BREEDING AND GROWTH 415 



with Han-ner's work. Risting (1929) has further pubHshed a brief paper on the same 

 subject in Den Norske Hvalfongst Tidende. 



Among the sources of information on breeding mentioned above are the repro- 

 ductive organs and the occurrence of foetuses. The latter, which are perhaps the 

 most important of all, are comparatively simple to deal with, for, by plotting out the 

 foetuses found according to their length and the date on which they occurred, we have 

 important evidence at once as to the rate of growth and the probable seasons of pairing 

 and parturition. 



Owing to the fact that the pairing season is prolonged over a considerable period, the 

 points so plotted are very much scattered, so that the best one can do is to construct a 

 curve which seems to represent as accurately as possible the mean rate of growth 

 throughout gestation. This, however, is not a serious difficulty. The main weakness of 

 this method (at least of fixing the pairing season) lies, as already mentioned, in the 

 uncertainty of the rate of growth in the early stages of development. It is here that 

 observations on the genitalia of whales are needed, to fix, for example, the exact season 

 at which pairing takes place, and to investigate the details of the oestrous cycle, the 

 sexual season of the male, etc. In this connection the most important whales to 

 examine are the adult females, and naturally the most important observations are to 

 be made at the period when pairing and parturition mostly take place. One of the 

 most serious obstacles to the work on whales at whaling stations has been the difficulty 

 of finding adult females at this particular time of year. As will be explained later, the 

 seasons of pairing and parturition fall in the southern winter or autumn, when whaling 

 closes in the Dependencies and opens at South African stations. It is at the latter, there- 

 fore, that the most important observations on the breeding processes are to be sought. 

 It has already been mentioned that at Saldanha Bay the number of adult whales caught 

 is unfortunately small and the conditions appear to be much the same at other African 

 stations, though the percentage of mature whales appears to be somewhat higher at 

 the Durban stations. At present it is impossible to say what becomes in winter of the 

 numerous adult whales which frequent the neighbourhood of South Georgia in summer. 

 This question, however, will be considered again later on. 



SEXUAL MATURITY 



In an investigation of the breeding of whales, the first fact to be sought, if possible, for 

 every whale is whether or not it is sexually mature. Sexual maturity is to be distin- 

 guished from full (physical) maturity. The latter may take place long after the former 

 is attained, and it can sometimes, though with difficulty, be distinguished by the degree 

 of ossification of the vertebral epiphyses. 



An accurate diagnosis of sexual maturity is of special importance for several reasons. 

 In the first place, the proportion of immature whales among those which are killed is of 

 fundamental importance in any consideration of the effect of whaling on the stock of 



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