194 ESSAY ON CLASSIFICATION 



side. We are therefore compelled to consider the solid arches of 

 Crustacea only as analogous structures to Vertebrae and not as ho- 

 mologous with them, the more so since these arches enclose not only 

 the nervous system, as in Vertebrates, but all the other viscera be- 

 sides. The system of articulation in Articulates exhibits, therefore, 

 a branch analogy with the vertebral system of the Vertebrates, but 

 there is no true homology between them. The class of Fishes is emi- 

 nently characterized by the presence of gills, and so have Crustacea 

 gills, and so also the Cephalopods, a large number of Gasteropods, 

 and most Acephala. But the structure of these gills is widely differ- 

 ent in these different classes, and their presence only constitutes class 

 analogies and is no indication of a real affinity; while the so-called 

 lungs of the land Gasteropods have the closest structural resemblance 

 to the gills of the other Mollusks, thus showing a real affinity between 

 them, while their air sacs, on account of their gill-like structure, 

 constitute only an analogy between them and the other air-breathing 

 animals. We may go on testing in this way the analogies and homolo- 

 gies in all their degrees and combinations throughout the animal 

 kingdom and be sure to arrive at satisfactory results, provided we 

 remember that analogies are features of one group combined with 

 the characteristic features of another group, and not, like homologies, 

 circumscribed within one and the same group. 



SECTION X 

 CONCLUSIONS 



The importance of such an investigation as the preceding must 

 be obvious to every philosophical investigator. As soon as it is un- 

 derstood that all the different groups introduced into a natural sys- 

 tem may have a definite meaning; as soon as it can be shown that each 

 exhibits a definite relation among living beings, founded in nature 

 and no more subject to arbitrary modifications than any other law 

 expressing natural phenomena; as soon as it is made plain that the 

 natural limits of all these groups may be ascertained by careful in- 

 vestigations, the interest in the study of classification or the system- 

 atic relationship existing among all organized beings, which has 



