10 



I have said that we cannot imagine — or that we can do so oulj^ with 

 difficulty — that the Limfjord-phinktou, as it is different l'rom that of the North 

 Sea, can have come from tlie North Sea in the form in whieh it occurs in 

 the fjord. For we could imagine this to be the case onl}-, b}' snpposing 

 that the plankton in tlie North Sea, every time the investigations were 

 undertakeu, some days before liad been like tlie Limfjord-plankton, but then 

 again had changed before the day of the investigation; this should have taken 

 place all three times. Nothing is known of such sudden changes in the plank- 

 ton of the North Sea; on the coutrary, it has been proportionally very little 

 changed even at the different times of the year (See Ihe table), so that it may 

 be left quite out of consideration that the plankton of the Limfjord on these 

 three excursious might be imagined to hail from the North Sea. But we might 

 imagine tliat some single specimens of all the species that occur in the plank- 

 ton of the fjord, though extremely few in numbor and therefore not observed 

 in these samples, are carried in from the sea, so that we might saj' that the 

 »germs« at auy rate hail from the sea, whence they must come again every 

 year, carried in, consequently, by the curreut. The Limfjord then should 

 have the power to develop some species in great masses and to destroy others; 

 but it should not be able to develop auy diatoin-plankton whatever, if only 

 pure water without any diatom- »germs« poured into it. Although I do not 

 believe in this possibility, I must ackuowledge it as such. The question may 

 be settled perhaps by germiuation experiments with the diatom-germs contained 

 in the bottom-clay of the Limfjord or in the North Sea water. Should it be 

 proved then that the water of the North Sea, whicli appareutly contains but 

 a very meager »styli-plankton«, by being exposed only to soraewhat changed 

 physico-chemical conditions (by getting into the Limfjord) can change its 

 plankton, so that it must be characterized as a »didymus-plankton« (See later 

 on), then it shows that it is not so mueh the direction in which the volume of 

 water moves, which the plankton follows, as the physico-chemical conditions 

 of the volume of water itself, and that the plankton in au ahnost chameleonic 

 way can change its appearance. 



With respect to the difference between the plankton in and outside the 

 Limfjord on our first excursion Mr. Gran says, that the North Sea has a »warm 

 north- Atlantic plankton with neritic North Sea forms in small quantities (Bi(J- 

 (Jidphia mohiliensis) = oceanic 'sfi/li-jilmiJdon' «, while the Limfjord has a »ne- 

 i-itic plankton with Chætoceros dehile vastly predominant, and as subordinate 

 component parts Ch. didymum and Bhisosolenia setigera.«. He says that »this 

 plankton might be cailed tltc Bidymus-planlcton of the Limfjord, if by this 



