Man's Work on the Farm 341 



the soot. Lichens have also disappeared from Kew 

 Gardens, and are rare in Epping Forest. 



Changes of this sort are, however, very small com- 

 pared with those which have followed the destruction 

 of primeval forests, the introduction of agriculture, 

 the wanderings, and even wars, of the human race, 

 the voyages of traders, and the discovery of new lands. 



Those who are accustomed to cultivate the soil, in 

 however rude and primitive a fashion, naturally carry 

 with them on their wanderings the seeds of such 

 plants as they have found useful. This is true even 

 of such nomads as the gipsies, whose attempts at 

 agriculture have been necessarily of a very limited 

 description, since their crops had to be sown and 

 reaped during -the short time that they remained 

 stationary. 



But besides carrying some seeds purposely, travellers 

 of all sorts, whether nomads or not, frequently carry 

 others by pure accident, and quite unconsciously. 

 They mean to take with them only cultivated plants, 

 but somehow or other, in this way and in that, the 

 weeds contrive to get taken too. It is often quite 

 impossible to say how they have been brought, but 

 there they are. This is especially the case with what 

 may be called cultivated, weeds — those which are found 

 in cultivated fields and nowhere else ; for, unless great 

 care is taken, their seeds are very apt to be mixed 

 with those of the crop, and then both are sown to- 

 gether. Clover and grass seeds are especially liable 

 to adulteration of this sort ; broom-rape and dodder 

 are conveyed in the one, and worthless grasses, and 

 many another weed, sneak in with the others. 



Many seeds are conveyed, also, in forage; and in 



