DYNAMICS OF DISTRIBUTION 347 



Propositions concerning variable vertical migration 



In a former section we have seen that a vertically migrating animal may remain 

 higher in the water in the daytime on some occasions than on others; and whilst the 

 evidence is not conclusive, there is some indication that this difference in level in the 

 daytime may be due to differences in the phytoplankton concentrations in the upper 

 layers. So similar is the natural grouping of the different plankton animals in respect on 

 the one hand to their horizontal distribution in relation to the phytoplankton (as revealed 

 by phosphate values), and on the other to their vertical migration, see pp. 312 to 313, 

 that we are led to the conclusion that in some way the two are connected. In this section 

 we will assume the hypothesis that the animals migrate more deeply in the daytime in 

 areas of dense phytoplankton than in weak, and see what would be the effect upon their 

 distribution if they are migrating between layers of water differing in speed. // should 

 be noted that similar effects will be produced if the animals remain down longer in time 

 rather than descending more deeply. Again, whilst we are assuming the phytoplankton 

 hypothesis for the sake of simple description, it must be kept in mind that other factors, 

 chemical and physical, may be producing similar results, so that for all the propositions 

 outlined when phytoplankton is mentioned as a possible cause modifying vertical 

 migration we should interpolate or some other factor bringing about a similar change 

 in the vertical migration. Our series of propositions is based on the assumption that 

 the animals come into the surface layer at night, but descend to different depths during 

 the day. Exactly similar but reverse effects will be produced if the animals descend to a 

 constant level but rise at night to different heights under different conditions; dif- 

 ferences in the density of the water might bring about such differences in the extent of 

 upward movement. 



Let us first consider in Fig. 177 the 

 movement of an animal a migrating be- < x 



tween two water masses X and Y going 2 a'- 



at equal speeds in opposite directions 

 separated by the line ZZ'. Let X be 



moving to the left and Y to the right. 



Let us observe the movement of a from 



a point outside the system, say on the lg- I77 ' 



sea-bottom at P, and let us assume that a makes a migration downwards at 6 o'clock in 



the morning and upwards at 6 o'clock in the evening, and let us begin our observations 



at midnight. During the 24 hours a will take up the points a , a", a'", and come back 



to the point a ; at a' it will be crossing from the water mass X to the water mass Y at 



6 o'clock in the morning, and at a'" it will be returning from Y to X at 6 o'clock in 



the evening. 



Now let us observe the same phenomenon from within the surface layer at S and for 

 simplicity let us regard the layer X as stationary and the layer Fas moving. In Fig. 178, 

 starting to observe again at midnight, a will follow the path a, a', a", a'" to a x in the 

 first 24 hours, at the end of the second day it will be at a 2 , and at the end of the third 

 day it will be at a 3 . 



