PELAGIC PLANT LIFE 349 



large amount of southern neritic species (Didymus-plankton), 

 made up to a great extent of forms of distinctly foreign origin. 

 As the dark months of winter approach, however, their numbers 

 rapidly decline. 



In the open sea, too, our investigations appear to indicate Flora of the 

 that the southern forms reach farthest north in the autumn, say °pensea. 

 about November, while during the months of spring, from 

 April to May, northern forms extend very far south. We have 

 not as yet made investigations at different seasons in the tropical 

 parts of the Atlantic ; consequently we cannot say whether there 

 is an annual cycle of plant-development in a region where the 

 conditions of existence seem to vary so little. It would be an 

 excellent thing if researches of this nature could be undertaken. 



Supposing that the ocean -currents do exercise a direct Ocean- 

 influence upon the character of the plankton in the tropics, it is J^hrpSnkton 

 fair to imagine that it must be in the direction of periodicity. 

 Lohmann has put forward the suggestion that the changes in 

 pelagic animal life near the coasts of South Europe are connected 

 with a cyclic movement of the water-masses. When these 

 reach their northernmost point the conditions of existence will 

 affect the organisms, so that the water-masses that pass through 

 this region in the winter are likely to have a different fauna 

 from that of the water passing through in summer. Elsewhere 

 it is very difficult to tell what changes in the plankton are 

 due to the direct influence of ocean-currents, and what changes 

 are the result of altered conditions of existence partly due 

 to ocean -currents and partly to other causes. It has often 

 been observed, not only by Cleve and Hensen, but also 

 during previous researches made by the " Michael Sars " and 

 during the "Challenger" and " Valdivia " Expeditions, that the 

 plankton changes its character the moment one passes the 

 boundary between two currents. Thus an examination of the 

 plankton may serve as a check on purely hydrographical 

 investigations, which aim at ascertaining the boundaries of 

 currents by means of observations of their temperatures and 

 salinities. Perhaps the best guiding forms are the species of 

 Ceratiwn, and strangely enough it is very often the species 

 that systematically are the nearest related, which replace each 

 other as we pass from one area to another. Many of them 

 are so closely related that it is only for the sake of con- 

 venience that we regard them as distinct species, and there 

 is always the possibility that they may be able to pass directly 

 from one form into the other, even if we cannot actually prove 



