METHODS OF PLANKTON RESEARCH. 515 



the second catch (where in proportion to the diatoms and 

 other forms they seemed abundant), and again very rare 

 in the third catch (where the C. macroceros has appeared 

 so abundantly). In reality, however, the number of 

 C. fusus has remained the same. An observer estimating 

 by relative frequencies would have constructed a table 

 and curve showing a great increase at Station 2, and then 

 sought for an explanation of this increase, which in 

 reality did not exist. If plankton tables are to be con- 

 structed for a large sea area, in order to compare the 

 plankton at different places under different conditions of 

 salt contents, temperature, currents, and other changing 

 conditions in the sea, quite false results would be obtained 

 from the method of estimation without counting. 



Moreover, the reliability of such estimations is not 

 good. In order to determine this, Apstein and one of his 

 colleagues took four catches and first simply estimated 

 them in the usual way, and then counted and estimated 

 by the Hensen method (14). A section of the table will 

 show the results. The first column gives Apstein's 

 estimate, the second gives that of his colleague, and the 

 third gives the true number present as found by counting 

 the various forms present and then using letters derived 

 from the frequencies determined by the counting, in order 

 to compare with the other two columns. 



A., by 

 estimation. 



Rhizosolenia alata r 



,, semispina 



,, shrubsolei c 



,, stolterfothi r 



,, styliformis -f 



Ceratium tripos cc 



,, longipes cc 



,, furca + 



,, fusus r 



Cyphonautes 



Limacina r 



Molluscan larvae + 



Oikopleura c 



KK 





By 



R.,by 



counting 



estimation. 



method. 



+ 



rr 



rr 



rr 





c 



r 



4- 



T 



c 



cc 



c 



+ 



cc 



c 



cc 



r 



+ 



r 



r 



+ 



c 



4- 



c 



T 



c 



