4] DISPERSAL AND MIGRATION II9 



from the indigenous flora only by their known history, we see all 

 manner of degrees of success in establishment. Some aliens flourish 

 for a time and then disappear, while others, after manv years of 

 restriction to one locality, suddenly burst forth all over a countryside. 

 A notable example of the latter category is the Oxford Ragwort 

 {Senecio squalidus), which was introduced into the Oxford Botanic 

 Garden late in the seventeenth century but scarcely spread at all 

 until late in the nineteenth century, when it started migrating along 

 the railways. Thereafter, migration proceeded so extensively that by 

 the nineteen-twenties it became known from the vicinity of railways 

 in other counties, and when, in the nineteen-thirties and -forties, the 

 present writer was in charge of the botanical collections at Oxford, 

 it was apt to be sent in from quite remote districts of Great Britain 

 as a curiosity or for identification. 



It will be suflicient — without going into detailed examples which 

 could fill whole chapters — to indicate here some of the main methods 

 by which Man introduces plants to new- areas and, often, new 

 countries and even continents (for it is said that the majoritv of 

 alien plants in Australia and New Zealand come from Europe). 

 In addition to intentional transport of desirable plants for agri- 

 cultural, horticultural, forestral, medicinal, or other purposes, weeds 

 are often dispersed unwittingly with the seeds of vegetables, cereals, 

 and garden flowers, as well as with pot plants and in making trans- 

 plants. All manner of disseminules and whole plants are dispersed 

 accidentally (but quite commonly) by land or water traffic, garbage 

 removal, and in baggage and soil transportation, while admixture 

 in animal fodder, litter, and manure are other extensive means of 

 transport. Dispersal used to be widely effected in ships' ballast and 

 still is in many packaging materials, often to the far corners of the 

 earth, as it is also in bird-seed and building material, or as algal 

 growth attached to ships' hulls. Other sources of dispersed dis- 

 seminules are timber and drug and spice materials — such as Caraway 

 seeds imported by the Danes to Greenland for flavouring bread, with 

 the result that the plant, Carum carzi, is now common around many 

 of the settlements. Indeed, very many kinds of commercial export- 

 import traffic must involve the carriage of disseminules, some of 

 which evidently lead to fresh introductions ; the same is true of 

 personal travel, for people often carry a considerable range of seeds 

 and fruits about their clothing, and, doubtless, greater numbers of 

 microscopic spores. 



In this connection air travel may be particularly effective, for one 



