150 THE DISTRIBUTION OF THE PLANKTON [PART II 



plankton of the sea above them by the addition of swarms 

 of larvae emitted at their spawning periods. I have seen prac- 

 tically pure collections of larval creatures taken in the tow-net 

 (spatangid plutei, cirripede nauplii and cypres, crab zoeae and 

 megalopae and so on) in inshore waters just as the net happened 

 to encounter a swarm of these larvae resulting from the simul- 

 taneous hatching of great numbers of eggs just previous to the 

 time of collection. All these causes must lead to irregularity 

 in the distribution of the inshore plankton : currents by con- 

 centrating larval swarms; temperature and salinity changes j^er se — 

 how exactly we do not know ; the pollution of the sea by rivers 

 and sewers adding food-stuffs; and the rich benthic fauna by 

 adding spawn and larvae. Out at sea the continual movement 

 of the water smooths down these irregularities of distribution, but 

 in inshore waters we expect to find, and do find, a plankton which 

 is often highly variable from place to place, and from time to time. 

 Thus the physical conditions in a sea area within twenty 

 miles of the land are very different from those without this limit. 

 In the latter case the conditions are much more uniform and so we 

 have a more uniform plankton. Still further away from the land, 

 in an oceanic area we find still greater uniformity ^ The winds 

 are more constant; and as they are unaffected by immediate proxi- 

 mity of land we find that the air temperature is more constant. 

 The sea there receives no contribution from the land that can be 

 recognised. Tidal streams are absent or are slight. Therefore 

 because the physical conditions are so uniform we find that the 

 irregularities of the plankton which are so apparent near the land 

 do not exist, and the unequal catches w^hich are made by a vertical 

 plankton net in such a situation are probably due almost entirely 

 to the errors of experiment. 



Plankton types. Nevertheless in a wide oceanic area such as 

 the North Atlantic we do find that there is a gradual change in the 

 nature of the plankton from place to place. This change is due to 

 the existence of physical changes in the sea such as are caused by 

 ocean currents. Even if the Atlantic were a streamless ocean we 

 should still find a gradual change due to the change of climate 



1 See Appendix. 



