GLEASON: THE PLANT ASSOCIATION 479 
to be the climax of that region. Such is the case with the oak- 
hickory forests of the gravel hills of southern Michigan. Even if 
the cumulative effect of exceedingly slow physiographic and biotic 
processes should accomplish this result and lead to their succession 
by the beech-maple forests, the same fate is not necessarily in 
store for similar forests of western Iowa, where neither beech nor 
maple occur. It is always possible, also, that the future may 
bring an effective change of environment, no indication of which 
is at present visible. A slight change of rainfall, for example, 
might lead to the extension of the pine forests of the Rocky Moun- 
tains over the high plains of eastern Colorado. The use of the 
term climax is accordingly largely a matter of convenience, and 
it will be applied broadly or narrowly, depending on the viewpoint 
of the ecologist. 
Since physiographic processes tend toward stability, succes- 
sional series tend also toward the establishment of associations of 
greater duration and the ultimate appearance of a climax. In 
some cases, as the succession of forest by prairie, this involves a 
process which is not usual for our climate, or not commonly ob- 
served, and which has been termed regressive or retrogressive. 
In other cases also, an actual reversal of the ordinary direction of 
succession may be seen, as in the establishment of a pond upon a 
sand dune (6, p. 111-116). Clements denies the existence of 
reversed successions, and attempts to exclude described cases by 
definition (3, p. 145, 146). Measured by the behavior of and 
effects upon individual plants, however, the processes are precisely 
the same as in the usual types of succession: a change of environ- 
ment, the gradual death of the original flora, and the gradual 
entrance of the new, with the simultaneous revision of environ- 
mental control. 
28. Great climatic changes in a region, when they occur, are 
of course productive of proportionately great changes in the 
vegetation, involving ultimately perhaps the complete replace- 
ment of all the original associations by new ones. Without the 
arrival of a new flora in the region, the change in the original 
vegetation as the result of climatic change can consist only of the 
extinction of unadapted species, their replacement wholly or in — 
part by species of recent evolution, and of readjustments in the 
