. PHYSICAL CABEIERS. 879 



country from another, constitutes an important barrier to unrestricted distribution. 

 Even narrow straits form an insuperable obstacle to any mode of dispersion which 

 proceeds step by step, whilst broad seas also interfere with the dissemination f&r 

 saMwm, which is accomplished by roving animals and by currents of air and water. 

 The number of species capable of being transported across the sea by birds is so 

 small, that the dispersion of plants as a whole is not appreciably affected by this 

 process. The same remark applies to dissemination by water. It is well known 

 that fruits and seeds of American plants are occasibnally conveyed to Europe by 

 the Gulf Stream, and Linnaeus tells us how the seeds of the West Indian Filbert 

 {Entada Oingolobium) germinated after being stranded on the coast of Norway, 

 There is no need to point out that tropical plants of the kind would not be able to 

 €stabhsh themselves permanently in Western Europe were it only for the nature of 

 the climate. But even amongst other American plants to which the chmate would 

 be no drawback, not a single species is known to have come to Europe by water 

 without human intervention. Nor has any fruit or seed achieved the crossing of 

 the ocean to Europe through the medium of the air. America possesses a large 

 number of Willows, Composites, and Onagraceae of her own, which have their fruits 

 and seeds exquisitely adapted to aerial flight, and are themselves well fitted to 

 thrive under the climatic condition^ of Europe. Nevertheless not a single instance 

 is recorded of such a plant migrating from America to Europe through the agency 

 of the wind. The Compositse and Onagracese, which have become naturalized in 

 Europe since America was discovered (e.g. Erigeron Ganadense, Galinsoga parvi- 

 Jiora, Solidago Canadensis, Stenactis bellidiflora, CEnothera biennis, &c.), were 

 introduced in other ways, and would neither have established themselves nor have 

 been disseminated in Europe without human intervention. 



The fact that a considerable number of American plants have found a home in 

 Europe through the agency of man alone, and independently of the movements 

 of birds or currents, is of great interest in connection with the present subject, 

 inasmuch as it shows that the limits of distribution imposed by the sea are only 

 temporary, that is to say, they are only maintained so long as the present distribu- 

 tion of land and water remaras unaltered. If Europe and America were to become 

 connected by a bridge of land, the possibility would arise of a gradual or sudden 

 migration across the bridge, and such plants as have been conveyed from America 

 to Europe by human agency would be able to immigrate without such assistance, 

 .and to disseminate themselves over Europe. The external conditions would offer no 

 impediment to their naturalization in Central Europe any more than they now do 

 to the installation of the same species when introduced by man. As the sea limits 

 the distribution of land-plants, so the dry land restricts the dispersion of marine 

 plants. The larger the expanse of land between two seas, the more difficult is it for 

 the plants which inhabit them to exchange their homes. But here again the barrier 

 is merely temporary; for were the land to sink in any part so as to become sub- 

 merged, and the two seas thus become confluent, there would be nothing to prevent 

 the plants living in them from passing from one to the other. 



