THE EVOLUTION OF MAN 



vided with a valve for its regulation. Many fish 

 therefore retained an open connection between 

 the swimming bladder, the intestines and the 

 mouth for the purpose of inhaling or exhaling 

 air. This is the starting point of the lungs. The 

 balloon or bladder in the vicinity of the aesopha- 

 gus being filled or closed at will, it served at the 

 same time to feed the arteries of its walls with 



A SHARK. 



The sharks are a primeval group of ancestors of fishes and 

 are related to the family tree of man. 



oxygen. Once it had come into operation, it 

 could under severe conditions be used as a sub- 

 stitute for the gills in times when water was 

 scarce. When these conditions continued for a 

 long period of time, the swimming bladder as- 

 sumed this role permanently and became a genu- 

 ine lung, while the gills atrophied until nothing 

 remained of them but traces in the embryo. Thus 

 the land animal sprang into being, or to express 



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