IMMUNITY AN ADAPTATION 49 



Here then is a perfectly clear cut example of direct individual 

 adaptation in the higher animals fulfilling wholly the essential 

 conditions that — 



1. We deal with the acquirement of a new property ; the 



acquirement cannot possibly be regarded as the calling 

 into activity of a property previously possessed, either 

 by the individual or by the species : this power to 

 neutralize ricin is something absolutely new. 



2. The acquirement is something positive, something addi- 



tional ; there can here be no alternative hypothesis of 

 loss of inhibiting factors. 



3. There can here be no possibility of ascribing the new 



property to the persistence of a chance variation : the 

 power to develop antiricin and discharge it into the 

 blood can be produced in any mouse or rabbit with 

 absolute certainty. 



4. There is here no alternative explanation of the survival 



of the fittest. 

 I give this as a first example because in it we have no addi- 

 tional factor of prolif erative activities on the part of the microbes 

 which in the case of the development of immunity against 

 bacteria and their products may, to those not familiar with 

 bacteriology, be apt to confuse the picture. But what is true 

 with regard to these phytotoxins may be exactly paralleled in 

 the reaction that can be induced in the higher animals against 

 bacterial poisons — bacteriotoxins. We do not in nature find 

 guinea-pigs, for example, subject to diphtheria : by cutaneous 

 inoculation of pure cultures of the diphtheria bacillus these 

 animals can be given a fatal form of the disease, and what is 

 more, the ectotoxins, the substances given off by these bacilli 

 when they are grown in fluid media, are intensely poisonous for 

 these little animals : Roux and Yersin found that 0*0002 mg. of 

 the dried — and impure — toxin, obtained by salting it out from 

 the fluid of growth, would surely kill a guinea-pig of 250 grams 

 within three days, or otherwise one gram would kill five million 

 guinea-pigs of like weight. Now exactly as with ricin the exhibi- 

 tion of minute, progressively increasing doses leads to the pro- 

 duction of a high order of immunity, and with this the blood 

 serum is found to contain the antitoxin in relatively very large 

 quantities. In other words, the tissues have elaborated, and 



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