30 COMMUNITY FUNCTIONS— DYNAMICS OF BIOTIC FORMATION 



ment varies inversely with the factors controlling stability. Where 

 wave action is strong, surface for attachment c^uite overshadows the 

 physical factors of both land and water; where it is reduced to a 

 minimum consistent with the deposition of sand, barnacles attach to 

 small pebbles and sea mussels form mats upon which a community 

 usually characteristic of rock may be essentially complete. However, , 

 the building-up of a muddy bottom, as in estuaries and land-locked 

 bays, gradually withdraws the shore from the tidal belt and initiates 

 a halosere from brackish water to land; a somewhat similar transi- 

 tion to land is exhibited by coral atolls and volcanic islands. Small 

 bays may undergo succession to land directly from salt water through 

 the invasion of halophytes, Salicornia, etc. (McLean, 1935). During 

 the past, epirogenic agencies were active in the production of such 

 habitats, especially on the continental shelf, but also in the midst of 

 continents, as illustrated by the withdrawal of the great Mediterranean 

 of North America during the late Cretaceous. 



Causal Sequence. Inherent in the very name itself is the basic 

 principle of ecology that the habitat is the complex of factors or 

 causes. Ecology is not merely the science of the habitat, but pecu- 

 liarly also of the cause-and-effect relation between this and the biotic 

 community, whether on land or in the sea. Some have assumed that 

 its attention was focused so largely or exclusively on the habitat as 

 to preclude any interest in the life found in it, while others have 

 thought that the study of communities was paramount, with little 

 or no consideration of the habitat. The two views are equally in- 

 complete, and the essence of ecology lies in its giving the fullest 

 possible value to the habitat as cause and the community as effect, 

 the two constituting the basic phases of a unit process. The assump- 

 tion that the habitat is entirely subordinate to the community in 

 value and interest appears to be current in plant sociology, but so far 

 as there is any difference between this and ecology, it resides in the 

 fact that ecology is primarily concerned with causes, but solely by 

 reason of their effects on life. For such researches, the use of quanti- 

 ties is imperative, and hence the cardinal points of ecology, as distinct 

 from its parts, have come to be measurement, experiment, and devel- 

 opment, applied to habitat and biome as inseparable cause and effect. 



In the plant matrix of the land biotic community, the causal 

 sequence is a fairly simple cycle. The action of the habitat as ex- 

 pressed in stimuli gives rise to responses on the part of the plant or 

 community. These in turn operate on the habitat, producing reactions 

 that modify it, and then again in turn its action on i)Iant life follows. 

 Embraced within this primary cycle is a secondary one of interaction 



