. OA—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—i—1 1925 1930 1935 1940 1945 YEARS Text-fig. 32. Variation in the mean monthly foetal lengths over the period 1925-58 (from International Whaling Statistics, Sandefjord). period. In January 1935 and February 1953 for instance, where there is a large deviation from the overall mean for the period, the calculation of the mean ±2 shows that these particular values deviate significantly by more than 0-25 m. and about 0-5 m. respectively, from the means for the full period. Deviations of this order could represent displacements of the mean date of con
. OA—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—1—i—1 1925 1930 1935 1940 1945 YEARS Text-fig. 32. Variation in the mean monthly foetal lengths over the period 1925-58 (from International Whaling Statistics, Sandefjord). period. In January 1935 and February 1953 for instance, where there is a large deviation from the overall mean for the period, the calculation of the mean ±2 shows that these particular values deviate significantly by more than 0-25 m. and about 0-5 m. respectively, from the means for the full period. Deviations of this order could represent displacements of the mean date of conception of about i-2| weeks (from Text-fig. 30), but a number of complicating factors need to be considered in assessing this temporal deviation. Have there, for instance, been any changes during this 30-year period in the rates of foetal growth ? Have changes in the age structure of the populations of mature females been sufficient to produce an apparent change in the pairing season? It is not possible to provide conclusive answers to these questions, but such changes would probably be progressive and would not account for variations from year to year. It may be that a slight increase in the rate of foetal growth, and a shift towards lower ages in the age structure of the mature females, could produce an apparent advancement or retardation of the pairing season. On the evidence available it seems probable that there has been no progressive change in the pairing
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