American art from the 20th and 21st centuries is broader, and better than previously acknowledged, especially by museums. As these institutions struggle to become more inclusive than before, and give new prominence to neglected works, they rarely act alone. Essential help has come from people like William Arnett and his exemplary Souls Grown Deep Foundation. Their focus is the important achievement of black self-taught artists of the American South, born of extreme deprivation and social cruelty, raw talent and fragments of lost African cultures.