VISION. 85 



harmonies to be yet discovered, exhibiting an 

 orderly evolution of structure in the animal king- 

 dom ; but it is difficult to see what help it can give 

 to those who must needs have the evolutions of 

 organization accounted for by natural selection. In 

 the very early vertebrate embryo the hollow of the 

 interior of the brain was an open groove, and thus 

 the epithelium lining it, including the bacillary 

 layer of the retina, is originally continuous with 

 the cells of the cuticle all over the body. The lens 

 in both vertebrates and invertebrates is cuticular 

 growth ; and thus in a sense the whole eye is a 

 superficial development in both. 



But the distinction remains that the vertebrate 

 eye is derived from two separate hollows placed 

 apex to apex, and one folded round the other, 

 while the invertebrate eye represents only one 

 of them. Having regard to certain transparent 

 tunicates and to Kowalevsky's observations on the 

 development of the ascidians, in which the eye takes 

 origin from the walls of an originally open neural 

 sac, it seems even possible that the cerebral part of 

 the vertebrate eye is the part homologous with the 

 invertebrate eye, while it is the vertebrate lens which 

 is the superadded structure. But such a suggestion is 

 little more than a speculation. What is important is 

 this, that the lowest vertebrate has no eyes properly 



