HYDROGRAPHY 169 



6. Sunlight is all-important in connection with photo- 

 synthesis by Diatoms and other plants in the sea. Its effect 

 is also evident in the heliotropic movements of Copepoda 

 and many other free-swimming animals, and in the vertical 

 rise and fall of plankton. All the energy made use of by- 

 organisms is ultimately derived from the energy of solar 

 radiation. There appears to be some connection between 

 the periodic changes in solar energy indicated by '' sunspots " 

 and variations in the strength of oceanic currents, and these 

 in their turn affect some of the periodic fisheries, such as the 

 great Norwegian cod fisheries at Lofoten. 



6. In addition to these large and obvious factors affecting 

 the distribution of marine organisms, it seems probable that 

 some very slight modifications in the physical condition of 

 sea-water may have a curious effect upon their life and 

 prosperity. Some animals will live healthily in one tank in 

 a biological station and not in another; the proximity of 

 other animals may in some cases be an advantage and in 

 others the reverse ; it is even possible that meteorological 

 conditions may exercise some subtle influence upon animals 

 on the sea bottom through several fathoms of water, as in 

 the following case, which seems well established : — Crabs and 

 lobsters at Port Erin are never caught in quantity during 

 northerly to easterly winds and in cold dry weather, but if 

 the wind goes round to the south-west and it becomes warmer 

 and damper, the crabs " travel," as the fishermen say, and 

 are then caught in the creels in abundance. 



