42 THE SOCIAL LIFE OF ANIMALS 



belongs. It is interesting that animals whose struc- 

 ture forces them to the sort of compulsory mutual 

 aid that automatically follows such structural con- 

 tinuity have never progressed far either in social 

 achievement or in the evolutionary scale. When 

 higher animals, such as the lower chordates, show 

 this development they are usually regarded as de- 

 generate members of their general stock. These 

 colonial animals are seldom dominant elements in 

 the major communities of which they are a part. 

 One comes to the conclusion that the more nearly 

 voluntary such co-operation is, the greater its ad- 

 vantage in social life. It might on the other hand 

 be pointed out that when an animal has achieved 

 social organization and division of labor low in the 

 evolutionary scale, the resulting colonies are so well 

 adapted to their environment that there is not suffi- 

 cient pressure to cause evolutionary changes. 



A second type of aggregation occurs when animals 

 are forced together willy-nilly by the action of wind 

 or tidal currents or waves over which they have no 

 control, and whose effects they cannot resist. Many 

 of the masses which lend color to wide patches of 

 the ocean surface are brought together by tempo- 

 rary or permanent currents. Often animals so dis- 

 tributed are thrown down more or less by chance 

 on types of bottom on which they can develop, and 



