26a DISCOVERY REPORTS 



extent of this migration varies very considerably in the two species, and in different 

 years. The tendency seems to be for whales to arrive in batches, more or less closely 

 aggregated, with one species or sex, or with immature whales predominating. While 

 those of the main stock are doubtless guided in their search for food by complex 

 environmental factors, in the operation of which the spring temperature seems to play 

 an important part (Harmer, 1931, p. 131), the behaviour of immature and lactating 

 whales is somewhat different. This is most probably correlated with their physical 

 inability to respond to the changes influencing their movements with the same speed as 

 the larger or unhampered whales (Mackintosh and Wheeler, 1929, p. 462). 



With a cold spring Blue whales are generally more numerous than Fin whales during 

 the first half of the season at South Georgia. With the advent of large-scale pelagic 

 whaling along the ice-edge, it has become well established that the centre of distribution 

 of Blue whales lies to the south of that of the Fin whales, since the greater part of the 

 catch of the pelagic boats is made up of Blue whales. This is in agreement with the 

 observation that Blue whales became relatively scarce on the South Georgia grounds 

 during the later part of the seasons studied for this paper. There is commonly a tre- 

 mendous influx of immature whales, mainly Fin, assumed to have lagged behind the 

 main southern migration, in January and February. 



Kemp and Bennett (1932) have recently made an exhaustive study of whale con- 

 centrations and movements on the South Georgia and South Shetland grounds, based 

 on data supplied by the whaling companies. Off South Georgia the general trend of 

 movement appeared to be easterly, with an additional southerly trend in the case of 

 Blue whales. During the first half of the season the data indicated so much northerly 

 movement among Fin whales as to lead the authors to doubt its value. The distributional 

 data clearly indicated that the centre of concentration shifted to the south in January and 

 February, and to the south-east towards the end of the season. 



It is obvious that the general idea gained from these more or less direct methods of 

 observation can be usefully supplemented by a consideration of the external parasites 

 of the whales. Thus it seems safe to assume that whales parasitized by Pennella and 

 Cirripedia, or bearing the partially healed pits in the blubber observed by Mackintosh 

 and Wheeler (1929, pp. 373, 374), are comparatively recent arrivals from warmer waters. 

 Conversely, as previously shown, the presence of diatom film indicates that the whale 

 has spent some time within Antarctic waters. 



Corroborative evidence can also be obtained from the condition of the whales. The 

 increase in proportionate thickness of the blubber during the southern feeding migra- 

 tion, and its rapid decrease in the warmer waters to the north during the southern 

 winter, have been admirably demonstrated by Mackintosh and Wheeler (1929, pp. 

 368-72). 



The impression gained from all these lines of evidence does not appear trustworthy 

 unless comparatively large numbers of whales are considered. It is easy to find indi- 

 vidual exceptions to every rule. It is for this reason that in considering the numerical 

 data relating to diatom film the whales examined at South Georgia have been treated in 



