226 L. Agassiz on the Natural Relations between 



as an air-bladder. But all classes have both apparatuses in an 

 inverse state of development, and thus fishes are as fully 

 constructed on the plan of the higher Vertebrata 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 exist in any Invertebrates as 1 have shewn elsewhere, 

 but throughout the Vertebrata including Fishes, shew that 

 the whole type of the Fishes, have to be viewed in the same 

 light as Reptiles, Birds, and Mammals, 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 

 tracheaa of Insects, and lungs of spiders, are only modified 

 branchiae of the type of Articulata, just as much as lungs of 

 Pulmonata are modified branchise of the type of Mollusca, 

 while gills and lungs in Vertebrata are parallel systems, both 

 co-existing in all of them, and only acquiring respectively 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, Mol- 

 lusca, and Radiata, shew plainly that the aquatic, marine, or 

 fluviatile, and terrestrial mode of life are introduced through- 

 out 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 organization. But taking in 

 future into consideration all these peculiarities, we shall 

 rapidly proceed towards the full understanding of all the re- 

 lations 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 con- 

 sidered as a general law ruling the whole animal kingdom, 

 it was necessary that they should be treated in a special 

 manner through the innumerable types of Invertebrated ani- 



