ll] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 323 



It is a commonplace that units of vegetation, left to themselves, 

 tend to change in a particular direction— usually from less complex 

 communities of small plants to more complex ones dominated by 

 larger plants of higher life-form (or, at all events, greater competition- 

 impress). The change is continuous, recognizable ' stages ' being 

 mere nodes of vegetational expression. Such is succession, the de- 

 velopmental series of communities constituting a sere and leading 

 up to a state of relative stability and permanence known as the climax. 

 It should, however, here be admitted that not all ecologists accept 

 the idea that vegetation can be widely interpreted in terms of develop- 

 ment and equilibrium, while some, such as Professor Hugh M. Raup 

 {in lift.), seem to doubt the validity of some of the basic assumptions 

 involved — at least for those parts of the world in which they have 

 themselves worked. Certainly, many of the beliefs involved are 

 mere presumptions, or true only in some degree : thus successions 

 may proceed only in relation to preceding and following stages, and 

 climaxes are only relatively stable. This is often expressed by saving 

 they are in ' dynamic equilibrium '. Nor is it for us to write into 

 Nature's book meanings which she does not intend, or to attempt 

 to inculcate for our own convenience an orderliness of pattern which 

 does not exist. But if we deny the existence of seres and climaxes, 

 we do away with two of the most stimulating concepts and useful 

 tools of our trade, and so with this reservation it seems best to pro- 

 ceed to use them. In doing so we ought also to bear in mind that 

 many of the principles with which we are concerned have emerged 

 from work carried out in temperate regions, and that in the Arctic 

 (for example owing to frost action) and in the tropics (where there 

 is often no clear dominance) things may be very different. 



We shall deal a little later with the typical stages of some charac- 

 teristic seres, and, in the next section, with the main types of climax. 

 With the reservations expressed in the last paragraph, some under- 

 standing of these and allied concepts seems essential for an appreci- 

 ation of the mosaic which is vegetation, and whose study, at least 

 in terms of distribution, is the mainstay of modern plant geography. 

 But first we should outline the component (often more or less con- 

 tinuous) actions of a sere, which normally may be considered as 

 follows : (i) niidation (the production of a bare area) is the initial 

 prerequisite, whether it be by emergence or submergence, glacial 

 recession, erosion, deposit, climatic change, or biotic agency. There- 

 after follow (2) plant migration (including initial colonization) ; (3) 

 ecesis (successful establishment) ; (4) aggregation of germiiles to 



