ANIMALS AND THEIR ENVIRONMENTS ADAMS. 525 



mals, especially the relatively sedentary kinds, and with their environ- 

 ment of plants and the inorganic factors. The relatively sedentary 

 animals are correspondingly bombarded by all elements of their en- 

 vironment. The association, as a whole, is thus in a continuous proc- 

 ess of bombardment and response from every possible angle, and just 

 as the individual animal is stimulated and responds, so all the mem- 

 bers of any association are stimulated and respond in a similar man- 

 ner. It is by this form of activity that animals not only maintain 

 themselves but exert a radiating influence. 



It will assist in realizing the constant pressure exerted by animals 

 if we compare their activity to the flow of a stream. The pressure ex- 

 erted by the stream may be realized if by a dam or similar means the 

 current is resisted. Think for a moment of the amount of energy 

 which would be transformed in an effort to prevent animals (or 

 plants) from taking possession of a favorable habitat. Imagine an 

 area 10 feet square and think of the effort it would require to prevent 

 animals permanently from invading and establishing themselves in 

 this habitat if no barriers were interposed, and if the means of de- 

 struction of the invaders were not so drastic that they materially 

 changed the character of the habitat. Increase the size of the area and 

 the difficulties will increase in geometrical ratio, and the utter futility 

 of such an undertaking will soon be realized. The spreading processes 

 of the gypsy moth in Massachusetts, and of the San Jose scale and the 

 cotton 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. 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 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 land snails, 

 diplopods, and certain insects, can not maintain themselves upon a 

 mineral soil, rock, or clay. As such a forest area becomes reforested 



