394 Agassiz on the Relations between Animals, §c. ‘ 
tion, is developed to be a permanent organ of respiration, while 
it is reduced 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 organ- 
ically in a rudimentary state, 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 Verte- 
brata as the aérial 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, haye 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 aérial types, and not the latter as a 
higher degree of the former. For tracheze of Insects, and lungs 
of Spiders, are only modified branchie of the type of Articulata, 
just as much as lungs of Pulmonata are modified branchie of the 
type of Mollusca, while gills and lungs in Vertebrata are paral- 
lel systems both coéxisting 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 Radi- 
ata, show plainly, that the aquatic, marine, or fluviatile, and ter- 
restrial mode of life ave introduced throughout the animal king- 
dom by special adaptations of peculiar different systems of organs 
performing analogous functions; and that the failure of intro- 
ducing the consideration ‘ef 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 peculiari- 
able 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. . : ais 
