ELEMENTARY VITAL PHENOMENA 141 



green plants. Other forms of Bacteria cannot exist without 

 organic food. 



To glance at the more special nutrition of animals, as regards 

 the organic food-stuffs a considerable difference prevails 

 between individual species. There are remarkable adapta- 

 tions to single food-stuffs. Thus, the caterpillar of the fur-moth 

 lives exclusively upon the hairs of fur, which consist of pure 

 keratin. Keratin, which is closely allied to proteid, is, therefore, 

 capable of furnishing all the elements for the formation of the 

 living substance of the fur-caterpillar. In other cases, e.g., in 

 carnivora, proteid alone suffices to supply all the elements 

 necessary to the formation of the body ; and lately Pfltiger ('92) 

 has shown by detailed experiments that even dogs, when forced to 

 perform hard labour daily, can live continually upon pure proteid 

 food. In such experiments, after a short time the dogs lose almost 

 all their body-fat, but remain abundantly capable of work, strong 

 and healthy. On the other hand, it is impossible to maintain an 

 animal's life with carbohydrates or fats solely, or even with the two 

 together. In spite of an abundance of such food, the animals 

 consume their own body-proteid, as shown by the continual excre- 

 tion of nitrogen in the urine, and finally grow weaker and die. 

 The reason for this is evident, for, since the living substance is 

 constantly breaking down of itself in a definite quantity, it must 

 constantly be reconstructed if the animal is to live. But this 

 cannot happen if no nitrogen, which is lacking in carbohydrates 

 and fats, be given to the animal. Since, however, as has been seen, 

 animals cannot take up nitrogen from inorganic compounds, it- 

 follows that proteids, which alone represent the nitrogenous food- 

 stuffs, are absolutely necessary for the maintenance of animal life. 

 Hence we arrive at the important fact that of all organic sub- 

 stances proteids alone are indispensable to the nutrition of 

 animals, and in certain cases also they alone suffice to maintain 

 the animal's life. Pfliiger, therefore, distinguishes proteid as the 

 primitive food from the carbohydrates, fats, etc., which act only 

 as substitute foods. 



In addition to food proper in the narrow sense, all organisms 

 take in oxygen a process that is termed respiration. Of course all 

 organisms do not receive oxygen in the same form and from the 

 same source. Terrestrial organisms take it in the form of gas 

 from the air ; aquatic organisms use the oxygen dissolved in the 

 water; and the tissue-cells of animals that are provided with a 

 blood-circulation, as well as many parasitic organisms, withdraw it 

 from chemical compounds the tissue-cells from the haemoglobin 

 of the blood, with which it is loosely combined, and certain para- 

 sites from relatively fixed combinations. All organisms take only 

 a certain quantity of oxygen, even when more is offered ; their 

 consumption of it is not essentially increased in a medium of pure 



