THE STRUCTURE OF CLIMAXES 243 



degrees of influence, the term arthropod influent may well be applied 

 to the numerous insects, arachnids, etc. (Shelford and Olson, 1935). 



Structure of the Climax. Each climax is the product of climatic 

 differentiation operating upon an original community of vast extent 

 and fairly uniform composition. Such a climax under the compulsion 

 of climatic shifts became a panclimax comprising two or more cli- 

 maxes. The best illustrations of this process today are to be seen in 

 the circumpolar tundra, coniferous forest, and prairie-steppe pan- 

 climax or panformation, each of which is divided into an old and a 

 new world climax. Such a process of community evolution has op- 

 erated alike upon the plant dominants and the animal influents. 

 The old and new world biomes are as closely related in one as in the 

 other respect, while the community bonds in terms of coaction espe- 

 cially are very similar, when not identical. 



Major Units of the Biome. The climatic factors that produced 

 climaxes or formations have continued to act, with the consequence 

 that each biome has been further differentiated into divisions known 

 as associations. The common origin of the associations of a forma- 

 tion is still attested by the fact that the dominant genera are largely 

 the same throughout and that several species serve as "binders" (or 

 pcrdominants) between the various divisions and especially the con- 

 tiguous ones. This principle is well exemplified by the grassland 

 formation in which the dominants are certain species of Stipa, Bou- 

 teloua, Sporobolus, Agropyrum, Koeleria, and Andropogon that occur 

 over most of the climax area, while such dominants as Stipa comata, 

 Koeleria cristata, Bouteloua gracilis, and Sporobohis cryptandrus re- 

 cur in nearly all six associations. The distribution of influents is 

 comparable to a large degree, as is indicated by such animals as 

 Bison bison, Antilocapra americana, Canis nubilus, Taxidca taxus, and 

 species of Citellus, Dipodomys, and Geomys. 



In their turn, associations are divided into faciations, on the basis 

 of subclimates as reflected in the entrance or disappearance of one 

 or more dominants. Although faciations have now been recognized in 

 most of the major associations of the continent, such analysis has been 

 carried out most completely in the mixed prairie, which is thought to 

 represent the original matrix of the grassland climax. The presence 

 of Buchloe dactyloides as a major dominant marks a central faciation, 

 Hilaria janiesi and Stipa pennata a southwestern one, and Festuca 

 ovind, one at high altitudes. To what extent animal influents are to 

 be correlated with such units is at present uncertain, but there are at 

 least some examples of agreement. The faciation itself may be char- 



