CHAP. XXXV FORMATIONS 143 



Mixed vegetation : Quite different from the complex, several-storeyed 

 formations just mentioned is mixed vegetation which consists of small 

 patches of different formations, occurring close together, but each retaining 

 its own individuahty as a pure formation. This is specially the case 

 where the terrain varies suddenly and greatly, and thus causes the 

 oecological conditions to do likewise. In many mountainous districts, 

 rocks, small bushlands, grasslands, perhaps pools of water, and the like, suc- 

 ceed one another within a limited area, without one formation perceptibly 

 affecting another, and without any principle, other than chance, revealing 

 itself in the admixture. The more level and uniform does soil remain 

 over a wide area, the larger and more uniform are the formations ; the 

 more uneven and variable is it, the more mixed is the vegetation. It 

 is impossible to draw a sharp distinction between a vegetation consisting 

 of several pure formations mixed together, and one consisting of a single 

 complex formation ; for the smaller the patch of a mixed vegetation 

 is the more are the species influenced in their biotic conditions by other 

 adjoining ones. One condition especially responsible for change of 

 this kind is human intervention. 



By the mixing of formations, especially if these be extensive, there 

 come into existence various physiognomical types of landscape which 

 differ according to the constituents. 



Fixity of formations. ' Fixed physiognomic character ' is a part 

 of Grisebach's definition of a plant-formation. Recent writers, like 

 Beck \ and Drude -, emphasize the fact that ' fixity ' is a character 

 essential to the concept of a formation. We may paraphrase Drude 

 as follows : 



A vegetative formation is an independent community of first rank, 

 which consists of like growth-forms or of such as are necessarily asso- 

 ciated, and is confined by its natural boundary to a site determined 

 by the prevalence of identical conditions of existence ; it is thus assumed 

 that without external interference no real change in the nature of any 

 community occupying the site can set in the community is ' fixed '. 



This fixity can be regarded as being only relative, as Drude distinctly 

 agrees ; for, as the external conditions change, every formation will 

 be able to undergo modification, and will always do so in the course 

 of time ^ : there are formations that may have remained, and perhaps 

 will remain, unchanged for thousands of years tropical rain-forest, 

 for example. There are others that will soon be exterminated in the 

 places they now occupy, and driven to other sites. 



Secondary formations. By the term ' secondary formations ' we 

 indicate formations which have arisen through human interference.'' 

 There are various formations of this kind that are changed only in their 

 flora : such may be termed semi-cultivated '" formations, and are exempli- 

 fied by North European heaths, which have been modified by the browsing 

 of animals, and by other agencies. But there are entirely new formations 

 resulting from man's activity in destroying forest, in farming, and in 

 otherwise utilizing the soil ; of such a nature are the ' sibljak ' in Servia '^, 



' Beck von Mannagctta, 1902, ' Drude, 1896, p. 286. 



^ Compare Section XVII. 



* Warming, 1892. They are the ' Substitute Associations ' of \V. G. Smith, 1905, 

 p. 62. * Krause, 1892 a; see also Grabner, 1909. ' According to Adamovicz, 1902. 



