278 PLANT SOCIOLOGY 



meadow in the subalpine levels of the mountains of central France is 

 based upon the yarding method. From the phytosociological view- 

 point this process is a continuous battle against the extension of the 

 Genista pilosa-Calluna association, leading to the climax, which takes 

 possession of all dry pastures. In order to stop this climatically 

 induced degradation of the pastures, every square foot of the ground is 

 subjected to several weeks of cattle yarding at intervals of from fifteen 

 to twenty years. The temporary enclosure in which the cattle spend 

 the nights is moved from time to time (Fig. 143) . As soon as pasturing 

 is discontinued, everywhere the natural tendency toward the climax 

 resumes its sway. 



2. MAN 



There is no sharp Hne of distinction between the influence of man 

 and animals upon vegetation except in man's use of fire. Man's 

 influence is often exerted through his domestic animals. 



The effect of man's destructive hand upon vegetation is visible 

 everywhere, from the tropics to the poles, and from the valley floors 

 to the mountain tops. If we speak today of "untouched vegetation," 

 we exaggerate greatly; Chevalier (1925) reminds us that even the 

 concept of "virgin tropical forest" is a mere myth. 



In all records of vegetation one should first attempt to account for 

 the degree of human influence. Even our paleolithic ancestors of the 

 early and middle Quaternary, with their primitive implements, knev/ 

 fire, and they may have encouraged certain types of vegetation 

 (meadow, steppe) at the expense of others (virgin forest). With the 

 rise of commerce and transportation, agriculture, and stock farming 

 in the neolithic period, the era of vegetational change begins. Great 

 plant migrations which began under the influence of the "lords of 

 creation" have not yet reached their conclusion (see particularly 

 Thellung, 1915; Hauman, 1928). 



No other factor of vegetation has been investigated and written 

 upon so much as the influence of man. In fact, hundreds of volumes 

 in agriculture and forestry have reference to it. We are obliged to 

 limit ourselves to a few points and to refer, in general, to the texts on 

 agriculture and forestry. 



Fire. — The most remorseless associate of man in the destruction of 

 native vegetation is fire. While prairie and forest fires may occa- 

 sionally be caused by lightning, that is the exception rather than the 

 rule. In 90 out of 100 cases they are caused by man, either willfully 

 CT accidentally. Contrary to the opinion of some American investiga- 

 tors, therefore, fire is to be classed among the anthropogenous factors. 



