CH. VIl] THE DISTRIBUTION OF THE PLANKTON 155 



species. The neritic plankton-types are so called because they 

 seem to be in some way dependent on the sea bottom in shallow 

 water seas. In the latter, just as in the open ocean, we may have 

 well-marked plankton-tjrpes which have often well-defined limits 

 of distribution. If the reader will refer to Fig. 29, which is a 

 reproduction of Cleve's chart of the distribution of the plankton of 

 the North Sea in January 1897, he will see that this was the case 

 then. The central part of the North Sea was covered with Tripos- 

 plankton. Near the land on both the British and continental sides 

 was a broad edging of Concinnus-plankton ; and between this and 

 the area of Tripos-plankton was a band of Halosphaera-plankton. 

 Then in the Skagerak Arctic plankton seems to have prevailed for 

 that area was covered with the Sira- and Tricho-types. 



We have considered these types as if they were always well- 

 defined and easily recognisable plankton groups, but a little con- 

 sideration will shew that this is not always the case. If the nature 

 of the plankton is intimately dependent on the physical conditions 

 of the water in which it occurs then we should expect to find that 

 wherever there is a mixture of the water coming from two different 

 currents there we should have a mixture of the plankton which 

 normally inhabits those streams. Thus we have mixed plankton- 

 types in which species of widely ditferent sources of origin are to 

 be found. Tben where an oceanic current, such as the European 

 Stream, or the Labrador current, flows through an extensive sea- 

 area, it will encounter very different climatic conditions as the 

 latitude changes, and so it may happen that a gradual modification 

 of the type of plankton, which was originally characteristic of the 

 stream, takes place with the change of latitude. Thus we find that 

 though the water of the European stream from the Azores to the 

 Faeroe Channel, and then to Cape North, is always the same and 

 is unmixed usually with water from any other source yet there is 

 a gradual reduction of temperature, and other meteorological con- 

 ditions change also. Therefore the organisms of the plankton, 

 which were originally sub-tropical ones, find a gradual change of 

 life conditions and so those which are most susceptible to change 

 die out as the stream passes to the extreme north-east. So also 

 with the southerly flowing Arctic stream which passes into warmer 

 latitudes. 



