EXTERNAL CHARACTERS OF BLUE WHALES 277 



or three readings only and owing to the individual variation which naturally occurs, 

 are not to be depended on for purposes of accurate comparison. 



It is seen from the table that the only marked difference in bodily proportion between 

 the sexes is that shown by measurement No. 13, in which the mean distance between 

 the anus and reproductive aperture works out at more than twice as great in the male 

 as in the female. In the other measurements the differences are comparatively insig- 

 nificant, though there is a very slight indication that in males the head is relatively 

 slightly larger than in females, while in the latter the tail measurements are slightly 

 greater. It will be shown below, however, that as a whale grows larger the head becomes 

 slightly larger in proportion to the tail and the slight difference in these measurements 

 is very probably due to the fact that females are on the average a little larger than males. 



As a result of this comparison between the sexes it may be said that no distinction 

 is apparent in respect of the bodily proportions except in the case of the interval 

 between the anus and reproductive aperture. This distinction of course is simply due 

 to the fact that the penis occupies a more anterior position than the vulva. 



A comparison between the general average measurements from South Georgia and 

 South Africa shows a corresponding difference between the relative sizes of the head 

 and tail which in this case is much more marked. The mean value of all the anterior 

 measurements is distinctly greater in the South Georgia whales, while the posterior 

 measurements are, with one exception, greater in the South African whales. This 

 exception is found in No. 24 — "Depth of tail at dorsal fin" — which is, on the average, 

 greater for South Georgia whales. A possible explanation of this is that, in general, the 

 blubber of whales taken at South Georgia tends to be relatively thicker than at South 

 Africa, and since the blubber is very massive in the mid-dorsal and ventral lines on 

 the tail, the effect of an increase in thickness would be quite likely to exaggerate this 

 particular measurement. 



The fact that the tail is, on the average, relatively large in South African whales, and 

 the head relatively large in South Georgia whales, is undoubtedly due to the difference 

 in the average sizes of the whales from the two localities. 



The manner in which the bodily proportions vary according to the size of the whale 

 may now be considered. It has been found that there is a very definite variation of 

 this description, and it follows that the value of the general average measurements, 

 which include whales of any and every size, is much reduced for purposes of comparison. 



In the following tables the mean value of each (percentage) measurement has been 

 worked out as far as possible for each metre of whale-length, foetuses being included. 



P. 



