MATHEMATICAL CONTRIBUTIONS TO THE THEORY OF EVOLUTION. 281 



R, ilv = '1875, B,,, = - 1. 



By aid of these we find the following table of error correlations : 



Now this table enables us to draw some remarkable conclusions with regard to 



O 



enteric fever. We see at once that no random selection of a group of individuals, 

 which has any single characteristic differing from that of the general population will, 

 except in the case of mean age of incidence and skewness, leave the other charac- 

 teristics unmodified. Thus the most probable result of any selection which alters the 

 nature of the distribution of enteric fever can be predicted. The reader will possibly 

 appreciate this better, if we replace the above table by another giving the absolute 

 progressions in years, number of cases per thousand, &c. 



PROGRESSION TABLE. 



* The frequency of incidence in the modal year = y X ^ since the unit is five years = 1894-57 X i 



To make this 1000 we must multiply by 



1000 x 5 



1894-57 



year. Thus we have to replace Ay by i5 ^? ( l^S 1000 ] = -^7^7- (error per thousand in modal 



1000 \ *'- 

 year of incidence). 



VOL, CXOI. A. 2 O 



Similarly A?/ X | = error in incidence of modal 

 1000" 



