32 6 DISCOVERY REPORTS 



ment upwards is in response to light, we do not know that it is to intensity of light. 

 Russell's and Nicholls' results quoted above appear to conflict with its being due to in- 

 tensity, and Clarke (1930) in his laboratory experiments showed that for Daphnia it was 

 not actual intensity but changing intensity that produced the upward movement. 

 Nevertheless we see that such indications as there are of some of these organisms being 

 higher in the water in the regions of lower phytoplankton values are largely due to 

 stations taken in March or May, i.e. Parathemisto, Thysanoessa, and Ctenocalanus. These 

 results may be due to the small quantity of phytoplankton at these stations, they may be 

 due to a difference of light intensity, or they may be just due to chance sampling. For 

 Euphausia superba, both including and excluding the March and May stations, there is 

 an indication that they may be higher in the water in the lower ranges of phytoplankton 

 values; reading from left to right in order of increasing phytoplankton values, but 

 omitting the March and May stations, we see the daytime depth distribution as 

 follows : 



0-5 m • 0160 1 0000 000 00010 00 



Av. 58 m.: o 41 678 554 2 o 2 264 3 2 285 . o o o 00 



Av. 128 m.: o 140 28 41 1 o . 52 1 8 521 o o o o 179 o 



Table LXVII shows that these organisms may be as high in the water at night (2000 

 to 0400 o'clock) in regions of rich phytoplankton as in poor. In Tables LXVIII 

 and LXIX the data for copepods in Group B are similarly arranged, and in Tables 

 LXX and LXXI those organisms which appear to "prefer" regions of richer phyto- 

 plankton concentrations: Antarctomysis and the copepods of Group C. These tables are 

 summarized in Table LXXII, which tabulates the mean values for the three different 

 groups under the headings of poorer and richer phytoplankton. The actual individual 

 meanvaluesareof no significance in themselves, but considered together in a group they 

 do seem to indicate as far as our very limited data go a difference in daytime depth 

 distribution within the two ranges of phytoplankton values. If it is legitimate to include 

 the stations in March and May they give an indication that the organisms showing 

 exclusion effects may be higher in the water in daytime in ranges of poor phytoplankton 

 than in rich ; and that the animals which are found in larger numbers at stations richer 

 in phytoplankton do not show this but is anything slightly the reverse. Whilst no definite 

 conclusions can be drawn from such limited material, the fact that, in spite of the small 

 number of stations we are dealing with, more evidence in support of a difference in 

 depth in relation to phytoplankton is not found would seem to indicate that if the 

 hypothesis is correct the operation of the principle must depend rather on the length of 

 time the organisms come into the phytoplankton zones than on the depth to which they 

 sink away from it. Another way of presenting the data, which includes the time factor, 

 is shown in Fig. 170. The two diagrams show the positions of stations, marked by crosses, 

 in relation to time of day and phytoplankton values, the latter shown on a logarithmic 

 scale. The time scale ranges from midnight to noon and back to midnight up and down 

 the same scale; thus stations occurring at 0500 and 1900 o'clock would lie opposite the 

 same point on the scale. Against the position of each station in the upper figure are 



