40 DEGENERATION : I 



Ascidian tadpole. As the tissues of this ancestral 

 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 r) and 

 a skin-vesicle (Fig. 23, B e, I), 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 or- 

 ganisms in question, as in Sacculina, in the Barnacle, 

 and in the Ascidian, or they are cases where the com- 

 parison of the degenerate animal with others like it 

 in structure, but not degenerate, renders the hypothesis 

 of degeneration an unassailable one. Such cases are 

 the Acarus or mite, and the skin-worm (Demodex). 



We have seen that degeneration, or the simplifica- 

 tion of the general structure of an animal, may be due 

 to the ancestors of that animal having taken to one of 

 two new habits of life, either the parasitic or the im- 

 mobile. Other new habits of life appear also to be 

 such as to lead to degeneration. Let us suppose a race 

 of animals fitted and accustomed to catch their food, 

 and having a variety of organs to help them in this 

 chase — suppose such animals suddenly to acquire the 



1 I do not at the present time (November 1889) attach great im- 

 portance to the above suggestion. The facts admit of other possible 

 and plausible interpretations. 



