144 THE ACTION OF THE LIVING CELL 



hand, if the pigments so developed were colorless, 

 but absorbed light of the near ultra-violet portion of 

 the spectrum, v^e might be equally well protected from 

 the sun; yet in all probability we should be in a quan- 

 dary as to the mechanism by which increased resistance 

 to sunlight is accomplished. Similarly, one is mysti- 

 fied by the vast majority of changes to which many 

 animals, and man, are resistant or may become ac- 

 climatized. The observed resistance or acclimatiza- 

 tion is usually attributed to natural resistance or adap- 

 tation — admittedly and necessarily vague terms. 



However, although we may not completely com- 

 prehend the underlying mechanism by which in- 

 creased natural resistance takes place, it seems neces- 

 sary to postulate that such a physiological mechanism 

 is absolutely essential to life. 



While the tissue culture experiments previously re- 

 ferred to have shown life to be potentially immortal 

 if provided with an adequately controlled environ- 

 ment, the existence of animals and plants in their natu- 

 ral environment is sooner or later terminated by death, 

 natural or otherwise. This is necessarily so, because 

 an organism's ability to increase its natural resistance 

 towards all sorts of changes is not unlimited — if it 

 were, we might reasonably expect all animals to be 

 immortal. 



The study of disease has disclosed a number of im- 

 portant facts concerning natural resistance, or im- 

 munity, as this is termed by medical men. 



Following Pasteur's classic experiments on immu- 

 nization to the virus of rabies, and Behring's discovery 

 of diphtheria antitoxin, it became apparent that some 

 organisms, notably the bacteria, when introduced into 



