CLASSES OF ANIMALS. 221 



ner in which its plan is executed, or to the plan itself, 

 as a comparison between Bats and Birds, between Whales 

 and Fishes, or between Holothuriaiis and Worms, may 

 easily show; fifthly and lastly, with reference to its last 

 finish, to the execution of the details in the individual 

 parts. 



It would not be difficult to show that the differences 

 which exist among naturalists in their limitation of classes 

 have arisen from an indiscriminate consideration of the 

 structure of animals in all these different points of view, 

 and an equally indiscriminate application of the results 

 obtained, to characterizing classes. Those who have not 

 made a proper distinction between the plan of a structure 

 and the manner in which that plan is actually executed, 

 have either overlooked the importance of the great fun- 

 damental divisions of the animal kingdom, or have un- 

 duly multiplied the number of these primary divisions, 

 basing their distinctions upon purely anatomical consider- 

 ations, that is to say, not upon differences in the character 

 of the general plan of structure, but upon the material 

 development of that plan. Those, again, who have con- 

 founded the complication of the structure with the ways 

 and means by which life is maintained through any given 

 combination of systems of organs, have failed in establish- 

 ing a proper difference between class and ordinal charac- 

 ters, and have again and again raised orders to the rank 

 of classes. For we shall presently see that natural orders 

 must be based upon the different degrees of complication 

 of structure, exhibited within the limits of the classes, 

 while the classes themselves are characterized by the man- 

 ner in which the plan of the type is carried out, that is to 

 say, by the various combinations of the systems of organs 

 constituting the body of the representatives of any of the 



