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THE STRUCTURE OF CLIMAXES 233 



appear to converge, the climax being one stage before the postclimax. 

 Watson (1925) has indicated the convergence of a postclimax in 

 northern Florida. 



Stability is one measure of a climax, except where, for example, 

 a rock substratum controls a scral community, such as rapid-water 

 communities or lichen communities on granite. The length of time 

 for a climax to develop on the non-rock substrata noted above renders 

 it necessary to appeal to indirect evidence of biotic stability. Such 

 evidence is supplied by the serai position in climatic seres by the 

 comparison of transition communities (ecotones) with the adjacent 

 formations, and by the presence of relicts, especially in the form of 

 those from drier periods (preclimaxes) and from moister periods 

 (postclimaxes). Where the community covers the general level and is 

 absent from the topographic extremes, it is to be regarded as the 

 climax proper. Some attention must be given to the meaning of gen- 

 eral level. For example, in southern Illinois the beech-sugar maple 

 climax covers the valleys, hilltops, and hillsides where the difference 

 in level is 500 to 600 feet and the annual rainfall 40 inches. AVith 

 less rain the climax would cover a much lesser relief and the meaning 

 of "general level" would be different. The seasonal distribution of 

 precipitation must also be considered. When one community is found 

 in small areas in narrow valleys, another on the large area of hillside, 

 and a third on hilltops 900-1000 ft. above the valleys, they are usually 

 considered as postclimax, climax, and preclimax, respectivel3^ Addi- 

 tional evidence may frequently be drawn from the trend of annua- 

 tion, in view of the fact that the difference between the wet and dry 

 phase of a cycle may exceed that of a full climate. Further testimony 

 can be secured from micro-climates (ecoclines) since the divergence 

 between northerly and southerly slope exposures often amounts to half 

 a climate or sometimes an entire climate. Observation in water is 

 much more difficult; convergence is less evident, and the criteria indi- 

 cated above can hardly be applied. 



Types of Climaxes and Climates. As has been previously indicated, 

 the inclusion of animals in the climax to constitute the biome renders 

 it necessary to broaden the definition of climate and to designate the 

 corresponding communities as climaxes. In consequence, three major 

 types are to be recognized and considered, viz. (1) terrestrial, (2) 

 fresh-water, and (3) marine. In respect to North America, the knowl- 

 edge of land climaxes has undergone progressive organization for 

 nearly two decades, and the plant matrix and its relations are fairly 

 well understood, though relatively less animal material has been pub- 

 lished. On the other hand, though much preliminary work has been 



