ago, if you were building a city park system, a college campus, a world's fair, a suburban community, or a new town, the first person you consulted was a landscape architect. Thanks to Frederick Law Olmsted—who built all of the above and virtually invented the discipline—landscape architects were the pre-eminent land planners of the day. Olmsted's so-called Emerald Necklace in Boston, for example, is a combination of park, parkway, and urban engineering that winds its way through several neighborhoods. That was the American city: a commercial free-for-all, given a human face by Arcadian stretches of parks, boulevards, avenues, and residential Elm streets.