In the early days of bacteriology it seemed to some workers a relatively simple matter to distinguish between different species of bacteria and either to assign to every organism its place in an established group or else to install it as a "new species." It soon became apparent, however, that the ordinary cultural characters afforded an insecure basis for such differentiation, since micro-organisms closely related in some of their growth phenomena showed in others wide divergence, and since also one and the same bacterium varied greatly in its manner of growth and in its chemical products under different conditions and at different times. Morphologic differences were, if anything, less to be relied on than physiologic. The biologic relationship of different micro-organisms consequently became, and to some extent still remains, a subject for speculation rather than for exact verification.
The discovery of the agglutinative reaction, now so widely used in the