February. If this migrates directly across the Gulf Stream, "it 

 most thus take .S months to perform the journey, and during that 

 period be exposed to great hydrographical changes. It has, however, 

 nerer as yet been observed on its way, so that there exists nothing 

 whaUover to allow of our assuming the probability of such a 

 migration. 



We, therefore, have come to the conclusion that the Plankton 

 Diatoms cannot be used as indicatory organisms in the comparison 

 of far separated places, as their migratory powers arc not great, 

 and the same kinds of Plankton can. in all probability, be deve- 

 loped at different places under like conditions. 



With the large animals of the Plankton, matters are, however, 

 very different. The Animal Plankton has been very thoroughly 

 gone in to by AfRivu.i.irs. who. especially in his latest great work 

 [98] gives a general account of their relation to the ocean currents. 



Ai'RiviLLirs bases his observations on the Plankton of the 

 Skagerak. which is partly endogenous, and partly allogenus. 



The allogcnus -pecies he separates into 5 groups. 

 1 Forms from the Jutland current. 

 2. Forms from the amalgamated waters in the Northern Ocean. 



3. Forms from the northern parts of the Gult Stream. 



4. Forms from the Arctic current. 



5. Forms from the Baltic water-. 



This classification has been partly founded on the distribution 

 of the species in the Northern Ocean Ac., which however, is to 

 -inn- extent but little known, and partly, and especially on their 

 periodical appearance in the Skagerak. 



Thus, for instance, the forms in the Jutland current attain 

 their maximum in September October; those of the Arctic current 

 in February. 



The Author, however, say-, (1. c. p. 127) "Wenn ferner die 

 Erscheinungszeiten der Fonnen einer jeden dieser funf binnen 

 Jahresfrist auftretenden Klassen unter sich verglichen werden, so 

 p-ht hervor. ilass sie nicht in der Meinung von einander gesonclert 

 sind, dass die cine auftritt, wenn die andere verschwindet oder im 

 Ruckgang begriffen ist, sondern vielmehr grerfen die Perioden in 

 einander fiber so dass glcichzeitig Ms auf vice Katciroricn vertreten 

 -in ki. nnen. So ist 2. B. im Januar der Fall, wo noch einige 

 Formen der Jfltschen Strcimung zu treffen sind, wahrend dass 

 gleichzeitig alle drei Gruppen nordlichen Ursprungs melir oder 

 weniger zahlreiche Vertreter haben." 



In this, in our opinion, lies the kernel of the question. We. 

 consider it as settled that, throughout the year, bodies of oceanic 

 water flow into th<- Ska-.'i-iak. especially along the coast of Jutland 

 where the strong current which pours in has lonj: lieen recognised. 



And the water must convey with it Oceanic organisms, provided 

 they have sufficient powers of resistance to withstand the changes 

 that occur in the surrounding wr.ter. 



But we regai d it as being doubtful, whether, from the solitary 

 species or group, it is possible to indicate with certainty the pale 

 of sea, or the ocean currents from which they originate. 



Two stretches of ocean or land, with not too different a climate, 

 own. as is known, most species in common, whilst but a minority, of 

 most often rare, sensitive forms, are typical of individual spheres. 



And even were it possible with certainty to say that certain 

 species but existed, for instance in the Polar current (such as 

 Ciilniins hi/jH'rlmrnu, Cliotie limacina), one cannot, therefore, either 

 in the North Sea or Skagerak point out certain layers of water 

 as being, or having been direct continuations of the Polar current. 



This current shews its influence by giving the northern por- 

 tion of the Gulf Stream a lower temperature and lesser salinity than 

 eUewhere. \\V saw on PI. 4, that the great development of the 

 Polar current, during the winter and spring of 1897, lowered the 

 maximum temperature on the line Sogne Fjord -Jan May en (12th 

 15th May) to 7 fl .2. whilst in 1896, at the same period it was 9.6. 

 The Polar current is checked to the north of the Shetland Islands, 

 but as all the Arctic organisms can thrive in water of o to 7 

 (see AURIVILLIUS 1. c. p. 87 91), they may he conveyed on in 

 the amalgamated layers to the North Sea and Skagerak. 



As AI'KIVILLIUS, generally rinds his geographical groups inter- 

 mingled, this surely likewise indicates that the waters become 

 mixed, and that it is, thereby, impossible to point out strata of any 

 certain origin. 



The oceanic waters which How into the Skagerak must pass, 

 partly, through the English Channel, partly through the northern 

 entrance to the North Sea, and the Norwegian Channel. The 

 nature of the in-flowing layers are therefore determined by the 

 hydrographical conditions existing in those parts. 



There, as everywhere else, the changes of the seasons affect 

 the ocean water and its organisms, so that the temperature, and 

 the Pelagic Fauna and Flora obtain an annual period. This compli- 

 cation alone can. in our opinion, cause great yearly changes in 

 the Plankton of the Skagerak. To this must be added the fact, 

 that the Gulf Stream during winter, in the northern portion of 

 the North Sea. becomes chilled by the proximity of the Arctic 

 current, and that, possibly, its fauna is supplemented with Arctic- 

 animals. 



The biological conditions can, thus, not affect the results of 

 our bydrographioal studies. The general result of the combined 

 Cations we may sum up in the following manner. 



