CHAP. III. NATURAL AND ECONOMICAL USES. 291 
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. 
_Secr. Il. Trees and Shrubs considered with 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 pn 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, 
