26 GENERAL HISTORICAL SUMMARY. 



(1) Soils of the Heath; (2) Dependence of Heath upon Climatic Conditions; 

 (3) Requirements of Heath Plants. The origin of the heath is dealt with 

 under the following heads: (1) Origin of Heath from Forest; (2) Origin of 

 Heath on Bare Sand; (3) Origin of Hfeath-moor or Moss-moor: (a) in water, 

 (6) on bare soil, (c) from forest; (4) Origin of Heath from Heath-moor. The 

 details of many of these developmental processes ^re quoted in Chapter VIII. 

 The utilization of the heath is discussed un4eF(l) afforestation, (2) cutting of 

 sods, (3) burning, and (4) meadow. 



Pound and Clements, 1898-1900.— Pound and Clements (1898:216; 1900: 

 315) also attempted to deal with the origin of formations in a general manner. 

 They distinguished formations as either primitive or recent, with respect to 

 origin. By the former was understood the origin in the geological past, while 

 recent origin has to do with development at the present time. Formations 

 were said to arise at the present time either by nascence or by modification. 

 Origin by nascence occurs only upon bare areas, while origin by modification 

 occurs through changes in existing communities. Formations regularly dis- 

 appear through the agency of fires, floods, man, etc., and in all such cases new 

 formations arise by nascence. ^wo sets of factors are concerned in the origin 

 of formations by modification, viz, natural and artificial. Natural factors 

 are either biological or physical; artificial factors ar e due to the presence o g/ 

 agency of man or animals. ('■©ilTtic-^ercesiSayTTanSfofm facies Or patches into 

 formations, or they may cnSmge the latter by bringing about the intrusion of 

 other facies. Patches (coloniesTlcre-iijvariably incipient formations, and in 

 many situations have become actual foiraa4ions. 



The physical forces are either meteorolo^i©-or"^ysiographic^ A rapid 

 change from one extreme to another affords the best example of the influence 

 of climatic forces. While the instances cited illustrate in a slight degree the 

 bearing of climatology upon formations, it is impossible to estimate fully and 

 accurately the influence of climatic changes operating through a long period, 

 or of a sudden reversal of such conditions. Modification of formations by 

 physiographic forces is illustrated in the canyons of the Niobrara, where the 

 sandy soil has become covered with a layer of loam. Modification due to 

 artificial factors is of several sorts. It may arise through the direct agency of 

 man, as in the case of culture formations, or through his presence, as in most 

 waste formations. The prairie-dog-town waste is an example of a formation 

 produced by animal agency. The origin and development of the vegetation 

 in blow-outs and sand-draws were described in detail (1898:258; 1900:365). 

 The same authors (1898^: 19) devised the quadrat method for the quantitative 

 study of plant communities, and of ecotones especially, and applied it as the 

 basic method for determining the structure and development of vegetation. 



Schimper, 1898.SchhapeT (1898) has distinguished two ecological groups 

 of formations, viz, "climatic or district formations, the character of whose 

 vegetation is governed by atmospheric precipitations, and edaphic or local 

 formations, whose vegetation is chiefly determined by the nature of the soil." 

 Climatic formations belong to one of three types, forest, grassland, and desert. 

 A good forest climate is regarded as consisting of a warm growing-season, a 

 continuously moist subsoil, and damp, calm air, especially in winter. A 

 climate with dry winters is hostile because trees can not replace the moisture 

 lost by transpiration. A good grassland climate consists of frequent, even 

 though slight, precipitations during the growing-season, so that the superficial 



