204 ORIGIN AND NATURE OF LIFE 



a first rough orientation and places the 

 organism in a group ; its identification then 

 follows from its chemical behaviour, and in 

 all chemistry there are no reactions so delicate 

 and specific as those induced by these minutest 

 of living organisms. 



Two micro-organisms so closely similar that 

 a skilled bacteriologist cannot distinguish 

 them by examination with the highest powers 

 of the microscope, when they invade the 

 body of man, or are inoculated into an 

 animal, produce two diseases absolutely 

 specific and quite different from each other. 

 The physician can diagnose the two diseases 

 with the greatest ease, and in all cases of 

 infection the one germ produces its own 

 disease, and the other quite as definitely its 

 special disease with definite symptoms. Each 

 micro-organism produces its own specific set 

 of chemical poisons called toxins, and these 

 toxins have special affinities for certain of 

 the tissue cells, on account of adaptation of 

 structure of the colloid molecule of the toxin 

 to the colloids of the cells of the affected 

 tissue. In fact, these two fit together almost 

 like a highly complex lock and key, while 

 other colloids of other tissue cells devoid of 

 such specific adaptation are left entirely 

 unaffected. This accounts for the favourite 



