selective nature of the death rate at the beginning of life he must make due allowances for these factors.
Snow attempts to correct for this environmental factor by using the deaths other than those for the infants born in the particular year under consideration as a measure of the stringency of its influence. The precise manner in which this is done need not concern us here, nor is it necessary to explain in detail the various ways in which the mortality of the first years of life was split up into earlier and later periods in order to ascertain what influence, if any, excessive mortality in the earlier period has upon the chances of survival in the later. Indeed, to discuss adequately all of the difficulties encountered and the highly complex methods by which they were largely overcome by Mr. Snow would treble the space which may be devoted to his research, and transform a review intended for the layman into a discussion comprehensible only to the trained statistician.
For present purposes, it is sufficient to say that (correction being made for environment), those districts in which the mortality for the first period was high had in general a low mortality in the second period.[1] Thus in the long run a high mortality in childhood follows a low mortality in infancy; low mortality in childhood follows high mortality in infancy—remembering always the correction for environmental factors which may hide the action of selection.
Natural selection, in the form of a selective death rate, is strongly operative in man in the early years of life.[2]
M. Greenwood and J. W. Bevan, "An Examination of Some Factors Influencing the Rate of Infant Mortality," Jour. Hyg., 12: 5-45, 1912, find some evidence of the selective nature of infantile mortality in the Bavarian data of
- ↑ All of Snow's results are not concordant. There are good reasons for believing that some of the series of public statistics analyzed by him are inadequate for so complex and delicate a biological problem as that of selective mortality. Those series of data which biologically and statistically may be regarded as most suitable and trustworthy evidence the most strongly in favor of the selective nature of infantile mortality. One's confidence in Snow's own interpretation of his results is strengthened by the fact that he has laid all his evidence—that which goes against his own general conclusions as well as that which supports his view—before his reader, believing it to be "more in accord with scientific spirit that the reader should be allowed to draw his own conclusions from the whole of the research, and to form his own opinion on the value of the material used and of the results deduced from it." It is a great pity that such merit should be so distinctive as to require comment, but to-day there is a most unfortunate tendency, among biologists at least, to pigeonhole the contra and publish the pro. Thus current and popular theories are often for a time bolstered up, when if all of the facts were brought forward their standing would be much less secure.
- ↑ The reader who goes thoroughly into these problems will read an editorial criticism of Snow's paper in Jour. Boy. Stat. Soc, 75: 133-135, 1911; also the reply by Snow in Biometrika, 8: 456-460, 1912, where the criticisms seem to be fully met.
