23G 



Part III. — Eighteenth An aval Report 



and that the decrease in the skull and body is accompanied by a less great 

 development of the reproductive organs than in the northerly plaice. 



Again, it has to be noticed that a similar alteration in form has taken 

 place in the Baltic group. This, so far as Duncker's results and tliDse 

 given here allow us to conclude, is more nearly allied in form and pro- 

 portions to the southerly North Sea group. Yet, on the other hand, 

 accompanying the alteration in form, the Baltic plaice also show a great 

 reduction in the number of vertebrae and fin-rays, and this reduction is 

 probably associated with the decrease in salinity of the waters in which 

 the adults live and the young develop. 



We may turn now to the evidence from other sources for 

 the existence of groups or races of plaice in the North Sea. 

 Fulton*, as one of the results of his great work on the migration 

 of fishes and the directions of the currents in the North Sea, 

 has shown that even for the slow-moving plaice there is a definite cycle of 

 changes on the east coast of Scotland. Beginning with the spawning of 

 the plaice off the coasts of Aberdeenshire and Kincardineshire, he has 

 shown that the eggs are drifted along by the surface currents— which are 

 not simply superficial — until the young are found on the southern shores 

 of Scotland. There they pass the early stages of life, probably during 

 two and three years, according to the results of Dannevig (I.e.), and then 

 begin a northward migration along the shores until at the end of their 

 third, or perhaps fourth, year they reach as far north as where they were 

 spawned, and thence at the period of maturity they go out into deeper 

 waters to spawn. These facts show how definite is the life-cycle, and in 

 a former paper* it has been shown how the young immature plaice of St. 

 Andrews Bay could be detected as a portion of the Aberdeen group by an 

 inspection of their characters. 



The facts displayed in this present paper, along with this life-cycle 

 demonstrated by Fulton, show how the northern plaice of the North Sea 

 are distinct from the southern, and show also that the separation is a 

 consequence of this life-cycle, and requires no barriers of any sort in the 

 sea. It is possible, again, that the plaice off the English, German, and 

 Dutch coasts are similarly divided into groups or races, but there is no 

 satisfactory evidence as yet from either side — that of the life-cycle or 

 that of the characters. Tt may be that as we pass from north to south 

 the differences in characters between the northerly and southerly plaice 

 of the North Sea shown here gradually diminish, so that the differences 

 in the averages are too small to be detected and spoken of with any 

 certainty. Perhaps with a very large number of specimens these finer 

 differences will show themselves, but for the present we can only say that 

 the groups may be present, and that the life-cycle of the one group may 

 be passed under very nearly the same conditions as the life-cycles of the 

 other groups, so that the characters which vary under the stimulus of those 

 conditions are similarly closely related. 



The evidence of a definite life-cycle may likewise be brought to bear 

 upon the problem of the plaice in the Baltic. So far as direct evidence 

 is concerned, Petersen's view that they may spring continually from the 

 plaice of the Kattegat, just as those of the Limfjord do from the plaice of 

 the North Sea, is just as correct as Heincke's that the plaice in the Baltic 

 form a distinct race. But the probability seems to me in favour of 

 Heincke, because we have no evidence that the characters of the plaice 

 can change so quickly, and because the evidence of a race remaining 

 distinct and separate off the east coast of Scotland appears conclusive. 

 The plaice of the Baltic may do so likewise, and the eggs and larvae that 



* Fulton, T. W. : Eleventh Report, Scottish Fishery Board, pp. 176-186. 



f 11 Extension of the Method of Treating Variations, etc.," Natural Science, Dec. 1899. 



