166 AFFINITIES AND GEOGRAPHICAL 



the tail, after the manner of a family of animals equally low 

 in the mammalia. 



In the present state of this inquiry, it is impossible to give 

 an entire genealogical tree of Being. Much must remain 

 obscure and unindicated. Even of what is set forth, some 

 parts must be held liable to correction under better light. 

 Enough, however, is done for the present object, if such 

 fragments of the great composite chain be shown, as afford 

 proof that there is such a thing in nature, and that the idea 

 of genetic succession of advancing forms is in harmony with 

 it. In the Fishes, we have one of the obscurer portions of the 

 animal kingdom. The classifications of Cuvier and Agassiz 

 are neither of them admitted to be natural ; it is therefore 

 not to be expected that any general student should be able to 

 display the class in all its genetic relations, however confident 

 he may be, from what he sees elsewhere, that such relations 

 exist. We find, however, three advances made to its lower 

 confines from the invertebrata namely, by the cephalopodous 

 mollusks, by the annulose animals (annelides), and by the 

 echinodermata. And we see advances made in its upper 

 confines to the next higher class, the REPTILIA, which suc- 

 ceed it in the strata and chronology of the earth, as in orga- 

 nization. 



It is also sufficiently clear, that the succession of fishes 

 upon our globe was in conformity with the embryonic deve- 

 lopment of the individual fish of a high order. This has been 

 denied ; but against all inferior authorities that of M. Agassiz 

 must on such a point be held incontestable. He expressly 

 affirms it as proved "that the embryo of a fish during its deve- 

 lopment, the class of living fishes in its numerous families, and 

 the fish type in its planetary history, in every respect go through 

 analogous phases" The want of substantial vertebrae in the 

 Devonian fishes is found in the last gradation of the class of 

 fishes, among the Cyclostomes. He has reason to think that 

 the internal case for the brain in the Devonian fishes was 

 cartilaginous. So it is in the sturgeon ; so it is in all em- 

 bryos. Certain arrangements of the fins, as well as the hete- 



