The Breath of a Bird 179 



A simple experiment will show what fishes have a 

 canal, or duct, leading from the throat to the swim-blad- 

 der and what have not. If a goldfish and a perch or 

 sunfish be placed in a bowl of water and the air exhausted, 

 the two latter will be forced to the surface, while the gold- 

 fish will soon eject a few bubbles of air, or gas, from its 

 mouth and stay at the bottom. Thus we can see the ad- 



FIG. 134. Diagram of growth of lungs. X, the lower part of the primitive diges- 

 tive tract, divides into two parts, XX, the lungs. 



vantage of such a canal in enabling the fish to regulate 

 the amount of gas in the bladder. 



When the fish-like creatures of old took to living on 

 land, the change from swim-bladder and gas to lung 

 and air was a remarkable example of change of function 

 of an organ, and the more we learn of the lungs of living 

 creatures the more marvellous does this transformation 



