156 THE DISTRIBUTION OF THE PLANKTON [PART II 



As the oceanic currents approach the land their characteristic 

 13lankton changes, not only because the water of the currents 

 becomes mixed with water derived from the land, and many organ- 

 isms which are susceptible to changes in the salinity of the water 

 in which they live die out; but also because a new plankton — 

 a neritic type — which is more adapted for life in shallow water 

 already exists there. In the struggle for existence, that is for 

 the always scanty food-stuffs which are dissolved in the sea, this 

 neritic plankton, which is better adapted for the conditions of life 

 in shallow water, predominates, to the partial or entire exclusion of 

 the oceanic forms. 



Thus owing to the mixture of the different currents with each 

 other and with the coastal waters, all kinds of mixtures of the 

 plankton may be produced, so that sub-types can often be recog- 

 nised. Seasonal changes in the life of the great currents may be 

 observed, and there are also seasonal changes in the composition of 

 the neritic plankton. Add to these causes, which produce mixed 

 plankton types, those periodic seasonal changes in the direction 

 and time of maximum flow of the currents, and also the unperiodic 

 changes to which I have already referred, in the case of the flow 

 of the Atlantic stream into North European seas, and we will see 

 that changes of the greatest complexity may take place in the 

 composition of the plankton inhabiting any one sea area. 



An endemic plankton which neither receives immigrant forms 

 from without the area in which it is found, nor sends emigrant 

 species to the outside of its own area, is seldom found. Only 

 in the '* Halistatic," or streamless sea areas, do we find such a 

 plankton. Such halistatic sea areas are found wherever, as in the 

 case of the Sargasso Sea, we have a true circulation round a centre, 

 so that in the middle of this swirl of water there is a part which 

 does not move ; or in a lake or an almost land-locked sea ; or 

 in such places, as in the Irish Sea between the Isle of Man and 

 Ireland, where there is no definite current, and towards which the 

 tides set from opposite directions so that they interfere with each 

 other and so produce a streamless region. In such areas the 

 physical conditions change only with the seasons, and the plankton 

 changes are also so determined. 



