The Nature of Animal Life. 



substance. One of these is the amoeba, a minute speck of 

 jelly-like life-stuff, which lives in water, and tucks in a bit 

 of food-material just as it comes. And there are certain 

 degenerate organisms which have taken to a parasitic life, 

 and live within the bodies of other animals. Many of 

 these can absorb the material prepared by their host 

 through the general surface of their simple bodies. But 

 here, again, though there may be no special organs set 

 apart for the preparation, absorption, and digestion of 

 food, the process of feeding is essential to the life of all 

 animals. Stop that process for a sufficient length of time, 

 and they inevitably die. 



4. They grow. Food, as we have just seen, has to be 

 taken in, digested, and absorbed, in order that the loss of 

 substance due to the chemical changes consequent on 

 respiration may be made good. But where the digestion 

 and absorption are in excess of that requisite for this 

 purpose, we have the phenomenon of growth. 



What are the characteristics of this growth ? We 

 cannot, perhaps, describe it better than by saying (1) that 

 it is organic, that is to say, a growth of the various organs 

 of the animal in due proportion; (2) that it takes place, 

 not merely by the addition of new material (for a crystal 

 grows by the addition of new material, layer upon layer), 

 but by the incorporation of that new material into the very 

 substance of the old ; and (3) that the material incor- 

 porated during growth differs from the material absorbed 

 from without, which has undergone a preparatory chemical 

 transformation within the animal during digestion. The 

 growth of an animal is thus dependent upon the continued 

 absorption of new material from without, and its trans- 

 formation into the substance of the body. 



The animal is, in fact, a centre of continual waste and 

 repair, of nicely balanced constructive and destructive 

 processes. These are the invariable concomitants of life. 

 Only so long as the constructive processes outbalance the 

 destructive processes does growth continue. During the 

 greater part of a healthy man's life, for example, the two 



