220 Marsh — Limnetic Crustacea of Green Lake. 



where the numbers of a species were very large, would the fact 

 that a considerably larger number were found in one collection 

 than in another be any conclusive argument against the practi- 

 cal uniformity of distribution. Nor, on the other hand should 

 it be assumed, because two or three successive hauls show the 

 same, or nearly the same numbers, that the distribution is 

 therefore uniform, because this could be easily explained by sup- 

 posing that the swarm was of considerable extent or remained 

 stationary for a considerable period. 



My collections made in 1893, which were reported in the for- 

 mer paper, were made almost continuously in the course of two 

 days. Now if the plankton is uniformly distributed, those collec- 

 tions should show a practical uniformity of numbers, and 

 the more numerous a species was, the less should be the pro- 

 portional variation. Yet the collections of Diaptomus, the most 

 abundant genus, varied from 291 to 2,966. In many of the col- 

 lections made in the fall of 1894 on the same day, or successive 

 days, there was a marked uniformity in the numbers of Diaptomus, 

 as for example, nos. 4. 94, 5. 94 and 6. 94 show a range of num- 

 bers only from 4,171 to 5,630. If one were to base his conclu- 

 sions on a small number of observations, he might well say that 

 here was clear evidence of uniformity. Yet a few hours later in 

 the same place I found only 2,023; with a difference as great as 

 this, we certainly cannot speak of the Diaptomi as being uniform- 

 ly distributed. In hauls 21.94 and 22.94, made in the forenoon 

 of October 25, there was in one case 1,917 and in the other 

 3,823 — twice as many. Still more marked was the difference 

 in two collections, one made at about six p. m., and the other 

 between ten and eleven p. m., November 8. In the six o'clock 

 collection there were 884, while in the evening collection there 

 were 6,447. Such an enormous difference as this is certainly 

 not consistent with any theory of uniformity of distribution. In 

 these same two collections of November 8, Cyclops fluviatilis 

 showed a similar wide variation, — the numbers in the six o'clock 

 collection being 1,912, and in the evening collection being 564. 

 October 24 I found between ten and eleven o'clock in the even- 

 ing 1,241 C. fluviatilis, and yet the next morning between six 

 and seven o'clock, I found only 618. 



