FISHES. 297 



became differentiated, and scales around the margins of the jaw became teeth. With 

 the materials for great diversity, great diversity ensued. The class of Selachians 

 had come into existence, and long reigned the highest of Vertebrates, and still remain, 

 represented by two divergent groups ; on one hand the Holocephals, on another the 

 Sharks, with the Notidanids nearest the ancestral types, and the Rays most distant 

 and specialized. 



From a type most resembling the Holocephals of familiar forms, another group m 

 time originated. A concrescence of the scales took place forwards, in the form of 

 plates on the head and behind the head, which dipped inward upon the skull and 

 shoulder-girdle, and, still later, became in their turn covered by the skin, and mem- 

 brane bones were constituted, the branchial skeleton became concentrated in the 

 back of the head. The class of Fishes was then definable. Increased diversity 

 became manifested ; a diverticulum originated from the floor of the stomach or 

 (Esophagus, and this was utilized for the respiration of air, and various forms came 

 out of the waters, and on the land sought for food and safety. A development of the 

 bones which supported the pectoral and ventral fins was concomitant with the change, 

 and enabled them to slowly progress on land, where few, if any, enemies could cope 

 with them. The Dipnoans and bichirs are modified survivors of this group. We have 

 in them parents at once of the Teleostomous fishes and the amphibians. 



For a certain tendency was exhibited by some forms, and the diverticulum lost its 

 inchoate lung-like function, and assumed a hydrostatic one, moving round and 

 upwards till it reached the roof of the visceral cavity under the backbone, and estab- 

 lished a shorter communication with the uppermost arch of the intestinal canal. The 

 air-bladder was thus developed. The type so brought into being increased and multi- 

 plied, and, even as early as the cretaceous period, had almost become the predominant 

 group of fishes ; the brain and heart, as well as other parts, had become modified, and 

 the great horde of Teleosts was outlined. The structure and habits of those 

 fishes were in full accord with the conditions of sea and river ; not so those of the 

 older forms. The consequences of the new order of things became evident as time 

 rolled on. From small beginnings the Teleost crowd continued ever swelling 

 down the stream of time, crowding out their parent stocks, and their older relatives 

 and progenitors gradually faded out, or were forced out of existence. Coats of mail, 

 even, were of little account ; and the heavily armed Ganoids were unable to maintain 

 their race against their light and active rivals. Most of the new stock developed 

 innumerable eggs, or took active care of their young, and in other respects became 

 accommodated to the changing conditions. These hunted the eggs or young of 

 the older stock ; and such raids, in connection with various elements of unfitness, 

 resulted in the gradual diminution and final extinction of the older fishes. A few 

 isolated groups were at last alone left of the mighty races of the olden times. On 

 the other hand, the Teleosts invaded all waters, — the coasts, the open seas, the deep 

 seas, the torrid seas of the equator, the ice-covered seas of the jjoles, the inland 

 streams and lakes, — and adapted themselves to all conditions. Swiftness was gained 

 by one, power of concealment by another ; one took to the most limpid streams, an- 

 other accommodated itself to muddy and stagnant pools ; some dove down into the 

 greatest depths, others sought escape by flight into the air from the waters when 

 death threatened, and still others left their native element for a time to seek safety or 

 sustenance on land. A riotous variety supervened, intense specialization in various 

 directions became manifest, and the means by which such specializations were effected 



