16 STUDIES IN IMMUNITY. 



We believe, then, that we are justified in repeating what we wrote 

 three years ago on the increase in virulence of bacterial infective 

 agents:* "When we inject vibrios subcutaneously in a vaccinated 

 guinea-pig an emigration of leucocytes rapidly takes place. The 

 first white corpuscles to arrive at the point of inoculation find a 

 considerable number of adversaries, of which they can destroy only 

 a few. It would be sufficient for a few of these vibrios to be endowed 

 with a slightly more intense power of attraction than their fellows, 

 to cause the phagocytes to direct themselves by preference toward 

 these organisms and to take them up. Very slight, almost inappre- 

 ciable differences will consequently predestine certain bacteria to 

 rapid phagocytosis. In the same way an inferiority in secretion of 

 toxic products, however slight, will cause a predisposition to rapid 

 destruction. 



"In a word, leucocytes kill first those organisms that are less 

 resistant, and the culture inoculated will be freed first of those 

 individuals which either form less poison than their fellows or 

 attract the leucocytes more. 



"In the meantime the vibrios that have been left alone will divide 

 and produce new organisms which, in turn, will be exposed to the 

 attack of phagocytes. These latter will again suppress those indi- 

 viduals that are most poorly armed for the struggle, and will leave 

 only those that possess in highest degree the two characters just 

 noted. Thanks to this process of selection, new generations of 

 organisms, like those represented by the cultures used in our experi- 

 ments, will be derived for the most part from those bacteria which 

 have been endowed with certain advantages." 



It has already been well established that virulent strains of an 

 organism attract leucocytes less strongly than do attenuated 

 strains. 



This phenomenon of selection certainly comes into play in mixed 

 infections. Phagocytes may direct their efforts particularly toward 

 one of the invaders and so neglect the other. 



A diversion would thus be caused which would favor the develop- 

 ment of the more dangerous infection. As we have given a very 

 clear experimental example of this fact, it may be assumed that 

 phenomena of the same sort occur under natural conditions. 

 * See "Adaptative changes of bacterial cultures, etc.," p. 6. 



