SEX-RATIO OF THE DEER-MOUSE. l6l 



our doubts are further increased by the lack of a uniform grada- 

 tion when the broods are grouped according to size. 



7. When the number of each possible combination of males 

 and females, in broods of each size, is compared with the num- 

 ber expected according to chance, the conformity is found to 

 be, on the whole, very close. For example, the number of all- 

 male and all-female broods (excluding broods of one) was 276, 

 the " expected " number being 274. There is thus no preponder- 

 ant tendency toward the production of homosexual litters, and 

 thus no likelihood that polyembryony or true twinning is at all 

 common in these animals. 



8. In our material, the sex ratio is lower for the earlier broods 

 (91.7) than for later broods of the same mother (103.3). The 

 numbers are so small, however, that the difference is probably ac- 

 cidental. 



9. Likewise, inbreeding and outbreeding seem to have had no 

 influence upon the relative numbers of males and females, within 

 the limited material available for this comparison. 



10. Similarly negative results were obtained from a compari- 

 son of the offspring of meat-fed individuals with the offspring 

 of those whose diet was strictly vegetarian. Here again, the 

 numbers were too limited to permit us to regard this experiment 

 as decisive. 



n. The most significant result of all, statistically speaking", 

 and one which is at present utterly inexplicable, is the fact that the 

 sex ratios for the seven different years included in our records show 

 a wide range of variation. The extreme figures are those for 

 1916 and 1917, the ratios being 125.36 7.82 and 70.56 4.70, 

 respectively. These figures are based upon 471 and 423 indi- 

 viduals, respectively. The difference is 54.80 9.1 (i.e., differ- 

 ence 6 X P- E.). The likelihood of obtaining such a result by 

 " accident " is less than one in 40,000. We have furthermore 

 determined that this difference is not due either to the seasonal 

 distribution of births, to the preponderance of hybrid births in 

 one year, as compared with another, or to the operation of any 

 of the other factors previously considered. 



Mention was likewise made of the occurrence of similar annual 

 differences in the sex ratio of man as revealed bv the tables of 



