In the retina the rods are more numerous than the cones, while the macula lutea only appears in the Primates in connexion with binocular vision.

In order then to avoid diplopia the macula lutea moves to where the retinal image is formed.

If this did not happen, if this eye remained also immovable, the retinal image would deviate outwards more and more from the macula lutea and diplopia would arise.