102 THE EVOLUTIONIST AT LARGE. 



supposition, and for this reason : In our 

 heads, and those of all vertebrates, there is a 

 curious cross-connection between the eyes 

 and the brain, so that the right optic nerve 

 goes to the left side of the brain and the left 

 optic nerve goes to the right side. In higher 

 animals, this ' decussation,' as anatomists call 

 it, affects all the sense-organs except those of 

 smell ; but in fishes it only affects the eyes. 

 Now, as the young ascidian has retained the 

 ancestral position of his almost useless eye so 

 steadily, it is reasonable to suppose that he 

 has retained its other peculiarities as well. 

 May we not conclude, therefore, that the 

 primitive vertebrate had only one brain-eye ; 

 but that afterwards, as this brain-eye grew 

 outward to the surface, it split up into two, 

 because of the elongated and flattened form of 

 the head in swimming animals, while its two 

 halves still kept up a memory of their former 

 union in the cross-connection with the oppo- 

 site halves of the brain ? If this be so, then 

 we might suppose that the other organs 



