140 



THE LIVING WORLD. 



their digestive organs, etc. Even the barnacles may 

 be included under this head, for although not para- 

 sitic, they have acquired stationary habits, and have 

 lost the locomotive organs they once possessed. 

 While, then, the general history of the animal king- 

 dom has been an advance, this advance is compatible 

 with many cases of retrogression. All groups ad- 

 vance to a culmination, and then decline through 

 decaying forms, and innumerable instances of para- 

 sitism have produced thus their inevitable degrading 

 effects. 



