PLANKTONIC STUDIES. 621 
individuals can in none of these regions be called 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. 
D,.—CURRENTIC PLANKTONIC DIFFERENCES, 
By far the most important of all the causes which determine the 
changing and irregular distribution of the plankton in the sea are the 
marine currents. The fundamental importance of these currents for all 
planktonic studies is generally recognized and has lately been men- 
tioned many times and explained by Murray (6) and Chierchia (8). Even 
the zodlogists 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, Hensen 
mentions the “extraordinarily large plankton 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 pelagic treasures of the strait (3, p. 172). 
The periodical strong marine current, which there is known to the 
Messinese under the name of the current or the Rema, 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 Jater 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 Greeff, has very vividly 
described these marine currents as “animal roads” (18, p. 307). Dur- 
ing my numerous peiagic 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. Itseems tome, we must here, with reference to our theme, 
particularly distinguish (1) halicurrents (the great oceanic currents); 
(2) the bathycurrents (the manifold deep currents or undercurrents); 
