148 ESSAY ON CLASSIFICATION. Part L 



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

 to which Cuvier has already called attention, though his warning 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 Acalephoe, than that of Acalepha? 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, characteristic idea. But 

 this idea is exhibited in nature under the most different forms, and expressed in 

 different ways, by the most diversified 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 modifications 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 different ways in 

 which the fundamental thought embodied in it is manifested 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 structure, not the structure itself. This distinction leads to 

 an imjDortant practical result. Since, in the beginning of this century, naturalists 

 have begun, under the lead of the German physiophilosophers, to compare more 

 closely the structure of the different classes of the animal kingdom, points of 

 resemblance have been noticed between them which had entirely escaped the atten- 

 tion of earlier investigators, structural modifications have been identified, which, at 

 first, seemed to exhibit no similarity, so much so, that step by step these com- 

 parisons have been extended over the whole animal kingdom, and it has been 

 asserted, that, whatever may be the apparent differences in the organization of ani- 

 mals, they should be considered as constructed of parts essentially identical. This 

 assumed identity of structure has been called homology.^ But the progress of 

 science is gradually restricting these comparisons within narrower lunits, and it 

 appears now, that the structure of animals is homologous only as far as they belong 

 to the same branch, so much so, that the study of homologies is likely to afford 

 one of the most trustworthy means of testing the natural limits of any of the 



' CuTiER, Regn. An., 2d edit., p. 48. ^ See Chap. I., Sect. 5. 



