THE EVOLUTIONARY HISTORY OF EYE ~OVE~E~TSl

The nature of eye ~lovements is discussed from the evolutionary point of view. Their origin lies in the need to keep an object fixed on the retina, not in the need to scan the surroundings. Distinctions are made between egocentric and oculocentric directions in their dependence upon voluntary and involuntary eye movements. The possibility, in Man, of retinal correspondency between two regions of the same eye is discussed. RCum&-La nature des mouvements des yeux est discutte d’un point de vue &olutionniste. Leur origine provient de la n&essit& de conserver la fixit& d’un objet sur la r&tine, et non du besoin de balayer les environs. On fait la distinction entre les directions egocentrique et ~ulocentrique, en liaison avec les mouvements volontaires et involontaires des yeux. On discute la possibilit6, chez l’homme, d’une correspondance rbtinienne entre deux regions du m&me oeil. Zusammenfassung-Es wird die Natur der Augenbewegungen vom entwicklungsgeschichtlichen Gesichtspunkt betrachtet. Sie entspringen der Notwend~gkeit, Bilder von Sehobjekten auf der Retina zu fixieren, nicht der Notwendigkeit, das Umfeld abzutasten Es wird unterschieden zwischen egozentrischen und okulozentrischen Richtungen in AbhPngigkeit von willkiirlichen und unwillkiirlichen Augenbewegungen. Ferner wird die MGglichkeit einer retinalen Korrespondenzzwischen zwei Bereichen des nsmlichen Auges beim Menschen diskutiert. OF ALL the questions that may arise about eye movements, probably the least likely one is the simple question: “Why?” Optometrists, ophthalmologists and neuroanatomists seem never to wonder why our eyes should move at all. I am none of these, but a backslid zoologist, with my attention still-from force of habit-upon the evolutionary backgrounds of things. Looking at any biological system, I may wonder about it in several ways; but one of my wonders will always be, how did it come to be as it is-how did it get this way? Man seems to have to have eye movements. I have a fovea in each retina. With it, I see fine detail in a patch of space as big as three or four full moons. To see this well here and there, I must aim the fovea here and there-and it is more economical to turn an eyeball than a whole head. Again, unless my convergence could be altered I would see most objects double. Here, head movements could serve in place of eye movements only if the head were carried toward or away from the object until the latter was imaged upon both foveas. Lacking eye movements, then, Man would require to have far more elaborate head-movements, and he would really be better off with one eye than he is with two. But, there are other animals, such as the owls, which have both foveas and binocular vision and yet have no eye movements. On the other hand there are many animals, including most fishes, which have 1 Essentially as read before the American Academy of Optometry in its annual meeting; San Francisco, 10-13 December 1960.