11 



desiguation \ve mean warm iieritic harvest- plankton«. Also on the other 

 excursions he finds a great difference between the plankton of the Limfjord 

 and that of tlie North Sea. — 



If the )vhole Limfjord had been formed by a series of brackish lakes 

 with stagnant water, I should not have been aslonished to find a different 

 plankton in the different lakes; but when we see a current which generally 

 runs east, or which at any rate much oftener runs east than the opposite 

 way, euter through the western opeuing of the Limfjord and go on through 

 the whole fjord, with so great speed that the narrow parts remind us of slowly 

 running rivers, against whose current sailing vessels can but beat up with 

 difficulty, then we should think, certainly, that every trace of differences in 

 the plankton must disappear. We should expect the same result for all 

 organisms as for Nodiluca and Pilema odopus: that the single specimens are 

 driven through the whole fjord by the current from the North Sea, or are killed 

 by the brackisli water if they enter the more closed coves; but it appears 

 that the diatoms are capahle of forming independent floras in the water while this 

 is moving through the fjord, so that a diatom-flora occurs already in Nissum- 

 Bredning, reaches its maximum in Sallingsund, and dies away in the vicinity 

 of Løgstør. Organisms of so short deration of life as the diatomaceæ are there- 

 fore, evidenthj, onJy to a certain degree fit to folio w the currents of the sea through 

 Jonger jyeviods or through longer distances. The more uniform the natural con- 

 ditions in the course of such a current are, as for iustance in certain great 

 oceauic currents, the more unchanged its diatom-flora will be sure to keep; 

 but any eun'ent of somewhat greater length will, as a rule, be subject to chan- 

 ges, changes of light among others, and the diatomaceæ are highly sensitive 

 to even the shghtest of these changes. On the maps of the north-Atlantic 

 plankton, published by Professor P. T. Cleve in his fine work: »A Treatise on 

 the Phytoplankton 1897«, we see, for instance, that the iilankton-types on the 

 whole follow certain sea-currents, I suppose as long as the chemico-physical 

 conditious of the water are the same, and I am sure that, withiu certain limits, 

 we can here conclude from uniformity in the diatom-plankton to uniformity 

 in the chemico-physical conditions of the water; but simply to conclude from 

 uniformity in the diatom-plankton of two voluraes of water to a yearly repeated 

 current-connection between them seems to me to be unjustified. Just as fresh 

 waters without any connection with one another can have uniform diatom- 

 floras, so it is quite likely also that this eau be the case with the seas. — As 

 some authors seem to have found it difficult to explain the occurrence of cer- 

 tain arctic shore-diatomaceæ in the Baltic Sea, I shall as to this question refer 



