122 THE PRINCIPLES OF HEREDITY 



antagonistic to the toxins, and so bring about immunity. The 

 theory of chemical neutralization is plainly at fault. 



199. Another of Pasteur's experiments may afford some 

 light. He submitted the bacilli of anthrax to an abnormal 

 degree of heat, and found that they generally lost their viru- 

 lence. Ordinary anthrax bacilli are very fatal to sheep, but 

 Pasteur made the highly important discovery that if the 

 sheep were inoculated first with bacilli of little virulence, 

 next with those of greater virulence, and lastly with those of 

 great virulence, the animals could be rendered immune to 

 bacilli of the greatest virulence. This result could be ob- 

 tained only by observing the j steps of the process ; an 

 immediate passage from very weak to very virulent bacilli 

 resulted in the death of the sheep. Here again the theory 

 of chemical neutralization breaks down entirely. The 

 anthrax bacilli are introduced alone or with an insignificant 

 quantity of toxin. The antitoxins, whatever they may be, 

 are elaborated within the sheep, and it is difficult to under- 

 stand, if they are substances chemically antagonistic to the 

 toxins, why they should be elaborated only when the animal 

 has undergone Pasteur's treatment, and not at other times. 

 The truth appears to be, that, just as heating the bacilli of 

 anthrax progressively weakens their toxins, so drying the 

 cords of infected rabbits weakens the toxins contained in the 

 cords. It is not that the toxins are diminished in quantity, 

 but that they are altered in quality. Habit uat ion to a weaker 

 toxin places the individual in a position of advantage, from 

 which he is able to become habituated to a stronger toxin more 

 easily than he could otherwise have done. Progressive treat- 

 ment enables him to resist the strongest toxins and recover from 

 a disease which, without it, would have been inevitably fatal. 



200. Snake-venom (a toxin) may be swallowed in enor- 

 mous quantities, not only without ill effects, but with the 

 remarkable result that the individual is rendered immune 

 to venom injected under the skin. The Bushmen of South 

 Africa make a practice of eating the poison-glands of snakes, 

 and are thus rendered immune to the bite. 1 In this case the 

 swallowed venom enters the blood in a digested condition, 

 and thus supplies the stepping-stones. It has been found, 

 that if venom be altered by artificial means outside the body, 

 the antivenene when injected under the skin of a person 

 suffering from snake-bite may actually effect his recovery. 



201. Now, the presumption is that all the so-called antitoxins 

 are weakened toxins. A phagocyte does not kill a micro- 



1 Eraser, Address at the Eoyal Institution, March 30, 1896. 



