476 THE AMERICAN NATURALIST [Vol. LII 



expression of them. Limiting factors, because of their 

 intensity and repetition, tend to change the animal sys- 

 tem so as to minimize the external disturbance, or the 

 animal system tends to change in such a manner as to 

 minimize external disturbance, at a speed determined by 

 the intensity and repetition of the disturbance. The so- 

 called ' ' trial and error," or, better, trial method of be- 

 havior, is also an independent formulation of Bancroft 's 

 law. It seems probable that the modifiability of be- 

 havior, and even all methods of animal regulation, are 

 expressions of these lairs of interaction. 



The ''balance of nature" is a culminating phase of the 

 cycle of adjustment to strain. As expressed elsewhere, I 

 have said (Adams, '15, p. 14): "When a balanced con- 

 dition, or relative equilibrium, in nature is referred to, 

 we must not assume that all balances are alike, for some 

 are disturbed with little effort and others are exceedingly 

 difficult to change. This distinction is an important one. 

 Once the balance is disturbed, the process of readjust- 

 ment begins. This is a phase in the balancing of a com- 

 plex of forces. Just what stages this process will pass 

 through will depend, to an important degree, upon the 

 extent of the disturbance. Slight disturbances are tak- 

 ing place all the time and grade imperceptibly into the 

 normal process of maintenance, as when a tree dies in the 

 forest and its neighbors or suppressed trees expand and 

 take possession of the vacancy thus formed. Disturb- 

 ances of a greater degree, on the other hand, may only be 

 adjusted by a long cumulative process. This change can 

 progress no faster than the rate at which its slowest 

 member can advance. Thus a forest association of ani- 

 mals may be destroyed by a fire so severe that all the lit- 

 ter and humus of the forest floor is burned. The animals 

 which live in the moist humic layer as a habitat, such as 

 many land snails, diplopods, and certain insects, can not 

 maintain themselves upon a mineral soil, rock or clay. 

 As such a forest area becomes reforested, these animals 

 can only find the optimum conditions when the slow proc- 



