442 LIFE : OUTLINES OF GENERAL BIOLOGY 



The cobra's poison has no effect on other cobras of the same 

 species, and this is a general rule. But when a venomous snake bites 

 a relative that belongs to a different species, the result may be fatal, 

 as is generally the case with different kinds of vipers. A cobra is not 

 much affected by a viper's poison, while all vipers readily succumb 

 to a cobra's. All this shows how specific the immunity may be. 

 There arc many peculiarities of this sort which do not readily find 

 explanation at present. Thus if the blood of a crayfish be injected 

 into a mouse, it confers immunity against the poison of a scorpion! 

 Yet the crayfish itself is far more sensitive than the mouse to 

 scorpion poison. 



Artificial Immunity. — The immunity which is so well illustrated 

 by the hedgehog to adder toxin and by the mongoose to cobra toxin 

 is natural immunity; but the same quality of insusceptibility may 

 be more or less artificially acquired. One method is to begin by 

 injecting minute doses of the poison, and to continue with gradually 

 increased doses. The other method is to inject a serum preparation 

 of the blood of an animal that has been immunised by graduated 

 direct doses of the poison, or by non-fatal attacks of the virulent 

 microbes. Some animal, such as a horse, is rendered immune, and its 

 serum, abounding in anti-toxins according to the theory, is injected 

 into another organism. This method is less drastic than the other, 

 yet very effective. 



Another form of immunity is very familiar, namely that which 

 follows recovery from a disease. The individual is more or less 

 protected against taking the disease a second time. The immunity 

 may last for many years in the case of smallpox, but there 

 are records of two or even three attacks; it may be strong, as 

 in the case of scarlet fever, measles and mumps; it may be 

 transient, as in diphtheria. There are rare cases, such as pneu- 

 monia, where the immunity is at most very temporary; where 

 indeed susceptibility to the disease appears to be increased, not 

 decreased. 



There is evidence in some instances that an artificially immunised 

 rabbit or guinea-pig has young ones which are bom immune. The 

 same is asserted in regard to smallpox, that the immunised human 

 mother may confer immunity on her offspring; but it is difficult to 

 prove this satisfactorily. If it occurs, it may be due to specific 

 anti-toxins passing through the placenta from the blood of the 

 mother into that of the unborn child. 



When there is evidence of immunity as a racial character, as in the 

 case of negroes, who are relatively immune to yellow fever and 

 malaria, or in the case of Algerian sheep, which are relatively immune 

 to splenic fever or anthrax, the explanation is probably that a 

 constitutional variation, like the hedgehog's, arising apart from 

 infection or other poisoning, has become hereditary, whereas those 



