132 THE POPULAR SCIENCE MONTHLY 



far as possible to correct for them. His method may be described as 

 follows : 



The mortality of children born in any year, say 1903, in as large a 

 series of districts as possible 14 is followed year by year. Working with 

 this series which is homogeneous with respect to year of birth, 1 * the 

 problem to determine whether, allowing for environmental influence, 

 the death rate of, e. g., the first year, or of the first two years, has any 

 influence upon that of subsequent periods of life. 



Now this factor, which for convenience we have designated as 

 environmental, 16 is of great importance. The death rate differs widely 

 from district to district, and in response to many factors. 17 Thus 

 absolutely districts having a low mortality for the first year of life 

 might have a low mortality for the second to the fifth years of life; 

 districts having a high death rate for the first twelve months of life 

 would also have a high death rate for the thirteenth to the sixtieth 

 month of life, since many of the causes operating in the two cases 

 (bringing about high or low death rates) affect both age periods. 



Thus a high mortality of infancy does not necessarily imply a low 

 mortality of childhood or a high military efficiency. This is true 

 because any influence of selection is largely obscured by such factors as 

 ethnic or social composition or physical environment in the various 

 districts. Before one can say anything at all concerning the possible 



M It is very important that the subdivisions be as numerous and as homo- 

 geneous within themselves as is consistent with data for trustworthy death rates. 

 For, obviously, the death rate in one district in a given year may be abnormally 

 high (or low) because of purely local and transitory conditions. These are 

 precisely the factors which make for a high or low selective death rate. By 

 lumping a number of districts together one may cancel out the very terms he is 

 seeking to investigate! The value of some of the published work is nullified by 

 the neglect of this point. 



" Obvious as the importance of this point is, it has been entirely overlooked 

 or disregarded by some. To determine that the death rate of children to 1 

 year of age and that of those 1 to 5 years of age in a series of districts are 

 correlated for a given year, say 1905, proves nothing at all concerning a selective 

 death rate. The infants exposed to conditions (in the various districts) resulting 

 in high and low death rates for their first year of life in 1905 are being com- 

 pared with those exposed to the action of selection under what may have been 

 widely different conditions in 1904, 1903, 1902 and 1901. The whole factor of 

 the variation in mortality from year to year due to epidemics, meteorological 

 conditions, economic changes, etc., is thus left entirely out of account. 



18 Local influence might have been a better term, since racial composition 

 as well as environment may play a part. 



11 The fact that the death rate is to so great extent within the control of 

 the sanitary and charity boards is sufficient general evidence for this statement. 

 A quantitative demonstration is seen in the fact that a correlation is found 

 between, e. g., the birth rate and infantile mortality; also between artificial 

 feeding rate and infantile mortality. See Greenwood and Bevan, Jour. Hyg., 

 12: 5-45, 1912. 



