32 ESSAY ON CLASSIFICATION 



arrangement of each great type will be more perfected, the greater 

 is likely to appear the difference at the two ends of the series which 

 are ultimately to be brought into connection with those of other 

 series, in any attempt to establish a single series for all animals. 



I doubt whether there is a naturalist now living who could object 

 to an arrangement in which, to determine the respective standing 

 of Radiata, Polyps would be placed lowest, Acalephs next, and 

 Echinoderms highest; a similar arrangement of Mollusks would 

 bring Acephala lowest. Gasteropoda next, and Cephalopoda highest; 

 Articulata would appear in the following order: Worms, Crustacea, 

 and Insects, and Vertebrata, with the Fishes lowest, next Reptiles and 

 Birds, and Mammalia highest. I have here purposely avoided every 

 allusion to controverted points. Now if Mollusks were to follow 

 Radiata in a simple series, Acephala should join on to the Echino- 

 derms; if Articulata, Worms would be the connecting link. We 

 should then have either Cephalopods or Insects, as the highest term 

 of a series beginning with Radiata, followed by Mollusks or by 

 Articulates. In the first case Cephalopods would be followed by 

 Worms; in the second, Insects by Acephala. Again, the connection 

 with Vertebrata would be made either by Cephalopods, if Articulata 

 were considered as lower than Mollusks, or by Insects, if Mollusks 

 were placed below Articulata. Who does not see, therefore, that in 

 proportion as our knowledge of the true affinities of animals is im- 

 proving, we accumulate more and more convincing evidence against 

 the idea that the animal kingdom constitutes one simple series? 



The next question would then be: Does the animal kingdom con- 

 stitute several or any number of graduated series? In attempting to 

 ascertain the value of the less comprehensive groups, when com- 

 pared to one another, the difficulties seem to be gradually less and 

 less. It is already possible to mark out with tolerable precision the 

 relative standing between the classes, though even here we do not 

 yet perceive in all the types the same relations. Among Vertebrata 

 there can be little if any doubt that the Fishes are lower than the 

 Reptiles, these lower than Birds, and that Mammalia stand highest; 

 it seems equally evident that, in the main. Insects and Crustacea are 

 superior to Worms, Cephalopods to Gasteropods, and Acephala and 

 Echinoderms to Acalephs and Polypi. But there are genuine Insects, 

 the superiority of which over many Crustacea would be difficult to 



