THE MUTUAL RELATIONS. 307 



important changes. We are obliged to resort here almost exclusively to 

 experiments with animals. Certain observations are also a result of 

 Nature's physiological experiment, pathology. In all such investigations, 

 we are dealing with indirect methods of proof. Experiments with animals 

 rarely lead to other than indirect results. These are not entirely con- 

 clusive; they are merely more or less convincing, and in most cases are 

 dependent upon the personal interpretation of the investigator. This is 

 a weak point in nearly all biological investigation, which undeniably gives 

 it a certain amount of fundamental uncertainty. For this reason, it is 

 difficult to form a positive decision from the numerous experiments which 

 have been performed in the attempt to decide the question as to the 

 conversion of one class of food-stuffs into another. We can merely 

 enumerate here the more important and best substantiated conceptions 

 and those experiments which have been carried out in the most convinc- 

 ing manner. On the other hand, we would be committing a grave error 

 if we were to consider only those processes and changes in the organism 

 as proved for which we have a purely chemical explanation, and which we 

 are able to repeat, where possible, outside of the living cell. We would 

 thus be making a restriction, which would hinder the further develop- 

 ment of biology, and we should then be forgetting that biology has a 

 distinct field of its own, with its own peculiar methods of investigation. 

 Ultimately, of course, we must always depend upon the exact sciences, 

 chemistry and physics, and consider a biological problem as fully settled 

 when these sciences supply the key-stone. 



Let us begin first of all with the carbohydrates and their conversion into 

 the other two groups of food materials, fats and albumins. The formation 

 of albumin from carbohydrates in vegetable organisms comes into con- 

 sideration only to the extent that the plant cells utilize the carbon chains of 

 the sugars in syntheses together with the nitrogen, which is either assimi- 

 lated by the roots from the ground, or, as in rare cases, is obtained directly 

 from the air. We do not know anything definite about these processes. 

 In the animal organism, we can deny the possibility of such transforma- 

 tions. It is otherwise, however, as regards relations of the carbohydrates 

 to the fats. One of the first observations in this connection was the 

 formation of fat in ripening rape-seeds, and in the pulp of the olive. We 

 know that unripe rape-seeds contain large amounts of carbohydrates, but 

 practically no fat. During the ripening of the seeds we find that the 

 carbohydrates gradually diminish, while fat takes their place. This obser- 

 vation is very significant, for the seed is unable to give off its carbohydrate 

 externally, or to take up fats. The changes in nature of a food material are 

 also attested by the metabolism. The respiratory quotient changes. Ger- 

 ber 1 has shown that the ratio of carbon dioxide produced to the oxygen 



1 C. Gerber: Compt. rend. 125, 658 and 732 (1897). 



