SEX-RATIO OF THE DEER-MOUSE. 



133 



Two other methods of subdividing our material have been em- 

 ployed, these giving conflicting results. When we plot separately 



Jan Feb Mar Apr M<vy Jun Jul 



Sep Oct Nov J)ec 



FIG. 3. Seasonal variations in the sex ratio, for three groups of years, 

 treated separately. The continuous line (1917-1918) is based upon the largest 

 number (1,917). Ordinates indicate sex ratios. Numbers along graphs indi- 

 cate numbers born in each month. Certain of the larger fluctuations will be 

 seen to be due to limited numbers of individuals {e.g., in 19141916). 



the hybrid mice and those of pure race (Fig. 4), we find that 

 each of these groups displays a pretty well marked biennial 

 rhythm. This is, however, much more pronounced in the for- 

 mer group than in the latter, and it is also to be noted that the 

 ' fall maximum " occurs in one case in August, in the other in 

 October. In passing, let us point to the distinctly higher sex 

 ratios shown by the hybrids throughout most of the year, a fact 

 to which we shall refer a^ain. 



