I20 



THE CHAIN OF LIFE. 



Besides the inferior tribes alread}' referred to, the modern 

 seas and rivers present four leading types of fishes : — first, the 

 ordinary bony fishes (Teleostians), such as the Cod, Salmon, 

 and Herring ; secondly, the Ganoid fishes, protected with bony 

 plates on the skin, as the Bony-pike^ and Sturgeon ; thirdly, the 

 Sharks and their allies, the Dog-fishes and Rays ; fourthly, the 

 peculiar and at present rare group of semi-reptilian fishes to 

 which the name of Dipnoi has been given, on account of their 

 capacity for breathing both in air and in water. 



Of these four types the first is altogether modern and 

 includes the great majority of our present fishes. It does not 

 make its appearance till the Cretaceous age, and then is at 

 once represented by at least three of the modern families, those 

 of the Salmon, Herring, and Perch. The history of the other 

 three groups is precisely the opposite of this. They abound 

 exceedingly at an early period, and dwindle to a much smaller 

 number in the modern time. This is especially the case with 

 the Ganoids and the Dipnoi. It is also remarkable that thest 

 groups of old-fashioned fishes^ are in some respects the highest 

 members of the class, approaching the nearest to the reptiles ; 

 but this accords with a well-known palaeontological law, namely, 

 that the higher members of low groups give way on the intro- 

 duction of more elevated types, while the lower members may 

 continue. Thus the decadence of these higher fish begins with 

 the incoming of the reptiles, just as the decadence of the higher 

 Mollusks and predaceous Crustaceans began with the incoming 

 of the fishes. Further, the modern Ganoids and Dipnoi are 

 mostly fresh-water animals, though the Sharks are largely pelagic. 

 In the Palaeozoic there seem to have been abundance ot 

 marine species of all these types ; but though marine, they 

 probably flourished most in bays and estuaries and on shallow 

 banks ; and the existence of these implies continental masses of 

 land. This explains the curious coincidence that the intro- 

 duction of fishes and of an abundant land flora synchronise, 

 * Lepidosteus. ^ Palccichthyes of Giinther. 



