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Part III. — Eighteenth Annual Report 



The advantage of this method lies in the clear and simple manner by 

 which it shows the proportions of the mature to immature at the different 

 sizes. Theoretically, if there is a true average at first maturity in nature 

 — above which more are mature than immature, below which more 

 immature than mature — this can only be where 50 per cent, of the one 

 coincide with 50 per cent, of the other. Hence, if these percentages be 

 plotted out as two curves, the point where the curve of immaturity crosses 

 that of maturity is the point required at which the plaice are ripe on the 

 average for the first time. In practice, however, this point is difficult to 

 obtain. In theory, trustworthy curves and averages could only be obtained 

 if a very large number of specimens at each size was examined, and it is 

 further necessary that these should not be specially selected. Again, the 

 specimens should be taken from the same region and at the same time- 

 viz., the spawning season. If these conditions are not fulfilled then certain 

 irregularities may appear in the curves, and it will be difficult to frame any 

 conclusion. Such irregularities, however, do not destroy the efficacy of 

 these curves ; on the contrary, by means of the curves not only are the 

 irregularities shown, but their causes and thence their explanation as well. 



In the data given by Cunningham, for example, the percentages of the 

 immature start from 100 and descend to almost nothing, whilst those of the 

 mature start from 0 and mount up to 100. (Vide Plate IX.). This is the 

 general trend of the curves, but it will be remarked that both for the males 

 and females a fluctuation occurs so that the course of the curves is not quite 

 uniform. This is partly due to the smaller number of specimens that have 

 been examined at certain sizes than at others. "Where the specimens are 

 fairly numerous, as between 10-12in. for the males and 10-13in. for the 

 females, the trend of both curves in each case is uniform. After these 

 sizes the numbers decrease and irregularities appear. In such cases it is 

 evident that the general trend of the curves has to be taken into considera- 

 tion, otherwise no conclusions could be drawn. A further discussion of 

 Cunningham's data and the curves that result therefrom will be entered 

 into later, when the comparison is made between them and my own. It 

 may be said at present, however, that, so far as numbers are concerned, 

 Cunningham's data are better than mine, but the relative numbers at the 

 different sizes are not so well proportioned. 



In the following Tables the specimens from Heligoland, Helder, 

 Lowestoft, and Grimsby are included together. The relatively small 

 numbers from each place necessitated this \ but the fact that the characters 

 so far examined for the purpose of detecting races are almost identical in 

 all four groups, as well as that the spawning season seems to be the same 

 in all, makes this course a reasonable one. The inclusion of these four 

 groups, on the other hand, will give a common average for the plaice of 

 the southerly portion of the North Sea — south of the Dogger Bank — and 

 this is an advantage if any common size-limit is wanted for legislative 

 purposes. 



These specimens are therefore considered as coming from one and the 

 same biological region — i.e., one in which the surrounding conditions are 

 for the time being thought to be the same. On the other hand, the 

 specimens from Aberdeen are separated from these because the characters 

 differ, as will be shown later. 



Something further may be said concerning the method. The range of 

 variation within which the specimens fall lies between 240mm. and 

 380mm. (9*5 to 15in.), and it is divided into seven portions of 20mm. 

 each. The specimens whose lengths fall within each division are 

 considered to be half-way between the limits — i.e., at 250, 270, and so on. 

 By this method a more exact result is obtained than when the different 

 lots are taken to the nearest inch. When the percentages of immature 



