5 3 DEGENERA TIO N. 



where the comparison of the degenerate animal, 

 with others like it in structure, but not degenerate, 

 renders the hypothesis of degeneration an unassail- 

 able 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 nev/ habits of life, either the parasitic 

 or the immobile. Other new habits of life appear 

 also to be such as to lead to dec^eneration. 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 power of feeding on the 

 carbonic acid dissolved in the water around them 

 just as green plants do. This would lead to a dege- 

 neration ; they would cease to hunt their food, and 

 would bask in the sunlight, taking food in by the whole 

 surface, as plants do by their leaves. Certain small 

 flat worms, by name Convoluta, of a bright green 

 colour, appear to be in this condition. Their green 

 colour is known to be the same substance as leaf- 

 green ; and Mr. Patrick Geddes has recently shown 



