418 University of California Publications in Zoology [Vol.22 



after the haul was concentrated the material was divided into two 

 portions as nearly equal as possible, one of which was removed with a 

 pipette! The remaining portion was shaken together again and half 

 of it removed, if there still were too many organisms left to make 

 fairly rapid counting possible. This procedure was repeated as often 

 as was necessary in order to reduce the amount to be gone over to a 

 point where enumeration would not be an unreasonable burden. In 

 the final counting the numbers were increased in proportion to the 

 numbers of parts into which the whole haul had been divided ; that is, 

 if the haul had been divided into eight parts, the number in the one- 

 eighth counted was multiplied by eight to give an estimate of the total 

 number in the haul. Before any division of the collection was made it 

 was looked over for rare or unusual forms. Note was also taken of 

 the relative numbers of copepods that usually occurred, and if it was 

 apparent that one sort was abundant while another was scarce, the 

 number of the latter was estimated upon the basis of a larger fraction 

 of the whole (if the entire number was not counted) than was used in 

 the case of the more numerous form. 



It is fully realized that this method of counting is inaccurate so 

 far as giving the numbers of animals actually present in a collection is 

 concerned. That, however, was not the information most desired when 

 the work was planned. The aim was to ascertain the relative numbers 

 of different organisms under different conditions in regard to years, 

 seasons, months, times of day, temperatures, salinities and other pos- 

 sible factors. Those of us who have been concerned with the project 

 from the beginning believe that this method of enumeration is 

 adequate to throw a good deal of light on the problem of variations 

 in numbers of the plankton copepods; and other members of the 

 staff concur in this view. Even when every allowance is made for 

 errors in counting, the number of hauls and of animals taken is so 

 large that the errors are bound to balance one another. The crudity 

 of the method is fully understood, and the numbers of animals set 

 down in the tables are not thought of as important except in indicating 

 relative abundance. The method does make it possible to work fairly 

 rapidly and with a minimum of fatigue. There is no reason to suppose 

 that any errors that may have occurred in the counting are systematic, 

 though the extent of any one error may be large as regards actual 

 numbers of animals involved. If one accidental error was made, others 

 of the same sort must have occurred many times. But accidental 

 errors tend to become eliminated as the number of observations in- 

 creases, while, in the long run, systematic errors do not disappear. 



