CONDITIONS OF EXISTENCE 43 



of conditions. The principal environmental rhythms are daily, 

 seasonal, weather, and lunar, and, in the sea, tidal. 



Rhythms of fresh-water organisms have been but little studied. 

 From the seasonal standpoint it has been observed that some organ- 

 isms tend to do certain things even though the external conditions 

 which usually accompany them are delayed, thus showing that the 

 environmental rhythms have been impressed upon the organism. 

 The best examples of this have to do with the tide and thus do not 

 belong to fresh water. Bohn found that there are rhythms of 

 activity related to tide. The green flatworm (Conwluta roscojfensis) 

 comes to the surface of the sand at low tide and descends as the 

 tide comes in. The worm continues to ascend and descend at 

 tide time for several days after having been removed from the sea 

 and kept in an aquarium. 



One of the best known rhythmic movements in fresh water is 

 the daily depth migration of Crustacea. Whether they show any 

 tendency to make such movements when placed under uniform 

 conditions is not known. Lunar rhythms likewise appear to have 

 been little investigated among fresh-water organisms though Kofoid 

 noted rhythmic monthly increases of Illinois River plankton. The 

 best examples of these are found among the marine worms. The 

 Atlantic palolo swarms within three days of the last day of the last 

 quarter of the June 29 to July 28 moon (Mayer), the swarming 

 taking place under the influence of the light of the moon. 



Various single factors have been regarded as of prime importance 

 in the control of organisms. Thus many writers emphasize food, 

 others temperature, etc. Merriam has maintained for years that 

 the total of temperature above an arbitrary minimum during the 

 growing season controls the distribution of life in North America. 

 Sanderson has shown that for some insects and some horticultural 

 plants winter temperatures are more important, just as may be 

 the case with organisms like fresh-water sponges and bryozoans 

 having winter bodies, and aquatic plants with seeds and spores. 

 Marine workers emphasize salinity and density. Birge and Juday 

 emphasize oxygen. All these ideas have important bearings on 

 questions of aquatic biology but no one of them is adequate. 



Dormancy sometimes makes otherwise insignificant conditions 



