THE CATEGORIES OF ANALOGY. 281 



system of Articulation in Articulates exhibits, therefore, a 

 Branch analogy with the vertebral system of the Verte- 

 brates, but there is no true homology between them. The 

 class of Fishes is eminently characterized by the presence 

 of gills, and so have Crustacea gills, and so also the 

 Cephalopoda, a large number of Gasteropoda, and most 

 Acephala. But the structure of these gills is widely 

 different 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 Gastero- 

 pods have the closest structural resemblance to the gills 

 of the other Mollusks, thus showing a real affinity between 

 them, while their air sacks, 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 homologies in all their 

 degrees and combinations throughout the animal king- 

 dom, 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 preced- 

 ing must be obvious to every philosophical investigator. 

 As soon as it is understood that all the different groups 

 introduced into a natural system may have a definite 

 meaning; as soon as it can be shown that each exhibits a 

 definite relation among living beings, founded in nature, 



