86 GRADATION AMONG ANIMALS 



the Nautili may be drawn completely within the 

 shell. If this view be correct, these two types 

 occupy an intermediate position between the 

 highest and the lowest divisions of the Animal 

 Kingdom, but stand on equal ground when com-^ 

 pared with each other. 



Another, though a less direct, evidence that 

 there is no absolute structural superiority or in- 

 feriority between these two types as a whole 

 may be found in the fact that the most pro- 

 found naturalists who have attempted a serial 

 arrangement of the whole Animal Kingdom have 

 differed in their estimate of these two divisions, 

 some placing the Mollusks highest, while others 

 have given the ascendency to the Articulates. 



But is there a transition from Radiates to 

 Mollusks, or from Articulates to Vertebrates, or 

 from any one of these divisions into any other ? 

 Let us first consider the classes as they stand 

 within their divisions. We have seen that there 

 are three classes of Radiates, — Polyps, Acalephs, 

 and Echinoderms ; three classes of Mollusks, — 

 Acephala, Gasteropoda, and Cephalopoda; three 

 classes of Articulates, — Worms, Crustacea, and 

 Insects ; and, according to the usually accepted 

 classification, four classes of Vertebrates, — 

 Fishes, Reptiles, Birds, and Mammalia. If there 

 is indeed a transition between all these classes, 

 It must become clear to us, when we have accu- 

 rately interpreted their relative standing. 



