CHAr. III.

NATURAL AND ECONOMICAL USES.

221

The same may .be said with reference to different species of reptiles. In Bel-gium and part of Holland, the caterpillars of some species of motlis are soabundant in the woods at a particular season every year, that it is a part ofthebusiness of the government police to see that they are destroyed. Hencethe advantage of knowing what trees and shrubs are obnoxious to particularinsects, and what insects attack trees and shrubs generally. The total destruc-tion of herbaceous plants in dense forests teaches us, that, where \ve wish thegrass or other herbage under trees to thrive, we rnust piant the trees thinly ;and the influence of the decay of leaves, branches, and trees, in adding to thesoil, teaches us how barren soil may be improved by trees; and this naturaleffect has been imitated by trenching down entire plantations of Scotch pine,grown on extreniely poor soils, in some parts of Scotland. Undrainedwoods, and especially copse woods, are known to retain the water whichfalis on them much longer than open groves or plains ; and, as increased ex-halation and evaporation must be going on from such woods during theperiod of retention, and increased moisture must be thus produced in theatmosphere, the circumstance may demand consideration in planting extensiveshmbberies near dwelling-houses; and, more especially, in planting such asare intended, by frequent digging, always to present a surface of naked loosesoil. The influence of trees in modifying both the temperature and- moistureof the atmosphere, in civilised countries, and in artificial scenery, is generallyknown; and this knowledge should not be lost sight of in the disposition oftrees and shrubs about a house, more especially in low situations. Thereisgreat reason to believe that many country residences in England, naturallyhealthy, are rendered unhealthy by the superabundance of trees and shrubs,and by the quantity of dug ground close to the house. The insects whichinfest the rooms of a house are also very much increased by the proximityof wood.

From trees and shrubs in a wild state we can only truly learn their aboriginalnatures; because plants, like animals, begin to change their habits as soon asthey are taken into cultivation. The fact that this change takes place oughtto be borne in view, when speaking of the native soils and situations of differentspecies; because, if it is desirable to improve these species, it may be necessaryor advantageous, for that purpose, to place them in a different soil or situationfrom that in which they are found in a wild state. There are certain soils andsituations, however, in which plants are found in a wild state, that can hardlybe improved by art; these are peat bogs, or peaty soils, such as are found inNorth America, and in alpine situations. We mention these particulars merelyas a few, among a great number, to which attention ought to be directed ingiving the history of particular species of trees and shrubs, and in treating oftheir introduction into useful or ornamental plantations.

Sect. II. Trees and Shrubs considered with Reference to Man.

So various and so important are the uses of trees and shrubs to man, thatto say much on the subject here is altogether unnecessary. It must be obvious,that to state what these uses are, in the case of every particular treeand shrubtreated of in this work, will form an important part of the information givenrespecting it. It is not necessary, in every case, to mention how the differentkinds of wood are used in particular arts or manufactures ; but it is necessaryto know, not only .the particular sorts of timber, but what modifications ofthese sorts are best for particular purposes. For example, in the case ofship-building, it is not only necessary to know the different kinds of trees indemand by ship-builders, but the different purposes for which different parts°f a tree, and different forms of its trunk or branches, are adapted, and towhich they are applied.

Though the timber is the principal part of trees and shrubs which is em-ployed in arts and manufactures, yet, in many cases, the other parts of a tree,