CHAPTER IV.— CATAGENESIS. 



WE HAVE been principally occupied so far with 

 progressive evolution or anagenesis. Reference 

 has, however, been made to retrogressive evolution or 

 degeneracy, in Chapter III., in describing the evolution 

 of the Vertebrata, and will be in Chapter V., under the 

 caption ' ' Disuse in Mammalia. " Degeneracy has, how- 

 ever, played a more important part in creation than 

 would be suspected from these references, and I pro- 

 pose in the present chapter to go more fully into its 

 phenomena, which, in the broadest sense, I have called 

 collectively Catagenesis. 



As evidence for degeneracy as a factor in evolution 

 we naturally appeal first to examples in the life histo- 

 ries of plants and animals which are known to us; and 

 then examine the records of the past, in the light thus 

 gained, for evidence of degeneracy in vegetable and 

 animal phylogeny. In both directions we are met by 

 an embarras de richesse, and a few conspicuous cases 

 will have to suffice. 



The parasitic copepod Crustacea undergo a retro- 

 grade metamorphosis, which commences at different 

 periods of the growth history of different genera. Says 

 Claus : "Many parasitic Copepoda, however, pass 



