292 C. U. Ariüns Kappers 



Evidently this reasoning is teleological and is right from the darwinian 

 point of view regarding the survival of the fittest. It does not however 

 explain the way in wliich this fittest arrangement is made ^. 



As it is consistent with hodogenetic questions to look out for a 

 causal explanation, we may start from the fact that simultaneously 

 stimulated nerve-tracts show a tendency to run together and to the 

 same centre. Now, when the eyes are both directed frontally, their 

 axes not converging, but running parallel to eachother (this being a 

 rule in passive, binocular sight as exhibited by some mammals and 

 preceding active, binocular convergency), a lateral object will simul- 

 taneously stimulate the nasal retinal fibers of one eye and the tem- 

 poral fibers of the other (figure 2 2 A and B), in consequence of 

 which these fibers will join each other running to the same or adja- 

 cent cells. 



This unión of the temporal fibers of one eye with the nasal fibers 

 of the other might be eñ'ected however in two ways, either temporal 

 fibers may arrange themselves with nasal fibers of the other eye by 

 taking a homolateral course or the nasal fibers may acquire a homo- 

 lateral course and associate with temporal fibers of the other eye. — 

 Since we know that only temporal fibers change their course in a ho- 

 molateral one whilst the nasal fibers remain unchanged, we must explain 

 why especially the temporal fibers change their central behaviour, eñ'ec- 

 ting the unión by taking a homolateral course along nasal fibers of the 

 other side. 



Every attempt to give a causal explanation of the partial decussa- 

 tion must start from the temporal fibers, as it is here that the change- 

 occurs, whilst in the course of the nasal fibers there is no difierence be- 

 tween animáis with a total and a partial decussation. 



Xow it seems possible to find the solution of this question in the fol- 

 lowing facts: 



A frontal object, lying in the median line of the head will project itself 

 in both the eyes on t/ie temporal fields of the retina, when the eyes are 

 directed frontally with parallel axes, without active convergency, yet with 

 ^partial (passive) binocular sight (fig. 22 C). 



' The same may be said of all teleological explanations. That something 

 useful will remain is conceivable. 



How does it come to existence however? 



