or on seeing the slightest movement or the passing of a shadow. But still 

 more curious, after the animals had been fed the poison in this way for a 

 considerable time, they became unable to live without it. If the drug was 

 omitted from their daily rations, they quickly died. 



The rabbit's protoplasm adjusted itself to new surroundings. The proto- 

 plasm became able to live under conditions that would normally destroy it. 

 In experiments with bacteria similar results were obtained. Bacteria of va- 

 rious species were placed in dishes with the usual food materials, but with 

 the addition of a small amount of phenol or other germicide. When the 

 colony had about used up all the food in the dish, some bacteria were trans- 

 ferred to a similar dish containing a slightly greater concentration of the 

 poison. This was repeated several times. In the end there was a growth of 

 bacteria that could tolerate much more poison than would normally kill 

 their ancestors. 



Persons suffering from malaria are systematically treated with quinin to 

 keep the parasite in check. After a long and seemingly successful treatment 

 a patient sometimes relapses. It has been suggested that in such cases the 

 malaria parasite has become able to tolerate relatively large quantities of 

 quinin, so that it is useless to drug the patient further. 



Such observations suggest that while each particular kind of protoplasm 

 thrives best in a particular set of conditions, it is able also to adjust itself 

 to different conditions — provided they are not too different. It is not clear 

 just what change takes place in the protoplasm itself under such circum- 

 stances. 



Antitoxin Different kinds of bacteria produce substances that act as 

 poisons in the bodies of animals. Such protein poisons, or toxins, are found 

 also in the venom of various snakes and in the tissues of various higher 

 plants. When some toxin gets into living tissue, it stimulates the protoplasm 

 to produce specific neutralizing, or counteracting, substances. The reaction 

 of the invaded protoplasm may be compared to some of the chemical proc- 

 esses that bring about homeostasis — the release of acid under the stimulus of 

 alkali, and vice versa (see page 193). The reaction of protoplasm to the 

 toxins is apparently much more complex, however. The counteracting sub- 

 stance produced by living cells under the influence of a toxin is called an 

 antitoxin, and it is always specific. That is, it will neutralize the poison 

 under whose stimulation it was produced, but no other. 



Among the best-known toxins are those produced by the bacteria that 

 cause lockjaw and diphtheria. When a quantity of toxin, not enough to 

 kill, is injected into the blood of a healthy animal (a young horse, for 

 example), the cells begin to produce and excrete antitoxin. They will pro- 

 duce more than enough antitoxin to neutralize the poison received by the 

 body, and the surplus antitoxin remains in the blood. This surplus can then 



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