340 ADAPTATIONS TO SPACE AND MOTION 



functional and provided with a lens and a fairly fine-grained retina in 

 Sphenodon and in some lizards. 



One can understand why the eventual single parietal eye of the rep- 

 tiles should have ceased to be an eye, and disappeared, in their avian 

 and mammalian descendants; for, being unprovided with lids, it could 

 not clear a way for its operation through the shrubbery of feathers or 

 hair. But a good question which has never been answered — ^perhaps never 

 even raised before — is : why did the median eyes ever lose their bilateral 

 paired condition, and why was one member of the lamprey's tandem 

 combination eliminated by higher forms which perfected the eyes but 

 kept only one of them? 



This question is not of any real importance; but it is an interesting 

 one, and perhaps we can answer it in the light of the foregoing discus- 

 sion of the universal fusibility of the binocular images of the lateral eyes. 

 It seems quite possible that the dorsal eyes, being less fortunate in their 

 cormections within the brain, yielded sensory impressions which were 

 incapable of any sort of fusion — just as our two hands, separately and 

 simultaneously touching steel and leather, give us normal impressions 

 of those two materials and not of a single hybrid substance of inter- 

 mediate or summated properties. 



If no fusion could be accomplished between the members of a pair of 

 dorsal eyes, no harm was done as long as the eyes were not capable of 

 seeing pictures. But as their lenses evolved and their retinae improved, 

 this point of perfection was reached and difficulties arose. So far as we 

 can tell, none of these median eyes ever had any muscles to move them, 

 or indeed any accessory organs of any kind. With rivalry or diplopia 

 occurring in each pair, the number of pairs was radically reduced to two, 

 and one member of each pair was discarded (Fig. 54b and c, p. 126). 

 When at last the eyes became such good ones that diplopia between the 

 unconvergible tandem eyes became intolerable, one of them had to go. 

 The solitary remaining median eye could then be perfected to any degree 

 by the ancient amphibians and early reptiles, without further diplopic 

 trouble or even any danger of its field of view overlapping into the fields 

 of the lateral eyes. 



The reptilian lateral eyes are such very fine visual organs, however, 

 that in this group the median eye lost most of its importance. Gone in 

 the turtles, gone in many lizards and in all snakes, it was already well 

 on its way out of the vertebrate picture even before it was finally buried 

 beneath the plumage and fur of the birds and mammals. 



