No. 571] 



TERRESTRIAL ASSOCIATION 



417 



direct plant influence are infrequent. Certain plants 

 which become more abundant as a result of close grazing 

 are equipped with spines, or have acrid or pungent 

 juices; and many other characters may be correlated 

 with animal influence. The structural modifications are 

 most evident in the adjustment of the plant to external 

 conditions, though these are accompanied by physiolog- 

 ical characters which are also in harmony with the en- 

 vironment. 



C. Constitution of the Animal in Relation to 

 Envibonment 



The animal, like the plant, selects an environmental 

 complex which is of three kinds: (1) physical, (2) plant, 

 (3) animal. Different animals show extreme variation 

 as to the degree in which the different parts of the en- 

 vironment are important to their existence. Endopara- 

 sites, for example, are most directly concerned with the 

 animal part of their environmental complex. 



The existence of any animal is dependent upon a num- 

 ber of physical factors, all of which must be present in 

 proper degree or quantity. Minimal and maximal quan- 

 tities of any one of several factors mark the limits of 

 existence of any animal {A: 598 — law of toleration of 

 physical factors). It is not necessary to consider these 

 factors in detail. The animal reacts to physical environ- 

 ment most evidently by its behavior : psychological char- 

 acters restrict activities more narrowly than do those of 

 other types. They are accompanied by structural and 

 physiological characters; hibernation, storage of food, 

 etc., are biographical characters correlated with seasonal 

 changes in physical environment. Animals which are 

 subjected to very severe physical conditions may produce 

 a larger number of offspring than those to which physical 

 conditions are favorable.^ This is an example of corre- 



ment. 



The plant environment reacts upon and modifies phys- 

 ical and animal environments, and has also direct influ- 



