=299.= Generally speaking, all successions on denuded soils are secondary. When vegetation is completely removed by excessive erosion, it is an open question whether the resulting habitat is to be regarded as new or denuded. Erosion is rarely so extreme and so rapid, however, as to produce such a condition, even when it results from cultivation or deforestation. It is, moreover, especially characteristic of newly formed soils, and in studying succession in eroded habitats, it is fundamentally important to determine whether erosion has produced denudation, or has operated upon a new soil. The great majority of secondary successions owe their origin to floods, animals, or the activities of man, and they agree in occurring upon decomposed soils of medium water-content, which contain considerable organic matter, and a large number of dormant migrants. These successions consist of relatively few stages, and are rarely of extreme character. =300. Succession in eroded soils.= Eroded soils show considerable differences, as they arise in consequence of erosion by water or by wind, though the initial stages of revegetation derive their character more from the aggregation of the soil than from the nature of the erosive agent. Eroded soils are as a rule xerophytic. In the case of erosion by water, dysgeogenous soils are readily worn away in consequence of their lack of cohesion, as in sand draws, etc., while eugeogenous soils are easily eroded only on slopes, as in the case of ravines, hillsides, etc. In the former, the extreme porosity and slight capillarity of the sand and gravel result in a low water-content. In the finer soils, the water-content is also low, on account of the excessive run-off, due to compactness of the particles and to the slope. The erosive action of winds upon soils bearing vegetation is not very general; it is found to some extent in more or less established dunes, and exists in a marked degree in buttes, mushroom rocks, and blowouts. The first two are regularly xerophytic, the last as a rule, dissophytic. The early stages of successions in eroded soils are composed of xerophytes. In loose soils, these are forms capable of binding the soil particles together, thus preventing wash, and increasing the accumulation of fine particles, especially of organic matter. In compact soils, the effect is much the same; the pioneers not only decrease erosion, but at the same time also increase the water-content by retarding the movement of the run-off. =301. Succession in flooded soils.= The universal response of vegetation to floods is found in the amphibious plant, which is a plastic form capable of adjustment to very different water-contents. Floods are confined largely to river basins and coasts. In hilly and mountainous regions, where the slope is great, any considerable accumulation of flood waters is now impossible, although of frequent occurrence when land forms were more plastic.