M. Agassiz on Fuhes. 335 



The better to comprehend the general results -which I propose to 

 present, it will be necessary to say a few words on the existing 

 species. 



We are now acquainted with about 8000 species of fishes. Of this 

 number more than three-fourths belong to two orders of that class, 

 the presence of which has not yet been discovered in formations older 

 than the cretaceous one, viz to the Cyclo'ides and to the Ctenoides; 

 so that, truly, there is nothing an;dogoas to them in the whole series 

 of the secondary rocks, even to greensand ; whilst the remaining fourth 

 belongs to the orders Placoides and Gano'ides, not at all numerous now, 

 but which existed alone, during thcM-hole period which elapsed from 

 the time the earth began to be inhabited, till the moment in which 

 the nnimals found in the greensand appeared. The same precise pro- 

 portion existing in the orders of the class to which attention is more 

 especially solicited in this work, is a fact which is truly remarkable; 

 it is almost inconceivable, but still undoubted, since it is a mere mat- 

 ter of calculation ; and moreover, we may remark, it is not only in 

 general that this regular arrangement of the groups may be noticed ; 

 but in each order, and in each family even, the genera produce in their 

 affinities analogous series, so that the differences of organization be- 

 come the distinctive characters for the geological epochs, even in those 

 species which are seen for the first time. I can now state this result 

 with confidence, after having reviewed the general conclusions to 

 which I have arrived in the study of fossils, and supported by the 

 examination of 250 new species discovered in British Collections, 

 ■without having met with a single exception in the 800 species with 

 which I am now acquainted. These essential organic differences 

 have an especial reference to the nature of the integuments, and to 

 the mode in which the vertebral column terminates in the caudal fin, 

 in other words, to the relations which subsist in the animal to the ma- 

 terial world which surrounds him and the structure of that organ which 

 is essential to his locomotion. I shall now very briefly point out those 

 distinctions, and, at a future time, will fully enumerate all the fishes 

 belonging to each great formation ; for in thus presenting a general 

 description, it will easily be understood that I cannot enter largely 

 into detail. 



That we may appreciate at its just value the study of fishes in 

 general, and of fossil fishes in particular, we ought never to lose 

 sight of the true position of this class in the scale of living beings. 

 Placed higher than the radiata and the mollusca, they present peculia- 

 rities of organization more numerous, and also subject to greater 

 differences ; and in them also we remark, within narrower geological 

 limits, more marked differences than among animals lower in the 

 scale. In the class of fishes, we do not see genera, nor even fa- 

 milies, run throughout a whole series of formations with species which 

 often differ but very little in appearance, as happens in the Zoophytes j 

 on the contrary, this class, from one formation to another, is succes- 

 sively represented by very distinct genera, referable to families 

 which themselves soon vanish, as if the complicated apparatus of a 

 superior organization could not be long perpetuated without intimate 

 modifications ; or rather, as if animal life had a more rapid tendency 

 to change in the higher orders of the animal kingdom, than in those 



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