104 ANIMAL LIFE AND SOCIAL GROWTH 



If another and stronger light is thrown on such a 

 worm at right angles to its former course, it will 

 turn immediately and swim directly towards the 

 new source. There is no trial reaction in this 

 process which seems to be as precise a mechanism 

 as one would expect to find in as complicated an 

 organism as a free-swimming worm. 



Another, and indeed more common method 

 of forming these aggregations, is shown by such 

 animals as land isopods which, if the surroundings 

 are dry, tend to collect in the more moist regions. 

 These are found not by direct orientation and 

 moving in a straight line to a nearby damp spot 

 but by a sort of random movement which can 

 but remind the observer of the human method 

 of trial and error. Eventually this behavior 

 results in the selection of the least stimulating 

 spot which for these land isopods is usually the 

 dampest place found in their wanderings. 



When there is no difference in dampness in 

 different parts of their environment and when it is 

 not so dry that the isopods are so stimulated that 

 they find it impossible to come to rest, then they 

 tend to wander about until one of them stops 

 for some reason unknown to a non-isopod and 

 then in time all the rest collect on or near this 

 quiet animal. After many have collected, some 

 disturbance may cause the whole group to break 



