ABSTRACT 1872 normal mice bred during November 1922-October 1925 had an average litter-size of 6·18, and a male percentage of 51·7 ± ·77, both of which figures are lower than the corresponding ones found for 1921–2. The annual variation in the sex-ratio is, however, not very appreciable, the figures for the four years running 54·2 ± 1·04, 50·4 ± 3·22,52·2 ± I-18, 51·4 ± 1·09. The average size of litter during these four periods ranged from 6·65 to 5·93. Considerable seasonal variation in both fertility and sex-ratio occurred, the highest figures being fertility 6·46 and male percentage 55·9 ± 1·83 in the October-December quarter, and the lowest being fertility 5·82 and male percentage 48·2 ± 1·46 in the April-June period. Litter-size appeared to be uncorrelated with any sort of definite variation in the sex-ratio. Recent work on the mouse is discussed.
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A. S. Parkes (1926) studied this question.
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