CH. VI l] THE DISTRIBUTION OF THE PLANKTOX 153 



Remember that there are two main sources of water in the North- 

 western Ocean: (1) water derived from the two great Equatorial 

 streams, and (2) water of Arctic origin. Then we have (3) mixed 

 water at the places where the offshoots from these two streams 

 meet ; and (4) near the land we find littoral water. Now it is 

 easy to see from the chart that the area covered with Styli- and 

 Chaeto-plankton corresponds roughly with the area covered with 

 the Gulf Stream drift in the summer, that is at a time when the 

 extension of this water towards the north-west coasts of Europe 

 is at its minimum. Now Styli- and Chaeto-plankton are the 

 types of the plankton of the sub-tropical North Atlantic and a 

 detailed consideration of the species of the latter will shew that 

 there are many which are common to it and the plankton of 

 the Gulf of Guinea, and even of the Indian Ocean. These are 

 carried far from the regions in which their distribution is normal 

 and thus the occurrence of sub-tropical forms in the cold water of 

 the Faeroe channel — a peculiarity of distribution which is so 

 puzzling to many zoologists — receives a very simple explanation. 



Then note that the water off the coasts of Greenland and 

 Iceland, and in Davis Straits, contains Sira- and Tricho-plankton. 

 This is the type of plankton which has been described by several 

 expeditions as occurring abundantly in the Arctic seas. When we 

 remember that the southerly flowing currents of Arctic water 

 pass to the east and west of Iceland, and down from Davis Straits, 

 and that a well-known stream of cold water — the Labrador current — 

 flows to the south along the coasts of North America, then it is 

 easy to see that the distribution of these plankton types is just 

 that of the southerly flowing Arctic water which enters the North 

 Atlantic. To the south of Greenland the two plankton types mix 

 and this mixed pelagic life is that which is characteristic of the 

 Labrador stream as it flows along the shores of Newfoundland. 



Two other plankton types are shewn on the chart. The Desmo- 

 plankton contains as its principal form the alga Trichodesmium, 

 a plankton form which we recognise as being characteristic of 

 tropical waters. It is found in the Atlantic in the Antilles current^ 

 and also in the Brazil current — both offshoots from the South 

 Equatorial stream. Tripos-plankton is the oceanic type which is 

 characteristic of the north-eastern part of the Atlantic and we 



