128 F. B. SUMNER, M. E. MCDANIEL AND R. R. HUESTIS. 



statistically more certain than any of the others which appear in 

 our records. Such a relationship is not, of course, an ultimate 

 fact, incapable of further analysis. But it does not seem to be 

 dependent upon any of the other agencies for whose influence we 

 have positive evidence, nor are we at present able to offer even 

 a plausible explanation of it. 



\Ye shall consider, in turn, the supposed influences which have 

 been enumerated above. 



SEASON. 



Before proceeding- to discuss the possible influence of season 

 upon the sex ratio of Pcromyscus, it should be stated that the 

 mice in question have, with a few exceptions, been born and 

 reared under atmospheric conditions closely approaching those of 

 the outside world. The building ("murarium") used for the 

 purpose was specially constructed with a view to securing this 

 result throughout the year. 



Our studies of the relation between season and the sex ratio 

 emphasize the danger of basing conclusions of this sort upon in- 

 adequate statistical data, even though the results may at first seem 

 to be " significant," according to accepted standards. Before we 

 included the records for the last two years (1920 and 1921), the 

 evidence of a well-marked biennial rhythm in the sex ratio of 

 Pcroin\scus seemed fairly conclusive. The seasonal cycle, when 

 plotted graphically, was perfectly consistent, there being an un- 

 interrupted rise and fall twice annually. Likewise the differences 

 between the highest and the lowest ratios were of tolerably high 

 " significance." The inclusion of the data for these two later 

 years greatly weakens the evidence for a definite seasonal cycle 

 in the sex ratio. But the possibility still seems to be great enough, 

 and the facts, if true, of sufficient interest, to warrant our present- 

 ing the evidence rather briefly. 



The following table gives the sex ratio for each month of the 

 year, likewise (in parenthesis) the number of individuals upon 

 which this ratio is based. The table is based upon the combined 

 data for all of the years and for all the different series of mice. 



