302 GEOGRAPHICAL DISTRIBUTION [ch. 



may accidentally as it were offer some lure to birds. When 

 a fruit of Castalia alba bursts, some 1 600 to 1 700 seeds rise to 

 the surface, where they float for a day or two in a mass, looking 

 like a patch of fish spawn ^ and perhaps on this account attract- 

 ing the attention of birds. 



Ascherson^, who has given much study to the distribution 

 of marine Angiosperms, argues, from the occurrence of Zostera 

 nana in the Caspian Sea, that this water area must have been 

 in comparatively recent times connected with the Black Sea, 

 where this species is also found. However, in the light of the 

 part played by birds in the distribution of water plants, it is 

 probable that little stress can be laid upon such evidence. It has 

 been observed^ in Britain that Brent Geese feed on Zostera^ 

 and that these birds are almost confined to the parts of the coast 

 where the Grass-wrack is to be found. It is quite conceivable 

 that they may occasionally carry seeds or fragments of the plant 

 which would be able to take root on reaching salt water again : 

 by analogy we may suppose that birds might also be competent 

 to convey Zostera nana over the three hundred miles or so which 

 separate the Black Sea from the Caspian. 



Problems of plant distribution are often a good deal com- 

 plicated by the interference of man. This is less the case with 

 aquatic than with terrestrial vegetation, because, on the whole, 

 water plants are of no great utility to the human race, and are 

 seldom introduced intentionally. But the present distribution 

 of certain aquatics cannot be understood unless allowance be 

 made for the influence of mankind in their dispersal. Trapa 

 natans^ the Bull Nut or Water Chestnut, is an instance. 

 This plant now occurs over a considerable part of Europe, the 

 Caucasus and Siberia*. It has been used from early times for 

 food, medicine and magic, and is supposed to have been intro- 

 duced into Switzerland as long ago as the period of the lake 

 dwellings^. It is now nearly exterminated in that country, and 



1 Guppy, H. B. (1893). 2 Ascherson, P. (1875). 



2 Walsingham, Lord and Payne-Gallwey, R. (1886). 



4 Areschoug, F. W. C, (18732). ^ j^ggi, J. (1883). 



