60 INITIAL CAUSES. 



Bare areas with wet soils or water. — ^As indicated under topographic causes, 

 draining and flooding may bring two different areas to the same condition for 

 invasion. The habitats produced by both are similar in having a wet soil, 

 capable of colonization only by hydrophytes or marsh-plants, except in cases 

 where drainage is reinforced by rapid or excessive evaporation. This is true 

 of the canals and ditches, as well as of the areas actually drained or flooded, and 

 equally so of all canals and ditches, regardless of their purpose. Again, it is 

 unimportant whether flooding, for example, is brought about by the diversion 

 of a stream of water or by the construction of a dam. It is equally immaterial 

 whether the dam is built by man or by beavers. The essential fact is that the 

 water-content will be excessive and that the pioneer stages will consist of hydro- 

 phytes in all these cases. The effect of drainage, i. e., relative lowering of the 

 water-level, can be produced by filling, just as flooding can be caused by the 

 formation of a depression due to the removal of soil. An exceptional instance 

 of the former is furnished by the cases of coral reefs and islands (plate 10 b). 



PRIMARY AND SECONDARY AREAS. 



Distinction. — The whole course of succession rests upon the nature of the 

 bare area which initiates it. We have already seen that the essential nature 

 of a bare area is expressed in the amount and kind of water. Hence, in 

 attempting to group naturally all the foregoing areas, i. e., from the stand- 

 point of succession, it is necessary to recognize that water areas and rock areas 

 constitute the two primary groups. While these are opposed in water-content 

 and density, they agree in presenting extreme conditions in which develop- 

 ment is necessarily slow and of long duration. The denudation of either area 

 in the course of succession results in the sudden reappearance of earlier con- 

 ditions, which cause the repetition of certain stages. If denudation consists 

 of the destruction of the vegetation alone, the soil factors are changed relatively 

 little. The sere thus initiated is relatively short, consisting of fewer stages 

 and reaching the climax in a short time. If the soil is much distmrbed, how- 

 ever, the conditions produced approach much nearer the original extreme, and 

 the resulting sere is correspondingly longer and more complex. The degree 

 of disturbance may be so great as to bring back the original extreme conditions, 

 in which case the normal course of development is repeated. This amounts 

 to the production of a new area, both with respect to the extreme condition 

 and the lack of germules. Hence, all bare areas fall into a second basic 

 grouping into primary and secondary areas. Primary bare areas present 

 extreme conditions as to water-content, possess no viable germules of other 

 than pioneer species, require long-continued reaction before they are ready 

 for climax stages, and hence give rise to long and complex seres. Secondary 

 bare areas present less extreme conditions, normally possess viable germules 

 of more than one stage, often in large number, retain more or less of the pre- 

 ceding reactions, and consequently give rise to relatively short and simple 

 seres. From the standpoint of succession, secondary areas are related to 

 primary ones. In consequence, the most natural classification of all bare 

 areas seems to be into primary and secondary, with a subdivision into water, 

 rock, and soil (plate 11, a, b). 



Sterility of primary and secondary areas. — ^As stated above, primary areas, 

 such as lakes, rocks, lava-flows, dunes, etc., contain no germules at the outset, 



