THE UNITS OP VEGETATION. 117 



consists almost entirely of perennial herbs. In a general account of the forma- 

 tion of a flora, it wovdd be necessary to indicate the character plants and to 

 determine the species to which they owe their physiognomic features, which 

 are in no wise subjective. This is a task especially to be recommended to 

 travellers, since it can be carried out easily and thoroughly. These formations 

 repeat themselves everywhere in accordance with local conditions, but they 

 find their absolute, their climatic limits with the natural flora, which they con- 

 stitute. Just as far as forests of Pinus silvestris or heaths of Calluna vulgaris 

 ejctend, just so far does one find himself in the region of the middle European 

 flora. Even if a single species of one flora pass into another, a dominant 

 species of a group does not appear at the same time in two floras. Every forma- 

 tion, whose character and components are indicated with distinctness, con- 

 forms to the limits of its natural flora." 



From the above, it is obvious that Grisebaqh's conception of the formation 

 was essentially if not wholly physiognomic. This was also true of the idea 

 underlying Humboldt's use (1807 : 17) of the term association. While it is 

 possible to find much harmony between the use of this term by Humboldt 

 and by many modern writers, it seems obvious that Humboldt and Grise- 

 bach meant practically the same thing by their respective terms. Indeed, 

 Moss (1910 : 21, 28) has already pointed out this fact in the case of both terms. 



Drude's concept. — Drude (1890 : 28) has criticized Grisebach's concept and 

 has insisted upon the necessity of considering the flora as of more importance 

 than the physiognomy : 



" Grisebach's definition of the formation must be taken in its entirety. It 

 appears correct to regard the 'groups of plants, which bear a definite physiog- 

 nomic character' as classes of formations. The occurrence of these and their 

 extent permits one to distinguish the great vegetation zones of the earth, 

 but they throw no light upon the question of floristic. Definiteness can be 

 secured only by means of the latter, particularly if one considers that the 

 special physiognomy is due simply to the dominant species, and without invent- 

 ing a special physiognomic system. Therefore the essential task, in order to 

 secure a general survey of the formations of a flora, is to determine their domi- 

 nant species, and, one may add, to study their local conditions. Therefore 

 I view the concept of formation in this later sense with reference to a particular 

 flora. 



"Hence, I regard as a vegetation formation, within the limits of a definite 

 phytogeographic flora, each independent closed chief association of one or 

 several life-forms, the permanent composition of which is effected by the 

 definite conditions of the habitat, which keep it distinct from the adjacent 

 formations." 



Drude here clearly assigns a basic r61e to the habitat, but his actual delimi- 

 tation of formations is based primarily upon floristic. He (1896 : 281, 286) 

 further emphasized the necessity of taking the habitat into account in deter- 

 mining formations : 



"The division of the vegetative covering appears to be determined by the 

 arrangement of definite habitats, and coincides with the alternation of the 

 principal plant communities, in which the physiognomic character of the land 

 lies hidden. These concepts are designated as vegetation formations, which 

 are the botanical units of the vegetative covering of the earth. . . . ' Every 



