*30 HISTOEY OF ZOOLOGY. 



truths it brings into view ; but they ought to be very important and 

 striking ones, to entitle them to supersede those which led Cuvier to 

 his system. To this I may add, that the new ichthyological classification 

 does not seem to form, as we should expect that any great advance 

 towards a natural system would form, a connected sequel to the past 

 history of ichthyology ; a step to which anterior discoveries and im- 

 provements have led, and in which they are retained. 



But notwithstanding these considerations, the method of M. Agassiz 

 has probably very great advantages for his purpose ; for in the case of 

 fossil fish, the parts which are the basis of his system often remain, 

 when even the skeleton is gone. And we may here again refer to a 

 principle of the classificatory sciences which we cannot make too pro- 

 minent ; all arrangements and nomenclatures are good, which enable 

 its to assert general propositions. 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 respecting 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 Placo'idians 

 and Gano'idians, existed before the commencement of the cretaceous 

 formation : the third and fourth orders, the Cteno'idians and Cycloi- 

 dians, which contain three-fourths of the eight thousand known spe- 

 cies of living Fishes, appear for the first time in the cretaceous forma- 

 tion : and other geological relations of these orders, no less remarkable, 

 have been ascertained by M. Agassiz. 



But we have now, I trust, pursued these sciences of classification suf- 

 ficiently 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. 



[2nd Ed.] [I have retained the remarks which I ventured at first to 

 make on the System of M. Agassiz ; but I believe the opinion of the 

 most philosophical ichthyologists to be that Cuvier's System was too 

 exclusively based on the internal skeleton, as Agassiz's was on the 

 external skeleton. In some degree both systems have been superseded, 

 while all that was true in each has been retained. Mr. Owen, in his 

 Lectures on Vertebrata (1846), takes Cuvierian characters from the 

 endo-skeleton, Agassizian ones from the exo-skeleton, Linnsean ones 

 from the ventral fins, Miillerian ones from the air-bladder, and combines 

 them by the light of his own researches, with the view of forming a 

 system more truly natural than any preceding one. 



As I have said above, naturalists, in their progress towards a Natura. 



