PLANKTONIC STUDIES. 617 
flora. Here as there the explanation of the facts is above all to be 
sought in the influence of the sun, that “all-powerful creator,” which 
in the tropical zone conditions a much more lively interaction of the 
natural forces than in the polar zones. The “‘cyele of matter in the 
sea” (Stoffiwechsel des Meeres) is no less influenced by the perpendicular 
rays of the sun than is the terrestrial fauna and flora; and as in the 
tropics the quantity and the complexity of the terrestrial organic living 
forms is by far most highly developed, so is it also the case with the 
marine forms. 
Hensen places himself in remarkable opposition to this hitherto 
accepted view when in his account of the results of the National expe- 
dition he surprises us with the following statement: 
Although we have found plankton everywhere, the amount of it under and near 
the tropics was relatively small, namely on an average 8 times less than in the 
north near the Banks of Newfoundland. Each one of these hauls contained 
upwards of a hundred different forms; but the poverty of the quantity is still a 
remarkably apparent established fact (22, p. 245). 
In the notable account which KE. du Bois-Reymond (on January 23, 
1890) laid before the Berlin Academy upon the results of the National 
expedition, it was said concerning its scientific results that a complete 
account could not be given for three years, but then he added: 
Only one chief result may here be assumed beforehand. Contrary to all expecta- 
tions, established upon a theoretical basis, the quantity of plankton in the tropical 
waters is shown to be surprisingly small (21, p. 87). 
Since Hensen with this “chief result” of the National expedition 
stands in strong opposition to the familiar experience of the Challenger, 
of the Vettor Pisani, and of many other expeditions, we must first of 
all again examine the empirical foun dations upon which his assertions 
rest. For these he admits that he regards as such only the results of 
his “trial trip” through a part of the Atlantic ocean, in which the rest- 
dence in the tropics embraced scarcely two months. The results which 
he here draws from his plankton fisheries, which obviously turned out 
remarkably poorly as a result of accidental conditions, may contradict 
the results which were set up by the Challenger and the Vettor Pisani 
during a residence in the tropics of altogether four years, in different 
parts of three great oceans. It is not indeed saying too much, if we 
declare this kind of conclusion by Hensen as hasty, and the “exact 
method” which he wishes tu establish by computation as useless. 
My own comparative study of the rich planktonic collections which 
Murray and Rabbe have brought in from the different parts of the three 
great oceans, has convinced me that the tropical ocean is not only qual- 
itatively much richer (by the variety and number of planktonic spe- 
cies and genera) than the oceans of the temperate and cold zones, but 
that it also does not fall behind the latter quantitatively (in the abun- 
dant distribution and vast accumulations of individuals). To be sure, 
one ought not to take into consideration merely the surface of the trop- 
ical ocean (although this also is often extremely densely populated), but 
