480 GLEASON: THE PLANT ASSOCIATION 
plant associations concerned. Such events are so slow that they 
have never been described from observation, and it is extremely 
doubtful if the term succession would be applied to them if they 
were visible. 
The-actual effect of climatic changes as seen consists of the 
arrival in the region of a new flora, the establishment of successions 
between its associations and the original ones, and the gradual 
replacement of the latter. It is possible that temperature changes 
have never taken place in the past more rapidly than they are 
proceeding at present, and that is at a rate too slow to be measured. 
‘The effect of such changes, however, are seen in the northward 
advance of the hardwood forests at the expense of the conifers. 
As has been repeatedly shown by various authors, the successions 
involved here are invariably referable to one or another of the 
causes already discussed in these pages, and no direct effect of a 
temperature change is visible at all, so far as the structure or suc- 
cession of associations is concerned. 
, Great climatic changes, therefore, proceed at a much slower 
rate than the normal observable causes of succession. Those great 
successional movements which have marked the development of 
‘the flora of the continent through all the climatic and geologic 
vicissitudes of the epochs, and whose results are now exhibited 
by the geographical distribution of the flora, are merely the mass 
effects of small successions, coupled with the evolutionary phe- 
nomena of extinction and appearance of species. 
SUMMARY 
1, All phenomena of vegetation, 7. e., of numbers of individ- 
uals, depend upon the phenomena of the individual plant. 
2. The plant population of any area is determined by environ- 
mental selection of immigrants from the surrounding population. 
3. Because of similarity of environmental selection and o 
available sources of immigration, areas of uniform vegetation are 
developed, known as plant associations. 
4. Effective changes in the environment or in the surrounding 
population may lead to significant changes in the vegetation of an 
area. If these changes involve the establishment upon it of a new 
association, the phenomenon is known as succession. 
UNIVERSITY OF MICHIGAN, 
ANN ARBOR, MICHIGAN 
