No. 571] TERRESTRIAL ASSOCIATIONS 



42«J 



The motility of the animal allows change in stratum, 

 and to some extent and in some animals, in habitat, with 

 change in activity. The food-stratum and the food- 

 habitat are apparently of greatest importance in the rela- 

 tion of the animal to other organisms. 



B. Distribution in Time 



Physiological activities of the plants are subject to 

 diurnal variation, and are also greatly affected by varia- 

 tions in weather conditions. The greater part of the 

 animals of an association are active during the day. 

 Others are nocturnal. During the inactive period of the 

 day the animal rests in some more or less sheltered place, 

 perhaps in a burrow or nest. The inactive state is also 

 induced by unfavorable weather conditions. 



Seasonal changes in the association are very great in 

 temperate climates, particularly in treeless regions, 

 where the winters are severe. Seasonal changes in 

 the vegetation are marked, certain groups of the 

 plants appearing in successive periods during a sum- 

 mer season, giving four or five successive aspects to the 

 plant cover. A corresponding seasonal distribution is 

 observed among the animals of the association (cf. D: 

 175). 



Annual changes in the associations are indicated by 

 the very marked differences in the numbers of indi- 

 viduals, in certain species of plants and animals, in suc- 

 cessive years. This may be due (1) to fluctuation in the 

 numerical adjustment between different organisms, and 

 (2) to the effect of annually varying phenological condi- 

 tions upon the various organisms. 



Oscillatory irregularities in the association take place 

 at indefinite intervals. The causes and nature of oscilla- 

 tions have been thoroughly treated in several of Forbes 's 

 writings (1880, 1883, 1887). 



V. INTERDEPENDENCE OF TERRESTRIAL PLANT AND ANIMAL 



The thesis of the following section is that, in terrestrial 

 climatic or extensive environments, the relations between 



