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, flatworms, 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 
