TEE STUDY OF NATURAL SELECTION 133 



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 mor- 

 tality 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 aDd 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 trans- 

 form a review intended for the layman into a discussion comprehen- 

 sible 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. 18 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 environ- 

 mental 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. 19 



M. Greenwood and J. W. Bevan, "An Examination of Some Factors Influ- 

 encing the Rate of Infant Mortality," Jour. Eyg., 12: 5-45, 1912, find some 

 evidence of the selective nature of infantile mortality in the Bavarian data of 



18 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 inade- 

 quate 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. 



19 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. 



