THE FACTORS OF DISTRIBUTION 6r 



was not native, until recourse was had to artificial 

 impregnation. 



The dissemination of plants by animal agency belongs 

 to our next chapter. 



MAN AS INFLUENCING VEGETATION. No one agency 

 probably has so profoundly modified the distribution of 

 plants as has man. The clearing of forest, whether with 

 a view to agriculture or for timber, the careful removal 

 of " weeds/' drainage, the tillage, marling, and manuring 

 of the land, and the introduction of animals foreign to- 

 the various countries, have for ages altered the natural 

 balance of species, and have even altered climate. It 

 has been stated that the mean annual temperature of 

 England has probably been raised 2 F. by drainage; 

 and this increase of temperature has of course been 

 accompanied by a much lessened humidity of the 

 atmosphere. 



By intentional introduction of new species we have in 

 Britain considerable areas covered with Pine and Larch ; 

 the extensive fields in Prussia occupied by the American 

 Potato; millions probably of human beings in Africa 

 mainly dependent for their food upon the equally 

 American Maize; and vast areas in America and 

 Australia occupied by Wheat, Rice, and other Old World, 

 species. 



Even more striking are the effects of man's unin- 

 tentional introductions. Xanthium spinosum L., a 

 Russian Composite, is said to have been introduced into 

 Wallachia in 1828 in the manes and tails of Cossack 

 horses. Similarly it spread, with cattle and sheep, into 

 Hungary and Bavaria, and, reaching South Africa, 

 proved so detrimental to the wool as to necessitate 

 strenuous laws for its extirpation. The Milk Thistle 

 (Silybum Marianum Gaertn.) and the Cardoon (Cynara 

 Cardunculus L.) have spread over hundreds of square 

 miles of the Argentine Pampas; and these are but some 

 striking instances out of many. It should, however, be 

 noted that many plants introduced to the neighbourhood 

 of*ocks in ballast, to that of cloth-factories by the 

 combing of foreign wool, or to the site of international 

 exhibitions in the straw of packing-cases, do not survive 

 many years or succeed in establishing themselves. 



Many widespread weeds are self-pollinating, small- 

 seeded species ; and in the case of such plants, at least,. 



