16 



is for speaking of the responses of the individual. The association can 

 continue to exist indefinitely only in such environments as possess, in 

 available form, substance and energy for its individual members. The 

 activities of the individuals transform energy and substance, produc- 

 ing growth, development, multiplication, and behavior. The persist- 

 ence of an association in a given habitat brings about the formation of 

 certain waste products, which if not changed or transformed at a cer- 

 tain rate, or transported from the environment in some way, tend to 

 limit the optimum activity of the individuals and of the association. 

 In the association, as in the individual, there must be an internal rela- 

 tive balance before there can be such a surplus of energy that the asso- 

 ciation can radiate or exert outward stress or pressure. An association 

 which is only maintaining itself is not at an optimum, for in this latter 

 condition there is a surplus of energy, and the activity, rate of multi- 

 plication, and favorable development under normal conditions are fa- 

 vorable to the extension of the association. The pressure which such 

 an association exerts is shown by the progressive extension of its range 

 of influence. By the active movements of the animals, by the activity 

 of the environment, or by both together, they tend to invade other 

 habitats and areas, and in such of these as afford favorable conditions 

 they tend to survive and extend the area of the association. From the 

 standpoint of the association the behavior of these active pioneering 

 animals corresponds to the trial activities in the behavior of the indi- 

 vidual animal. These activities are not different in kind from those 

 which are invohed in normal maintenance. They are those which 

 form the initial stages in the establishment and extension of the asso- 

 ciation in a new locality or the re-establishment in an old one, and thus 

 lead to a sequence or succession of associations. Ecological succession 

 thus consists in an orderly sequence or series of associations which 

 occur successively and form a genetic series. 



9. ASSOCIATIONAI, SUCCESSION 



A succession of associations takes place either through the trans- 

 formation of older ones, or through the origin of a new one on a 

 surface which has been newly formed and has had no population. A 

 favorable habitat without a population of animals is comparable in 

 some respects to a vacuum ; it exists as a condition of unstable equi- 

 librium which tends to change toward a more stable state. The active 

 life of animals tends to lead them into all possible habitats, and where 

 they find the conditions favorable for existence they tend to survive 

 and thus bring about the establishment of an association. Each asso- 

 ciation, like the individual animal, has a certain amount of unity and 



