52 A HISTORY OF FISHES 



primitive respiratory sac, which itself originally arose as a 

 pouch growing out from the gullet. What, then, were the 

 causes that led to the origin of this air-breathing sac? To 

 answer this question it is necessary to go back to the early 

 Silurian and Devonian epochs when the evolution of this 

 organ probably took place. 



It may be assumed that the earhest fishes swarming in the 

 sea close to the shore found a sufficiency of oxygen dissolved 

 in the water to make air-breathing unnecessary, but in course 

 of time stress of competition must have led many of them to 

 ascend the rivers just as certain Sharks and Rays do to-day, 

 and to become permanent inhabitants of fresh water. In the 

 course of further penetration a number of species would find 

 their way into the smaller streams, ponds, marshes, and so on. 

 Here the water might be expected to be thick with sediment, 

 or, owing to the heat or to the decay of vegetation, poorly 

 aerated, thus making gill-breathing a matter of difficulty. 

 These conditions would necessitate the fishes coming to the 

 surface at intervals to gulp air, just as the Bichirs and Lung- 

 fishes do to-day when the water in which they are hving 

 becomes foul. At first, the air-breathing would perhaps be 

 performed by the walls of the gullet, but as the habit persisted 

 the process would be improved by the development of a more 

 or less definite pouch or reservoir. This would gradually 

 assume the appearance of a lung, and eventually divide down 

 the middle to form the double structure characteristic of the 

 higher vertebrates. In the early fishes which took up their 

 abode in the rivers and marshes the air-bladder probably 

 served merely as an accessory respiratory organ, the bulk of 

 the work of breathing falling on the gills. Conditions being 

 favourable to their progress, these fishes must have multiplied 

 rapidly. Later on, however, owing to a number of factors, 

 among which the arrival of fish-eating reptiles may be men- 

 tioned, a large number of them appear to have been driven 

 back to the sea, where, with an adequate supply of oxygen for 

 gill-breathing, the air-bladder gradually lost its respiratory 

 function to a greater or lesser extent, and either became modified 

 for totally different ends or disappeared altogether. 



That this invasion of fresh water by the early fishes was a very 

 important step in the evolution of higher vertebrates there 

 can be no doubt, for once the habit of breathing air had 

 become established, the abundance of food and the complete 

 absence of enemies would be an inducement for some species 



