THE VAKIATIOX, RACES AND MIGRATIONS OF THE MACKEREL. 275 



process of encroachment upon the lower part of the fin by the 

 surrounding tissues. I fancy, therefore, that as the fish grows, this 

 encroachment leads to the gradual covering up of the smallest rays 

 altogether, which would sufficiently account for the observed reduction 

 in the number of finrays as growth of the fish increases. As already 

 remarked, I cannot yet say whether the spines are ever actually 

 absorbed. From the nature of the case such a conclusion could only 

 be derived from an extensive study of macerated specimens of different 

 sizes, since mackerel will not live more than a few days in captivity, 

 and it has not been possible to devote the necessary time to such 

 an enquiry. 



§ 4. Racial differences. Owing to the reduction of the number of 

 finrays with growth of the fish, it is clearly impossible to use the 

 data given in Table E as a basis for conclusions as to racial peculiarities, 

 since no account has been taken in that table of growth-changes. If 

 any racial peculiarities exist, they can only be determined by comparing 

 fish of the same size from the different localities. Accordingly, as the 

 various localities are most uniformly represented by 13-inch fish (see 

 Table I, p. 246), I have compared the mean numbers of finrays in local 

 groups of fish of this size. 



The results are as follows : — 



The number of fish representing most of the localities is unfortunately 

 too small to yield very accurate results, but the general trend of the 

 differences is probably reliable. The Amewcan sample, as in other 

 cases, yields one of the extreme values, in this case the lowest; and 

 among European samples the Kerry and Channel values are widest 

 apart, as was also the case in regard to the transverse bars (Table B) 

 and intermediate spots (p. 266). The difference between the values for 

 Plymouth and the North Sea is certainly rather large, as also is that 

 between Kinsale and Kerry ; but, in view of the small available 

 numbers of fish of the proper size, I doubt whether any importance 

 can be attached to these differences. A more reliable conclusion can 

 probably be derived from the fact that the maximum value for the 

 Channel and North Sea is no higher than the minimum vahie for the 

 Irish coasts, and that Brest and Scilly yield a value which is inter- 

 mediate between the two. 



