220 L. Agassiz on the Natural delations between 



ground. If there is any natural connection, as I have at- 

 tempted elsewhere to shew that there is, between Scombroids 

 and Scomberesoces, and Esoces proper, it becomes plain at 

 once, that the latter are the higher from the abdominal posi- 

 tion of their ventrals, and they are a fluviatile family. Even 

 taking the Cycloids as a whole, we find among them the 

 lower families of Thoracici and Ingulares, as the families of 

 Cod and Scombrides, chiefly marine, whilst the family of Sal- 

 monidse and Cyprinidse are chiefly fluviatile. Among the 

 Gadoids, we have those with many vertical fins, as the true 

 Cod, marine, while those in which the dorsals and anals are 

 reduced, such as the genus Lota, are fluviatile. Even among 

 the Salmonidse, in the widest extension which this family had 

 formerly, we find the Scopelidse, with the inferior structure 

 of their jaws chiefly marine, while the Coracini and true Sal- 

 monidse are chiefly fluviatile. Everywhere, in fact, in each 

 minor group, the fluviatile representatives shew characters 

 indicating their superiority over their marine representatives. 

 Whatever exceptions might be found to this law, which, in 

 the outset, appears so general, I have no doubt will lead at 

 some future time to the discovery of some other principle as 

 yet unknown. 



2. Reptiles. — The class of Reptiles is one of the most in- 

 teresting in the point of view under consideration ; and each 

 of their types exemplifies in itself the law of the intimate con- 

 nection between animal types and the media in which they 

 live in the most striking manner, inasmuch as here the gra- 

 dation which might be inferred from structural and embry- 

 ological evidence, agrees most fully with the gradation of the 

 elements in which they live. Among Batrachians we have 

 chiefly fluviatile and terrestrial families. The Ichthyodes, 

 or Batrachians with permanent branchiae, are all aquatic, 

 and acknowledged the lowest in the class. Some of their 

 lowest representatives occur even in the brackish swamps, 

 and, as soon as attention is called to this subject, it cannot 

 fail to be perceived that the frogs, with their more or less 

 palmate fingers, and their more aquatic habits, rank lower 

 than the toads, with their divided fingers and terrestrial 

 mode of life. Among Ophidians we have chiefly terrestrial 



