LEADING GROUPS OF EXISTING SYSTEMS 189 



upon analogy. The entire difference of structure between the organs 

 of flight in these two classes of animals forbids our considering the 

 resemblance which exists between them as homological, for they are 

 not built upon homologous structures. But there is analogy between 

 them, inasmuch as the peculiar structure characteristic of two differ- 

 ent types is worked up into organs that appear the same because they 

 perform similar functions. 



Admitting these distinctions to be correct, the categories of anal- 

 ogy 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 analo- 

 gies becomes very difficult and highly complicated; and these com- 

 plications have, no doubt, been the source of most errors and inac- 

 curacies in the attempts that have been made to classify the animal 

 kingdom. 



Branch analogies. The plan[s] of structure characteristic of the 

 four branches of the animal kingdom are so peculiar that we nowhere 

 find analogies of this kind extending from one branch to all the rep- 

 resentatives of another branch. On the contrary, they extend gener- 

 ally 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 unmis- 

 takable analogy between the structure of the solid frame of Echino- 

 derms (especially in the star-fishes) and the plan of structure of the 

 Articulates; so much so that Oken does not hesitate to refer the 

 Echinoderms to the type of Articulates, mistaking their analogy for 

 true homology. 



Class analogies. The ways in which and the means by which the 

 plan of structure of one class is carried out, as compared with another 

 class, frequently produce striking analogies. For instance, among 

 Vertebrates the whole class of Birds is winged; and wings constructed 

 like the wing of Birds exist in no other class. Yet the bats are also 



