vegetation of that province and represented on the sand areas of Illi- 

 nois, this is ecologically the best fitted to meet the environmental con- 

 ditions under which it lives. Such associations have been called by 

 Cowles cliina.v associations (iSpp: 374, iQOi: 80, 81), a term which 

 is both logical and expressive and which has been generally adopted 

 by American ecologists. Some associations, however, which are rel- 

 atively stable and consequently more nearly permanent, may under 

 certain conditions give way to others, and to this type may be given 

 the name temporary climax, introduced by Cowles {ipoi: 88) to 

 cover a somewhat different case, but applicable here as w'ell. 



Within every vegetation province there is one climax association, 

 which tends to displace every other association with which it comes 

 m contact. For the Prairie Province this seems to be the prairie-grass 

 association (Pound and Clements, i8g8: 389), which is vei-y poorly 

 iTpresented in the areas under discussion. In the Havana area it 

 tends to come in at the bottom of extinct blowouts, which have 

 I'eached a depth sufficient to expose moist strata of sand (Hart and 

 Gleason, ipoy: 168). In the Hanover area certain ti-acts of bunch- 

 grass occupying depressions between the dunes are composed of an 

 unusually dense and luxuriant covering of grasses in which Paniciiin 

 Scribnerianuiii occurs (see description of station i of the mixed 

 consocies, p. 66). This species is representative in Nebraska and 

 South Dakota of the prairie-grass association (Pound and Clements, 

 t8qS: 389; I poo: 348; Harvey, ipoc?.- 102), and may be considered 

 in our area as a pioneer invader in a prairie-gr-ass succession. The 

 environmental and vegetational differences betw'een the depressions 

 mentioned and the remaining stations of the consocies were not con- 

 sidered sufficient to wan-ant its separation as an example of the latter 

 association. In other cases where Panicnin Scrihnerianuni occurs in 

 the mixed consocies the usual bunch-grasses are so well developed 

 that there is no doubt as to the association concerned. 



When associations from two provinces come in contact, local con- 

 ditions, either climatic or edaphic or both, together with the stnicture 

 of the associations themselves, decide the supremacy, and one is re- 

 placed by the other. In the Illinois sand areas the associations of the 

 Pr-airie Province are surrounded by those of the Deciduous Forest 

 Province, and the Imnch-grass association is under certain conditions 

 succeeded by an oak forest. Certain physical conditions, in this case 

 wind, may also destroy the bunch-grass, and open the way for a 

 series of successions, which generally revert sooner or later to the 

 bunch-grass. The fundamental difference between these two tvpes of 

 succession is apparent. One consists merelv of changes witliin the 



