Watercolor is the oldest painting medium known. It dates back to the early cave dwellers 'who discovered they could add lifelike qualities to drawings of animals and other figures on the walls of caves by mixing the natural colors found in the earth with water. Fresco, one of the greatest of all art forms, is done with watercolor. It is created by mixing pigments and water and applying these to wet plaster. Of the thousands of people who stand under Michelangelo's heroic ceiling in the Sistine Chapel, very few are aware that they are looking at perhaps the greatest watercolor painting in the world. The invention of oil painting by the Flemish masters in the fifteenth century led to a decline in fresco painting, and for the next several centuries watercolor was used mainly as a medium for doing preliminary sketches or as a tool for study. It was not until the eighteenth and nineteenth centuries that English painters reinstated watercolor as a serious art form. The English have a notorious love for the outdoors and also great fondness for small, intimate pictures. The subdued tones of watercolor had a remarkably strong appeal for them. The popularity of watercolor continued to grow until in the twentieth century the United States passed England as the center for watercolor, producing such well - known watercolor artists as Thomas Eakins and Andrew Wyeth. What is the main theme of the passage?