216 Part III. — Eighteenth Annual Report 



Table V. 



Growth Variability in Vertebra? and Fin-rays. 

 M — Average ; P.F. — Probable fluctuation of average; No. — Number of specimens. 



Length in 

 Mm. 



No. 



Vertebra?. 



Dorsal Fin. 



Anal Fin. 



M. 



P.F. 



M. 



P.F. 



M. 



P.F. 



16- 25 



134 



43'05 



42-95 -43-15 



72*45 



71-69-73-21 



54-31 



53-70-54-92 



67-300 



113 



43-03 



42-93 -43-13 



71-83 



71-05-72-61 







53-30 



52-68-53-92 



300-464 



94 



43 01 



42-895-43-125 



72-53 



71-75-73-31 53-99 



53-37-54-61 



It is evident that the vertebrae remain the same throughout, because, 

 even when the simple probable error of the average is employed, the 

 fluctuations still overlap. The fin-rays, however, seem to show a decrease 

 and then an increase. In the case of the dorsal fin there is no difference 

 between the smallest and the largest lots, but the fluctuations of the 

 averages between the intermediate lot, from 67-300mm. and those on 

 either side, cease to overlap when twice the probable errors of the 

 averages are taken instead of five times. In the case of the anal fin, the 

 smallest and largest lots are again almost the same, but the averages of 

 the intermediate lot do not overlap with those of the first when four 

 times the probable error of the averages are used, and do not overlap 

 with those of the third lot when twice the probable errors are employed. 

 There would thus seem to be some evidence of a decrease in the numbers 

 of fin-rays during the intermediate stages of growth, but this is counter- 

 balanced by a subsequent increase. This process seems improbable, and 

 it is safer to conclude either that the intermediate lot is not representa- 

 tive or that a difference in the external conditions in the different years 

 has given rise to these observed differences. 



Duncker (I.e., p. 174), in his work on the flounder at Plymouth, 

 similarly divided up the specimens into different lots according to size, 

 but although there are slight differences in the averages at the different 

 sizes both for the dorsal and anal fins, they are too small and too irregular 

 to be recognised as differences due to growth-variability. Heincke found 

 also for the herring that there were no such differences, and this seems to 

 be the most reasonable conclusion for the plaice. This point will be 

 referred to again. 



Body-height. —A reference to Table XL, p. 230, will show that there 

 is very little growth-variability in this character. The difference that 

 there is tends to show that a slight increase takes place, but it is very 

 slight, and the amount of probability is small. It will be noticed, further, 

 from this Table that maturity has little or no effect on the body-height, 

 the averages for the southerly portion of the North Sea being practically 

 identical, whilst in the case where a difference is shown — viz., in the male 

 specimens from Aberdeen — the probability is very small in favour of a 

 decrease. 



Similar conclusions hold for the Tail (Table XII., p. 231); the changes 

 in the averages are irregular, and the differences too small to admit of 



