“Very primitive forms of life have been able to orient themselves to sunlight, through the development of light sensitive spots on their skin-surface, such as the light sensitive cells of worms. The sea star also has light sensitive spots on the ends of its star arms. Molluscs and ringworms have a more protected beaker-eye, whereby the light sensitive cells lie in an indentation in the skin. Inkfish even have two convex beaker eyes which are filled with sea-water. The beaker opening can be made larger or smaller through a circular muscle. Other kinds of eyes in primitive life forms are sealed over with mucus.

Higher life forms have transparent tissue over the opening, which is somewhat convex in order to collect the light waves on the back of the indentation (the retina); this is what is found in more highly developed sorts of inkfish and snails. The embyological development of vertebrate animals shows that the eye is formed by a bulge, the inside of a part of the central nervous system, pushed out, instead of the surface indentation pushed in of more primitive life forms. The sight organ has evolved, via primitive life forms, through the qualities of sunlight into the sight organ as we know it. Therefore, when we want to judge a color, we use light which has the qualities of sunlight, as we experience it after it passes through the atmosphere.”

—Franz Gerritsen, from his Theory and Practice of Color: A Color Theory Based on Laws of Perception, 1974, translation by Ruth de Vriendt.