THE EXPERIMENTAL BASIS. 53 



condition is xerophytic or at least considerably drier than the climax. The 

 nature and indicator value of hydroseres differ in accordance with their 

 origin in lakes and swamps, or in bogs or other poorly aerated wet soils 

 (oxyseres). Similarly, the indicator values of xeroseres vary with their origin 

 upon rock, dune-sand, or in saline areas (plate 10). 



Major successions as indicators. — The seres or unit successions discussed 

 above are themselves parts or stages of greater successions. The cosere is a 

 series of two or more unit successions in the same spot, and is best illustrated 

 by those peat bogs in which the remains of the various stages and seres are 

 accumulated in sequence and in position. In addition to the indications 

 furnished by each sere, the cosere always indicates one or more striking changes 

 of condition. When it exists over a wide area or recurs in the same relation in 

 several regions, it is an indicator of climatic change. An effective change of 

 climate is denoted by the occurrence of the peat formed by water-plants as the 

 layer above that which records the presence of the climax or subclimax trees. 

 Such coseres have been industriously studied by European investigators, 

 Steenstrup, Blytt', Lewis, and others (Plant Succession, 378), and their climatic 

 correlations established with much certainty. The record of a cosere is well 

 preserved in water and especially in peat-bogs, but the more or less fragmen- 

 tary records furnished by burns, dunes, moraines, and volcanic deposits are 

 often of great value. This is especially true of the deposits of periods of great 

 volcanic activity, such as the Miocene, as found in Yellowstone Park and the 

 John Day Basin (Plant Succession, 367). 



Major changes of climate are accompanied by the shifting of climaxes as 

 well as by the succession of seres in the same spot. The differentiation of 

 climates during the Paleophytic and Mesophytic eras led to corresponding 

 differentiation of vegetation with characteristic zones grouped around centers 

 of deficiency or excess. These zones were clearly marked out by the middle of 

 the Cenophytic era, since which time the major effects of climate have been 

 recorded in their shifting. It seems highly probable that the climatic cycles 

 which produced and characterized the glacial period were accompanied by 

 marked shifting of climax zones and that the close of the period left the primary 

 zones of continents and mountains much as they are to-day. Such zones are 

 the most striking and important of all climatic indicators, and their significance 

 has been appreciated and investigated for more than a century. Perhaps even 

 more important is the fact that such a series of shiftings or zones is a succes- 

 sional process by which it becomes possible to predict the general effect of any 

 climatic cycle. This relation has already been developed to some extent 

 (Plant Succession, 347, 364) and is further discussed in connection with paleo- 

 ecology (Chapter III). The greatest climatic changes of geological times are 

 thought to be indicated by the evolution of the great land-floras and their 

 differentiation into climax vegetations. Thus, the entire course of the devel- 

 opment of the earth's vegetation, which is called the geosere, is divided into 

 eoseres corresponding to the three great eras, and each eosere then exhibits 

 clisere shifting in response to lesser cycles. The use of zones as indicator 

 criteria is discussed in the next section. 



THE EXPERIMENTAL BASIS. 



Nature. — Indicators derive their importance chiefly from their practical 

 applications. For all practical purposes, indicator values must finally be 

 determined by experiment. The degree of their usefulness will depend mostly 



