DEGENERA TION. 49 



perfectly transparent and has its eye actually inside 

 its brain. The light passes through the transparent 

 tissues and acts on the pigmented eye, lying deep 

 in the brain. We are thus led to the conclusion — 

 and I believe this inference to be now for the first 

 time put into so many words — that the original Ver- 

 tebrate must have been a transparent animal, and 

 had an eye or pair of eyes inside its brain, like that of 

 the Ascidian tadpole. As the tissues of this ances- 

 tral Vertebrate grew denser and more opaque, the 

 eye-bearing part of the brain was forced by natural 

 selection to grow outwards towards the surface, in 

 order that it might still be in a position to receive 

 the influence of the sun's rays. Thus the very 

 peculiar mode of development of the Vertebrate eye 

 from two parts, a brain-vesicle (Fig. 23, A a, and 

 B p ?'), and a skin-vesicle (Fig. 23, B e, /), is 

 accounted for. 



The cases of degeneration which I have up to this 

 point brought forward, are cases which admit of very 

 little dispute or doubt. They are attested by either 

 the history of the individual development of the 

 organisms in question, as in Sacculina, in the 

 Barnacle, and in the Ascidian, or they are cases 



E 



