286 PALAEONTOLOGY. 



referred to the eocene stage of the tertiary period ; of these, 

 one half of the genera are now extinct, whilst in the newer 

 stages of this period nearly all the fossil species belong to 

 existing genera. 



In the strata anterior to the lias, almost all the fishes 

 had heterocercal or unilobed tails (fig. 194) ; that is to say, 

 the spinal vertebra were prolonged into the npper lobe of 

 that organ, as seen now only in the family of sharks, of which 

 the dog-fish of our coasts is an example. "Whilst from the 

 lias upwards the tails of the majority of fishes were bilobed 

 or homocercal. (Fig. 195.) 



TIG. 194. Heterocercal ; Shark. FIG. 195. Homocercal ; Herring. 



As the ancient fishes have left their remains in the rocks 

 of all epochs, they afford a satisfactory demonstration of the 

 law which limits fossil forms to different stages of the earth's 

 history. Of the one thousand fossil species at present 

 known, not one is identical with any species now living, and 

 none of the fossil species pass from one stage to another. 



The history of fossil fishes affords an unanswerable argu- 

 ment against the hypothesis, that a gradual development in 

 the species of animals has taken place during past ages ; an 

 idea now better known as the development theory. "We 

 have seen that the fossil fishes belonging to the placoid and 

 ganoid orders, which are in fact the most highly organised 

 types of the class, approached reptiles in some important 

 points of their anatomy; that these highest orders maintained 

 their dominion during the deposition of the paleozoic, triasic, 

 and oolitic strata, and preserved their typical structure 

 through all the changing conditions to which they were 

 exposed. The cycloid and ctenoid orders were not created 

 until towards the close of the cretaceous period ; their orga- 

 nisation being inferior to the placoids and ganoids, they 



