The periodical change of vegetation has often been discussed, and its process has been confirmed in every country. Thus Grisebach in his classical work"die Vegetation der Erde," relates the fact that in the valleys of Guiana a secular change has taken place between woods and grass. This is not an isolated fact; on the contrary, many more are extant, and of the most patent ones I I shall here put down a few.

In the lowest strata of the moor and peat lands of Jutland, that much split-up tongue of land, in Western Europe, trunks of pine trees, Pica excelsa, Lamb, have been found, proof of former existence of pine woods. That tree not only is nowhere to be found any more in Jutland, but not even tradition hands down any knowledge of its former existence there. On the top of this layer of pine trees, trunks of the German oak are found. There are but few isolated trees of that oak found in Jutland nowadays. The present woods are mostly Beech.

Another tree that has disappeared is Piceafuc-c'nifera, Rick; there must have been once vast forests of it on the eastern shores of the Baltic, and in some sections of Southern France. Its precious petrified rosin is our present amber.

The same process is now going on in New-Zealand with the Kauri fir, Dammara Australis, Lamb., it disappears and, spite of pains and trouble taken, will not succeed any more when planted. In places where that tree does not exist any more for a long time, clumps are found of its rosin, in a more or less hardened state. Wherever that fir disappeared, there appeared Pteris esculenla, the roots of which serve the Maoris as food. A poor substitute for the valuable ship-timber of the Kauri fir.

Another evidence of the law of vegetable change is the quick acclimatization and astonishing spread of plants which have migrated into distant countries. This is the case, notahly with Cynara scolymus, Linn., orartichoke, the seed of which is easily transmitted by the wind or by adhering to the coats of ani-mals. Thus, it was carried to the pampas of La Plata by a donkey, about the year 1769. Much to the chagrin of the Gauchos, and to the disad-vantage of their cattle industry, this plant now covers very many square miles. It seems, in fact, to have found there a most favorable soil, for its dimensions and development may be called gigantic when compared with its native ones. The traveler through such districts of the Pampas must not leave the narrow-trodden paths; if he does, he will be lost amongst the dense and growing artichokes.

Erigeron Canadensis has been imported into England in the body of a bird; has from there spread all over Europe, and is now one of the most troublesome weeds, found everywhere, even on roofs and old walls.

Xanthium strumarium, Linn., and X. echina-tum, Murr, came from the lower Danube, by droves of pigs, into Hungary, and now troubles all pastures as far as Nothern Germany.

That water pest, Elodea Canadensis, Mich., has multiplied enormously in the waters of England, Scotland, Belgium, Holland and Germany, often stopping up entirely drainpipes, and, in canals, driving before it all other vegetation.

The " Bulletin de la migration des vegetaux" make mention of Lindernia pyxidoeria, Linn., a scrophulanoe, as having covered toward the end of the last century the waters of the Sevre, near Nantes, where that river joins the Loire. It is five years ago that a botanist of Nantes found to his surprise that this Lindernia was driven out by an American llysanthes. M. Hedates found in 1869, a great many of these llysanthes, on the slimy shores of Mayenne, and amongst them choked the native Lindernia.

On the other hand, how many migrations of plants have not succeeded, or have occurred without further consequence ? The last Franco. German war furnishes such an instance. About 163 different species of plants were imported into France in the forage of German horses. Not finding favorable elements there, they gradually disappeared, all but seven of them, which are now citizens of French soil.

These phenomena of vegetation can be explained by physiological laws, by the eternal mutation of chemical and physical properties of the soil, and by climatic influences.

There reigns in nature a constant motion, a continual change, the laws of which are fixed and immutable. The change already of one single factor of the conditions of vegetation of a place or country produces a corresponding change in its plants or in their vitality. Take the trouble and mark on your next meadow the spot where a certain plant now grows, return to it after one or several years and you will not find it there any more, but replaced by some other one. The same takes place in the woods, only trees live longer, and so the change takes more years. Every pomologist knows that, in the place of a dead fruit tree, no new one of the same kind can be planted without giving it new soil.

Every plant, in accordance with its specific individuality, appropriates to itself such elements of the soil as are most suitable, returning to the soil the unsuitable ones, which, however, are absorbed in their turn by other and different plants.

This explains a steady change of our earth's green dress.

Greater attention to this process and more precise records of its details seem to be most desirable.