35° DISCOVERY REPORTS 



half the 24 hours they are in the layer W) ; now at a 3 , b 3 and c 3 in the region of greater 

 phytoplankton concentration they will be 2 units of distance apart, so that for every 

 unit of surface area in the region BC there will be half the number of organisms that 

 there are for every unit of surface area in AB. Again in the region CD the organisms will 

 be 3 units of distance apart, and there will be only one-third as many organisms per 

 surface area as compared with region AB. The greater the phytoplankton concentration, 

 assuming the hypothesis is correct, the fewer will be the zooplankton organisms per unit 

 of surface area. This is what we have found in our survey. The organisms, it will be 

 observed, are travelling faster through the regions of dense phytoplankton than through 

 the regions of low phytoplankton. The greater the difference in speed between the 

 water at the bottom of the organisms' migration and the water of the surface the greater 

 will be the reduction in the numbers of such organisms per unit of surface area in 

 regions of dense phytoplankton. Less commonly, perhaps under the influence of 

 contrary winds, the surface layer will be travelling more slowly than the lower layers in 

 relation to a fixed point on the sea bottom ; no matter, the effect will be just the same in 

 relation to the other layers, but in the opposite direction. In regions of still greater 

 phytoplankton than those considered or met with in our surveys, it might be possible 

 that organisms might be prevented from coming to the surface even at night, so that 

 they would travel still faster away from the region. 



Again, according to the hypothesis, 1 if we encounter a reduction in the phytoplankton 

 at the surface the reverse of the process just outlined would take place ; the organisms 

 would become nearer together again, so that there would again be more per surface area 

 than under regions of dense phytoplankton. 



If the phytoplankton increases gradually and evenly over the whole area under con- 

 sideration, the zooplankton would migrate more deeply, but the number of organisms per 

 unit area would not be reduced , and similarly if the phytoplankton decreases gradually and 

 evenly and the organisms migrate less deeply their numbers per unit area would not be 

 increased ; the reduction or increase of zooplankton organisms per unit of surface area 

 would, according to the hypothesis, be dependent upon the uneven concentration of 

 the phytoplankton in relation to movement of water layers. 



Now let us consider what would happen if we pass from a region of moderate phyto- 

 plankton to a region of very poor phytoplankton, so poor that the zooplankton organisms 

 we are considering may remain in the surface layers. For simplicity let us consider in 

 Fig. 182 only two water masses IFand X. Let us observe again at 5 in the surface layer 

 W, which we will consider as stationary and the layer X as moving. Let the phyto- 

 plankton in the region AB cause the organism a to migrate between the layers of W 

 and X, so that at the end of the fourth day it will reach a t , having followed the path 

 a,a 1 ,a 2 ,a 3 ,a i ; now let us suppose that at B it comes into the region BC, where the phyto- 

 plankton is so low that it remains in the surface layer W, migrating between the points 

 S and S'. Now in Fig. 183 let us consider four organisms a, b, c and d occupying re- 



1 Whilst the phytoplankton hypothesis is referred to throughout this argument for simplicity, the 

 qualifications set out on p. 347 must be kept in mind. 



