CH. Vl] QUANTITATIVE PLANKTON INVESTIGATIONS 119 



of the benthic population of the sea in this way, and it was 

 thought by Hensen and his pupils that a quantitative study of 

 the plankton would probably be attended with some measure of 

 success. 



The Hensen Methods. Hensen tells us that he sought to 

 devise methods by which the answers to two questions might be 

 given : (1) what quantities of living organisms in the form of 

 plankton does the sea, in a given area, contain at a certain time ? 

 and (2) how does this quantity of plankton vary from place to 

 place, and from time to time ? He studied the plankton because 

 it is here that the greatest mass of life is to be found, and 

 also because it is only the plankton w^hich is apparently capable of 

 receiving quantitative treatment. In order to supply the answers 

 to the questions propounded it was necessary to devise some means 

 by which it would be possible to filter a known volume of sea- 

 water so as to remove all the organisms contained in it, and to do 

 this expeditiously so that the experiments could be carried out 

 over a wide sea area. It was indeed possible to make comparative 

 estimates of the amount of plankton contained in the sea by the 

 use of the ordinary tow-net, but even at the best such estimations 

 can only be very imperfect ones. Plankton might be collected in 

 quantity and the quantity might vary from place to place and 

 from time to time, but a detailed consideration of the methods so 

 involved will shew that the quantity of material so collected in 

 the nets depends so much on uncontrollable circumstances that 

 even the comparative results so obtained may be of little value 

 from a quantitative point of view. 



It is obvious that the amount of material taken by a tow-net 

 will depend on the length of time that the net has been hauled 

 through the water. This can, of course, be regulated very exactly. 

 But the catch of the net depends on the amount of water w4iich 

 passes through the meshes, and this depends not only on the length 

 of time that the net is hauled but also on the velocity of the haul. 

 It is very difficult to cause a vessel of any size to travel through 

 the water at a constant rate unless she is going fast, and it is 

 necessary that the nets should be hauled slowly. If the vessel 

 travels slowly less water will pass through the meshes of the net, 



