294 BOTANICAL GAZETTE [MAY 
factors remain constant, the tension line does not change. In 
that case, for instance, the forest does not advance on the heath 
nor the heath on the forest. But, as will be shown in the dis- 
cussion of the historical factors, the conditions as a rule are 
changing constantly. Not only may the struggle be between 
the forest on the one hand and some other type of plant society 
on the other, but it may be between different kinds of forests. 
The struggle between plants and animals may have an impor- 
tant bearing on the explanation of any given floral region. In 
civilized communities man has changed the whole nature of the 
vegetation. His influence is seen where forests are cleared for 
timber and fuel, or for purposes of cultivation; also indirectly 
where forest and prairie fires are active. Again in regions where 
stock is raised trees are damaged or kept from spreading by the 
ravages of domestic animals. Insect life may also be an impor- 
tant factor in explaining the floral character of a given region. 
In concluding this discussion of the ecological factors, it is 
well to note again that the three great physical media—soil, air, 
and water—are all influential in bringing about certain plant 
physiognomies. These, together with the biotic factors, make 
that variety in the landscape of any region which is shown in 
the plant societies that are present. 
C. Historical factors. The third great set of factors that play 
a part in the understanding of plant associations has been desig- 
nated historical, for it involves the element of time. It means 
simply that the geological and physiographic forces have 
changed and are changing the factors so far considered as to 
make absolutely necessary a readjustment of plant formations 
and plant societies to meet the new conditions. Diastrophic 
movements may submerge large areas of land and thus destroy 
all terrestrial vegetation or force it to migrate to unsubmerged 
parts. On the other hand, when there is an addition to the land 
area a condition is obtained where there can be noted the suc- 
cessive stages in the reclamation of such an area by vegetation. 
Again, in the last glacial epoch the ice sheet in its advance des- 
troyed vegetation and modified the climate beyond the limits of 

