CYCLE OF CAUSE AND EFFECT 27 



solutes themselves, some can be used by the animal directly, while 

 others are available only, or usually, in combination. Substratum and 

 bottom are of much significance for great numbers of aquatic animals, 

 and soil is indispensable to most plants and of no little importance to 

 many land animals. 



It is a significant fact that, though the factor complex differs 

 greatly between land and sea, the same essential factors are present 

 in both. The characteristic distinction is one of degree or quantity 

 rather than of intrinsic constitution. The two extremes are repre- 

 sented by a single medium, air in the case of epiphytes and water for 

 all submerged aquatic organisms. In this respect, rooted plants are 

 more or less intermediate, the roots being essentially aquatic, though 

 imbedded in soil. Between the two extremes lie a series of habitats 

 with diminishing air and increasing water, exemplified by mesophytes, 

 amphibious, floating, and submerged hydrophytes. Amphibious ani- 

 mals exhibit an irregular alternation between the two media; for 

 intertidal plants and animals the alternation is regular, as it is like- 

 wise in the life cycle of many insects with aquatic larvae. 



Development and Cycles. On land, as in the sea and in some 

 bodies of fresh water, there are two major types of habitats; one of 

 these corresponds to the climax, the other to the sere. Obviously, 

 in the first, climatic factors are paramount; in the second, edaphic, 

 i.e., local, factors are more controlling. Apart from this major dis- 

 tinction, climates are of wide and often vast extent and relatively few 

 in number; conversely, serai communities are for the most part rela- 

 tively small in size and usually recur in great numbers. Both are 

 essentially dynamic rather than static, but edaphic changes are gener- 

 ally more tangible and progressive than variations of climate. This 

 is due primarily to the reaction of the community, as a consequence 

 of which this and the habitat develop pari passu from initial bare 

 area to a climax. In such a dynamic concept, habitat and sere are 

 regarded as correlated processes or stages of a unified development 

 that terminates in the relatively stable climate and biome (Clements, 

 1916:357). 



The sere represents the cycle of development of the community, 

 which resembles in many respects the life cycle of the species- 

 individual. Its duration varies within wide limits, from the few 

 years required for the shortest of subseres to the hundreds or thou- 

 sands necessary for priseres in lakes or on lava flows. The phase of 

 development is relatively brief, however, by comparison with that of 

 stabilization in the climax, except in those fairly frequent cases where 

 such an agency as repeated fire produces more than one sere on the 



