74 



THE AZIEBICAN NATURALIST [V{.l. LIII 



migration in relation to evolution because it suggests the 

 critical period at which the stress of the new environ- 

 ment may have its greatest direct influence upon the new 

 generation and thus influence its heredity. 



The next important category above the animal system 

 is the laiv of interacting systems. The main models of 

 interacting systems are : 



1. The physical model of interacting forces, recalling in 

 this connection the law of inertia, the tendency of a body 

 to continue in its present state at rest, or in motion, and 

 the law of reinforcement or acceleration. 



2. Bancroft's law is that a system tends to change in 

 such manner as to minimize external disturbance. This 

 should be applied to the interaction of all systems. This 

 is a law concerned with responses to stress and to the 

 process of adjustment, and it shows development or evo- 

 lution of equilibria. 



3. The phase rule applies to the result of responding 

 to stress or equilibria. This is thus complementary to 

 Bancroft's law; one is concerned with the condition of 

 stress, and the other with the condition of equilibria. 



These laws appear to be universal and not limited 

 solely to the non-living. Irritability may not be causally 

 explained, but it seems to obey these general laws in the 

 same manner as causal changes. Applying these laws to 

 animal migration, we see that the present geological age 

 is one of physical stress, and that the process of adjust- 

 ment to strain is now in operation. The physical stress 

 applies to the air, water, earth and to their interactions. 

 'This is an age of physical diversity— tending toward one 

 of simplicity and uniformity. With diversity there are 

 many local cycles of activity in all features of the en- 

 vironment. These cycles of circulation influence the 

 transportation of animals, and their active migrations. 

 By transportation and migration animals encounter new 

 conditions, new stresses, and change to minimize the dis- 

 turbance and acclimate themselves to the limit of their 



