and the Elements in which they live. 179 



duced and disappears in the higher classes in proportion as the 

 lungs acquire a greater development. In Fishes, on the con- 

 trary, the homologue of the lung remains functionally and orga- 

 nically in a rudimentary state, as an air-bladder. But all classes 

 have both apparatus in an inverse state of development, and thus 

 Fishes are as fully constructed on the plan of the higher Verte- 

 brata, as the aerial Invertebrata are on the plan of their aquatic 

 types. But the circumstances that Fishes have the double type 

 of respiratory organs, and that the pulmonary one, which by no 

 means exists in any Invertebrates, as I have shown elsewhere, but 

 throughout the Vertebrata including Fishes, show that the whole 

 type of the Fishes have to be viewed in the same light as Rep- 

 tiles, Birds and Mammalia, and must therefore be only considered 

 as a lower condition of these aerial types, and not the latter as a 

 higher degree of the former. For tracheae of Insects, and lungs 

 of Spiders, are only modified branchiae of the type of Articulata, 

 just as much as lungs of Pulmonata are modified branchiae of the 

 type of xMollusca, while gills and lungs in Vertebrata are parallel 

 systems both coexisting in all of them, and only acquiring re- 

 spectively a different degree of development in each of their 

 classes. These facts which I have traced in other papers through 

 a special comparison of all the homologies of the different types 

 of respiratory organs in Vertebrata, Articulata, Mollusca and Ra- 

 diata, show plainly, that the aquatic, marine, or fluviatile, and 

 terrestrial mode of life are introduced throughout the animal 

 kingdom by special adaptations of peculiar different systems of 

 organs performing analogous functions ; and that the failure of 

 introducing the consideration of the adaptation of animals to the 

 media in which they live, in the plan of their classification, must 

 be ascribed to the fact that these analogous structures were in 

 the beginning considered as identical features in the organiza- 

 tion. But taking in future into consideration all these peculia- 

 rities, we shall rapidly proceed towards the full understanding of 

 all the relations between the gradation of animals and the media 

 in which they live, as far as they are not yet fully understood. 



An extensive review of the Vertebrata might long ago have led 

 to such conclusions ; but before they could be considered as a ge- 

 neral law ruling the whole animal kingdom, it was necessary that 

 they should be treated in a special manner through the innume- 

 rable types of Invertebrated animals ; and we have seen that this 

 agreement is as close and as complete throughout the types of 

 Radiata, Mollusca and Articulata, as it is plain among Vertebrata ; 

 and the slight difficulties to which we have alluded, must proba- 

 bly be referred to the present state of our knowledge respecting 

 some of them, rather than to a departure from this law in any of 

 their types. 



