RISE OF MODERN FISHES 169 



animal form since the earliest researches of Carl Gegenbaur, 

 of Heidelberg, who sought to derive the lateral fins from a 

 modification through a profound change of adaptation (func- 

 tion) of the cartilaginous rods which support the respiratory 

 gill arches. While palaeontology has disproved Gegenbaur's 

 hypothesis that the Hmbs of the higher vertebrates, including 

 those of man, are derived from the cartilaginous gill arches of 

 fishes, it has helped to demonstrate the truth of Reichert's 

 anatomical hypothesis that the bony chain of the middle ear 

 of man has been derived through change of adaptation from a 

 portion of a modified gill arch, namely, the mandibular carti- 

 lage of the fish. 



The cycle of shark evolution in course of geologic time 

 embraces a majority of the swift-moving, predaceous types, 

 which radiate into the sinuous, elongate body of the frilled 

 shark {Chlamydoselache) and into forms with broadly depressed 

 bodies, such as the bottom-living skates and rays. Under the 

 law of adaptive radiation the sharks seek every possible habitat 

 zone except the abyssal in the search for food. The nearest 

 approach to the evolution of the eel-shaped type among the 

 sharks are certain forms discovered in Carboniferous time. 



Rise of Modern Fishes 



By Upper Devonian time the fishes in general had already 

 radiated into all the great existing groups. The primitive 

 armored arthrodires and ostracoderms were nearing extinc- 

 tion. The sharks were still in the early lappet-fin stage of 

 evolution above described, a common characteristic of the 

 members of this entire order being that they never evolved a 

 solid bony armature, finding sufficient protection in the sha- 

 green covering. 



The scaled armature of the first true ganoid, enamel-cov- 



