328 ADAPTATIONS TO SPACE AND MOTION 



(c) If they see the object singly with two separate eyes, they must 

 have fusion of the two images of the object. Now: 



(d) If they have fusion, they have parallactic localization of the object 

 in space; and 



(e) If they have fusion of their disparate right- and left-eyed views of 

 a solid object, they have a percept of its solidity. 



But if all these ifs are true, they still leave unexplained why the optic 

 nerves became incompletely decussated in the mammals. If our reasoning 

 so far is correct, partial decussation is no prerequisite at all for fusion 

 and stereopsis. What, then, does depend upon it, and what special 

 ability has it given to the mammals which lower forms do not possess? 



It has already been pointed out that partial decussation is associated 

 with conjugate eye movements and with corresponding retinal points or 

 fixed local signs of direction. If a fish with unconjugated eye movements 

 can look at an object now with the eyes in one position, now in another, 

 there are surely no fixed local signs of direction in the retina of the fish, 

 and no corresponding points. And, if a dog can see a rabbit singly with 

 one degree of convergence at first, and with another degree when he has 

 become excited by the chase, there would seem to be no corresponding 

 points involved. But where the eyes have become completely conjugated 

 in their movements, so that looking to a certain distance always means 

 a particular degree of convergence, then fixed local signs of direction, 

 or correspondency of points, could logically be evolved and could never 

 be detrimental so long as the conjugation remained perfect. 



When conjugated eye movements were evolved by the mammals, this 

 led to the freezing of local directional signs, which then ceased to depend 

 upon the position of the eye in the orbit. This in turn made the mammals 

 dependent upon the system of corresponding points for the maintenance 

 of fusion. How, then, did the conjugation itself arise? Well, if we wanted 

 to revise the nervous system of a fish or a bird to facilitate conjugation 

 of the eye movements, we could not do better than to connect each retina 

 to each of the nerve centers which in turn are connected with the muscles 

 of both eyes. Then, community of vision of the two eyes could be most 

 conveniently made to result in community of action. With the two eyes 

 seeing the same thing, it is optically desirable that they each face it 

 squarely. If, when one eye aims at and accommodates for a particular 

 point in space, the other eye automatically aims at and accommodates 

 for that same point (even if covered, or even after the eye-muscles have 



