662 GEOGRAPHICAL AND GEOLOGICAL BOTANY. 



Icebergs, Glaciers. Icebergs probably have some share in diffusing 

 plants, since they are often found loaded with masses of earth containing 

 seeds, which they occasionally cast upon strange shores. There is reason 

 to believe that this kind of influence was far more actively at work in 

 North Europe in the geological period preceding the present. 



Animals, Sec. The transport by animals takes place partly through 

 migrating birds, partly by quadrupeds which have wandering habits. 

 Among birds, many of the omnivorous kinds (for instance the Thrushes) 

 migrate from north to south in autumn, at the time when berries and 

 similar fruits are ripe, and they often void the seeds of these fruits little 

 altered. Animals which, like the northern Reindeers and Buffaloes &c. 

 of other lands, travel from place to place in troops in search of food, 

 doubtless often carry seeds and fruits adhering to their fur into new 

 localities. Flocks of animals transported through human agency become 

 more fruitful sources of this influence. 



Animals may likewise greatly affect vegetation in the way of limitation. 

 The invasion of a region by flocks of graminivorous quadrupeds, the sudden 

 appearance of plagues of locusts or any other of the numerous insect 

 enemies of vegetable life, may quite change the character of the vegeta- 

 tion of a district, somewhat in the manner in w r hich a totally different 

 assemblage of plants springs up on the ground cleared by burning the 

 primaeval forests of South America. 



Effects of Human Interference. 



Migrations. The results of the activity of man display them- 

 selves in both extensions and limitations of the distribution of 

 species. 



From the time of the earliest migrations of the human race, cer- 

 tain plants must have been in a condition of constantly increasing 

 diffusion. The native countries of our Cereal grains are not satis- 

 factorily known : they have been or are cultivated wherever they 

 will grow in Europe and the adjoining parts of the other continents. 

 Doubtless many of what we call weeds have shared in the transport 

 of the seeds of useful plants, having been either mixed with them 

 or accidently adherent to the clothes, goods, or domesticated ani- 

 mals of the wandering races. 



Transport of this kind would be still more active as agriculture 

 extended ; and wars, the improvement of navigation, the discovery 

 of the New World, and countless minor events contributed to 

 increase the interchange of the natural products of different regions, 

 either intentionally or by accident. 



Cultivation, &c. Systematic cultivation of land is peculiarly favourable 

 to the intermixture of new elements in the floras of particular regions, 

 from the fact that seeds of many varieties of useful plants are constantly im- 

 ported from climates where they ripen better or can be kept more easily 

 at a higher standard of excellence. 



