COMMUNITY FUNCTIONS 55 



in water often give aspect to marine communities because of a pre- 

 ponderance of one type over another (cf. page 323). Sucii less active 

 groups also show greater growth-form differences than the more active 

 ones. For example, Baker (1928) finds a river form and a lake form 

 of fresh-water mussels, and Humphrey and Macy (1930) report dif- 

 ferences of form and size in tide-pool snails. 



The more active animals present walking and running, flying and 

 gliding, hopping and looping, burrowing and swimming, and creeping 

 and crawling life forms. These are rather uniformly distributed 

 throughout the phyla, as well as in water and on land. The presence 

 of segregated groups of these types often gives some character to 

 certain communities, though the use of life habits appears more use- 

 ful, owing chiefly to the heterogeneity of adaptation characters. This 

 difficulty is well brought out in a series of papers devoted to the 

 adaptations of mammals to arboreal, cursorial, fossorial, and aquatic 

 life (Osburn, Dublin, Shimer, and Lull, 1903). These discussions 

 indicate that the layer adaptation may be effected in a variety of 

 ways, thus placing the emphasis upon activity rather than structure. 

 Again, since the primary adaptation is to layer or level, this also 

 tends to assign a subordinate role to form. Furthermore, some species 

 lack definite adaptations altogether, while more telling is the fact 

 that certain striking modifications have little ecological meaning, and 

 the activities rather than the structural adaptations are of signifi- 

 cance. In the study of coactions, it is evident that activity must be 

 taken into account with structure, and may often outweigh it. 



Forbes (1914) indicated another relation of structure to activity 

 in the food-getting apparatus and digestive tract of fishes. He found 

 that the fresh-water fishes of Illinois begin life by feeding upon 

 Entomostraca ; during development from a very early stage to the 

 adult form some become mud-eaters and others insectivorous or pisciv- 

 orous, while only a few continue to feed upon plankton. In each type, 

 the digestive tract and gill rakes become adapted to the special food 

 habits, the mud-eaters developing very long intestines, 



COMMUNITY FUNCTIONS 



Nature and Significance. The development and structure of the 

 biome are due to activities that may be properly regarded as func- 

 tions of the community. This is likewise true of its subdivisions, 

 both climax and serai, in which the nature of the processes is even 

 more clearly discernible. The group of organisms which constitute 

 the community is acted upon by the habitat, producing social response 



