1034 OYSTERS, AND ALL ABOUT THEM. 



Stream waters poured in through the Straits of Dover and 

 by the Orkneys, while on the German coast it is excep- 

 tionally low, on account of the masses of water bordering 

 on freezing point received from the Rhine, the Weser, the 

 Elbe, and the Baltic. This state of things continues till 

 April, in which month the temperature of the coast waters 

 begins to rise. The rivers w r arm as summer approaches, 

 and during July, August, and September, the southern parts 

 of the North Sea are considerably warmer than those fur- 

 ther north. In April, May, and June, when the coast tem- 

 perature is gradually rising, the Dogger remains cool, and 

 after the beginning of October the cooling process sets in 

 rapidly on the German coast, until the state of matters that 

 existed at the beginning of the year is restored. (h) 

 Generalising these observations still further, it may be 

 stated that in winter, with the exception of a small area 

 near the Straits of Dover, the shallow southern part of the 

 North Sea is cold, while the surface waters of the north 

 are relatively warm ; whereas in summer the southern 

 waters are warm and the northern surface waters rela- 

 tively cold. The temperature of the northern part of the 

 North Sea is much more equable than that of the southern 

 part. 



It was held by naturalists so experienced and observant 

 as Edward Forbes and Sir Wyville Thomson that the dis- 

 tribution of marine animals is determined almost exclusively 

 by temperature. Currents act as means of transport, but 

 temperature fixes the limits of habitableness. 



Sir Wyville Thomson was of opinion that distribution 

 of marine animals was probably determined by extremes of 



(h) Karsten, in Report of German Commission (trans, in U.S. 

 Commissioners' Report for 1879.) 



