46 COMPARATIVE ZOOLOGY. 



Another source of demand for food is the production of 

 germs, to propagate the race, and the nourishment of such 

 offspring ui the egg and infantile state. This reproduc- 

 tion and development of parts which can maintain an in- 

 dependent existence is a vegetative phenomenon (for plants 

 have it), and is a part of the general process of Nutrition. 

 But it will be more convenient to consider it hereafter 

 (chapters xix., xx.). Still another necessity for aliment 

 ,among the higher animals is the maintenance of bodily 

 heat. This will be treated under the head of Respiration. 



For the present, we will study Nutrition, as manifested 

 in maintaining the life of an adult individual. 



In all animals, this process essentially consists in the in- 

 troduction of food, its conversion into tissue, its oxidation, 

 and the removal of worn-out material. 



1. The food must be procured, and swallowed. (Inges- 

 tion.) 



2. The food must be dissolved, and the nutritious parts 

 separated into a fluid. (Digestion.) 



3. The nutritive fluid must be carefully taken up, and 

 then distributed all over the body. (Absorption and Cir- 

 culation.) 



4. The tissues must repair their parts wasted by use, 

 by transforming particles of blood into living matter like 

 themselves. (Assimilation.) 



5. Certain matters must be strained from the blood, 

 some to serve a purpose, others to be cast out of the sys- 

 tem. (Secretion and Excretion.) 



6. In order to produce work and heat, the food must be 

 oxidized, either in the blood or in the tissues, after assimi- 

 lation. The necessary oxygen is obtained through expos- 

 ure of the blood to the air in the lungs. (Respiration in 

 part.) 



7. The waste products of this oxidation taken up by 

 the blood must be got rid of ; some from the lungs (car- 



