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Part III. — Eighteenth Annual Report 



Barra mackerel overlap in respect to the fluctuations of the Means of the 

 characters, lateral length of the head (11 H), distance of the first dorsal 

 fin (d 1 D), and length of the skull (Z Sk) ; while the Aberdeen and Barra 

 specimens overlap in the length of the pectoral fin (I Pf) and distance 

 of the second dorsal fin (d 2 D). 



The liability of the distances of the first and second dorsal fins (d 1 D 

 and d 2 D) to alteration through the contortion of the fish has been 

 already referred to, and it may be well not to press the agreements and 

 differences in these characters. Leaving these characters aside, the 

 evidence points to the conclusion that m the lateral length of the head 

 and the length of the skull it is probable that the mackerel of the Clyde 

 and those of Barra and Stornoway resemble each other more closely than 

 they do those of Aberdeen ; while, as regards the length of the pectoral 

 fin, Aberdeen and Barra and Stornoway mackerel would appear to be more 

 closely allied to one another than they are to the mackerel of the Clyde. 

 The nature of the relationship consists in the Clyde and Barra mackerel 

 having a slightly shorter skull and lateral length of head than the 

 Aberdeen forms ; the means of these dimensions in the case of the fishes 

 of the former districts lie between 16*01 and 16*27, and between 21*91 

 and 22 '41, while the fluctuations of the means for the Aberdeen district 

 are 16*37-16*42, and 22*51-22*62 respectively. In the Clyde specimens 

 the pectoral fin is found to be slightly longer than in the Barra and 

 Aberdeen examples. The means for this character lie between 10*87 and 

 11*07 in the Aberdeen and Barra specimens, and between 11 "19 and 

 11*32 in the Clyde fishes. 



In order to show how great the differences between the groups are in 

 respect to the dimensions of the selected characters, and to the number of 

 fin-rays and unlets, a typical mackerel has been constructed for each 

 district from the average percentages given above. The averages for the 

 males and females combined are used. The average length of all the 

 mackerel is 33*4cm. The dimensions of a theoretical mackerel of a 

 length of 33cm. (13 inches) are given in Table VI. All the dimensions 

 are in centimetres, and it is therefore possible to see exactly by how 

 much the averages are separated from one another. 



Conclusion. 



Of the varied characters which have been considered, there are, with the 

 three exceptions referred to above, probably none which differ in the 

 three groups to an extent that would permit stress to be laid upon them, 

 when the shortcomings of the dimensions in respect to accurate measure- 

 ment are taken into account. In the case of the fin-rays, finlets, and 

 vertebras, no differences which would on Heincke's method admit of their 

 separation -into races were found. Between the mackerel, then, which 

 were caught at the times recorded in the Clyde, at Barra and Stornoway, 

 and off Aberdeen, the differences in the lengths of the head, skull, and 

 pectoral fin may with some probability be granted racial distinction. The 

 shorter length of head and skull, in which the Clyde and Barra and 

 Stornoway lots more closely resemble one another, separate them slightly 

 from the Aberdeen collection. And in respect to a shorter pectoral fin the 

 Aberdeen and Barra and Stornoway fish are separated by a small interval 

 from the Clyde individuals. With regard to the remaining characters I 

 consider that the evidence does not prove them of racial distinction. 



[Table. 



