The general dictionary definition of "illustration" is "to clarify, explain, or make vivid." In art, the illustration focuses on subject rather than form. The style began in early cave paintings, grew in the 15th century when books began to be published with wood-cut prints, and became quite popular in the early years of the 19th century, when illustrations were published in serial novels that were widely circulated. Illustrative art can be produced in watercolors, oil, pen and ink, pencil, engravings, and pastels. Popular illustrators include John James Audubon and Leonardo da Vinci.