2^5 DISCOVERY REPORTS 



which will be described in a later paper.) It should be noted that this conclusion is in agreement with 

 that reached bv Mackintosh and Wheeler (1929) on similar grounds. 



A second straight line of different slope can be drawn through the blocks marking the means plus 

 or minus two standard errors shown for January, February, March and April in Text-fig. 8. When 

 extrapolated this gives an impossibly long gestation period. In any case the true monthly means of 

 the population are probably well within the range of four standard errors, and in addition it can be 

 shown that the samples for March and April are biased towards lower values, owing to differential 

 migration of females from the area (discussed below). A logarithmic curve has therefore been fitted 

 to the mean lengths for December, January and February. When extrapolated this attains the mean 



MONTHS 



Text-fig. 9. Monthly foetal length frequencies of southern hemisphere fin whales, Balaenoptera physalus ; 

 class interval 0-2 m. The estimated mean growth curve is taken from Text-fig. 8. 



birth length (6-4 m., Mackintosh and Wheeler, 1929) in mid-May, giving a gestation period of just 

 over 1 1 months. It is considered that this estimate is as accurate as the data permit and that it is 

 unlikely to be more than plus or minus 1 5 days in error. 



The reasons for concluding that the fin whale foetal length samples for March and April are biased, 

 must now be presented. 



First, it should be noted that almost all the material comes from antarctic waters. In Text-fig. 9 

 the monthly foetal length frequency distributions are set out in 0-2 m. groups. This figure gives a 

 better idea of the individual variation than the same data presented in Table 2, and it will be noticed 

 that the shape of the frequency distributions changes from month to month and that the relative 

 numbers of larger foetuses appear to be reduced in March. The April sample is really too small to 

 show such a tendency. 



