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 sucha suggestion is 
little more than a speculation. What is important is 
this, that the lowest vertebrate has no eyes properly 
