If the Mall in Washington, D.C., is the "nation's lawn," then the institutions that line it must be the national lawn ornaments. It is a motley architectural collection, ranging from standouts such as John Russell Pope's National Gallery of Art and I.M. Pei's East Building to the distinctly pedestrian museums housing natural history, American history, and "air and space." There are at least two garden gnomes (the Smithsonian "Castle" and the polychrome Arts and Industries Building), a concrete birdbath (the Hirshhorn), and even a pink flamingo (the National Museum of the American Indian). This heterogeneous assortment is not what the designers of the Mall, Charles Follen McKim and Frederick Law Olmsted Jr., had in mind, but the result is actually appropriate for a large and varied democracy. Two weeks ago, the Smithsonian announced an architect to design the National Museum of African-American History and Culture, the latest addition to this display. Is it a nymph, a gnome, or another flamingo?