436 DISCOVERY REPORTS 



may itself take some little time and an even longer period may elapse before the con- 

 dition of the mammary glands has finally reverted to the normal. Thus in any case we 

 may expect to find lactating whales long after December, but even so the most important 

 cause of the irregularity in the appearance of lactating females is that their distribution 

 and movements are probably different from other "classes" of Blue whales. It is not 

 unlikely that they seek seclusion of some kind during a large part of the nursing period 

 and it is probable that if the catches of the whaling stations constituted a representative 

 sample of the whole stock, the lactating whales would regularly be found to be more 

 numerous in the earlier part of the southern summer. 



We have seen then that the nursing period in Blue whales appears to occupy about 

 seven months, on the average from about May to December, and that during this 

 period the length of the calf becomes more than doubled. 



Fig. 150 has been constructed for Fin whales in just the same way as Fig. 149, but 

 the records of calves are scarcer than in the case of Blue whales. This is no doubt due 

 to the fact that the calves in this species, being weaned when much smaller than those 

 of the Blue whale, are from the whaler's point of view not worth taking. All that 

 can be said is that there is a group of large calves occurring mostly about October 

 which were presumably born in the previous autumn (i.e. between about April and 

 July). The mean growth curve is therefore continued upwards so as to pass through the 

 middle of this group. It is then found to reach the 12 m. level opposite the month of 

 December. Thus for Fin whales about six months is estimated for the nursing period 

 which, on the average, should last from June to December. It will be noted that the 

 rate of growth during this period is appreciably slower than that of Blue whale calves. 



As to the occurrence of lactating whales much the same comments apply to Fin as 

 to Blue whales, but lactating Fin whales appear to have been spread a little more evenly 

 over the season than lactating Blue whales. 



In considering now the subsequent growth of the young whale. Blue whales must 

 again be taken first. Up till now the rate of growth of the calf has been extremely rapid, 

 but there is some evidence that after weaning the rate of growth slows down consider- 

 ably. This fact together with the almost certain intervention of different individual 

 rates of growth would obscure any evidence from the plotting of larger whales according 

 to length and date, and in order to find the rate of growth over the next period of 

 development, that is from weaning to sexual maturity, different methods have to be 

 employed. 



By a kind of statistical analysis of the catches of whales it can be argued that in all 

 probability the period which elapses between weaning and sexual maturity is rather 

 less than two years, or rather more than two years from birth. The evidence from this 

 cannot perhaps be regarded as conclusive, but receives support from evidence from 

 certain other sources. 



It is necessary first to consider some aspects of the migrations of Blue and Fin whales. 

 It has been shown that the period of lactation mostly covers the winter and early part 

 of the southern summer. During the winter the whales are to the north in warmer 



