CH. VIl] THE DISTRIBUTION OF THE PLANKTON 143 



The Kiel school of planktologists have always maintained that 

 wherever in a sea area the physical conditions (temperature, 

 salinity, &c.) are uniform there the plankton is also uniform in 

 its distribution. Plankton organisms, they say, have little powers 

 of locomotion, certainly not such as will enable them to segregate 

 themselves, and they are drifted about in the sea quite passively. 

 The sea is in continual motion. The tidal streams, even at some 

 distance from the land, have a considerable velocity and affect the 

 water down to a fair depth. For instance in the Irish Sea the 

 tide in the middle of the Channel will cause a floating body to be 

 carried about 6 miles each way by the ebb and flood, and about 

 9 miles each way during the springs; while nearer the land the 

 velocity of the tidal streams is greater still. Violent winds pro- 

 duce a considerable drift of the surface water, so much so that the 

 effect of the tidal streams may completely be masked. The wave 

 motion is felt down to a depth of from 30 to 40 fathoms. Storms 

 and variable winds will thus mix up the water very thoroughly. 

 Also sudden changes in the temperature of the air caused by 

 changes of wind will set up convection currents which further mix 

 the water. Because of this continual circulation of the water in 

 a sea area the physical conditions tend to become very uniform 

 unless there are strong currents entering it from outside. Large 

 areas of the Clyde sea-estuary are thus practically homothermic^, 

 and I hav^e seen that over wide areas of the Irish Sea the temper- 

 ature may change very slowly and uniformly. If then the physical 

 condition of the sea can be so uniform over wide areas it is not 

 unreasonable to assume also that the plankton would be as uniform 

 in its distribution, that is to say from place to place in a horizontal 

 direction. 



Experimental errors. It is not, of course, absolutely uniform : 

 that is it is not possible to employ a quantitative plankton net 

 twice in succession so as to obtain precisely similar catches in the 

 same area at the same time. Hensen^, on the first day of the 

 Plankton-Expedition, made experiments with the object of testing 

 the uniformity of the plankton. Two quantitative nets of exactly 



^ Physical conditions of the Clyde sea area; Fauna, Flora and Geology of the 

 Clyde Area, British Association, Glasgow Meeting, 1901. 

 2 Ergehn. PlanJcton-Exped. Bd. i. 



