THE LIVING ORGANISM 203 



exceedingly minute forms, some quite harm- 

 less and even beneficial, and others the 

 exciting causes of more than half the ills 

 that flesh is heir to. These tiny organisms, 

 sometimes form practically structureless col- 

 loidal globules, as far as the highest resolving 

 powers of the microscope reach, which are 

 known as micro-cocci. Yet each one of these 

 minute dots forms a little microcosm, made 

 up in each species of micro-coccus of a 

 highly specific grouping of complex colloidal 

 molecules, with a commerce of chemical 

 exchanges entirely its own, and unlike that 

 of any of the other species of micro-cocci. So 

 that in the majority of cases it is by its 

 effects and bio-chemical reactions alone that 

 a micro-coccus can be distinguished from 

 others microscopically indistinguishable from 

 it. The advance of modern bacteriology has 

 made it absolutely indispensable to the 

 bacteriologist to possess a highly special 

 training in bio-chemistry, and micro-organisms 

 are nowadays distinguished far more by the 

 reactions which they induce in culture media, 

 or in the fluids taken from man or another 

 mammal, or by the specific changes which 

 they produce in the body as a whole, than by 

 examination with the microscope. The micro- 

 scope, as a rule, gives the bacteriologist only 



