i68 



THE ORIGIN AND EVOLUTION OF LIFE 



branch off into the elongated eel-shaped forms of the Car- 

 boniferous. 



The prototype of the shark group is the Cladoselache (Fig. 

 49), a fish famed in the annals of comparative anatomy since 

 it demonstrates that the fins of fishes arise from lateral skin 



ORIGrN AND ADAPTIVE RADIATION OF THE FISHES 



Fig. 50. Origin and Adaptive Radiation of the Fishes. 



This chart shows the now extinct Siluro-Devonian groups, the Ostracoderms and Arthro- 

 dires, in relation to the surviving lampreys (Cyclostomes) ; sharks and rays (Elasmo- 

 branchs); sturgeons, garpikes, and bowfins (Ganoids); bon}' fishes (Teleosts); primi- 

 tive and recent lung-fishes (Dipnoi); and finally the fringe-finned or lobe-finned Ganoids 

 (Crossopterygii) from the cartilaginous fins of which the fore and hind limbs of the 

 first land-living vertebrates (Tetrapoda) were derived. Dotted areas represent groups 

 which still exist. Hatched areas represent extinct groups. Prepared for the author 

 by W. K. Gregory. 



folds of the body, into which are extended internal stiffening 

 cartilaginous rods (Fig. 49). In course of evolution these 

 rods are concentrated to form the central axis of a freely jointed 

 fin, while in a further step of evolution they transform into the 

 cartilages and bones of the limb girdles and limb segments of 

 the four-footed land vertebrates, the Tetrapoda. 



The manner of this fin and limb transformation has been 

 one of the greatest problems in the history of the origin of 



