84 LIFE IN THE SEA [CH. 



towards and then away from a fried-fish shop in just 

 the same way. Now there is another manner of 

 description which will appeal with greater force to 

 some minds, and which makes use of effective con- 

 sciousness as an agency of regulation of the move- 

 ments of an organism. An animal in good health has 

 superabundant vitality, and it moves about continually 

 in all directions, it may be in search of food, it may 

 be quite capriciously. It is adapted to live best in 

 certain optimal conditions and if it happens to move 

 into a region where the conditions are unfavourable 

 it will endeavour to avoid this region. Its method is 

 that of 'trial and error.' For instance, plaice of over 

 one and under two years of age have for their optimal 

 conditions those of the sea in shallow water where 

 the temperature is rather high, where there is plenty 

 of light, and where there is plenty of food in the shape 

 of small shell-fish. As the fish moves about it must 

 often enter the region of darker, colder, and deeper 

 water offshore, but having done so both the decrease 

 in the intensity of light, and either the increasing 

 pressure of the water or the increase of salinity will 

 affect it in some of the ways suggested, and then it 

 will turn this way and that, testing the conditions all 

 round it by means of its receptor organs, until finally 

 it reverses its former path and re-enters the former 

 region of optimal conditions. If a rapidly growing 

 bed of young mussels forms anywhere within the area 



