No. 624] MIGRATION A FACTOR IN EVOLUTION 73 



IV. Summary and Conclusions 

 The animal should be looked upon as a dynamic system 

 which tends to continue in its course of action until 

 changed from within or until diverted by external inter- 

 ference with its system, and until a condition of relative 

 equilibrium is developed by balancing all influences. The 

 behavior of animals should be viewed as a process of 

 rhythmical activity. 



The cycle of activity of the animal agent is a unit of 

 fundamental importance. To study cycles, their dynamic 

 status, their degree of relative equilibrium must be de- 

 termined. In this manner the conditions of stress, the 

 processes of adjustment to strain, and the conditions of 

 relative equilibrium may be recognized and determined. 

 These determinations should be applied to all cycles of 

 activity, that of the life history, and all others. The use 

 of these ideas enables one to apply Bancroft's law— that 

 a system tends to change to minimize external disturb- 

 ance—to animal activities, and thus one is enabled to ex- 

 plain a large number of diverse observations. Supple- 

 mentary to Bancroft's law are the influences which tend 

 to accelerate or reinforce, without other change, the con- 

 dition of the animal. 



The activities of animals cause them to collide with 

 their environment. Conditions under which animals 

 have become accustomed or attuned are those of relative 

 equilibrium. With departure from these conditions, the 

 animals are stimulated, their system is interfered with, 

 and the animal tends to change until the interference is 

 minimized. The hindrance thus placed upon animal 

 activities are its ''limiting factors," and these are to be 

 viewed according to Bancroft's law. This law is not 

 limited to the actions of the individual animal. l)iit in- 

 cludes also the race, and those of aiiiiiial associations. 

 The Vernon-DeVries law of the dlm}}iish-nni mfiurynr of 



