=324.= The investigation of succession has so far been neither sufficiently thorough nor systematic to permit the postulation of definite laws. Enough has been done, however, to warrant the formulation of a number of rules, which apply to the successions studied, and afford a convenient method for the critical investigation of all successions upon the basis of initial causes, and reactions. Warming has already brought together a few such rules, and an attempt is here made to reduce the phenomena of succession, including its causes and effects, to a tentative system. At present it is difficult to make a thoroughly satisfactory classification of such rules, and they are here arranged in general conformity with the procedure in succession. I. Causation. The initial cause of a succession is the formation or appearance of a new habitat, or the efficient change of an existing one. II. Reaction. Each stage reacts upon the habitat in such a way as to produce physical conditions more or less unfavorable to its permanence, but advantageous to the invaders of the next stage. III. Proximity and mobility. (1) The pioneers of a succession are those species nearest at hand that are the most mobile. (2) The number of migrants from any formation into a habitat varies inversely as the square of the distance. (3) The pioneer species are regularly derived from different formations, as the latter nearly always contain permobile species capable of effective ecesis. (4) The plants of the initial stages are normally algae and fungi, with minute spores, composites, and grasses, which possess permobile fruits, or ruderal plants, on account of their great seed production. IV. Ecesis. (1) All the migrants into a new, denuded, or greatly modified habitat are sorted by ecesis into three groups: (1) those that are unable to germinate or grow, and soon die; (2) those that grow normally under the conditions present; (3) those that pass through one or more of the earlier stages in a dormant state to appear at a later stage of the succession. (2) Wherever ruderal vegetation is present, it contributes a large number of the pioneer species of each succession, on account of the thorough ecesis. In other regions this part is played by subruderal native species. (3) Annuals and biennials are characteristic of the early stages of secondary successions, on account of their great seed production and ready ecesis. (4) In layered formations, heliophytes appear before sciophytes; they ultimately yield to the latter, except where they are able to maintain a position in the primary layer.