34 Probable number of Fossil Fishes. 



gical epoch to which they belong, it is not with living animals, 

 as a whole, that we ought to compare them, but rather with 

 an assemblage of species living within analogous limits, and 

 under analogous conditions, in the existing creation. An 

 example will explain my idea more accurately. If I sought 

 to determine approximately the number of fossil species of 

 tlie period of the deposition of the chalk or plastic clay, I 

 believe that I should choose a very bad method of attaining 

 my object by computing the lists of fossils of all the geolo- 

 gical deposits considered at present as belonging to these 

 geological horizons, and then comparing the sum obtained 

 with the sum of living species. We should certainly approach 

 much nearer the truth, by studying as completely as possible 

 the fossil fauna of some well explored localities, as, for ex- 

 ample, the deposits of chalk around Paris, or the plastic clay 

 of the Thames basin, and then comparing these lists of fos- 

 sils with the living animals of some gulf or some shore in the 

 present creation, which shall present most analogy with the 

 extent and conditions in which we may suppose these deposits 

 to have been formed. We shall thus obtain true foundations 

 to fix the numerical relations of the whole of these creations 

 compared with the actual creation. 



By following this process, and comparing successively the 

 ichthyological faunas of dififerent formations, in which I have 

 recognized different assemblages of fishes, with the ichthy- 

 ological faunas of the present creation, confined to analogous 

 limits, I have arrived at the result (a distressing one for 

 the actual state of our palaeontological knowledge, if it be 

 admitted to be correct), that the strata which constitute 

 the crust of our globe, considered as a whole, ought to con- 

 tain at least twenty-five thousand species of fossil fishes. 

 In this calculation, the grounds of which I think it unne- 

 cessary to specify in this place, I have carefully taken into 

 account the greatest uniformity which ancient contemporary 

 faunas present. Similar calculations, made with the same 

 precautions, raise the number of mammifera we may yet ex- 

 pect to discover to about 3000 ; that of reptiles to about 

 4000 ; and that of shells to at least 40,000. I am even of 

 opinion, that very few years will elapse before we shall have 



