FISHES 155 



reptiles consisted of a pair of amphibians that reached 

 special conditions, and so their offspring formed a 

 new class. These amphibians lived about the end 

 of the Carboniferous period. Their ancestry coincides 

 with that of the other amphibians living at the time, 

 whose descendants are found in our frogs and 

 salamanders ; earlier still, it is a pair of fish ancestors 

 that led to the beginning of the amphibians in a 

 particular locality. 



This similarity of organisms is a hopeless puzzle 

 for the theory that all our animals were created 

 separately. On that theory it is unintelligible why 

 some animals so closely resemble others in structure, 

 others still more, and others not at all. It can give 

 no explanation of these facts. Moreover, it cannot 

 explain why certain species seem to be in a much 

 worse position than others. Thus, for instance, the 

 circulation of the blood is clearly worse in the frog 

 than in the bird. The frogs have only one chamber 

 to the heart, and this has to receive both the used- 

 up blood from the body and the fresh blood from the 

 lungs with its new oxygen. The blood mixes in the 

 single chamber, and the body is not supplied entirely 

 with fresh blood, but with the mixed fluid, which 

 passes from the heart into the body. In the bird 

 the chamber is divided by a partition ; the fresh 

 blood from the lungs passes into the left half, and 

 is conveyed in its purity from this to the body, while 

 the right chamber receives the used - up blood and 

 drives it to the lungs to be renewed. 



The birds, with their constant supply of pure blood, 



