444 DISCOVERY REPORTS 



In Fin whales it has already been seen that the rate of foetal growth is slower and 

 that the foetus is born at a shorter length than that of Blue whales. The difference 

 in length between the two species is thus marked from the start (by the slower 

 growth rather than by the birth at a smaller size). 



The breeding season among Fin whales appears to be less definite than that of 

 Blue whales. As a consequence the length groups tend to coincide and this is 

 presumably the explanation of the normal frequency curves shown in Fig. 153 instead 

 of the trimodal curves of the Blue whales. As the growth rate of Fin whales is 

 unlikely to be substantially different from that of Blue whales a provisional curve for 

 the former can be constructed by analogy from weaning at 12 m. to sexual maturity 

 at 20 m. for females and 194 m. for males (Fig. 155). 



It may appear that a good deal has been taken for granted in the construction of this 

 curve for Fin whales, but it must be remembered that the curves of growth subsequent 

 to weaning are intended in both species only to represent the most probable rate of 

 growth, as indicated by such evidence as is available. The important point is to find 

 out whether two or three years or, say, five or six years are passed before maturity is 

 reached, and the details of the curve are relatively insignificant. 



At this point the steps by which the whole growth curve is built up from conception to 

 the attainment of sexual maturity may be briefly recapitulated. The records of foetuses 

 show that the greater part of foetal growth takes place during the southern summer, 

 and this, coupled with the occurrence of very small embryos, evidence in the ovaries 

 of ovulation and in the testis of a male sexual season, enables us to fix the middle of the 

 breeding season about June or July. It is known that birth takes place in Blue whales 

 at about 7 m. and in Fin whales at about 6*5 m., and by drawing a line to represent as 

 nearly as possible the slope of the plotted foetuses we are able to complete the curve 

 of growth during gestation, which gives us a period of about ten months. For the nursing 

 period exactly the same method is used. The length of the calf at weaning is known to 

 be in the neighbourhood of 16 m. in Blue whales and 12 m. in Fin whales. The gestation 

 curve is extended over the nursing period, and, guided by points plotted to represent 

 records of calves, reaches the length at which weaning takes place some six or seven 

 months after birth. This brings us over the southern winter to the early part of the 

 summer, the mother and calf having presumably migrated southwards during the spring. 

 As to the rest of the curve of growth, the fact that adolescent Blue whales at South 

 Georgia tend to approximate to one of two lengths is attributed to the production of 

 annual batches of calves, and from this, the length at maturity being known, it is 

 estimated that maturity is reached some two years after birth. Thus the whole curve 

 is built up by ascertaining the lengths at which the important "landmarks", such as 

 birth, weaning and sexual maturity, are reached, and by filling in the rate of growth 

 between by whatever evidence is available. 



The earlier part of the curve is based on the soundest evidence and it becomes more 

 speculative towards the end. Perhaps the most important point which emerges is the 

 very short period which elapses between birth and the attainment of sexual maturity. 



