PLANKTONIC STUDIES. 621 



individuals can in none of these regions bo ciiUed absolutely greater 

 than in the others, since the quantitative development is very depen- 

 dent upon local and temporal conditions and, according to time and 

 place, is on the whole extremely irregular. Estimation of individuals 

 can in this relation prove nothing. 



U. — CrUHEXTIC PlANKTONIC DiKl r>KKNCES. 



By far the most important of all the causes which determine thfe 

 changing- and irregular distribution of the plankton in the sea are the 

 marine currents. The fundamental imi)ortance of these currents for all 

 planktonic wStudies is generally recognized and has lately been men- 

 tioned many times and explained by Murray (0) and Chierchia (8). Even 

 the zo()logists of the plankton expedition of Kiel have not been able to 

 close themselves to this intelligence. Brandt calls special attention 

 to "the importance of the marine currents as a means of, and limit to, 

 the distribution of the planktonic organisms," so that in the various 

 Atlantic currents numerous forms continually appear which were want- 

 ing in the regions previously traveled" (23, p. 518). Thus, Heuseu 

 mentions the "extraordinarily large i)lankton catches, which were 

 transported by various currents." 



I learned thirty years ago to recognize the great importance of the 

 marine currents and their direct influence upon the composition of the 

 plankton, when at Messina I went out almost daily in the boat for 

 six months to secure the rich i^elagic treasures of the strait (3, p. 172). 

 The i)eriodical strong marine current, which there is known to the 

 Messinese under the name of the current or the Eema, enters the harbor 

 twice daily and brings to it inexhaustible treasures of pelagic animals 

 which since the time of Johannes Miiller have aroused the wonder 

 and desire for investigation of all naturalists tarrying there. Not 

 less important did I find later the planktonic importance of the local 

 marine currents (at Lanzarote), when the "Zain" current of the Canary 

 Sea in like manner brought with it an extraordinary wealth of pelagic 

 animals. My companion on the trip, Richard Greefr', has very vividly 

 described these marine currents as "animal roads" (IS, p. 307). Dur- 

 ing my numerous pelagic journeys on the Mediterranean it was always 

 my first care to investigate the conditions of the currents, and on the 

 most different parts of its coast (from Gibraltar to the Bosporus, from 

 Corfu to Rhodos, from Nizza to Tunis, I have always been convinced 

 of the determining influence which they exerted upon the composition 

 and distribution of the plankton. 



Although the fundamental importance of the marine currents for the 

 diverse questions of oceanography are now generally recognized, still 

 very little has been done to follow out in detail their significance for 

 planktology. It seems to me, we must here, with reference to our theme, 

 particularly distinguish {1) Judicurrcnts (the great oceanic currents) ; 

 (2) the hathycurrents (the manifold deep cuiTents or undercurrents); 



