HYPOTHESIS OF ANIMAL EXCLUSION 



3i3 



enter the phytoplankton zone before the others do so. The only macroplankton 

 animal which we observe having a vertical migration approaching that of these two 

 copepods is the carnivorous amphipod Parathemisto gandichaudi (Fig. 121); but its 

 arrival at the surface in large numbers is some two hours later in the evening. It 

 is interesting to notice that the modes of the curves of vertical migration of a number 

 of species balance at a point in time later than midnight. Whilst this might be due to 

 the time taken for these species to reach the surface, it might also suggest that it may 

 possibly be due to the time taken in the dissipation of some harmful influence produced 

 by the phytoplankton in the surface layers during the hours of daylight. 



These facts support the hypothesis that vertical migration is bound up with the 

 principle of animal exclusion. We see that the animals fall into the same groups in 

 relation to the two phenomena, suggesting that the former is likely to be concerned 

 with the latter, in that the correlation or non-correlation of the distribution of the zoo- 

 plankton with that of the phytoplankton (indicated by phosphates) is likely to be 

 brought about through the presence or absence of vertical migration. The process of 

 vertical migration then appears likely to be the means by which these passively drifting 

 organisms are distributed in relation to phytoplankton. For this to be so it is clear 

 that the extent of the vertical migration must vary according to the concentrations of 

 the phytoplankton, otherwise no such horizontal distribution in relation to the phyto- 

 plankton would be possible. This will be further discussed in the next section. 



Table LX 



Correlations of the more important species of Copepoda 1 on the "G" line of the South 



Georgia survey, January 1927, with phosphate values and phytoplankton expressed 



in millions of cells 



1 The Copepoda are those taken in the N 70 V nets expressed as number per 50 m. haul for the top 

 250 m. 



Numbers of organisms shown in italics are those which are less than half the average of the values for all 

 the stations on the line, except for Oithona frigida where they are less than approximately three-quarters of 

 the average, there being none less than a half. 



