BREEDING AND GROWTH 437 



water, but in the spring the southward migration begins and from the fact that weaning 

 appears to take place about December, it is to be supposed that the mother with her 

 calf migrates southwards in the beginning of the summer in order to wean the calf on 

 the feeding grounds of the Dependencies. Probably the migration of the mother is 

 more leisurely than that of the main body of whales travelling to the south. This is 

 in itself probable since the calf cannot be expected to swim as fast as the adult. The 

 suggestion is also supported by other facts. For instance there is a phenomenon which 

 appears to be quite regular from year to year at South Georgia. If reference is made 

 to Plates XLIII and XLIV, it will be seen that in the second half of the South 

 Georgia season there is a regular influx of smaller whales, and that many of these (unless 

 growth has slowed down to an improbable extent) can hardly have been weaned more 

 than a few months, and are thus the season's new batch of whales. 



There is one point, however, which does not appear to agree very well with the theory 

 that the mother and calf regularly migrate southwards towards the end of the nursing 

 period and that is that there are numerous small whales off the South African 

 coast in winter, many of which are obviously too small to have migrated south and back 

 again to warmer waters since they were weaned. It is difficult to say what relation 

 these small South African whales bear to the main stock. It is possible that in 

 some cases the calf is weaned on the small and rather scarce krill in those waters and 

 remains in the northerly regions for the first summer. 



Up to the present the story of the whale's growth may be summarized as follows. 

 Impregnation in both Blue and Fin whales may be expected to take place about June 

 or July and the calf is born on the average about the beginning of the following May 

 after the parent has made a southward migration to feed during gestation and returned 

 to the warmer waters towards the north. The calf is born at 6-5 to 7 m. and during a- 

 nursing period of some six or seven months it grows to about 16 m. in the case of Blue 

 whales and 12 m. in the case of Fin whales. At this stage the summer is reached and 

 the calf is weaned when it has migrated with the parent to the southern feeding 

 grounds. 



It is now necessary to find the rate of growth from this point up to sexual maturity, 

 and to throw light on this we must, as mentioned above, examine the catches from a 

 statistical point of view. 



The following table shows the length frequencies of all the whales examined in the 

 course of our work. That is to say it shows, for various periods, the numbers of individuals 

 which have occurred at different lengths, successive metres of length being taken as the 

 most convenient length groups. Separate figures are given for separate seasons, but the 

 second half of the 1924-5 season (when work was started) and the 1925-6 season are 

 amalgamated in one column as the constitution of the whale population of South 

 Georgia was somewhat similar in these two seasons, whereas it was quite different in 

 the 1926-7 season. 



The result of this analysis of the catches can be examined more satisfactorily in a 

 graphic form. In Figs. 151, 152 and 153 the figures are plotted in charts which show 



