l6o F. B. SUMNER, M. E. MCDANIEL AND R. R. HUESTIS. 



the probabilities claimed by us are correspondingly lower. Ac- 

 cording to this safer criterion, some of the most interesting of 

 our differences are not decisively " significant." 



4. Considering our aggregate material, there seems to be a 

 definite seasonal cycle in the proportion of males and females born. 

 The sex ratio presents two annual maxima, in March-April and 

 August-October, respectively; and two annual minima, in winter 

 and summer, respectively. The lowest figure is 78.79 (6.93), 

 in November and the highest 113.04 (8.04) in April. This 

 difference is about 3^4 times its probable error, and would ordi- 

 narily be regarded as " significant." A well-marked biennial 

 rhythm is shown by both pure and hybrid stock, taken separately, 

 though the fall maximum occurs in different months in the two 

 cases. On the other hand, when our material is subdivided in 

 certain other ways, the results are in some cases highly contradic- 

 tory. We cannot, therefore, regard the existence of a seasonal 

 cycle in the sex ratio of Peromyscus as being proved conclusively 

 by our data. 



In any case, the position of these annual maxima and minima, 

 as found by us, does not correspond with those which have been 

 reported 'for the white rat or for man. Indeed, the conditions in 

 Peromyscus are very nearly the reverse of those described by 

 certain other authors. 



5. Subspecific hybrids (1,722) give a mean ratio of 104.76 + 

 3.41. Mice of " pure " race (2,930) give a mean ratio of 93.27 

 2.32. Considering these figures alone, the difference is barely 

 significant statistically, but its reality is borne out by various other 

 considerations. Furthermore, such an effect of hybridization 

 upon the sex ratio was to be expected in view of the findings of 

 various other biologists, working upon widely different organisms. 

 It is possible, also, that the several subspecies of Peromyscus differ 

 inter sc to some extent. 



6. A positive correlation exists in our material between the 

 size of the brood and the sex ratio. Broods containing one to 

 three individuals give a mean sex ratio of 94.85 2.94, ones 

 containing four to nine individuals give a mean sex ratio of 

 102.42 3.01. The magnitude of these probable errors raises 

 the suspicion, however, that these differences are accidental, and 



