152 ESSAY ON CLASSIFICATION 



the study of the animal kingdom that a knowledge of their essential 

 features is rightly considered as the primary object of all investiga- 

 tions in comparative anatomy, are generally represented as exhibit- 

 ing each some essential modification of the type to which they belong. 

 This view, again, I consider to be a mistaken appreciation of the 

 facts to which Cuvier has already called attention, though his warn- 

 ing has remained unnoticed.^ There is in reality no difference in 

 the plan of animals belonging to different classes of the same branch. 

 The plan of structure of Polypi is no more a modification of that of 

 Acalephas, than that of Acalephae or Echinoderms is a modification 

 of the plan of Polyps; the plan is exactly the same in all three; it may 

 be represented by one simple diagram and may be expressed in one 

 single word, radiation; it is the manifestation of one distinct, char- 

 acteristic idea. But this idea is exhibited in nature under the most 

 different forms and expressed in different ways by the most diversi- 

 fied combinations of structural modifications and in the most varied 

 relations. In the innumerable representatives of each branch of the 

 animal kingdom it is not the plan that differs, but the manner in 

 which this plan is executed. In the same manner as the variations 

 played by a skilful artist upon the simplest tune are not modifica- 

 tions of the tune itself, but only different expressions of the same 

 fundamental harmony, just so are neither the classes, nor the orders, 

 nor the families, nor the genera, nor the species of any great type, 

 modifications of its plan, but only its different expressions, the differ- 

 ent ways in which the fundamental thought embodied in it is mani- 

 fested in a variety of living beings. 



In studying the characteristics of classes we have to deal with 

 structural features, while in investigating their relations to the 

 branches of the animal kingdom to which they belong we have only 

 to consider the general plan, the framework, as it were, of that struc- 

 ture, not the structure itself. This distinction leads to an important 

 practical result. Since in the beginning of this century naturalists 

 have begun, under the lead of the German physiophilosophers,^ to 



^ Regne animal (2d ed.), I, 48. 



® [Agassiz is commenting on the early nineteenth-century views of organic creation 

 identified primarily with German speculative biology and also known as Naturphilo- 

 sophie. Goethe and Oken were leading advocates of this idealized view of nature, and 

 Agassiz was very much impressed by it while a student in Germany. In later life he 

 was critical of this viewpoint as tending to support a concept of development from 

 lower to higher forms in a unified fashion. This ambivalence can be seen in his subse- 

 quent analysis of Oken's system of classification.] 



