PLANKTON 255 



three possible sources of error in the quantitative methods : — 



1. The imperfections of the net as a filtering apparatus. 

 These of course apply to all nets and are generally admitted, 

 and improvements and substitutes, such as pump and filter 

 and centrifuge, have been proposed and used. Kofoid finds 

 that the coefficient of the net may vary from 1-5 to 5-7, 

 according to its condition, and that it may retain anything 

 from J to 4Vth of the solid contents of the water filtered. 



2. The vertical haul may defeat its object by mixing 

 zones of plankton which ought to be sampled separately. 

 Closing quantitative nets have been devised to meet this 

 difficulty, but Paulsen has shown recently that these vertical 

 nets may fish while being lowered down, as well as when 

 coming up, and therefore are not reliable. 



3. The irregularity in distribution of the plankton. 

 No device can get over this difficulty. The only remedy is 

 more frequent sampHng and more accurate and detailed 

 determination of the characters, both physical and biological, 

 of the various areas, currents and zones of water making 

 up our seas — and all that is being done, and must be done 

 in still greater detail, by oceanographers all over the world. 



We need not, however, fail to appreciate the labours of 

 the plankton school at Kiel, or be at all hopeless as to science 

 attaining to a more exact knowledge of the populations 

 of the oceans. The leading idea of quantitative estimation 

 is a good one, the implements devised are very ingenious, 

 and the long-continued laborious computations of some of 

 the German professors have been most praiseworthy. But 

 the method is still open to serious objections, the most 

 fundamental of which is the obvious irregularity in the 

 distribution of the plankton — horizontally, vertically and 

 chronologically— an irregularity which must vitiate any 

 calculations based upon comparatively few and distant 

 samples. Marine biologists will probably do better to 

 concentrate their efforts upon the intensive study of small 

 areas before trying to estimate the contents of an ocean. 



