GENERAL BIOLOGY 763 



cod (see Fig. 557) plainly show winter-growth along their 

 edges, that is to say, vigorous summer-growth had not yet set 

 in, and as a matter of fact the temperatures were low (between 

 2 and 4 C, see Station 72, Fig. 95, p. 1 10) just where the cod 

 were taken. 



These variations of growth put their stamp on the fish, the 

 shape of which depends on its growth-history. And in waters, 

 like those off the Norwegian coast, subject to great variation 

 and extending south and north through so many degrees of 

 latitude, an infinite variety in growth-types appear as a natural 

 consequence. Some of these types may perhaps, through 

 generations, be subjected to the accumulating influence of 

 surroundings, thus possibly giving rise to races. Other and 

 minor variations in growth may perhaps be considered as 



A B 



Fig. 558. 



Interoperculum of plaice {Pleiironectes platessa). A,$ 21 cm. long, North Sea, three years old; 

 /), 9 21 cm. long, Baltic Sea, si.\ years old. (From Heincke. ) 



temporary or individual variations due to surroundings only, 

 and not subject to the laws of heredity. 



The way in which individuals vary according to surroundings 

 might profitably be studied by experiments in transplantation 

 and marking of various types. Heincke^ has made some very 

 interesting investigations on the growth of the plaice, and 

 found that in waters so widely different as the North Sea and 

 the Baltic the growth of the plaice varied greatly. Fig. 558 

 shows the gill covers of two plaice of the same size, both 21 

 cm. long ; the North Sea plaice is only 3 years old, while the 

 Baltic plaice is no less than 6 years old. Similar distinct types 

 of growth have been discovered in the herring during the 

 international investigations, Dahl having first drawn attention 

 to the existence of such types ; Lea continued these investiga- 

 tions with a large amount of material, and claims that among 

 others two growth-types may be recognised, one belonging to 

 the north-eastern part of the North Sea (the Norwegian 

 west coast), and the other to the Kattegat (see Fig. 559). 



^ Die Beteiliguug Dent schlaiids an der intenmlionaleii Meeresforschiing, IV.- V. Jahresbericht, 

 Berlin, IQOS. 



