constant) the survivors of an infantile population subjected to environmental conditions making for a high death rate should in later years show a lower mortality than the survivors of a population subjected to less stringent conditions of life.
The Possible Selective Element in Infant Mortality.—That the preservation of the weaker children may result in a population of adults below the maximum physical fitness is an idea as old as the study of Spartan history. The wide acceptance of the Darwinian theory and the modern reduction in the death rate—accomplished largely by the saving in the early months of life—have combined to give it considerable prominence in recent years.
To solve the problem one must find a series of districts differing as much as possible in the mortality of the early months of life, and determine whether those which have a lower mortality in infancy have a higher proportion of men unfit for military service or a higher adult mortality.
Such attempts have been made, for instance, by Rahts, Kruse, Gruber, Koeppe, Prinzing, Elben and others. The indeterminateness of these studies is apparent not only from the discordant conclusions but also from the obvious inadequacy of the statistical technique.
To Yule and Snow belongs the credit of having first applied the modern statistics to this problem. Yule's data and methods, however valuable they may be from the standpoint of the relationships between the mortality of early and later life in a series of districts, seem quite inadequate to the solution of the problem of the selective or non-selective nature of infantile mortality.
A first great merit of Snow's laborious study is that he fully recognizes the multiplicity of disturbing factors and has attempted in as
- Snow (see below) is quite right in insisting that the question as to what proportion of the general death rate is selective should be answered on national mortality statistics. From the point of view of evolution, or of sociology, such data are of far more value than the more complete records which can be secured in individual pedigrees, for to be of evolutionary or of national social importance the intensity of the selective death rate must be measured on a perfectly general population.
- Examples are given in subsequent footnotes.
- G. U. Yule, "On the Possible Selective Influence of Mortality in Infancy on Mortality in the Next Four Years of Life," Supplement to the Report of the Medical Officer of the Local Government Board (Great Britain), 1910, Cd. 5,263.
- E. C. Snow, "The Intensity of Natural Selection in Man," Drapers' Co. Res. Mem. (Univ. Coll., Lond.), Stud. Nat. Det. 7, p. 43, London, Dulan & Co., 1911.
- This is clear from the criticisms brought forward by Snow. Practically as much has been admitted elsewhere; see Jour. Roy. Stat. Soc, 75: 133-135. In his study Yule shows a caution in interpretation of results which is not as evident in the main body of the medical officer's report.