CYCLE OF CAUSE AND EFFECT 31 



or coaction between species and between individuals, both plant and 

 animal, well exemplified by plant parasites and saprophytes. The end 

 results of these processes are still other reactions, especially on the 

 soil. Animals too are acted upon directly by the habitat and then 

 react upon it in some degree, but their energy relations are primarily 

 a matter of food supply. In consequence, coaction becomes a response 

 of paramount importance, and plants as middlemen between the 

 supply of solar energy and animals may be regarded as constituting 

 a group of secondary or intermediate causes. Plants likewise exert 

 coactions upon animals, and in the case of lethal parasites these lead 

 to soil reactions through the decomposition of organisms. Hence, the 

 complete cycle of causes, and of effects that become causes in their 

 turn, includes the action of the habitat followed by the responses 

 thereto, which in turn become causes of further change. 



On land, the plants as dominants and subdominants play the major 

 role in reaction; in the large bodies of fresh water and in the sea 

 the situation is more or less reversed. In the intertidal and subtidal 

 belts of the continental shelf, the animal dominants assume the lead, 

 except occasionally where attached algae become the codominants. 

 In the open ocean, the reactions of the phytoplankton and of the ani- 

 mals are not readily separated or evaluated, as for example in their 

 effect on light. In ponds and shallow lakes, plants are usually the 

 chief reactors. It is obvious that the medium water as the seat of 

 the reaction has much to do with its nature and degree. This sub- 

 ject is discussed in considerable detail in the succeeding chapter. 



Adjustment and Adaptation. In the case of plants, the immediate 

 response to the action of the habitat is a quantitative change in one 

 or more functions, which is often followed in time by a more or less 

 evident modification of structure or form. The first phase of this 

 process has been termed adjustment; the second, adaptation (Clements, 

 1907) . Growth is a complex of functions and hence it is to be regarded 

 as adjustment, but when the intensity or duration of a factor is suf- 

 ficient, it results in a change of behavior or structure. In sessile ani- 

 mals, the processes are similar to those of plants (Wood-Jones, 1910). 

 In motile animals, this relatively simple sequence is modified by vir- 

 tue of a more or less effective regulatory mechanism, but it is also 

 possible to trace a connection between habitat and behavior. 



In spite of the fact that the plant is often more closely dependent 

 upon the land habitat than is the animal, both exhibit much the 

 same or equivalent general adjustments. This is true in broad terms 

 not only of function, growth, and behavior, but also of time of ap- 

 pearance (Clements and Long, 1923), numbers, grouping, and so forth. 



