36 THEORY OF RETENTION 



poison on the organism producing it. The theory of immunity 

 based on these facts is obvious. Bacteria growing in the body 

 will yield substances inimical to the continued growth of the 

 organism, so that they will die out and recovery ensue, and the 

 body will remain immune as long as these substances are retained 

 therein. This theory accounts well for the production of immunity 

 by injections of the toxins and other soluble products of bacteria. 

 It is negatived by the fact that bacteria may grow in the blood and 

 tissues of immune animals, and is improbable if we consider that 

 immunity may last for many years, and that it is extremely 

 improbable that substances (necessarily soluble) should be retained 

 in the body for so long a time. 



We shall now proceed to a study of the more modern views, 

 and in doing so it will be convenient to deal with the subjects of 

 immunity to toxins and immunity to bacteria in separate sections. 

 Of course, in most cases they run parallel to one another : an 

 animal contracts a disease because its fluids and tissues cannot 

 kill the pathogenic bacteria offhand, and because its cells are sus- 

 ceptible to the action of the toxin, and vice versa. This is not 

 necessarily the case, however, and the two phenomena may be 

 entirely independent. 



The subject of immunity of toxins is on the whole the more 

 important of the two, the simpler (though complex enough), and 

 the best understood. It will be best to deal with it first. 



