144 ADAPTATIONS. OECOLOGICAL CLASSIFICATION sect, hi 



oak-scrub in Jutland, and the Tristegia glutinosa-grasslands in Brazil ^ : 

 these formations arise and can only be preserved by cultivation, and 

 must be described as secondary formations. 



Sub-formations. There are various formations which are so immense 

 in extent and range, and display certain minor oecological distinctions, 

 that it is advisable and convenient to subdivide them into sub-formations ; 

 for example, plankton-formation, coniferous forest, and dicotylous forest, 

 can thus be subdivided. 



In deahng with forest one character for consideration is its deciduous 

 or evergreen nature, another concerns the question as to whether the 

 ground-vegetation consists of the same or different growth-forms ; the 

 differences in the ground-vegetation in forests may be so great that they 

 give an entirely different appearance to these, which must therefore 

 be accounted as sub-formations. The existence of sub-formations must, 

 however, be justified on oecological grounds (by the depth, water-content, 

 or kind of soil, or by other factors). Increase in our knowledge of 

 oecology will shed light upon these questions. - 



C. ASSOCIATIONS* 



The same formation, in different districts and locahties, or even 

 at different seasons of the year, may be composed of different species, 

 and is perhaps mostly so. Plankton-formation in certain months may 

 be composed of species different from those in other months ; reed- 

 formation in various places has Phragmites communis as the dominant 

 species, in others Scirpus lacustris or Typha, yet it remains the same 

 formation. Beech-forest and oak-forest are specifically different forms 

 of the same formation the summer-green (deciduous) dicotylous forest 

 in the temperate climate. Coniferous forest in one place may consist 

 of Pinus sylvestris, at another of P. montana or P. halepensis, or in 

 North America of entirely different species of Pinus ; or again, it may 

 be formed by species of Picea or Abies ; furthermore, it may be an ad- 

 mixture of coniferous species. A cornfield is a ' cultivated formation ' 

 of annual or biennial species ; but the maize-field, rye-field, wheat- 

 field, and buckwheat-field are different forms of it.* 



Such smaller, often more-localized subdivisions or kinds of the forma- 

 tion may be distinguished most correctly by the general term association, 

 which has been employed in this sense by various botanists.^ 



^ Warming, 1892. 



^ It is a matter of great difficulty to find suitable names for the various kinds 

 of formations. Some of the many words in common use have been utiUzed as 

 scientific terms, such as steppe, prairie, tundra, Caa Tinga, alang-alang, savannah, 

 and others (see Warburg, 1900), but many popular words are unsuitable. Other 

 scientific terms, such as plankton (Hensen), and garide (Chodat) are of recent 

 introduction. 



' Humboldt's ' plantes associees '. See p. 137. 



* In regard to Woodhead's ' complementary ' and ' competitive ' associations, 

 see p. 95. 



* Including C. Schroter und Kirchner, Ganong, Kearnley, Cajander, Adamovicz, 

 and partly W, G. Smith. Hult's, 1885, 'formations' (see p. 140) are associations, 

 and so also apparentlv are Drude's ' unit formations '. Discussion of the different 

 significations attached to the term ' association ' by authors is impossible here. 



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