410 HISTORY OF ZOOLOGY. 



ficatory sciences, which we cannot make too promi- 

 nent; all arrangements and nomenclatures are 

 good, which enable us to assert general proposi- 

 tions. Tried by this test, we cannot fail to set a 

 high value on the arrangement of M. Agassiz ; for 

 propositions of the most striking generality respect- 

 ing fossil remains of fish, of which geologists before 

 had never dreamt, are enunciated by means of his 

 groups and names. Thus only the two first orders, 

 the Placoidians and Gano'idians, existed before the 

 commencement of the cretaceous formation : the 

 third and fourth orders, the Ctenoidians and Cy- 

 clo'idians, which contain three-fourths of the eight 

 thousand known species of living Fishes, appear for 

 the first time in the cretaceous formation : and other 

 geological relations of these orders, no less remark- 

 able, have been ascertained by M. Agassiz (R). 



But we have now, I trust, pursued these sciences 

 of classification sufficiently far; and it is time for 

 us to enter upon that higher domain, of Physiology, 

 to which, as we have said, Zoology so irresistibly 

 directs us (s). 



