NATURAL ROTAT^^ N AMONG FOREST TREES. 305 



luxuriant green crops and a great bulk of straw, but give a coarse thick- 

 skinned grain, more or less imperfectly filled. Alter them further by a 

 dressing of clay, or keep them in arable culture, and stiffen them with 

 composts, and they will be converted into rich and sound corn-bearing 

 lands. 



5°. In the waters that gush from the sides of lime-stone hills — on the 

 bottoms of ditches that are formed of lime-stones or marls — and in the 

 springs ihat have their rise in many trap rocks, the water-cress appears 

 and accompanies the running waters, sometimes for miles on their 

 course. The mare's-tail {equisetum), on the other hand, attains its largest 

 size by the marshy banks of rivulets in which not lime but silica is 

 more abundantly present. So the Cornish heath {erica vagans) is found 

 only o!%r the serpentine soils of Cornwall, and the red broom rape 

 {orobanche rubra, Hooker's Flora Scotica), only on decayed traps in 

 Scotland and Ireland. 



These facts all point to the same natural law, that where other circum- 

 stances of climate, moisture, &c., are equal, the natural vegetation — that 

 which grows best on a given spot — is entirely dependent upon the chemical 

 constitution of the soil. 



But both the soil and the vegetation it willingly nourishes, are seen 

 to undergo slow but natural changes. Lay down a piece of land to grass, 

 and, after a lapse of years, the surface soil — originally, perhaps, of the 

 stitfest clay — is found to have become a rich, light, vegetable mould, 

 bearing a thick sward of nourishing grasses, almost totally different from 

 those which naturally grew upon it when first converted into pasture. 

 So in a wider field, and on a larger scale, the same slow changes aYe 

 exhibited in the vast natural forests that are known to have long covered 

 extensive tracts in various countries of Europe. 



Thus it is a matter of history that Charlemagne hunted in the forest 

 of Gerardmer, then consisting of oak and beech — though now the same 

 forest contains only pines of various species. On the Rhine, between 

 Landau and Kaiserlautern, oak forests, of several centuries old, are seen 

 to be gradually giving way to (he beech, while others of oak and beech 

 are yielding to the encroachments of the pine. In the Palatinate, the 

 Scotch fir {pinus sylvestris) is also succeeding to the oak. In the Jura, 

 and in the Tyrol, the beech and the pine are seen mutually to replace 

 each other — and the same is seen in many other districts. When the 

 time for a change of crop arrives, the existing trees begin to languish 

 one after another, their branches die, and finally their dry and naked 

 tops are seen surrounded by the luxuriant foliage of other races [Le Ba- 

 ron de Mortema*t de Boisse, Voyage dans les Landes, p. 189.1 These 

 facts not only show how much the vegetable tribes are dependent upon 

 the chemical nature of the soil — they indicate, likewise, the existence 

 of slow natural changes in the constitution of the soil, which lead neces- 

 sarily to a change of vegetation also. 



We can ourselves, in the case of ancient i^rests, eflfect such changes. 

 When in the United States a forest of oak or maple is cut down, one of 

 pine springs up in its place ; while on the site of a pine forest, oak and 

 other broad-leaved trees speedily appear. 



But if the full time for such changes has not come, the new vegeta- 

 tion may bej^yp^^^^^en, and smothered by the original tribes. ..Ti|^i% 



