120 THE PRINCIPLES OF HEREDITY 



195. The various vegetable poisons are in a real sense 

 toxins. Like the microbic poisons they protect the organ- 

 isms producing them from other organisms to which they are 

 liable to fall a prey. A tobacco-smoker or an opium-eater, if 

 he begins with small doses, is able to inure himself to im- 

 mensely larger doses to doses great enough to be fatal many 

 times over to a beginner. At any rate, though sequelae occur, 

 as in the case of disease, from smoking tobacco and opium, 

 he is able to inure himself against the immediate effects of 

 the poison. No one will venture to suggest that he owes his 

 immunity to the formation of antitoxins which chemically 

 neutralize nicotine and opium. The case of arsenic, a mineral 

 poison, is similar. In each instance the individual simply 

 becomes habituated to the poison. It is not necessary to 

 explain, indeed it is beyond our power to explain, how this 

 habituation is brought about. We know only that it does 

 occur, and we have no more reason to suppose that it is due 

 to the formation of a substance chemically antagonistic to the 

 poison than we have to suppose that the habituation of the 

 muscles of a trained athlete is due to the formation of a sub- 

 stance chemically antagonistic to fatigue. We can only say, 

 in a very vague way, that it is due to vital changes in the 

 cells, that it is part of that general capacity for undergoing 

 beneficial change by means of which the higher animals adapt 

 themselves to changes of circumstances. Now we have in- 

 controvertible evidence that acquired immunity to diseases is 

 due, to some extent at least, to a similar habituation to the 

 toxins. If we inject doses of toxin into a horse at a certain 

 very rapid rate, we are able to render his blood very poison- 

 ous to other animals. At the same time he retains his own 

 health. Clearly, then, in this case, since the toxins are not 

 neutralized, he owes his immunity to habituation. 



196. However, it may be argued: "All this does not im- 

 pugn the doctrine of neutralization. Habituation is possibly 

 part of the phenomenon. It is true that an individual may 

 become habituated to opium, nicotine, arsenic, or to a toxin, 

 by the use of gradually increased doses. But the question of 

 antitoxin stands on a different plane. If we mix a small 

 dose of opium or nicotine with a large dose, the latter is not 

 thereby rendered less poisonous. We cannot cure a person 

 poisoned by opium or nicotine by giving him additional small 

 doses. But, if we mix sufficient antitoxin with toxin, we render 

 it harmless, and a dose of antitoxin tends to cure a person suf- 

 fering from the corresponding toxins. Clearly an antitoxin 

 is not a small dose of a toxin, but quite a different thing." 



