Of these, five belong to the same succession, and it is possible to indicate the development of the spruce-pine forest by arranging these five formations in their proper order in a succession herbarium, as follows: (1) the gravel slide formation, (2) the half gravel slide formation, (3) the pine formation, (4) the balsam-spruce formation, (5) the spruce-pine formation. DEVELOPMENT AND STRUCTURE =247. Vegetation an organism.= The plant formation is an organic unit. It exhibits activities or changes which result in development, structure, and reproduction. These changes are progressive, or periodic, and, in some degree, rhythmic, and there can be no objection to regarding them as functions of vegetation. According to this point of view, the formation is a complex organism, which possesses functions and structure, and passes through a cycle of development similar to that of the plant. This concept may seem strange at first, owing to the fact that the common understanding of function and structure is based upon the individual plant alone. Since the formation, like the plant, is subject to changes caused by the habitat, and since these changes are recorded in its structure, it is evident that the terms, function and structure, are as applicable to the one as to the other. It is merely necessary to bear in mind that the functions of plants and of formations are absolutely different activities, which have no more in common than do the two structures, leaf and zone. =248. Vegetation essentially dynamic.= As an organism, the formation is undergoing constant change. Constructive or destructive forces are necessarily at work; the former, as in the plant, predominate until maturity, when the latter prevail. Consequently, it no longer seems fruitful to classify the phenomena of vegetation as dynamic or static. The emphasis which has been placed upon dynamic aspects of vegetation has served a useful purpose by calling attention to the development of the latter. Although it is a quarter of a century since Hult, and more than a half century since Steenstrup, by far the greater number of ecological studies still ignore the problem of development. This condition, however, can be remedied more easily by insisting upon an exact understanding of the nature of the formation than in any other way. It is entirely superfluous to speak of dynamic and static effects in the plant, and the use of these terms with reference to the formation becomes equally unnecessary as soon as the latter is looked upon as an organism. The proper investigation of a formation can no more overlook development than structure, so closely are the two interwoven. Future research must rest squarely upon this fact.