INFLUENCE OF LAWS OF DEYELOPMENT. 663 



Clover-seed, for instance, is largely imported into Britain from France 

 and Germany ; many of the weeds of our arable lands have doubtless been 

 introduced with foreign seed, such as Adonis autumnalis, Veronica Bux- 

 baumii, Papaver somniferum, &c. Ballast-heaps at sea-ports, where 

 vessels returning home from foreign countries discharge their ballast, have 

 become frequent sources of new importations ; and merchandise, such as 

 cotton, but especially seeds, fruits, dye-stufts, &c., often contains seeds of 

 plants, some of which now and then acquire a footing. 



Horticulture is so evidently one of the most important influences in 

 the diffusion of plants, that it is scarcely requisite to dwell upon it. 



Extermination of Plants. On the other hand, human industry lias a 

 great tendency to exterminate particular forms of vegetation, or, at least, 

 to greatly affect their relative predominance in a given region. The 

 destruction of forests for the purpose of clearing land for cultivation 

 changes the whole face of vegetation, and even, to some extent, 

 affects the local climate, as also do drainage operations in their degree. 

 Instances of change of this kind might be furnished from almost eveiy 

 part of the globe. North Europe w T as clothed in earlier times of the pre- 

 sent period with dense forests, long since cleared away to give place to 

 cultivated plants and a multitude of wild plants suited to the different 

 conditions of the soil. The forests of North America are, in like manner, 

 disappearing by degrees under the hand of man. 



The change is not merely one kept up by a continual effort of cultiva- 

 tion; the original vegetation does not always reestablish itself when the 

 region is deserted. New kinds of plants spread over the cleared ground, 

 and new animal inhabitants come to check the efforts of the old forests to 

 renew themselves. 



Sect. 2. INFLTJENCE OF THE LAWS OF DEVELOPMENT OF PLANTS. 



Plants of Wide Distribution. If we leave out of view the 

 question of the origin or creation of plants, there is no reason why 

 any given species should not exist in all places where the climate 

 is suitable. 



Some kinds of plants are, indeed, very widely spread over the surface of 

 the globe. Hooker has enumerated upwards of 30 species of Flower- 

 ing plants common to Northern Europe and the Antarctic regions. A 

 considerable number of North-European species extend round the globe 

 in northern latitudes, in the colder parts of North America and Asia. 

 Not a few of our plants occur also on the Himalayan Mountains. JEpi- 

 lobium tctraf/onum, a British species, is found in Canada and in Tierra del 

 Fuego. Oiir white Hedge-Convolvulus, with some other British plants, 

 occurs in the Galapagos Islands. Many of the Falkland-Island plants are 

 met w r ith also in Iceland. Plantago maritima, a common sea-side plant 

 with us, is found at the Cape of Good Hope and at the southern extremity 

 of America. 



Representatives of some few natural orders occur in all regions, e. g. 

 Composite, Leguminosse, Cyperaceee, Filices, Equisetaceae, and a few 

 others. A few genera also, such as Senecio, Rubus, Plantago, Oxalis, have 

 also representatives in all regions. Very few species are thorougly cos- 



