The lifetime of forest and thicket stages of successions is ascertained by counting the annual rings of the stumps of facies. This is a perfectly feasible 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 OF THE FORMATION =331.= Since all the structures exhibited by formations, such as zones, layers, consocies, etc., are to be referred to zonation or alternation, these principles are first considered in detail. This, then, constitutes the basis for a consideration of the structure of a normal formation, with special reference to the different parts that compose it. The investigation of formational structure, 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 Tournefort[40], 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 quantitative 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 determined 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 continental climate, however, often results in the interruption of these, with the consequence that these belts of vegetation are not always continuous. _CAUSES OF ZONATION_