CHAP. 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 moths are so 

 abundant in the woods at a particular season every year, that it is a part of 

 the business of the government police to see that they are destroyed. Hence 

 the advantage of knowing what trees and shrubs are obnoxious to particular 

 insects, and what insects attack trees and shrubs generally. The total destruc- 

 tion of herbaceous plants in dense forests teaches us, that, where we wish the 

 grass or other herbage under trees to thrive, we must plant the trees thinly; 

 and the influence of the decay of leaves, branches, and trees, in adding to the 

 soil, teaches us how barren soil may be improved by trees ; and this natural 

 effect has been imitated by trenching down entire plantations of Scotch pine, 

 grown on extremely poor soils in some parts of Scotland. Undrained 

 woods, and especially copse woods, are known to retain the water which falls 

 on them much longer than open groves or plains ; and, as increased exhal- 

 ation and evaporation must be going on from such woods during the period 

 of retention, and increased moisture must be thus produced in the atmo- 

 sphere, the circumstance may demand consideration in planting extensive shrub- 

 beries near dwelling-houses ; and, more especially, in planting such as are 

 intended, by frequent digging, always to present a surface of naked loose 

 soil. The influence of trees in modifying both the temperature and moisture 

 of the atmosphere, in civilised countries, and in artificial scenery, is generally 

 known ; and this knowledge should not be lost sight of in the disposition of 

 trees and shrubs about a house, more especially in low situations. There is 

 great reason to believe that many country residences in England, naturally 

 healthy, are rendered unhealthy by the superabundance of trees and shrubs, 

 and by the quantity of dug ground close to the house. The insects which 

 infest the rooms of a house are also very much increased by the proximity 

 of wood. 



From trees and shrubs in a wild state we can only truly learn their aboriginal 

 natures; because plants, like animals, begin to change their habits as soon as 

 they are taken into cultivation. The fact that this change takes place ought 

 to be borne in view, when speaking of the native soils and situations of different 

 species ; because, if it is desirable to improve these species, it may be necessary 

 or advantageous, for that purpose, to place them in a different soil or situation 

 from that in which they are found in a wild state. There are certain soils and 

 situations, however, in which plants are found in a wild state, that can hardly 

 be improved by art ; these are peat bogs, or peaty soils, such as are found in 

 North America, and in alpine situations. We mention these particulars merely 

 as a few, among a great number, to which attention ought to be directed in 

 giving the history of particular species of trees and shrubs, and in treating of 

 their introduction into useful or ornamental plantations. 



SECT. II. Trees and Shrubs considered ivith Reference to Man. 



So various and so important are the uses of trees and shrubs to man, that 

 to 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 tree and shrub 

 treated of in this work, will form an important part of the information given 

 respecting it. It is not necessary, in every case, to mention how the different 

 kinds of wood are used in particular arts or manufactures ; but it is necessary 

 to know, not only the particular sorts of timber, but what modifications of 

 these sorts are best for particular purposes. For example, in the case of 

 ship-building, it is not only necessary to know the different kinds of trees in 

 demand by ship-builders, but the different purposes for which different parts 

 of a tree, and different forms of its trunk or branches, are adapted, and to 

 which 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, 



