THE EFFECT OF MAN 539 



extension of farm areas, the amount of land formerly used for farming 

 and allowed to revert to "forest land" is larger 1 and our forested 

 areas now show a slight net increase. 



When further need of agricultural land arises, the draining of 

 swamps and marshes and the irrigation of arid districts is begun, or 

 so-called dry farming is improved and extended. Open land, however 

 acquired, is transformed into tilled fields, made fruitful by cultivation 

 and fertilization, and uniformly planted with useful grains or other 

 domestic plants. The growth of plants, which, as "weeds," compete 

 with the cultivated ones, is more and more reduced by careful cleaning 

 of the seed, plowing, and cultivation. Fields with a uniform growth 

 are the result. On account of being harvested at stated times, such 

 fields are an unsuitable habitat for many animals. The composition 

 of the plant associations in grassland is influenced by the repeated 

 cutting and by manuring; under these conditions wild prairie is most 

 closely represented by pastures and hay meadows. Special biotopes are 

 more and more sacrificed to civilization. In well-populated countries, 

 continuous forest persists only in districts unsuited to agriculture, such 

 as barren sands and the steep slopes of mountains. 



The uniformity of the vegetation over large areas is characteristic 

 of civilized countries. This is especially true of cultivated fields, whether 

 of grain, turnips, potatoes, sugar cane, rice, cotton, or tobacco. In a 

 smaller degree, the same is true of meadows and pastures. Such uni- 

 formity of food plants makes for uniformity of animal life. Each 

 species of plant has a series of animal dependents which are more or 

 less overlapping. With many cultivated plants the number of animals 

 feeding in part or wholly upon them seems unduly large; more than 

 100 species of animals feed on sugar cane, more than 200 on corn, the 

 same number on clover, and over 400 attack apple trees. 



Nevertheless, the number of species among the inhabitants of such 

 an environment is always less than that of more varied habitat con- 

 ditions. In compensation, certain species appear in great abundance 

 of individuals in such uniform cultivated areas. This phenomenon is 

 connected with the superabundance of food; succeeding generations 

 find it unnecessary to migrate. When weather conditions are favorable 

 and the mortality is low, such pests appear as the Phylloxera in south- 

 ern France, the Colorado potato beetle in potato fields and the chinch 

 bug (Blissus leucopterus) in the grain fields of the United States, the 

 tussock moth (Lyjnantria monacha) , and other insects, in European 

 pine plantations. A small number of species goes hand in hand with a 



