14 



boll weevil, show us in terms of human experience something of the 

 energy expended by these radiating animal activities even when there 

 are strong human economic inducements against such invasions. 



When a balanced condition, 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 readjustment begins. This is a phase of the 

 balancing of a complex 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 taking 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. Disturbances 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 animals may be destroyed by a fire so severe that all the litter 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 process of 

 humus formation reaches a certain degree of cumulative development. 

 Under such circumstances this later stage must be preceded by ante- 

 cedent processes, and restoration of the balance is long delayed. Some 

 adjustments take place so quickly that little can be learned of the 

 stages through which they pass. There are, however, many slow proc- 

 esses which afford an abundance of time for study; in fact some are 

 too slow to study during a lifetime. The processes which are moder- 

 ately slow are often particularly illuminating because all stages are 

 frequently so well preserved that comparison is a very useful method 

 of study ; the slowness of a process has a certain resolving power, as 

 it were, recalling the influence of a prism upon a beam of white light, 

 which reveals many characteristics obscure to direct vision. A study 

 of the processes of adjustment among animals is a study of an im- 

 portant phase of the problem of maintenance. The continued process 

 of response will, if circumstances permit, lead to a condition of rela- 

 tive adjustment, or to a balancing among all the factors in operation. 



7. ECOLOGICAL, UNITS FOR STUDY 



In the study of animal responses many different units are avail- 

 able, and a brief consideration of these will aid in an vmderstanding of 



