432 University of California Publications in Zoology [Vol. 22 



actual numbers of animals. The aim was to study daily, seasonal, and 

 yearly fluctuations, and to discover whether they were related to 

 variations in environmental conditions; and, if there were such a 

 relation, to ascertain the effect of changes in the different factors. 

 It is believed that the samples obtained are adequate and repre- 

 sentative, and that the methods employed are sufficiently reliable 

 to reveal certain broad, fundamental facts. It is uncertain how 

 generally applicable the results are, for in other localities other con- 

 ditions would prevail. But the investigations have undoubtedly given 

 us a trustworthy view of what may be expected at the pier of the 

 Scripps Institution. As regards methods and suggestions about 

 procedure, it would be difficult to add to what has been said by Allen 

 (1921, pp. 538, 541, 553). It seems perfectly evident that the prob- 

 lems connected with study of the marine plankton cannot be solved 

 until collecting has been done continuously over a long period of years. 

 While our series, which runs through two years is valuable, it is not 

 extensive enough. I believe that when such work is undertaken it 

 should be planned to cover at least ten years. It is doubtful whether 

 there is any need of taking as many hauls in a day as we did, but I am 

 convinced that one haul during daylight and one during the night, at 

 given times, is the least that can be expected to give adequate returns. 

 Collections made at such intervals would undoubtedly be enough to 

 enable us to detect unusual occurrences which might be missed if 

 hauls were taken less frequently. Furthermore, hauls made at some 

 level below the surface, at the same time that the surface hauls are 

 taken, would be extremely valuable. But the most important thing is 

 an unbroken continuity of collecting after the series is once started. 

 Time of collecting, location of stations, kinds of nets, methods of 

 dealing with the data — all these are of secondary importance in com- 

 parison with continuity and extent of the series. 



SUMMARY OF RESULTS 



1. The series of plankton hauls extends over two full years and 

 consists of over 3400 catches. All were surface hauls. 



2. The copepods were more abundant at the surface during the 

 second year, that is, from September, 1917, through August, 1918. 



3. Two species of Acartia, clausi and tonsa, are sharply set off 

 from each other in seasonal abundance. Clausi is most abundant in 

 March and April, tonsa in July and August, while clausi is least 

 abundant in July, August and September, and tonsa is least abundant 

 in the early part of the calendar year. (See table 3.) 



