PLANKTONIC STUDIES. 617 



flora. Here as there tlie explanation of tbe facts is above all to be 

 sought in the intiuence of the sun, that •'all-i)Owerful creator." which 

 in the tro})i('al zone conditions a niucli more lively interaction of the 

 natural forces than in the polar zones. The ''cycle of matter in the 

 sea" { Staff wechfiel defs Meeres) is no less influenced by the perpendicular 

 rays of the sun than is the terrestrial fauna and flora; and as in the 

 troi)ics the quantity and the complexity of the teriestrial 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 Ifational 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 apjiarent esta]>lished fact (22, p. 245). 



In the notable account which E. du Bois-Reymond (on January 23, 

 1890) laid before the Berlin Academy upon the results of the Xational 

 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 Xational expedition 

 stands in strong opposition to the familiar experience of the Challenger, 

 of the Veftor Pisani, and of many other expeditions, we must first of 

 all again examine the empirical foundations upon which his assertions 

 rest. For these he admits that he regards as such only the results of 

 his^frw? triiy throufjh a part of the Atlantic ocean, in which the resi- 

 dence in the tropics embraced scarcely two months. The results wliich 

 he here draws from Ms plankton fisheries, which obviously turned out 

 remarkably poorly as a result of accidental conditions, may contradict 

 the results which were set up by the ChalUn<jer and the Vcttor 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 to establish by computation as useless. 



My own comparative study of the rich planktonic collections which 

 Murray and liabbe have brought in from the different parts of the three 

 great oceaiis, 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 



