382 FISHES 



that it is the most probable explanation of the facts. There 

 was a natural tendency in the earlier days of the study of 

 evolution to regard existing animals as forming a series corre- 

 sponding to the series of steps in the evolution, and to overlook 

 the fact that forms now living side by side, although some may 

 retain more of the primitive and ancestral features than others, 

 cannot actually be in the relation of ancestors and descendants. 

 Thus although terrestrial vertebrates are doubtless descended 

 from ancestral fishes, we cannot conclude that a mammal such 

 as the dog is descended from a fish like the gold-fish, or that 

 the lungs of the dog are a modification of the air-bladder of 

 the gold-fish, which is not a respiratory organ, but an organ for 

 decreasing the specific gravity of the fish. 



Lungs are paired structures opening out of the ventral side 

 of the oesophagus : the air-bladder in the majority of fishes is 

 a single structure opening, when it is not closed completely, from 

 the dorsal side of the oesophagus. But we must endeavour to 

 trace the origin and history of the air-bladder in fishes themselves 

 by the kinds of evidence usually employed by the evolutionist, 

 by palaeontology, or the evidence of fossils, by comparative ana- 

 tomy, and by embryology or the evidence of development. It is 

 known that the fishes with closed air-bladders, classed together 

 by Dr. Giinther as Physoclisti, and comprising in Boulenger's 

 classification the Spiny-finned Fishes (Acanthopterygii) and a 

 few other Sub-Orders, are the latest fishes to appear in the pal- 

 aeontological record, and in all these the air-bladder arises in 

 development as an outgrowth of the gullet, the communication 

 with the latter being afterwards lost. The Soft-finned Fishes 

 (Malacopterygii) and a few other Sub-Orders, forming the old 

 group Physostomi which retain, e.g. the salmon and carp, the 

 communication of the air-bladder with the gullet throughout life, 

 are therefore more primitive, and their fossil representatives 

 are found in older geological strata. As the Spiny-finned 

 Fishes are characteristic of the sea ,so the Soft-finned Fishes for 

 the most part live in fresh water, or are like the salmon and the 

 eel inhabitants of fresh water during part of their lives. True 

 Malacopterygians begin to appear as fossils in the chalk, forms 

 closely allied to the smelt {Osmerus) and the anchovy (En- 

 graulis), being known from this formation. The Holostei, of 

 which the American bow-fin and bony-pike (Amia and Lep- 



