THE VARIATION, RACES AND MIGRATIONS OF THE MACKEREL. 285 



§ 2. European Mackerel. 



A subdivision of the mackerel which frequent the British coasts into 

 two principal races, an Irish race and a race frequenting the English 

 Channel and North Sea, appears to be demanded by the following facts 

 which have been elucidated by my researches : — 



(1) The identity of the Plymouth and North Sea fish, and the close 

 agreement between the Kinsale and Kerry fish in regard to the variation 

 in number of the transverse bars, and the emphatic difference between 

 the two former and the two latter groups in regard to the same 

 character ; 



(2) The close agreement between the Plymouth and North Sea fish in 

 regard to the frequency of intermediate spots among the bars, and the 

 emphatic difference between either of these groups and the Kerry fish 

 in regard to the same character ; 



(3) The correlation of a relatively low average number of first 

 dorsal Hnrays with a relatively high degree of spottiness in North Sea 

 and Channel fish, and the correlation of the highest observed average 

 number of finrays with the lowest observed degree of spottiness in the 

 Irish fish. These correlated differences could not be expected to occur 

 except in races of fish which had diverged to a different degree from 

 the primitive condition; 



(4) The correlated nature of the differences between the same two 

 groups in regard to the number of second dorsal finrays and dorsal 

 finlets. 



The discrimination of these races has been made exclusively on the 

 "round of structural differences and resemblances between the fish 

 coming from a number of chosen localities. If the differences revealed 

 by the present investigation should appear to some to be too small 

 to be significant, it should be remembered that large differences could 

 not in any case be expected to occur between the mackerel of any 

 two regions in British seas, partly because of the relative smallness 

 of the total area and the possibility of free intercourse between 

 its different waters, and partly because of the known activity and 

 wandering tendencies of the adult mackerel, in addition to its pro- 

 duction of freely fioating eggs. If, on the other hand, the differences 

 should be regarded by others as accidental, this idea may be negatived 

 at once by the general conformity of the results obtained for different 

 characters, and by the important fact that those local groups which are 

 shown to resemble one another most closely are exactly those which 

 might have been expected to do so from geographical considerations. 

 If there are no valid differences between any of the British groups 

 of mackerel, it is in the highest degree improbable, when the number of 



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