PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 261 



size-varieties or geographical races : (1) that their occurrence in the 

 plaice is only one instance among a number, several other northern 

 species, e.g., the Greenland bullhead, Cottus greenlandicus, and the 

 so-called Norway haddock, Sehastcs norvegicus, being very much larger 

 on more northern coasts than on British coasts or on the south coast of 

 Norway ; (2) the question whether definite structural peculiarities are 

 present, as well as mere size, to distinguish the geographical forms from 

 one another; (3) the question whether the differences are hereditary, 

 each race breeding and transmitting its peculiarities independently, or 

 whether the fish are the offspring of parents from other areas, and owe 

 their peculiarities merely to the conditions under which they have lived 

 and grown. 



With regard to the first point, we cannot say that the existence of 

 geographical races differing in size is peculiar to northern forms, 

 although it is to these that my attention has been principally directed. 

 It is probable enough that any wide-spread species may be found to 

 show the same state of things. At a certain part of its habitat it 

 appears that a species attains its greatest development, because there 

 the conditions, whatever they may be, are most favourable to it, and 

 at regions lying near the limits of its range it is less favourably 

 circumstanced, and is found in smaller numbers and of smaller size. 

 In Greenland it is stated that the short-spined bullhead attains to six 

 feet in length, although it is the same species as the Cottus scorpius 

 occurring on the east coast of Britain, where it never exceeds a length 

 of fifteen inches. It is very difficult to decide what are the favourable 

 and unfavourable conditions which cause the differences in size in such 

 cases, and the investigation of these conditions would be both im- 

 portant and interesting. With regard to the plaice and other northern 

 species, it might be supposed that a higher temperature was the chief 

 unfavourable condition, and it may probably enough be one of them. 

 We know that the water of the Channel is warmer than the bottom 

 water of the northern part of the North Sea, and this higher tempera- 

 ture extends for great part of the year to the southern narrower area of 

 the North Sea. But on the other hand, the Baltic, which contains 

 plaice of small adult size, is colder, except perhaps in the height of 

 summer, than the North Sea. Here it might be supposed that the 

 lower salinity was an unfavourable condition, but this would not apply 

 to the English Channel. It is possible that the amount of available 

 food, the extent of suitable ground, and the competition of other 

 species, have more influence on the size and general development of 

 a particular species than purely physical conditions such as salinity 

 and temperature. 



With regard to the second point, it is found in many cases that 



