PELAGIC PLANT LIFE 339 



in regular rotation with water-masses from the marine areas 

 to which these plankton-types belong. 



Subsequent investigations have shown that Cleve's view, 

 which he endeavoured to apply even more widely, was pre- 

 conceived. His eagerness to discover how far the distribution 

 of particular species depended on sea currents, made him apt to 

 forget that algae are living organisms which are incessantly in 

 process of formation. Accordingly, when the conditions of 

 existence in the flowing water-masses gradually alter, it is the 

 new conditions of existence that decide the character of the 

 flora, since the species best qualified to endure them will very 

 soon get the upper hand over the others. When, therefore, in 

 a sea like the Skagerrack we find northern and southern forms 

 alternating during the course of the year, we are not compelled 

 to assume that the flora is being periodically recruited from 

 different areas. The periodic alterations in the conditions of 

 existence, and particularly in temperature and sunlight, which 

 in our latitudes follow the course of the seasons, sufficiently 

 explain why at one time northerly species predominate and 

 thrive in low temperatures, and why southern forms succeed 

 them and benefit by the warmth which they find necessary for 

 their proper development. Of course it is absolutely essential 

 that germs should be present ready to develop whenever the 

 conditions of existence become favourable. A certain proportion 

 of these, no doubt, may be introduced by currents from else- 

 where, but there is nothing to prevent them from remaining in 

 a particular area, even though the water-masses are in constant 

 motion. Recent hydrographical researches have shown us that 

 eddies are far more common than was at one time believed. 

 Even in areas where huge masses of water are constantly 

 streaming in one direction, which one might naturally suppose 

 would carry away with them all germs belonging to a local flora, 

 these eddies act as a retaining factor, preventing any complete 

 replacement till germs sufficient to maintain the local flora have 

 been transferred to the supplanting water-masses. In coastal 

 seas, moreover, many of the species are able to evolve resting 

 bottom-stages, which lie waiting to reproduce the local flora, as 

 soon as the conditions of existence are congenial. 



Still Cleve's investigations have been of great value, and 

 his plankton-types provide us with a biological division of 

 species which is yet in the main quite serviceable. All that 

 we have to say by way of qualification is that Cleve looked 

 upon his types as representing communities of species limited 



