In a paper on “The Spread of Bacterial Infection” (Journal of Hygiene, xix. No. 4, March 22, 1921) Dr W. W. C. Topley has shown the results of many months' observation of the progress of an epidemic in a population of mice. The author thus describes the experiment: “This experiment lasted from May 21, 1919, until June 11, 1920.…The epidemic started about the middle of October, 1919, and continued in the manner to be described until the conclusion of the experiment.…The period May 21 to September 17 is omitted.…During this time there was no indication of the active spread of infection.…The whole experiment may be divided roughly into four periods. The first extends from May 21 to Sept. 17, and has been referred to already. From Sept. 18 and onwards, only normal mice were added to the cage. From this date until Jan. 5, 1920, the mice were added in such a way as to keep the total number in the cage roughly constant. From Jan. 6 to April 27, 1920, three normal mice were added each day, except on two occasions when none was added. The number of mice in the cage during this period varied from day to day, reaching a maximum immediately before each considerable wave of mortality, and falling to a minimum just before its cessation. During the final period from April 28 to June 11, 1920, two normal mice were added daily instead of three.”