LEADING GROUPS OF EXISTING SYSTEMS 149 



and an accurate knowledge of their anatomy the surest way to dis- 

 cover their natural limits. And yet, with this standard before them 

 naturalists have differed and differ still greatly in the limits they 

 assign to classes and in the number of them they adopt. It is really 

 strange that, applying apparently the same standard to the same ob- 

 jects, the results of their estimation should so greatly vary; and it was 

 this fact which led me to look more closely into the matter and to 

 inquire whether, after all, the seeming unity of standard was not 

 more a fancied than a real one. Structure may be considered from 

 many points of view: first, with reference to the plan adopted in 

 framing it; secondly, Avith reference to the work to be done by it, 

 and to the ways and means employed in building it up; thirdly, with 

 reference to the degrees of perfection or complication it exhibits, 

 Avhich may differ greatly, even though the plan be the same, and the 

 ways and means employed in carrying out such a plan should not dif- 

 fer in the least; fourthly, with reference to the form of the whole 

 structure and its parts, which bears no necessary relation, at all events 

 no very close relation, to the degree of perfection of the structure, 

 nor to the manner in which its plan is executed, nor to the plan it- 

 self, as a comparison between Bats and Birds, between Whales and 

 Fishes, or between Holothurians 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 w^ould 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 over- 

 looked the importance of the great fundamental divisions of the ani- 

 mal kingdom, or they have unduly multiplied the number of these 

 primary divisions, basing their distinctions upon purely anatomical 

 considerations, 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 confounded 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 



