262 PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 



minute structural peculiarities do co-exist in geograpliical races together 

 with limits of size. Such races, therefore, must be regarded as in- 

 cipient species ; they only differ from species in the minuteness of the 

 structural peculiarities and in the absence of definite limits between 

 one race and another, a continuous transition from one to the other 

 being observed in individuals and in intermediate areas. The study 

 of such geographical varieties is therefore philosophically important, 

 since in these cases we have actually the origin of species before our 

 eyes. 



Mr. Holt has previously written in this Journal concerning the 

 ciliation of the scales in the males of tlie dwarf variety of plaice in 

 the Baltic, and mentioned that these plaice have been stated to have 

 a smaller number of dorsal and ventral fin-rays. Dr. Heincke had 

 sucraested that the Heligoland plaice were smaller than those of the 

 western side of the North Sea, and probably formed a local variety. 

 Dr. Georg Duncker, at Heincke's request, has investigated,* by the 

 method applied by the latter to races of the herring, the distinguishing 

 characters of local varieties of the plaice. He examined separately 

 specimens from Greifswald, Kiel, the Cattegat, all localities in the Baltic, 

 and from the neighbourhood of Heligoland. It appears from Duncker's 

 results that ciliation restricted to the middle rays of the dorsal and oval 

 fins is more common than a greater extension of the condition. It was 

 more developed in specimens from Kiel and the neighbourhood than in 

 those from the Cattegat, and in specimens from Heligoland was found on 

 the body in two males out of 35, on the fins alone in 18 out of 35. But 

 unfortunately no examination was made of specimens from other parts 

 of the North Sea, and therefore it remains an open question whether 

 the plaice of the Heligoland Bight have the special characters of a local 

 race. Only a small number of specimens altogether were examined by 

 Duncker. It will be remembered that my own examination of the size 

 of mature specimens went to prove that the plaice of the Heligoland 

 region were not smaller at maturity than those of the north-western 

 part of the North Sea. The examination of specimens from the 

 different regions of the North Sea, for the purpose of ascertaining 

 whether constant structural differences can be found distinguishing the 

 local forms, is yet to be carried out. 



With regard to the third point, whether the peculiarities of local 

 races are hereditary, or are acquired by the individual in consequence 

 of the conditions under which it has lived and grown, to decide upon 

 this, it would be necessary to know in each case how far interchange of 



* Variation und Verwandtscliaft von Pleuronectes flems L. und PI. lilaUssa L. : 

 Wissensclwftlichc Mcercsunlcrsuchungen. Ncuc FoUje, Erster Band, Heft 2, 1S96. (See this 

 Journal, vol. iv. p. 293.) 



