OF EVOLUTION. 43 



but even here we find that this highest group was 

 immediately preceded in time by a type of ganoid- 

 plated fishes, the Leptolepidae, which in so far 

 partake of the characters of both ganoid and 

 teleost as to have induced naturalists to place 

 them alternately now in the one group, now in 

 the other. 



When we cast a broad glance over the existing 

 fish fauna of the globe, and compare it with that 

 of the earlier geological periods, we find that it 

 differs, not only in the introduction of types of a 

 higher grade of organization, but in the actual elim- 

 ination of the lower structural forms. The 

 ganoids, for example, which are numbered by hun- 

 dreds of species in the interval between the De- 

 vonian and Jurassic periods, are practically extinct 

 at the present day, numbering but a mere hand- 

 ful of species. A somewhat similar, although less 

 marked, elimination is also distinctive of the 

 selachians (sharks, rays). We thus find a com- 

 plete rotation marking the succession of these 

 animals. Evolution or transformism is the 



