PKESIDEXt's address. SECTION G^. 519 



but there are one or two matters of such peculiar importance as to be 

 specially worthy of mention. One is the theory of the mode of 

 appeariince and duration of disease. Some diseases, pulmonary tuber- 

 culosis, for example, have a kind of double incidence on the human 

 lace — that is, are dimodal in the frequency of death with age. The 

 infantile phase shows the greatest deatli-frequency at under twelve 

 months, while the adult phase has its death-maximum at twent^'-five 

 to thirty-five years of age. Both in the Commonwealth and the United 

 Kingdom the frequency curve for deaths of males is chai^acteristically 

 different to that for females. 



Statistics of the frequency of the duration of disease can scarcely 

 be said yet to exist, though such statistics would undoubtedly be 

 valuable generally, and would be of great economic importance. 



9. Theory of Epidemics. — In the theory of epidemics, the applica- 

 tion of the statistical method has indicated results of great value. 

 The late Dr. Farr, the eminent English statistician, had long ago re- 

 cognised that the distributions of death from epidemics follow 

 ordinary curve types, and in 1866, when a cattle plague was making 

 gi-eat havoc, and public fears were acute as to the limitless damage 

 it might do, Dr. Farr wrote a letter showing that from a proper 

 examination of the data the acme and decline of the plague might 

 Boon be expected.* 



Kecently Dr. Brownlee has shown from the statistical results of 

 epidemics that its course seems to depend on the reception of an 

 organism, which, at the point of time when the epidemic starts, has 

 a high degree of infectivity, but that this degi-ee of infectivity is 

 steadily lost till the end of the epidemic, at a rate approximately in- 

 creasing in geometrical progression. The increase of infectivity may 

 have an annual period, or (apparently) be without definite period. 

 The cessation of the epidemic depends upon the decline of infectivity 

 of the invading germ rather than upon any constitutional peculiarity 

 in the persons affected. 



In connection with the subject of epidemics and immunity there- 

 from, a recent study on phagocytosis as aft'ected by antiseptics, &c., 

 only disclosed its full significance on the application of the statistical 

 method, f 



10. Economic Aspect of Vital Phenomena. — One of the most 

 fundamental problems of economics is that which proposes to investi- 

 gate the economic efficiency of the human unit under given conditions — 

 in fact, any attempt to ascertain the population-carrying power of our 

 earth must attack this question as one element of the solution. The 

 enei'gy expended in the nurture, education, and general maintenance 

 of the human unit appears on one side of the ledger, his productive 

 activity on the other. Thi'ougli disease, however, man's activity as a 

 pi-oductive unit is reduced by total or partial incapacitation : and one 

 of the measures of this would be the extent to which he is affected 

 on the average during his life by various diseases. Statistics of this 

 character have not yet been adequately developed, but would bo 

 valuable, and could be obtained by what may be called an occasional 

 census. 



He virtually used the curve u = e 



— ax," + hx + c 



t Manwaring and Ruh, Rockfeller Institute VTII., 1908, pp. 473-48G. 



