212 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



particular insect, but they do not deny commerce with other 

 nations. No one who has seen the map and account of the range 

 of Bombus and Aconitum in Drude's PJlanzen Geographie'^ can 

 doubt that the range is, in some cases, influenced by the best 

 possible insect visitor. 



If we turn next to those plants which may be called the camp- 

 followers of man or his domesticated animals or plants, we know 

 that, whenever virgin soil is broken, a deadly struggle occurs 

 between the domestic plant with its weedy clients and the natural 

 vegetation. The seeds of Egyptian weeds have been found in the 

 prehistoric lake villages of Italy. I have gathered, e.g., in Tripoli, 

 to quote my herbarium book. No. 3196, Vrtica memhranacea, 

 Poir, " Aaron Arbib's orange gardens, rather sandy ground ; 

 sheltered. January 9th." This is an American weed brought 

 with the oranges. A very interesting discussion of this point is 

 to be found in the New Jersey Agricultural Experiment Station 

 report.^ Mr. Halsted shows there clearly how weeds of different 

 origins have traversed the country: Papaver duhiuni, Linn., from 

 Europe, travelling 25 miles in 15 years (I.e., p. 321). The 

 Argemone mexicana, Linn., has invaded the country from tropical 

 America. Abutilon Avicennce, Gaertn., has come from India and 

 is working westward, whilst Xantliium canadense, Mill, and 

 others are native plants which have become weeds of cultivation, 

 just as our common plants did long ago. 



I feel pretty certain that at least 100 dicotyledons of the 

 '■ London Catalogue," and fully half the grasses, are either arti- 

 ficially introduced by man, or have been brought accidentally. 

 This is about 12 per cent, of the flora, and probably a very long 

 way under-estimated. For all these the limit is human agriculture, 

 and, to a large extent, the skill in farming of a particular district. 



Again, plants of fresh water are very widely distributed. This 

 is probably through the fact of water birds carrying the seeds; 

 the extremely wide range of ducks, e.g., is notorious. It is 

 conceivable that a fresh-water plant might pass right round the 

 earth in four seasons if we take this into consideration. The 



1 Drude, PJlanzen Geographie, pp. 122 and 123. This is taken from 

 Kronfeld, Botan. Jahrhnch. jlir Systematik, Bd. XL, p. 19. 



2 Annual Keport, Botanical Department (New Jersey Agricultural 

 Experiment Station), 1891. 



