The question of variation, which has long attracted attention in connection with the biologic study of the most diverse forms of animal and plant life, has at length become a timely topic in bacteriology. The variability of micro-organisms in certain respects has, of course, been known since the days of Pasteur; for it was early realized that within the same species of bacteria, different races or strains may exhibit widely varying degrees of virulence. Such fluctuations have appeared to represent, to a large extent, degrees of adaptation on the part of bacteria to the conditions found in the living body. Not only, furthermore, can infectiousness be enhanced by the passage of micro-organisms through animals of certain species, but the virulence of bacteria can also be attenuated by laboratory manipulation.
The problem of variation in bacteria, however, is something more than one of alterations in the property of infectiousness which may