274 THE FORMATION 
method for many woodland formations where stumps already abound or 
where a fire has occurred, and it is but rarely necessary to cut down trees 
for this purpose. When trees or shrubs are present as relicts, the same 
method is used to determine the length of time taken by the development of 
the corresponding stages. 
THE STRUCTURE GF THE FORMATION 
331. Since all the structures exhibited by formations, such as zones, lay- 
ers, consocies, etc., are to be referred to zonation or alternation, these princi- 
ples are first considered in detail. This, then, constitutes the basis for a con- 
sideration of the structure of a normal formation, with special reference to 
the different parts that compose it. The investigation of formational struc- 
ture, since the latter is the result of aggregation, invasion, and succession, is 
accomplished by instruments, quadrats, etc., in the manner already indicated 
under development, and no further discussion of it is necessary here. 
ZONATION 
332. Concept. The recognition of vegetation zones dates from Tourne- 
fort!, who found that, while the plants of Armenia occupied the foot of 
Mount Ararat, the vegetation of the slopes above contained many species of 
southern Europe. Still higher appeared a flora similar to that of Sweden, 
and on the summit grew arctic plants, such as those of Lapland. 
As the historical summary shows, the concept of zonation is the oldest in 
phytogeography. Notwithstanding this, it has never been clearly defined, 
nor has there been any detailed investigation of the phenomenon itself, or 
of the causes which produce it. Zones are so common, and often so clearly 
marked, that they invite study, but no serious attempt has heretofore been 
made to analyze zonation, or to formulate a definite method of investigating 
it. Zonation is the practically universal response of plants to the quantita- 
tive distribution of physical factors in nature. In almost all habitats, one or 
more of the physical factors present decreases gradually in passing away 
from the point of greatest intensity. The result is that the plants of the 
habitat arrange themselves in belts about this point, their position being de- 
termined by their relation to the factor concerned. Close investigation will 
show that there is hardly a formation that is entirely without zonation, 
though in many cases the zones are incomplete or obscure for various reasons. 
Zonation is as characteristic of vegetation as a whole as it is of its: unit, 
the formation, a fact long ago recognized in temperature zones. A conti- 
!Relation d’un Voyage du Levant. 1717. 
