352 DEPTHS OF THE OCEAN 



Cleve, who looked upon the dispersal of organisms by currents 

 as the chief factor in affecting the character of the plankton, 

 was at first of opinion that he could fix the north-western 

 boundaries of the Gulf Stream by noting the distribution of 

 Rhizosolenia styliformis, the guiding form in his Styli-plankton. 

 But he, too, found that its area of distribution extends northwards 

 in the course of spring and summer, and that the swarms of 

 Rhizosolenia actually outdistanced the speed of the current. 

 The wider distribution of the algae was evidently, therefore, due 

 not alone to the increased volume of the current, but also to a 

 rapid propagation produced by summer warmth outside the 

 influence of the current, the algae apparently having been already 

 present in this area in small quantities. 



I may further instance the close agreement between oceanic 

 species in arctic and antarctic waters. Thalassiothrix longissima 

 and Rhizosolenia semispiiia [hebetata) are the two most character- 

 istic forms among algae along both the polar boundaries of the 

 Atlantic, though they have also been found in small quantities 

 at various localities in the tropics. I personally came across them 

 on several occasions during the "Michael Sars " Expedition, 

 and it requires, in my opinion, no special theories to account 

 for this " bipolarity." There is quite sufficient connection 

 between the two oceans to enable a few germs which are 

 exceptionally tenacious of life to pass from the one to the other, 

 and this would amply explain the agreement. Characteristically 

 enough there is no similar agreement between arctic and 

 antarctic waters when we come to the neritic forms, and this is 

 probably because they are less adapted to travel over such 

 immense distances. It may be, too, that their tendency to evolve 

 resting-spores is an obstacle to long passive wanderings. 



As a means of determinino^ the direction and velocity of 

 currents pelagic algae will be found of very little use. Their 

 continued existence during the progress of the current must 

 always depend upon their persistence in reproduction, and this 

 again is dependent upon conditions of existence and competition 

 with other species. It is not mere coincidence that the 

 microscopic flora of the warm Atlantic extends farthest north 

 during the dark winter months, when no other species are much 

 inclined to develop, and there is therefore no competition of 

 any consequence, the character of the flora consequently 

 remaining for a long time unaltered. Large animals, such as 

 medusae and salpae, or the larvae of bottom-animals like Phoronis, 

 will be found far better indicators of the currents. Ostenfeld 



