THE GROWTH OF BIOLOGICAL IDEAS 169 



From the thirties of the nineteenth century onward, thanks 

 largely to the work of the great German chemist Justus von 

 Liebig, proteins, fats, carbohydrates, salts, and water were 

 recognized as the main nutritional requirements of man and 

 other animals. So firmly did this idea take root that great 

 independence of mind was necessary in anyone who would 

 doubt it. Yet a Dutchman, G. Grijns, working inconspicu- 

 ously in the East Indies, did dare to doubt it; he even claimed 

 that men became ill and died just because proteins, fats, carbo- 

 hydrates, salts, and water were not enough. That was at the 

 very beginning of the present century, and not long afterward 

 the great Cambridge biochemist Sir Frederick Hopkins set 

 the study of vitamins on its feet by critical feeding experi- 

 ments on animals. 



We left the grand problem of respiration on page 153 with 

 Spallanzani's discovery that the reaction of combustible sub- 

 stances with oxygen occurs not in the lungs, as Lavoisier 

 thought, but in the various tissues of the body. This was not 

 definitely proved until the eighteen thirties, and at that time 

 it was still thought that the oxygen traveled from the lungs 

 to the tissues in simple solution in the water of the blood. In 

 the fifties people began to think that it must travel in loose 

 combination with some unknown substance. Today it seems 

 difficult to believe that it was not until the eighteen sixties 

 that this substance was shown to be hemoglobin, the familiar 

 red coloring matter of blood. The discovery was largely due 

 to the investigations of the great German biochemist F. 

 Hoppe-Seyler. Everything seemed straightforward. The 

 oxygen in the air of the lungs combined with the hemoglobin 

 in the red blood corpuscles and was carried in this combined 

 form to the tissues; it then escaped from combination, dif- 

 fused out of the blood into the cells, and there combined 

 with carbon and hydrogen to form carbon dioxide and water. 

 The energy produced by this combustion was the energy 

 necessary for life. 



The form in which oxygen travels in the blood stream had, 

 indeed, been discovered, but the manifold complications of 



