SEX-RATIO OF THE DEER-MOUSE. 153 



ORDER OF BIRTH. 



Both the order of birth, and the age of the parents have been 

 regarded by various writers as factors in determining sex. From 

 our data it would be difficult to separate the influence of these two 

 factors, had we reason to suspect that either of them had any 

 effect upon the sex ratio. Since, however, our findings here are 

 quite inconclusive, it is a matter of no importance that the pos- 

 sible influence of the two factors should be kept distinct. 



In 196 cases, it happened that the same parents had two or 

 more litters of young. The sex ratio for the 554 individuals of 

 known sex comprised in the first broods 1 is found to be 91.70 

 5.26. That for the 627 mice in the second broods is 104.91 5.65. 

 This would seem to be a considerable difference, but the numbers 

 are small and the probable errors correspondingly high. We are 

 not, therefore, warranted in attaching any significance to this 

 difference. When we add the very limited number of third and 

 fourth broods to the second ones and compute the sex ratio for 

 the 738 mice of known sex belonging to these " later " broods, 

 we have 103.31 5.48. The probability of an actual difference 

 in sex ratio between first and later broods is actually diminished 

 by this procedure. 



It is of some interest that the second broods averaged somewhat 

 larger than the first, the mean figures being 3.39 and 3.05 re- 

 spectively, based upon 196 broods of each class. This difference 

 is nearly three times its probable error. 



King and Stotsenburg report for the white rat a steadily de- 

 creasing sex ratio in passing from the first to the fourth litters 

 borne by the same mothers, the first figure being 122.0, the last 

 74.5. The authors recognize, however, that the number of broods 

 under consideration does not warrant any final conclusions on 

 this point. 



Punnett (1903) from an examination of Burke's "Peerage" 

 concludes that the first born, in man, are predominantly males. 



i By " first " broods we here mean the first which are known to have been 

 borne by a given mother. Since, in more than a third of the cases, the mothers 

 were wild mice, which had been trapped when nearly or quite mature, it is 

 likely that many of these had already given birth to young. In such cases, 

 the actual contrast is between earlier and later broods. 



