10 



the dynamic status. The law of conservation of energy teaches us 

 that energy can not be destroyed ; that it is transformed only, and thus 

 undergoes a cycle of changes. The animal or an animal community, 

 as a unit and as an agent or transformer, is constantly transforming 

 energy, setting it free. In this sense it originates, but not at a uniform 

 rate. At one time much energy may be transformed, and at another 

 very little. When a great amount of energy is being set free, when 

 the animal or community is exerting much influence, we may look upon 

 it as producing pressure or strain. A condition of stress is not a per- 

 manent one, because the pressure tends to cause such changes as will 

 equalize or relieve this condition. This is considered as the process 

 of adjustment to strain, and is called Bancroft's law ('ii). An ani- 

 mal in an unfavorable condition is stinuilated, its normal activities are 

 interfered with, and a physiological condition of stress is produced 

 which lasts until by repeated responses or "trials" the animal escapes 

 stimulation or succumbs and a relative equilibrium is established. An 

 area may become overpopulated and consequently there may be estab- 

 lished a condition of stress, which results in an adjustment by a reduc- 

 tion (through many causes) in the excess of population and a restora- 

 tion of the normal, or a condition of relative equilibrium. From these 

 examples it may be seen that the dynaiiiic status means the condition 

 of a unit or system with regard to its degree of relative eqnilibriuin. 

 The cycle of change may be considered to begin at any point. I have 

 taken as the initial stage of the cycle the condition of stress or pres- 

 sure, and have indicated how this condition tends to change in re- 

 sponse to pressure, bringing about the process of adjustment to strain, 

 and leading to the condition of adjustment to strain, or that of relative 

 equilibrium. The activity of the agent produces the condition of 

 stress, the process of adjustment to the strain follows, and this leads 

 to the product — the establishment of the condition of adjustment or of 

 relative equilibrium. 



These conceptions are very suggestive when applied to various 

 phases of organic activity, and aid greatly in utilizing the dynamic con- 

 ceptions which are in constant use in many of the physical sciences. 

 But we can not assume that these ideas will take definite form unless 

 the student makes some s])ecial effort to master the principles involved. 



5. ANIMAL, re;sponse;s 



The general character of the changes within the animal, which re- 

 sult in the transformations of energy and substance, or the process of 

 metabolism in its broadest sense, is the basis of all animal responses. 

 It is well known that growth, development, and behavior are condi- 



