342 DEPTHS OF THE OCEAN 



to an abundant plant-life exist in the Northern Atlantic, and 

 the somewhat exacting neritic species benefit accordingly. 

 This explanation, at any rate, seems to me the most reason- 

 able one. 



Another well-known instance is in the Polar Seas during 

 the summer. Close to the melting polar ice, where it meets 

 the warmer water- masses, a rich flora of neritic diatoms 

 sometimes develops, while littoral species form a brown layer 

 over the floes and broken lumps floating between them. 

 Blessing, who took part in Nansen's expedition during 1893- 

 1896, has given a good description of this latter phenomenon. 

 We must look upon the Polar Seas as coastal waters in 

 the biological sense. They have the extreme variations of 

 temperature and salinity, and probably also the abundant 

 supply of nourishment, that we would expect to find in a 

 coastal sea. The resting-spores are enclosed in the ice, as 

 I was able to show after examining the material collected 

 by Nansen. 



In the warmer parts of the Atlantic there are neritic 

 diatoms nearly everywhere, but never in any great quantity, 

 except where rivers enter the sea in the tropical regions. As 

 a rule, too, they are smaller and weaker in structure than 

 those we meet with in coastal waters under similar conditions 

 of temperature. The cell -walls are very often only slightly 

 silicated, and the form itself is so indistinct that it is difficult 

 to distinguish species, which in their properly developed 

 condition have unmistakable characters. It is not easy to 

 tell whether this degeneration is merely a sign of insufficient 

 nourishment, or whether other causes are also responsible. 

 Certainly in one case want of nourishment is not entirely to 

 blame. Out in the water-masses of the Atlantic to the south 

 of Iceland we get a community of neritic diatoms that occur 

 especially in the spring and autumn. Most of them are species 

 of Chcetoce7'as. The prevailing forms have been long ago 

 determined, and are undoubtedly C. schilttii and C. /aciniosum. 

 Still they are so dwarfed in structure, and so much the reverse 

 of typical, that one might very well say that they were separate 

 species (see Fig. 244). During this last expedition of ours we 

 succeeded in finding this diatom-flora again, though in smaller 

 quantities, in the Gulf Stream off the east coast of North 

 America, so that it is practically certain that the neritic diatoms 

 of the Atlantic south of Iceland are derived from the American 

 coastal sea. As they are borne passively northwards towards 



