AGREEMENT OF COMMUNITIES 305 



numerous marine animals, such as polyps, sea plumes, etc., which are 

 sessile, like plants. Sessile animals are probably all aquatic. Logically, 

 ecology cannot be divided into plant and animal ecology, but it may be 

 divided into the ecology of sessile and motile organisms. 



An appreciation of the likenesses and differences of sessile and 

 motile organisms is an important thing in ecology. The plant and the 

 animal groups contain both sessile and motile types together with types 

 intermediate between the two and thus taken as a whole plants and 

 animals are in agreement in the matter of response. However, since the 

 vast majority of animals with which we deal are motile, their activities 

 are evident because of their ability to move about. On the other hand 

 the majority of plants are sessile, and sessile individuals usually can 

 change the position of the whole or its parts only by growth. Changes 

 in the relation and character of parts are the results of the application of 

 stimuli to sessile plants. Movement is the chief result of the application 

 of stimuli to animals. Animal ecology has very much in common with 

 plant ecology. Diatoms, fiatworms, and many other marine animals 

 and plants meet the same conditions in the same or similar ways (72, 

 p. 121; 53a, p. 156; 536, p. 155). Sessile animals, such as reef-forming 

 corals, show growth form differences (193, 194, 195) under different 

 conditions, just as sessile plants do. Comparable plants and animals 

 show comparable responses. The physiological life history aspect of 

 plant ecology (52) is parallel with the same phenomenon in animals, 

 but the activities of motile animals correspond roughly to the growth 

 form phenomena in sessile plants (55, p. 593). 



All the way through the study of ecology we look for behavior or 

 activity difference in motile organisms (chiefly animals), when con- 

 sidering the species of two different habitats, while, when making a 

 comparison of the sessile organisms (chiefly plants) of two habitats, 

 we look for differences in form and structure. To be sure an occasional 

 sessile plant can move some of its parts and likewise some motile animals 

 change color, size, or form with differing conditions during development, 

 but these are of secondary rather than primary importance and we must 

 look mainly to form changes as "plant response" and behavior, or activity 

 changes as "animal response." 



2. AGREEMENT OF COMMUNITIES 



Are physical conditions sometimes similar when vegetation and 

 landscape aspect are very different ? That they are is clearly suggested 

 when we compare the forest and the shrub-covered bluff where forest 



