]5G 



HISTORY OF THE VEGETABLE KINGDO:\I. 



decomposition is called fermentation. Fermen- 

 tation never takes place unless vegetable sub- 

 stances contain a certain portion of water, and 

 unless they are exposed to a temperature at least 

 above tlie freezing point. When dry or freezing, 

 many of them continue long vi^ithout alteration. 

 There are three kinds of fermentation. The 

 vinous, where saccharine matter is converted into 

 intoxicating liquors, as wine, alcohol, &c. ; the 

 acetous, where fermented liquors undergo a fur- 

 ther change into vinegar ; and the panary, where 

 amylaceous matter is converted into bread. As 

 these different kinds of fermentation come to be 

 treated of at length under the heads of the vege- 

 table products which yield them, we shall re- 

 turn to the subject in a subsequent part of this 

 work. 



CHAP. XXII. 



GEOGKAPHICAL DISTKIBUTION OF PLANTS. 



The surface of the earth, with very few ex- 

 ceptions, presents the aspect of a natural garden, 

 teeming spontaneously with vegetable produc- 

 tions of every variety of form, of hue, and mag- 

 nitude. Notwithstanding the extremes of tem- 

 perature, from the fervid glow of the tropics to 

 the chiU atmosphere of the polar regions, there 

 are yet vegetable forms adapted to every climate; 

 and there is no region almost so cold, or arid, or 

 steeped in moisture, which has not its appropri- 

 ate vegetation. It has in consequence become a 

 question with the observers of nature, by what 

 means all these varieties of families and species 

 have obtained possession of their present locali- 

 ties ; and why it is that the banks of the Orinoco 

 are fi-inged with trees and herbs whose counter- 

 pai'ts we should in vain seek on the margin of 

 the Rhine ; that out of 7000 species of flower- 

 ing plants found wild in Europe, not a hundred 

 have been seen in Austi-alasia ; that the Alps of 

 Switzerland, and mountains of Nepaul, produce 

 perhaps not a greater number common to both ; 

 and in short, that every country of considerable 

 extent has certain species to distinguish it from 

 others. Investigations concerning the original 

 creation of plants, in the present state of human 

 knowledge, might be deemed by many at best 

 an idle waste of time; and even inquiries into 

 the means by which they occupy their present 

 situations, except in some few particular in- 

 stances, may truly seem a speculation not much 

 more profitable in itsell^ or likely to arrive at 

 ultimate success. 



This inquiry, nevertheless^ has occupied the 

 attention of many eminent botanists, and has led 

 to considerable diversity of opinion amongst 

 them ; one party supposing all plants to have 



originated in some central point, from whieli 

 they have been gradually spread over the earth's 

 surface ; others conceiving that several of such 

 centres must have existed ; and a third party be- 

 lieving species, for the most part, to have ori- 

 ginated where they now appear, as the natural 

 and untransported products of the soil and cli- 

 mate. Some again suppose that at first only 

 genera existed, species arising from generic ad- 

 mixture ; whUe others maintain that all vege- 

 table forms are modifications of each othei-, or 

 the result of a certain concun'ence of molecules 

 dispersed through matter, hence liable to be pro- 

 duced in any situation where the necessary con- 

 ditions of their existence occur. 



The causes, says Mr "Watson, now visibly ope- 

 rating in the extension of species, from one part 

 of the earth to another, afford us a more tangible 

 subject for inquiry. Millions of seeds are an- 

 nually ripened and dispersed abroad by the 

 agency of the winds, currents of water, or ani- 

 mal locomotion; and though doubtless a very 

 large proportion of them may be either entirely 

 lost, or being earned into situations unfavourable 

 to their development, may long remain un- 

 changed ; yet some among them must occasion- 

 ally be dispersed under moi-e favourable cu-cum- 

 stances, and the conditions requisite for their 

 vegetation being supplied, they are forthwith 

 developed, and add an additional species to the 

 flora of the district to which they have been car- 

 ried.- How much of vegetable distribution has 

 thus been effected it is quite impossible now to 

 estimate ; but any observer may have evidence 

 that such causes are still operative in our flora, 

 although, perhaps, not very materially altering 

 the range of its species, except when the inter- 

 ference of human agency is also introduced. 



When man transports the vegetable forms of 

 distant regions to his own home, then it may, 

 and does really happen, in Britain for instance, 

 that their ripened seeds, dispersed by the wind, 

 or carried to a distance by streams, spring up, 

 grow, and produce other seeds, to be again scat- 

 tered farther, until a species once unknown to 

 the country, next limited to one small spot only, 

 is spread over its surface, and at length comes to 

 be regarded as a part of its flora. The turnip, 

 parslej"-, canary grass, beech tree, and many 

 others, have been thus introduced to Britain. 

 Mr Winch enumerates nearly fifty specigs not 

 included in the catalogues of British plants, 

 which are nevertheless occasionally found wild 

 on the ballast hills of Northumberland and Dur- 

 ham, to which they have been carried by ship- 

 ping. The different kinds of corn, the grape, 

 the sugar cane, the bread fruit, the potatoe, aud 

 the coffee shi-ub, have thus been more or less 

 extensively spread over the earth ; and the wide 

 waste of waters, formerly bounding the progres- 

 sion of species, by the restless ingenuity of man 



