ADAPTATION 351 



more particularly because the broken integument opens the way 

 for invasions of bacteria. 



Immunity. — By slowly increasing the dosage, the tolerance 

 of the body for certain poisons can be gradually increased. An 

 animal can acquire an immunity to bacterial diseases by having 

 the disease itself. In either case there is a functional response 

 to the action of the toxic substance of the pathogenic agent 

 which results in an improvement of the defense mechanism of 

 the body. The usual explanation of what happens during the 

 recovery from a bacterial infection is that substances known as 

 antitoxins are formed and that these substances counteract the 

 effects of the bacterial toxins, with the result that the patient 

 for some time afterward is immune to further attack by the 

 particular organism concerned. Immunity against smallpox, 

 for example, may be established by having the disease or by sub- 

 mitting to vaccination with virus of a mild form of the disease. 

 In other cases, the practice of giving antitoxin as a prophylactic 

 or preventive measure is based upon the fact that an animal such 

 as a rabbit, guinea pig, or horse, if inoculated with disease-pro- 

 ducing organisms, reacts by producing a substance, antitoxin, 

 which tends to neutralize the injurious effect of the organisms. 

 Serum, the clear fluid obtained from clotted blood taken from 

 such an animal, contains the antitoxin, which if injected into 

 another animal or a human being, is capable of establishing 

 immunity in the latter against the particular pathogenic organism 

 involved. Immunity reactions and the power to regenerate lost 

 parts are excellent examples of purely physiological adaptation. 



Temperature. — The rate of metabolism varies with the temper- 

 ature of the animal body, within a range that is not the same for 

 all forms of life. The so-called cold-blooded animals have a body 

 temperature that is slightly higher than that of the surrounding 

 medium. Fluctuations in body temperatures follow fluctuations 

 in outside temperature, a condition described by the term poikilo- 

 thermous. All invertebrate animals and the lower vertebrates, 

 including fishes, amphibians, and reptiles, with the possible 

 exception of the turtle, are poikilothermous. Such animals lack a 

 mechanism for maintaining a constant body temperature and a 

 constant rate of metabolism. Some kinds of goldfish can be 

 frozen stiff in icy water and be recovered unharmed if the warm- 

 ing is done gradually. In the "frozen" state the temperature of 



