Page:Popular Science Monthly Volume 84.djvu/136

132 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 is followed year by year. Working with this series which is homogeneous with respect to year of birth, 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, is of great importance. The death rate differs widely from district to district, and in response to many factors. 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