BREEDING AND GROWTH 



427 



shifted to lateral intervals of one month so as to include all the plotted points. It will 

 be seen from these that, given that all foetuses of one species grow at an equal speed, 

 all the foetuses measured were conceived within a period of less than five months in 

 the case of Blue whales and less than seven months in the case of Fin whales. 



We are now in a position to draw curves showing the probable intensity of pairing 

 during the season, indicated by the material in question. For this it is only necessary 

 to count the numbers of plotted points between two parallel curves and one may plot 

 the number obtained against the month in which they were conceived as indicated by 

 the parallel curves, or better, one may divide the points between two curves into those 

 lying nearer the one curve and those lying nearer the other, and count the number for 

 each half-month. The results are shown for both species in the following table, and are 

 plotted in Fig, 147. 



Date of pairing 



May, 1st half 

 May, 2nd half 

 June, 1st half 

 June, 2nd half 

 July, I St half 

 July, and half 

 August, I St half 

 August, 2nd half 



Date of pairing 



September, ist half 

 September, 2nd half 

 October, ist half 

 October, 2nd half 

 November, ist half 

 November, 2nd half 

 December, ist half 

 December, 2nd half 



Blue 



Fin 



Thus in both species it appears that May, June, July and August are the months in 

 which the majority of pairings take place, the maximum falling at about the end of 

 June or beginning of July. In the case of Fin whales, the results indicate that pairing 

 may take place over seven or eight months and the fact that the Blue whale season 

 appears more restricted is probably merely because we have not so many foetal records 

 for this species. There would be no justification, however, for arguing from this that, 

 if the records were sufficiently increased, instances of pairing in every month of the year 

 would appear. 



The results of this investigation of the breeding season differ slightly from those of 

 Hinton and Harmer in that the period of maximum pairing is now set rather earlier 

 in the winter. This difference is mainly the outcome of correlating the evidence of 

 the reproductive organs with the curve of foetal growth. 



The question of the female sexual season, although intimately connected with the 

 breeding season in general, need not be considered here, as it has already been dealt 

 with in the section on the ovaries. It has been shown that whales are almost certainly 

 polyoestrous, and in view of the protracted nature of the breeding season one may 

 suppose that in many cases several dioestrous cycles may occur before pregnancy 

 supervenes. It is difficult, however, to express any opinion as to the length of the interval 

 between the successive dioestrous cycles. 



