THE EVOLUTION OF MAN 



and a backbone. Within this group of verte-X 

 ibrates we distinguish the fish, which breathe 

 ] in the water with gills instead of lungs; man 

 i breathes through lungs, therefore he is not 

 | a fish. Then follow the amphibians, that is to 

 I say, the newts and frogs that breathe alternately 



through gills and lungs. A frog, for instance, 

 i breathes through gills, when a tadpole, and ac- 

 quires his lungs later on. Human beings do 



t not have this double method of breathing. Fur- 

 I thermore, the reptiles, that is to say, lizards, 

 i crocodiles, turtles and related animals have blood 



which changes its temperature from warm to 



cold and vice versa. Their blood is cold when 

 ' the air which they breathe is cold, but it is warm 

 j when the sun shines upon them. These animals 

 , do not yet possess their own heating apparatus 



within them. The human body heats itself, it is 

 ! always warm, hence man is not a reptile. The 

 I two last groups of vertebrates are always warm. 



These groups consist of birds and mammals. 



Since we have to choose between these two, we 

 . must investigate further. No bird suckles its 

 ' young, but the human mother does that, and all 

 i mammals do, therefore we belong on the side 

 ' of the mammals. Now these mammals are again 

 I divided into two great sections. Those of one 



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