496 IMMUNITY. 



to toxic substances may explain the degrees of immunity 

 possessed by different animals, whilst in others immunity 

 may be due to special bactericidal powers possessed by 

 them. What these bactericidal powers really are cannot 

 be explained on any single theory. A vital activity of the 

 tissues and fluids is, no doubt, brought about by the pres- 

 ence of the bacteria, and this cannot be fully imitated in 

 experiments outside the body. The facts given above with 

 regard to the action of antimicrobic serum, show how com- 

 plicated a matter the bactericidal process may be. Further, 

 in natural immunity a direct killing of the organisms by the 

 fluids of the serum is not necessary. It may be sufficient 

 that their growth is prevented, so that they ultimately die 

 out or are taken up by the phagocytes. 



