CHAPTER II 



EVOLUTION AND PALAEONTOLOGY 



The extinct Ostracoderms. Fossil Elasmobranchs. Crossopterygians and 

 Dipnoi evolved in fresh water. Pedigree of Teleostomi. Relative abundance of 

 different orders in past and present times. 



IN a general way it is evident from the preceding summary 

 of structure and classification that the shark-like fishes or 

 Elasmobranchs are the most primitive of existing fishes, 

 and that the bony fishes can be derived from these by modifi- 

 cations of the various organs ; also that the Ganoids are the 

 more primitive of the bony fishes, and the Teleostei the most 

 modified. Among the Ganoids, the " fringe-finned " forms or 

 Crossopterygii are the most primitive, being nearest to the Elas- 

 mobranchs in the structure of the paired fins. The Dipnoi, or 

 lung-fishes, on the other hand, might be supposed to be the latest 

 stage in the evolution of fishes, especially on account of the ad- 

 vanced stage of evolution shown by the lungs in adaptation to 

 atmospheric respiration ; but on the other hand in structure the 

 Dipnoi show unmistakable affinities to the Crossopterygii. 

 In order, however, to discuss the evolution of fishes we must 

 consider the evidence afforded by the chronological succession 

 of fossil forms as well as the evidence of the structure of exist- 

 ing forms ; we must study palaeontology as well as comparative 

 anatomy, and include under the latter term the study of de- 

 velopment or embryology. We will proceed therefore to give 

 a brief outline of the most important facts concerning the pal- 

 aeontology of fishes. 



There is no evidence of the derivation of fishes with jaws 

 from those with suctorial mouths (Cyclostomes) resembling the 

 lampreys and hags. Minute fossils known as Conodonts 

 occur in the most ancient stratified rocks called by geologists 

 palaeozoic, from the Lower Silurian to the Carboniferous Lime- 



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