J 



D DEGENERATION. 



application of this hypothesis have not, I think, met 

 with the consideration which they merit. 



The statement that the hypothesis of Degeneration 

 has not been recognised by naturahsts generally as 

 an explanation of animal forms, requires to be cor- 

 rected by the exception of certain kinds of animals, 

 namely, those that are parasitic or quasi-parasitic. 

 With regard to parasites, naturalists have long re- 

 cognised what is called retrogressive metamorphosis ; 

 and parasitic animals are as a rule admitted to be 

 instances of Degeneration. It is the more remark- 

 able whilst the possibility of a degeneration — a loss 

 of organisation making the descendant far simpler or 

 lower in structure than its ancestor — has been ad- 

 mitted for a few exceptional animals, that the same 

 hypothesis should not have been applied to the 

 explanation of other simple forms of animals. The 

 hypothesis of Degeneration will, I believe, be found 

 to render most valuable service in pointing out the 

 true relationships of animals which are a puzzle and 

 a mystery when we use only and exclusively the 

 hypothesis of Balance, or the hypothesis of Elabora- 

 tion. It will, as a true scientific hypothesis, help us 

 to discover causes. 



