430 University of California, Publications in Zoology [Vol. 22 



It may be of interest to note that Herdman (1918, p. 197) found 

 that Acartia clausi is most abundant in the Irish Sea from May to 

 October, inclusive, generally reaching its maximum in June with a 

 smaller rise in September or October. At La Jolla this copepod is 

 most abundant in March, and not one specimen was recorded in July, 

 August, or September of 1917. It is hard to account for such striking 

 differences. The conditions are so complex that careful study of 

 hydrographic and hauling data, as well as knowledge of the life his- 

 tory of the animal, would be necessary to give an insight into the 

 matter. 



Such of our data as relate to seasonal distribution of the copepods 

 suggest that there is a natural sequence of events year after year. 

 The occurrence of the animals for the twelve months from September, 

 1916, to August, 1917, inclusive, is not strikingly different from that 

 in the year following. Herdman (1922, p. 145) found the same sort 

 of thing in his work. (See also Herdman and Scott, 1912, p. 154.) 

 While strict uniformity from year to year was lacking, the broad 

 features of the distribution were similar. In our pier series the 

 similarity of distribution in the two years extends even to correspond- 

 ing weeks, as has been shown in table 4. Our study might be carried 

 farther by ascertaining whether the proportions of different species 

 to the whole catch and to each other are similar at like seasons of 

 different years. If this should prove to be so, it would strengthen 

 the view that there is a rather regular recurrence of phenomena year 

 by year. 



Why should seasonal distribution of marine animals be essentially 

 different from that of terrestrial forms, such as birds? It is well 

 known that migrating birds appear in a given locality at about the 

 same time of year, season after season. Minor irregularities from 

 year to year are to be expected, and yet it is true that there is a 

 reasonably regular appearance of a given species. 



So in the case of the copepods or other organisms of the plankton, 

 the time of the maximum for one year may lag behind that of another 

 or may occur earlier, without upsetting the view that one year is 

 broadly like another. Surely it would never be found that Acartia 

 clausi would appear in greatest abundance in the fall or that A. tonsa 

 would reach its minimum in July or August. There is every reason 

 to believe that the times of the maximum and minimum of those two 

 species will be the same year after year. 



This series of collections has shown beyond a doubt that the Copep- 

 oda found at the pier are more abundant as a group on the surface 



