698 THE EYE IN EVOLUTION 



Newton (1704), WoUaston (1824), J. Muller (1826) and others. An alternative 

 view explained the phenomenon of binocular vision by denying its existence and 

 assuming that one eye only was able to see at a time ; this was originally offered 

 by Porta (1593) and elaborated by Gassendi (1658) and du Tour (1743), and in 

 miore recent times by such natural philosophers as Wundt (1862). A third view, 

 originating with Kepler (1611) and elaborated by Porterfield (1759) and main- 

 tained by such observers as Sherrington (1906) and Ovio (1927), postulated a 

 purely perceptual basis for the phenomenon ; so far as sensory perception is 

 concerned the chiasmal decussation or the laterality of the cerebral terminal is 

 immaterial, for a mental synthesis can deal with either topographical scheme. 



It would appear that a reservation may have to be made in this generaliza- 

 tion in the phenomenon of the interocular transfer of impressions. In man, 

 an eye trained to a task while the other eye is occluded can automatically 

 be replaced by the latter without detriment to his performance ; in infant 

 chimpanzees such a transference is not complete but the task can be re-learned 

 by the second eye very readily (Chow and Nissen, 1955). In fish, however, 

 Sperry and Clark (1949) found that this did not appear to be the case ; if gobies 

 (Bathygobius) were trained to swim towards the upper of two objects with one 

 eye occluded, occlusion of the other eye was followed by a large increase in 

 mistakes which were immediately rectified when the first eye was again occluded. 

 In pigeons, however, Seigel (1953) fovmd that they were able to effect immediate 

 transfer of a circle-versus-triangle discrimination from an eye used in training 

 to the other not so used ; such an immediate transfer occurs in cats even after 

 section of the crossed fibres of the chiasma (Myers, 1955). 



If decussation of the optic nerve fibres is without great sensory 

 significance, the occurrence of partial decussation in the Placentals and 

 the gradual increase in the number of uncrossed fibres until they reach 

 almost 50% of the total in the Primates — ^presumably a progressive 

 element in evolution — must receive some other explanation. The fact 

 remains that (with the exception of the presence of some non-decus- 

 sating fibres in snakes the significance of which is unknown) in animals 

 below Placentals decussation is complete no matter how large the 

 binocular field, in Placentals decussation is partial no matter how 

 small the binocular field. It is obvious that if a high degree of stereo- 

 scopic vision is to be attained, a mechanism of extreme exactitude 

 must be developed to ensure that, so far as it is possible, the two eyes 

 move as a unity, preserving a mutual relationship so that in all 

 positions the images of each object binocularly fixated will fall on 

 corresponding points of the retinse which have become functionally 

 associated with each other. If adequate motorial coordination is to 

 be attained it is essential that, the two eyes be controlled by the higher 

 centres as a unitary organ ; just as binocular sensations are regarded 

 introspectively as balanced in the median sagittal plane of the head, 

 the taxis of the eyeballs must be transferred each from its own sagittal 

 plane to the median sagittal plane of the body. As is seen in the limbs, 

 the taxis of the muscles situated (functionally) to the right (for example, 

 the right external rectus and the left internal rectus) is entrusted to 



