7IO 



DEPTHS OF THE OCEAN 



interest attaches to the fact that the immigra- 



into 



Norwe 



rapidly and the lower temperatures gradually descend towards 

 deep water during early spring and summer. Great changes 

 in specific gravity, viscosity, and light-intensity accompany 

 these changes in temperature ; in the very magnitude of 

 these changes we must look for the essential difference between 

 the tropical and subtropical conditions on the one hand, and the 

 arctic-boreal conditions on the other. 



The greatest 

 tion of Atlantic forms 

 season when the condi- 

 tions in the latter are 

 most similar to those of 

 the Atlantic. The in- 

 ternational investiga- 

 tions have contributed 

 to our knowledge on 

 this immigration. 



Schmidt,^ for instance, 

 in the Danish investiga- 

 tion-steamer " Thor," 

 had the opportunity of 

 studying the immigra- 

 tion of Salpae from the 

 Atlantic into the Nor- 

 wegian Sea, and writes 

 as follows : — 



" The organisms 

 concerned were the dis- 

 tinctly Atlantic Salpse 

 (especially Salpa fusi- 

 fornns), which are so 

 characteristic and which 

 were taken often in hundredweights in each haul of our 

 pelagic apparatus in the Atlantic beyond the looo- metres 

 line. The year 1905, during which we several times crossed 

 the North Sea, made two cruises to and from Iceland and the 

 Faroes, following approximately the looo-metres line, then 

 sailed southwards west of the British Isles to the Bay of 

 Biscay, was thus specially well suited to give light on these 

 conditions, as I have endeavoured to delineate on the accom- 

 panying Chart [reproduced in Fig. 510]. The shaded lines 



1 Jobs. Schmidt, "The Distribution of the Pelagic Fry and the Spawning Regions of the 

 Gadoids," etc., Rapports et proces verbatix dii Conseil International, vol. x., Copenhagen, 1909. 



Fig. 



510.— Drift ok Salp^ {Salpa fusiformis) 

 1905. (From Schmidt.) 



