along a definite line is a property of all zymotic diseases. During the cattle plague of 1865-6^ Mr. Lowe in the House of Commons (1866), predicted an epizootic of tremendous proportions, with a formidable rate of increase. His views were controverted by Dr. WiQiam Farr in a letter to the Daily News of February 17, 1866,^* in which it was main- tained that the rate of increase would begin to decrease rapidly at a certain point, after which it would go on decreasing until the rate of .incidence itself decreased. This generalization, the facts of which are not unlike the phenomena of depopulation in modern states, is known as Farr's law. It implies, as Farr says, that "the curve of an epidemic at first ascends rapidly, then slowly until it attains a maximum, then makes a turn and falls more rapidly than it mounted." To prove his case, Farr plotted a bell-shaped probability curve of the actual epidemic, based upon reported and calculated statistics, and predicted that it would have an early maximum with a rapid decline, ending in June, 1866. Actually, the epidemic rose to a maximum on February 24, a fortnight earlier than Farr had predicted, but subsided in the early summer, as he surmised, although at a slower rate than his curve indi- cated. Nevertheless, his calculations, in the face of the public alarm obtaining at the time, were a great advance in epidemiology, what Ross calls "the first a posteriori work on epidemics," in which it was at- tempted to work back inductively to underlying principles from ob- served and observable data. Farr also appUed his principle with suc- cess to a subsequent smallpox epidemic. The cause of the constantly decreasing increase has been sought in the gradual lack of susceptible or infective material, e. g., in the effects of vaccination on the Boston epidemic of smallpox in 1721, a view favored by Boss. Another cause, favored by Brownlee, is to be found in Pasteur's theory of attenuated viruses. Pasteur showed that the pathogenic properties of a virus may be increased or attenuated by successive passages through the bodies of appropriate animals, from which he reasoned that the origin or extinc- tion of an epidemic disease may be due to the strengthening or weakening of a virus by environmental conditions, either in external nature or in the bodies of animals. This seems borne out by the thermody- namic conditions governing the virulence of microorganisms. In the bodies of bacillus carriers, the typhoid baciUus is temporarily inactive or inactivated, for the nonce, an insulated adiabatic system, in that energy can neither go in nor out of it. In the body of a susceptible per- son, the same bacillus becomes activated and pathogenic, whence it is reasoned that a nonvirulent strain of a bacillus may become pathogenic under certain conditions in nature. In this way, Sudhoff has a1>- tempted to explain the origin of syphilis in Europe. Prior to its ap- pearance as such, in 1494, there had existed a class of lepra-like dis- eases yielding to mercury, as is shown by old Italian prescriptions of
iReprinted by Brownlee in Brit. M. J., Lond., 1915, II., 251.
