274 ESSAY ON CLASSIFICATION. 



blance which exists between them as homological, for 

 they are not built upon homologous structures. But there 

 is analogy between them, inasmuch as the peculiar struc- 

 ture characteristic of two different types is worked up 

 into organs that appear the same because they perform 

 similar functions. 



Admitting these distinctions to be correct, the catego- 

 ries of analogy must be like those of homology; either 

 analogies of branch, or of class, or of order, or of family, 

 or of genus, or of species ; and these analogies may either 

 be observed between different branches, classes, orders, 

 families, genera, and species; or features characteristic of 

 branch or of class may be limited to certain families, or 

 even to genera of other branches and other classes; so 

 that the study of analogies becomes very difficult and 

 highly complicated; and these complications have, no 

 doubt, been the source of most errors and inaccuracies in 

 the attempts that have been made to classify the animal 

 kingdom. 



Branch analogies. The plan of structure characteristic 

 of the four branches of the animal kingdom are so pecu- 

 liar that we nowhere find analogies of this kind extending 

 from one branch to all the representatives of another 

 branch. On the contrary, they extend generally to minor 

 divisions of some classes, and rarely to entire classes. Yet, 

 among Mollusks, all the Cephalopods have some analogy 

 with the Radiates in the arrangement of their arms around 

 the mouth. All the Bryozoa have a striking analogy 

 with the Polyps in the crown which spreads around their 

 upper part; and so it is with the tentacles of a large 

 number of the Dorsibranchiate Annelids. There is an 

 unmistakable analogy between the structure of the solid 

 frame of Echinoderms (especially in the star-fishes) and 



