THE SUCCESSION OF ANIMAL FORMS 185 



and the modern sharks have no swim-bladders. Possibly, 

 however, the sharks habitually haunted the open sea, and 

 only made occasional raids on the dangerous waters tenanted 

 by the ganoids. It is also true that only certain genera of 

 sharks are found to be represented in the carbonaceous shales, 

 and they may have differed in this respect from the ordinary 

 forms of the order. It has been suggested that only a small 

 change would be necessary to enable some of these lung fishes 

 to become Batrachians, and no doubt this is the nearest 

 approach of the fish to the reptile ; but we have not yet found 

 connecting links sufficient to bridge over the whole distance. 



The plate-bearing ganoids of the Silurian and Devonian, at 

 one time supposed to be allied to Crustaceans, but whose 

 dignity as " Forerunners of the back-boned animals " is now 

 generally admitted, 1 are clearly true fishes, and of somewhat 

 high rank, their strange bony armour being evidently a special 

 protection against the attacks of contemporary sharks and 

 gigantic crustaceans ; and if we may judge by the Colorado 

 specimens, their existence dates back almost to the close of the 

 Cambrian, and they were probably contemporary with small 

 sharks ; while as early as the Silurian and Devonian, if we 

 regard the scaly ganoids as a distinct type, we have already 

 four types of fishes, and these akin to those which in modern 

 time we must regard as the highest of their class. 



One very little fish of the Devonian, of which specimens 

 have been kindly sent me by a friend in Scotland, 2 the Palaeo- 



1 A. Smith Woodward, "Natural Science," 1892, and Annals and 

 Maga. Nat. Hist., October, 1890. This able naturalist, in introducing 

 his subject, remarks, from the point of view of an evolutionist : 

 "Whether some form of 'worm' gave origin to the forerunners of the 

 great back-boned race, or whether a primeval relative of the King-crab 

 turned upside down and rearranged limbs and head these are questions 

 still admitting of endless discussion, no doubt fruitless in their main object, 

 but desirable from the new lines of investigation they continually suggest." 



2 James Reed, Esq., of Allan House, Blairgowrie. 



