The discovery that different "species" of bacteria are not units but consist of a larger or smaller number of "types," each with its own pathologic implications, has had important practical consequences. Biochemical and serologic differences at first sight of minor importance have turned out to be of first class significance. The author of this useful little monograph, himself well known as a student of bacterial types, has brought together most of the well established facts in a clear and compact form. Although often treated rather summarily, as in the discussion of Bacillus botulinus (pp. 96 and 97), the material is on the whole dealt with critically and with an expert hand. The sections on streptococcus types are particularly good. The book will be of service to all students of one of the most vexed questions in modern bacteriology.