522 AisrisruAL report Smithsonian iisrsTiTUTioisr, 1917. 



of stress is not a permanent one, because the pressure tends to cause 

 such change as will equalize or relieve this condition. This is con- 

 sidered as the process of adjustment to strain, and is called Ban- 

 croft's law (1911). An animal in an unfavorable condition is 

 stimulated, its normal activities are interfered with, and a physio- 

 logical 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 over- 

 l^opulated and consequently there may be established n condition 

 of stress, which results in an adjustment by a reduction (through 

 many causes) in the excess of population and a restoration of the nor- 

 mal, or a condition of relative equilibrium. From these examples 

 it may be seen that the dynaTnw status means the condition of a 

 unit or system with regard to its degree of relative eqidUhrmni: 

 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 coTulition of adjustment to strain, or that of rela- 

 tive 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 

 conceptions 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 special effort to master the principles 

 involved.^ 



5. ANIMAL RESPONSES. 



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- 

 tioned bj^ certain metabolic processes, the rate of which are further 

 conditioned by the presence of certain substances, as enzymes (from 

 liver, etc.), and internal secretion (from thyroid, testes, adrenals, 

 etc.). The influence of certain physiological conditions or processes 

 is thus well known to affect the behavior of animals. The changes 

 of instinct through the removal of the testes or ovaries, may be cited 

 as examples of this influence. An animal whose metabolic processes 



' See Adams's Migration, etc. (loc. cit.), for an ampliflcation of dynamic principles, a 

 discussion of the relation of Bancroft's law to the phase rule, and tho biological signifi- 

 cance of these ideas. 



