430 DISCOVERY REPORTS 



impregnated earlier might also travel southwards earlier. Now as will be shown 

 in a later section the whale population at South Georgia undergoes considerable 

 changes during the season and it may be that the foetuses measured in the earlier 

 part of the South Georgia season were conceived earlier than those measured in the 

 later part of the season. The effect of this would be that the curve of mean foetal 

 growth, constructed as it is from the progressive increase in the lengths of foetuses 

 during the season, shows a slower rate of growth than that which actually takes place. 

 In other words, if the time of the southern migration is influenced by the time of 

 impregnation and if individual pregnant females stay in the vicinity of South Georgia 

 only for a comparatively short time, then the length of the period of gestation is somewhat 

 shorter than the period which has been estimated. However, there is no definite evidence 

 to show that this actually happens and in any case the difference may not be very great. 

 An important point to be considered at this stage is the length of the interval which 

 elapses between successive pregnancies in an individual. For this also the percentage 

 of pregnant females must be examined. If, for instance, pregnancy normally took place 

 every year we should expect to find nearly every adult female pregnant at almost any 

 time of year since the period of gestation lasts only a little less than a year, and it would 

 follow that a female would normally be impregnated long before she weaned her calf. On 

 the other hand, if pregnancy recurs every two years one would expect to find slightly 

 less than 50 per cent of the adult females to be pregnant at a given time or if every 

 three years something less than 33 per cent and so on. Calculations of this kind, how- 

 ever, involve a dangerous assumption, namely, that the whales actually brought to 

 the whaling station at South Georgia, or any other localities, constitute a representative 

 sample of the general stock of whales. This is a subject to be dealt with in a later section, 

 but it may be said at once that such an assumption is definitely not justified except so 

 far as certain approximate estimations are concerned. It is quite certain that the whales 

 caught oft' the south-west African coast are not representative of the whole stock, and 

 at South Georgia the whale population is perpetually fluctuating both as regards con- 

 stitution and numbers. However, as far as the latter locality is concerned some reason- 

 ably certain inferences may be drawn if they are based on observations covering several 

 seasons. The percentages of adult females pregnant vary mostly between 20 per cent 

 and 50 per cent in the case of Blue whales and 20 per cent and 70 per cent in the case 

 of Fin whales. Of all the adult females examined at South Georgia and Saldanha Bay 

 31 per cent of the Blue whales and 46 per cent of the Fin whales were pregnant. From 

 this alone it can be regarded as fairly certain that pregnancy does not normally recur 

 every year and the fact that out of the large number of lactating whales examined not 

 one was pregnant, clearly rules out the possibility of annual pregnancy so far as the 

 great majority of whales are concerned. The uterus in lactating whales has not always 

 been opened, but it has invariably been found in these whales that the ovaries have no 

 functional corpus luteum, i.e. of the type referred to in the section on ovaries as corpus 

 luteum a. There exist, however, one or two records of lactating females which were 

 pregnant. It appears that Barrett-Hamilton found one or two pregnant Humpbacks 



