CLASSES OF ANIMALS. 225 



distinct, characteristic idea. But tins 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 most skilful artist 

 upon the simplest tune are not modifications of the tune 

 itself, but only different expressions of the same funda- 

 mental harmony, just so neither the classes, nor the orders, 

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

 great type are modifications of its plan, but only its dif- 

 ferent expressions, the different ways in which the funda- 

 mental 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 important 

 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 attention of earlier investigators ; 

 structural modifications have been identified which at first 

 seemed to exhibit no similarity, so much so, that step by 

 step these comparisons have been extended over the whole 

 animal kingdom, and it has been asserted, that, whatever 

 may be the apparent differences in the organization of 



Q 



