188 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
zones probably coincide roughly with isothermal or 
isohaline regions, or with compromises of both factors. 
Indeed, it 1s even now possible to indicate these zones 
roughly. But, because of the irregular movement of the 
sea water, “islands” of plankton must be shifted from 
place to place, just as the temperature changes from place 
to place for the same reason. It will generally be practi- 
cably impossible so to control plankton fishing experi- 
ments as always to fish in water of the same origin. This 
must be the case in coastal waters (within the twenty- 
fathom line). Outside this limit the conditions are 
doubtless more uniform. 
It does not follow that there are no vertical movye- 
ments of the plankton because no well-marked thermal 
stratification of the sea is to be observed. Vertical 
planktonic migrations are probably set up both by 
changes in the intensity of light, and by convection 
currents, and the slight vertical changes of sea tempera- 
ture are caused by the latter. Such irregularity of the 
plankton can be eliminated by making all hauls strictly 
vertical ones, from bottom to surface. But, so to control 
plankton fishing that the results of hauls may be quantita- 
tively compared with each other, appears to me to be 
practically impossible, because of the complexity of the 
conditions characterising the coastal regions of the Irish 
Sea. It is, indeed, possible that there may be an 
approximate uniformity of the plankton in the central 
area South from the Calf of Man, and West from meridian 
4° W., or in the fairway of St. George’s Channel. But 
within the twenty-fathom line, the hydrographic con- 
ditions are so complex that the results of plankton 
experiments must be exceedingly difficult to interpret. 
