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- 
616 
(a) 
Fie. 134.—Diagram of growth of lungs. 1X, the lower part of the primitive diges- 
tive tract, divides into two parts, YY, 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 
