228 Part III. — Eighteenth Annual Report 



Table X. 



Fluctuation of the Averages of the Fin-rays. 



Region. 



Dorsal. 



Anal. 



Left Pectoral. 



Right Pectoral. 



Caudal. 



So. North Sea, 



72-03 -72-65 



53-64—54-10 



1074 —10-92 



11-11 -All -29 



19-99—20-09 



Aberdeen, 



72-12 —73-26 



53-84—54-72 



10-835—11-125 



11-39 —11-67 



19-98—20-11 



Solway Firth, - 



71-245—72-575 



53-78-54-64 



10-66 —11-02 



11-145—11-455 





Baltic, 



66-465—67-995 



49-37—50-59 



9'86 —10-60 



10-23 —10-95 



19-45—20-01 



Kattegat, 



71-31 —74-37 



53-92—56-20 









The number of fin-rays in the pectorals leads to the same conclusions. 



The groups from the North Sea and Solway Firth cannot reasonably 

 be separated from one another, but the Baltic group is again quite 

 distinct. For neither of the fins do the fluctuations of the averages 

 of the last group overlap with those of the other groups. 



The ventral fins show very little variation, but the left ventral fin — i.e., 

 that of the under side — shows a greater tendency towards a reduction than 

 the right. There is no difference in the different groups. In the caudal 

 fin, again, there is more variation than one would expect, and the average 

 for the Baltic group is somewhat under that for the North Sea groups. 

 The fluctuations of the averages show also that the probability is almost 

 1000 to 1 in favour of the difference being real. 



Considering the fin-rays as a whole, therefore, it is clear that the only 

 distinct difference that is demonstrably permanent lies between the 

 Baltic plaice and the plaice from the North Sea; the plaice of the latter 

 region cannot be separated into races by these characters, nor can they be 

 separated from the Solway Firth group. 



In comparison with the flounder, the plaice possesses a greater 

 variability in regard to its fin-rays. According to Duncker*, the standard 

 deviation (" index of variability ") is for the dorsal fin in the flounder 

 between 2 - 3118 and 2-4445; in the plaice it is 2-63. Similarly for the 

 anal fin ; but the pectorals possess an equal variability in both. This 

 large variability in the dorsal and anal fins shows how precarious any 

 attempt to found " races " on the numbers of fin-rays must be. Even 

 though the averages of two groups differed by one fin-ray the large 

 possibility of variation would show itself in the overlapping fluctuations 

 of the averages. There requires to be a difference of almost 2 in the 

 averages of a large number of specimens of both groups before conclusions 

 can be safely drawn. Further, this great variability probably means that 

 these characters are very sensitive to slight changes in the environment 

 when in process of formation, and if the average of these changes is not 

 the same from year to year, the average number of fin-rays will likewise 

 vary. This has been already referred to in the section on growth- 

 variability, and it probably accounts for the slight difference in the 

 averages given for the dorsal and anal fin by Duncker, Cunningham, and 

 myself. 



Those who are sceptical as to the existence of "races" and the 

 possibility of detecting them may perhaps think that this probable 

 change from year to year would explain the differences observed in other 

 characters, and would thence destroy the validity of any conclusions 



"Jour. Mar. Biol. Ass., Vol. V., 1898, p. 173. 



