154 DIRECTION OF DEVELOPMENT. 



Hole's view. — Hole (1911 : 13) defines progressive succession as follows: 



"A succession which thus proceeds from a xerophilous to a mesophilous and 

 finally a hygrophilous type of vegetation, i. e., from a simple to what must be 

 regarded as a more highly developed type, may be termed a -progressive succes- 

 sion. On the other hand, the reverse succession from a highly developed to 

 a more simple type may be termed regressive. An example of such a succession 

 is seen when mesophilous forest is cleared, or more gradually destroyed by fire 

 and grazing, the resulting erosion on steep slopes converting the area into a 

 rocky hillside only capable of supporting the poorest and most xerophilous 

 types of vegetation. Fire is a very potent factor in causing regressive succes- 

 sions, for it is not only capable of temporarily depriving the soil more or less 

 completely of its covering of vegetation, but it also directly dries the soil and 

 destroys the humus. Fire may in this way be responsible for the existence 

 of xerophilous grassland, or woodland, in locaUties which once supported 

 mesophilous or possibly hygrophilous, vegetation. Grazing again, by destroy- 

 mg the undergrowth and keeping a forest open, may so reduce the humus 

 content of the soil as to render unpossible the reproduction of the mesophUous 

 species constituting the forest and may thus cause a regressive succession 

 Coppice fellings m the middle of a forest may similarly cause a regressive 

 succession. 



"Finally there is a type of succession which we may distinguish as parallel 

 succession. Types of both grassland and woodland are found in all kinds of 

 habitats, ranging from the most xerophytic to the most hygrophytic, and it is 

 of great importance to realize that for each type of grassland there is as a rule 

 a corresponding type of woodland capable of thriving under similar conditions 

 of environment, seeing that this has a direct bearing on the afforesting of 

 grasslands. When a type of grassland, such as Munj savannah, is replaced 

 by a parallel type of woodland, e. g., dry miscellaneous forest of Acada, 

 Dalbergia, and others, we may therefore regard it as a case of parallel succession 

 to distinguish it from the progressive and regressive changes considered above. 

 Parallel changes can be effected more easily and rapidly than progressive 

 changes, and with reference to such questions as the afforestation of grass- 

 lands and the extension of woodlands, parallel changes are as a rule of more 

 importance." 



The author regards wet savannah, reed-swamp, and tropical evergreen forest 

 as hygrophilous formations. Of these, the reed-swamp is usually regarded as 

 hydrophytic, and, in extra-tropical regions at least, it never forms a final 

 stage in succession. While Hole is evidently seeking the climatic climax 

 in his definition of progressive succession, it seems doubtful that wet savannah 

 and reed-swamp can be regarded as such. His view that progression passes 

 through mesophytic stages to hygrophilous or hydrophytic ones is at variance 

 with that, of Cowles, in which mesophytic stages form the climax. While 

 Cowles also regards movement from hydrophytic to mesophytic communities 

 as progression. Hole does not consider this sequence at all. This conflict of 

 opinion serves to emphasize the necessity of dealing with development alone, 

 quite irrespective of the water character of the final stage. The author's state- 

 ment that the progressive succession "proceeds from a simple to what must 

 be regarded as a more highly developed type" is sound. But the types must 

 be arranged upon the basis of life-form or phyad, and not upon habitat-forms 

 determined by water. 



