``BECAUSE of its unique character, Mexican muralism is perhaps the only completely original contribution by American artists to the development of modern art,'' writes Mexican historian Lu'is Cardoza y Arag'on in the sumptuous catalog that accompanies the Diego Rivera retrospective on view here at the Philadelphia Museum of Art. While some might object to Mr. Arag'on's statement as hyperbolic or chauvinistic, few would deny him his point altogether. For it is true that the Mexican mural movement did not grow out of the usual European influences but sprang full blown, like Athena from the head of Zeus, from a combination of indigenous traditions and national necessity.
As the Mexican Revolution chipped away at the repressive oligarchic regime and gradually brought about agrarian, educational, and labor reforms during the second decade of this century, a wave of populist fervor swept over Mexico and found pictorial expression in the hands of the muralists -- Rivera, Jos'e Clemente Orozco, and David Alfaro Siqueiros. Rivera put their philosophy in the following perspective: ``Mexican muralism has not brought anything new to the universal plastic arts, nor to architecture, and even less to sculpture. But Mexican muralism -- for the first time in the history of monumental painting -- ceased to use gods, kings, chiefs of state, heroic generals, etc. as central heroes . . . . For the first time in the history of art, Mexican mural painting made the masses the hero of monumental art. That is to say, the man of the fields, of the factories, of the cities, and towns. When a hero appears among the people, it is clearly as part of the people and as one of them.''
Of the triumvirate, Rivera (1886-1957) had the greatest influence on North American artists, such as Thomas Hart Benton, Ben Shahn, and Works Progress Administration muralists during the 1930s, because of the several mural and fresco cycles he executed in the United States. The most celebrated of these are in San Francisco at the Pacific Stock Exchange, the City College, the Art Institute, and in Detroit at the Institute of Arts, which boasts a 27-panel tribute to the city's industry.