GERMAN PLANKTON INVESTIGATIONS. 381 



endeavours to estimate the number of fish already present 

 in the sea from the number of eggs produced by them, and 

 not as Haeckel supposes, the number of fish those eggs 

 will ultimately produce. In addition, Haeckel is of 

 opinion that the plankton of the warmer seas is richer 

 than that of the colder ; but it must be remarked that his 

 opinion — it is nothing more — is based on qualitative 

 methods, whereas the methods of Hensen are based on 

 quantitative experiments, carefully carried out, and their 

 results have therefore a more sure and firmer foundation. 

 Space does not allow me to enter into the discussion of 

 these and other points raised by Haeckel. It is perhaps 

 sufficient to say that his objections have been satisfactorily 

 answered by Hensen 1 >and Brandt. 2 



Kofoid 3 has recently published a criticism of the Hensen 

 method. He employed the method in the investigation 

 of the fresh water lakes in the district of Illinois, and he 

 found that for the silk bolting cloth No. 20 the meshes 

 were far too large and consequently most of the material 

 slipped through, and his quantitative results were value- 

 less. He further considers that the quantitative method 

 of fishing with the plankton net is entirely impracticable, 

 on account of the closure of the meshes by the captured 

 organisms, so that the filtration capacity of the silk cloth 

 and of the net itself varies in an uncontrollable manner. 

 Therefore it is impossible to estimate what volume of 

 water has been fished through, and the estimation of the 

 volume or the number of the plankton constituents is in a 

 like manner uncertain. On these grounds Kofoid wishes 



1 Die Plankton Expedition und Haeckel's Darwinismus. Kiel, 1891. 



^Haeckel's Ansichten iiber die Plankton-Expedition. Schrift. d. 

 Naturw. Vereins Schles-Holst. Bd. VIII. Heft 2. 1891. 



:; " On some important sources of error in the Plankton Method." 

 Science, N. S., vol. G, pp. 829-832. 



