7o8 



DEPTHS OF THE OCEAN 



encounter catastrophes which sweep them off in enormous 

 numbers. I come to this conclusion because our investigations 

 on the age-composition of various fish-species have proved the 

 frequency of the different year classes to be so variable (see 

 section on age and growth). 



As the Gulf Stream flows northwards its waters are 

 gradually cooled, partly because they give off heat to the cold 

 air, and partly because of the admixture of cold water. With 

 the cooling the southern forms disappear, and their place is 

 taken by entirely different boreal species ; very little is known 

 about the actual stages of this change. 



During the cruise of the "Michael Sars " from the west 

 coast of Scotland to Rockall, and north to beyond the Wyville 

 Thomson Ridge we found vast numbers of Salpa^ {S.foisiformis), 

 the great majority of which were wholly degenerated. Bjerkan, 

 who is examining our collection of Salpae, informs me that the 

 mantle and the muscular system of the specimens were generally 

 in a very ragged condition, in many cases only the intestine 

 being distincdy recognisable. Here then, on the border 

 between the Atlantic and the Norwegian Sea, it appears that 

 certain forms die in large numbers, while others degenerate. 

 Gran refers to the degeneration of certain coast diatoms found 

 drifting far out at sea (see p. 342). 



When organisms cannot within a certain time regain condi- 

 tions necessary for them, or to which they can adapt themselves, 

 they invariably die sooner or later. The isolated specimens of 

 such fishes as Argyn-opelecus found in the northernmost parts of 

 the Atlantic undoubtedly represent a few survivors of the 

 change. 



The boreal fauna which in northern waters replaces the 

 genuine Atlantic forms also belongs to a great current-cycle. 

 If we look at the current charts (Fig. 193, p. 284 and Fig. 

 508), we observe that the Gulf Stream receives admixtures 

 from boreal and boreo-arctic currents, which consequently carry 

 boreal organisms. As we have previously seen, we meet with a 

 wealth of boreal forms in deep water even in the Sargasso Sea, 

 and probably much farther south, living below the warm-water 

 fauna of the surface. 



The velocity of ocean currents is subject to many varieties 

 of periodical and non-periodical changes (see pp. 284-5). T^^e 

 annual changes are of peculiar interest, and are very noticeable 

 in northern waters, though also important in the Atlantic, 

 If we compare the two charts (Figs. 159, p. 227, and 160, p. 228) 



