THE AGE OF FEMALE BLUE WHALES 259 



years. This is in agreement with the results from the two large collections one year apart 

 with which we have just dealt. In the biological notes above attention was drawn to the 

 apparent failure of Wheeler's method for Fin whales when applied to Blue whales. If 

 the foregoing calculations are accepted an explanation of this failure is easy. If the 

 increment of corpora lutea is in the region of one per annum, instead of several as in 

 Fin whales, it is naturally impossible for the majority of whales to add a number of 

 corpora lutea which will stand out as peaks in a frequency graph. 



age and corpora lutea (summary). Three lines of approach have been explored in 

 the quest for a relation between age and number of corpora lutea : 



(1) It has been shown that the number of ovulations found in whales which had 

 not long since attained to maturity and which appeared to form an age group by them- 

 selves averaged 1-91. This figure is taken to be the average number of ovulations in 

 the first breeding season. 



(2) The subsequent rate of increase in one season measured by the number of recent 

 ovulations is 1-13, less than the first season but apparently consistent throughout the 

 adult life of the whale. The numbers of recent corpora lutea in ovaries which have 

 many old ones show that the tendency to ovulate is not absent from those whales which 

 may be said to be of advanced age. 



(3) The progress of peaks in the frequency curves indicates an increase of slightly 

 over one corpus luteum per year. As was shown in the argument leading to the result 

 expressed in (2) above, the increment depends to some extent on the percentage of 

 whales which have not ovulated in the breeding season prior to capture. It will probably 

 not be far short of the mark if we say that for every ten years of adult life after the first 

 Blue females produce eleven corpora lutea. 



It must be borne in mind that there are factors which may alter the proportions of 

 pregnant, resting, and ovulating whales, and that no positive proof can be advanced for 

 the conclusion that an average of one and a fraction corpora lutea are added every year 

 after the first adult season. In spite of these difficulties, an attempt will be made to 

 apply this conclusion, and the justification of such an attempt must lie in the im- 

 portance of the deductions to be made, even if they do no more than approximate to 

 the truth. It is indeed unlikely that any sample of biological material gathered from 

 animals of such wide distribution and imperfectly understood habits will ever yield 

 completely consistent results. In any case, it seems important to advance any deduc- 

 tions that can be made from the material as soon as possible (pp. 261 et seq.). 



THE AGE AT PHYSICAL MATURITY 



A remarkably close correlation was observed between the number of corpora lutea 

 and the onset of physical maturity. The occurrence of physical maturity was seen to 

 coincide with the presence of eleven to twelve corpora lutea. The closeness of the 

 correlation shows that whatever the potential variation in the numbers of corpora lutea 

 added in each breeding season, the average number is fairly constant. Converting the 

 number of corpora lutea into an indication of age, it appears that physical maturity takes 



