CHAPTER X 



EVIDENCES OF EVOLUTION 



Watch swimming fish in any pond or. aquarium ; even 

 goldfish in a glass jar will answer the purpose. Notice those 

 two small flaps, one on each side of the head. See them open 

 and shut, and open and shut again, with never-failing regu- 

 larity. They cover what are known as the gill-slits. These 



DOGFISH WITH GILL-SLITS WHERE THE ARROWS POINT 



slits open out from the gills, and gills are the breathing 

 apparatus of the fish. 1 



Notice also that the mouth of the fish is open, too, and that, 

 although he seems to be swallowing mouthful after mouthful 

 of water, he really gets nothing to eat. The truth is, he is 

 simply breathing with his mouth open. If he should shut his 

 mouth and keep it shut, he would die of suffocation. To keep 

 alive he needs oxygen as much as we do, and the only way 

 he can get it is by taking water into his mouth and sending 



1 Blood which circulates in the membranes of the gills takes oxygen from 

 water which passes over them, just as blood which circulates in the mem- 

 branes of lung cells takes oxygen from the air which enters the lungs. 



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