222 Part III. — Eighteenth Annual Report 



distinct races and thence species might arise. The individuals at the 

 extreme ends of such an area may and do differ greatly from one 

 another — in some characters as much as two separate " species " do — and 

 there may be all intermediate stages. On such an area the individuals 

 would probably aggregate into groups, and inter-crossing would be rare, 

 except between contiguous groups. The older notions of extensive 

 migrations amongst fishes are not now maintained, and hence races would 

 spring up simply because the various groups always remained in nearly 

 the same region, or because the spawning-period became extended over a 

 great part of the year, and broke up into two or more maximal periods. 

 It is evident that, so far as the environment is concerned, different races 

 and even species might arise, because the external conditions would not 

 be the same in the different regions or periods, and, again, if the external 

 conditions in any one region remained always about the same average so 

 would the group inhabiting that region. 



On the other hand, it might be held that since there are no real and 

 definite barriers in the sea the individuals of one group might pass by 

 some means or another into a neighbouring region at an early stage and 

 there assume the characters of the " race " of that region. This conception 

 has been put forward by Petersen in order to explain the differences in 

 the characters of the Baltic and Kattegat plaice. These differences, as 

 will be shown, are considerable, but even if this interchange does really 

 take place we should only have another example of the great variability of 

 the organism, which would not refute the evidence for " races " in other 

 regions. 



The matter is still very obscure, however, and more facts are wanted, 

 There has been a tendency — more especially in England* — to found 

 " races " upon little or no evidence, and without in any way showing what 

 was meant by " races." A few characters were taken, a little difference was 

 found, and " races " were discovered ! 



In the following pages it will be shown how, after the elimination of 

 the differences due to sex- and growth-variability, a difference determined 

 by probability as before, remains between certain groups which seems 

 due to race-variability as above defined. Whether such differences persist 

 from generation to generation it is of course impossible to tell from the 

 present facts, but other evidence will be shown which inclines one to 

 believe that such is really the case. 



The four groups of specimens from the southerly North Sea — i.e., from 

 Heligoland, Helder, Lowestoft, and Grimsby — are all taken together as 

 one. No satisfactory reasons could be found for regarding them as 

 separate. The evidence for this will be given in a later paper. Of 

 course, there may be two or more definite groups or races in this region ; 

 it is only that the evidence is insufficient to justify the declaration of 

 such. These four groups, considered as one, are compared with the 

 specimens from Aberdeen, the Solway Firth, the Baltic, and a few from 

 the Kattegat. 



Vertebrce. — In the two following Tables the averages and their fluctua- 

 tions are given for all the regions separately, except the Kattegat. In the 

 first Table the total number of vertebras is considered ; in the second, the 

 abdominal and caudal. That these two do not vary together can be seen 

 at once from the figures for the Baltic. Whilst the average for the 

 abdominal vertebras in the Baltic specimens is as high as in those from 

 Aberdeen, yet the average for the caudal vertebras is considerably lower. 

 Indeed, the abdominal vertebras differ little in the whole region concerned, 



* Vide Cunningham, T. C. : "On the Peculiarities of Plaice from different Fishing 

 Grounds,' Jour. Mar. Biol. Ass., Vol. IV., 1895-97, p. 315 Garstang, W. : "The Varia- 

 tion, ivaces, and Migrations of the Mackerel," Jour. Mar. Biol. Ass., 1898. 



