Emigration 361 



of competition, interference, or aggression always come into being. 

 However, a moderate amount of crowding may be beneficial to 

 members of a group, and this result may have favored the evolution- 

 ary development of integrated social organization in animals. The 

 fact that an increase in a population progressively curtails its own 

 growth makes possible a mathematical formulation of population de- 

 velopment and provides a basis for deducing the optimal yield that 

 can be obtained by exploiting a population. Numbers of a species 

 always fluctuate to a greater or a lesser extent. Every species has 

 definite space requirements, determined either mechanically or by 

 avoiding reactions related to home ranges, and overpopulation is re- 

 lieved by the destruction of the extra individuals or by emigration. 

 The presence of others of the same species thus forms a critical aspect 

 of the environment, and each organism is involved in many reciprocal 

 relations with individuals of its own kind. 



