open-ended question

write an essay that addresses the following open-ended question:
How has art changed over the period from the Early Renaissance to the Realist movement of the 19th Century?

Please include examples of four shifts that occur over this five hundred year period.  These changes might be in the area of technique, style, subject matter, meaning, and more. 
Please reference at least four works of art from the class as you provide your answer.  You do not need to provide images of the works of art in your essay.

Above is the instruction from the professor. Please use the four artworks that I choose.
1.Cimabue, Madonna Enthroned with Angels and Prophets  ca. 1280-1290.
2.Jan Van Eyck, The Arnolfini Portrait, 1434,
3.Parmigianino, Madonna of the Long Neck, 1534-35. 
4.Fragonard, The Swing, 1767, Oil on Canvas.