162 DIRECTION OF DEVELOPMENT. 



of water. It then spreads laterally over the level surface of the swamp, 

 always carrying larger quantities of water. 



"Since the tongue (of moss-turf) lies lower than the surface of the moor and 

 the Sphagnum holds the water so firmly that the surplus can soak into the soil 

 but slowly, the tongue is constantly dripping with water, and in most cases a 

 quantity of water flows away from it. The bottom once reached, the constant 

 flow furnishes abundant water for further development, unless, as is frequently 

 the case, a colony of Sphagnum has already occupied the bottom, as a conse- 

 quence of the accvimulating water. The soil is converted into swamp by the 

 Sphagnum and the air is driven out as a result." 



These are the precise consequences of ordinary flooding by water, and like- 

 wise lead to destruction of the grassland or forest. 



To sum up, while there is abundant evidence that forest is being changed 

 into scrub, heath, or grassland as a result of the action of artificial causes, 

 there is no convincing proof that such conversion can occur under existing 

 natural conditions. In all cases cited, disturbance by man is either a certain 

 or probable factor, or the destruction has been a consequence of topographic 

 or climatic changes. In no case is there clear proof, as a result of continued 

 quantitative investigation, that a forest produces changes inimical to its exist- 

 ence and favorable to a lower type of vegetation. 



Possibility of backward development. — In all cases of the change of forest 

 to scrub or grassland, even if they be admitted to result from artificial disturb- 

 ance in some degree, it would seem at first thought that the process is actually 

 a backward development, i. e., retrogression. In all the instances cited abov^ 

 however, as well as in all of those so far encountered, the only development is 

 that of a new community on ground left partially or completely bare by the 

 iforest. There is no difficulty at all in recognizing this when the ground is 

 entirely denuded by a fire, and but little when the trees are completely de- 

 stroyed by clean cutting. Similarly, when areas of some extent are cleared 

 in the forest, it is sufficiently obvious that the communities which appear in the 

 clearing are the result of the destruction of the former dominants, and of con- 

 sequent invasion into a sunny though localized habitat. When, however, 

 such areas are no larger than the space made by the fall or removal of a single 

 tree, the situation is more complex. A comparison of a number of such small 

 areas, alternating with each other as well as with clearings of various size, 

 would give the impression of an actual retrogression. This would be due to 

 the amount and kind of invasion in denuded areas of widely differing extent, 

 and the consequent persistence or adaptation of the original undergrowth in 

 varying degrees. Indeed, a general comparison of such areas can not be 

 expected to yield the real facts. It is only by the exact study of each cleared 

 area, large or small, that the true nature of the process stands revealed. Such 

 an investigation will invariably show that, no matter how small an area may 

 be, it has a progressive development all its own, but in every respect in essen- 

 tial harmony with the development in a large clearing of the same forest, or 

 in an extensive denuded area of the same type. In every case it is found that 

 there is no backward development, but merely a fictitious appearance of it due 

 to destruction of the dominants in large or small degree, and the immediate 

 invasion of species best adapted to the conditions of the new area. The care- 

 ful scrutiny and investigation of thousands of cases of local or minute denudar 

 tion in various associations permit of no other conclusion (plate 46). 



