1. The relation between hemolytic and non-hemolytic streptococci is orderly and arises from the fact that the former variety is unique while the latter is heterogeneous.
2. Analogous considerations of the classification, distribution, and pathogenicity of the streptococcus group and the pneumococcus and colon-typhoid groups show a definite parallelism.
3. These considerations suggest that unique varieties of bacteria associated with man are the more highly invasive, while the heterogeneous varieties are more saprophytic.