42 THE ORIGIN OF MAN 



seas the ancestors of the spinous sharks were 

 bound to be gradually changed into that of fresh- 

 water. There was thus set up a struggle for ex- 

 istence which it is thought eventually resulted in 

 the production of air-breathing animals. The 

 food of the freshwater fishes of the Devonian 

 time must have been water plants and algae as it 

 is known that there did not exist any water-living 

 plants nor invertebrates of any kind except bi- 

 valves and myriapods. 



There are many diverse methods in marine 

 animals for the extraction of oxygen from the 

 water but whatever the kind, it is always of the 

 same fundamental type, a localized organ or or- 

 gans having delicate canals with very thin mem- 

 branes in which the blood circulates, bringing the 

 red corpuscles into almost direct contact with the 

 surrounding water. These red corpuscles have 

 decided affinity for oxygen which they extract 

 from the water and carry through the circulation 

 of the blood to all parts of the animal's body. 



