The effect of elevation is not, in Europe, the same with all plants; there are many that grow indifferently upon the plains and upon mountains as high as perpetual snow. M. De Candolle speaks of 700 instances, with which he is acquainted, of the prevalence of this law. But, on the other hand, there are many plants, the limits of which are strictly circumscribed by elevation or equivalent temperature; as, for example, the chestnut does not rise higher in the Swiss Alps, in the parallel of 45°, than 2,400 feet; on Etna, in latitude 38°, it reaches no higher than 4000 feet. Many of the plants found on plains in the north of Europe occupy the mountains of the south. The olive, in 44° of latitude, its most northern range, will not grow at a greater elevation than 1200 feet, In general it is found that, as we approach the equator, vegetation becomes more and more affected by elevation; and that as we recede from it, the effects of elevation gradually

cease.

The cause of the influence of elevation upon plants is ascribed, in the first place, to reduced temperature; secondly, to a greater intensity of solar light; and, thirdly, to a decrease in humidity. The rate at which temperature decreases as we ascend from the surface of the earth varies according to latitude: Humboldt has shown that, in the temperate and torrid zones, the decrement of heat is essentially different. In the equatorial zone, the temperature of the region lying at the height of between 3000 and 6000 feet, on which the clouds repose that are visible to the natives of the plains,decreases much more slowly than either above or below that elevation; but, in the temperate zone, the decrease is more gradual. In proof of this the following table has been formed by Humboldt:

The diminution of the density of the air as we ascend, produces a corresponding increase in the intensity of the light; a circumstance in which high elevation again corresponds with high latitudes.

It is said that the humidity of the atmosphere decreases as we ascend, and that to this may be ascribed much of the effect produced upon vegetation by great heights. That the humidity of the atmosphere does much affect vegetation is not to be doubted; and if it were certain that the air became gradually drier as we ascend, a second cause, as powerful as that of temperature, would be found for the effects of elevation upon vegetation. But it is certain that the humidity of the air does not change gradually, as we ascend, with the character of vegetation; on the contrary, it has been found that atmospheric humidity is either uniform or increased to heights far beyond uniformity of vegetation, and then suddenly diminishes to a large amount, vegetation not suddenly altering with it; so that it would seem as if the atmosphere were composed of deep beds of air, suddenly differing from each other in the elasticity of their aqueous vapour.

From observations made by Capt. Sabine, with a Daniell's hygrometer, at Ascension, it appears that, on that island, at seventeen feet above the sea, the amount of dryness was 5°; and, at 2237 feet higher, was 3° 5′; so that, in this case, the air became more humid as he ascended. At Trinidad the amount of dryness on a level with the sea was 5°; at 1060

feet higher the air was saturated with moisture; in this instance, also, humidity increased with elevation. At Jamaica it was found that, on a level with the sea, the degree of dryness was 7°; at 4080 feet higher the air was saturated with moisture; but at 4580 feet the dryness was 16°. Hence it is to be inferred that, in these observations, the lower bed of the atmosphere was not passed through, either at Ascension or in Trinidad; but that, in Jamaica, it had been left below at the time the third observation was taken; and that, in that island the lower stratum of air is something more than 4000 feet deep. In Mr. Green's voyage the degree of dryness of the air, at an elevation of 9893 feet, was 5°, nearly the same as it was observed to be on the surface of the earth below at the same time; but, at 11,059 feet, it was 13°; and at 11,293 feet, the highest point at which an observation was made, it was still 13°; so that it would seem that the humidity of the atmosphere, at that time, did not vary through a bed of air rising perhaps 2000 feet beyond the highest limits of vegetation in Europe.

It must be confessed that these observations are by no means sufficiently numerous to become the foundation of any thing connected with the effect of elevation upon the characters of plants; but they, at least, answer the purpose of showing that, in the present state of our information, the effects of humidity are not appreciable in investigating the subject.

Whether the increased rarity of the air, as we ascend, has any effect upon vegetation, is not determined. It is not easy to say in what way it can act, according to any yet known physiological laws, unless, as M. De Candolle remarks, in supplying an insufficient quantity of oxygen for absorption. But, as we find plants of the plains grow indifferently on the highest mountains, it does not seem that there is any such diminution of oxygen as interposes with the operations of vegetation. The diminution of atmospheric pressure, which of course takes place at high elevations, may facilitate evaporation; but we have yet to learn in what precise way that phenomenon influences vegetation.

From what has now been said, all that is apparent is, that,

as we ascend in the atmosphere, temperature diminishes and light increases in a proportion corresponding, to a certain degree, with the climate of higher latitudes; but even to this there are exceptions, depending upon particular circumstances, and especially upon the amount of summer heat, of which more will be said presently. Thus, at Enontekissi, in Lapland, in 68° 30′ N. lat., at an elevation of 1356 feet above the sea, a climate which, from its situation, should be scarcely clothed with herbage, Von Buch found corn, orchards, and a rich vegetation.

Having now seen what great differences are produced in the characters of vegetation by elevation above the sea, let us next take a view of the influence caused by latitude. In the countries lying near the equator, the vegetation consists of dense forests of leafy evergreen trees, palms, and arborescent ferns, among which are intermingled epiphytal herbs and rigid grasses: there are no rich verdant meadows, such as form the chief beauty of our northern climate; and the lower orders of vegetation, such as mosses, fungi, and confervæ, are very rare: Myrtaceæ, Melastomaceæ, Musaceæ, Piperaceæ, Scitamineæ, and frutescent Compositæ abound. As we recede from the equator these gradually give way to trees with deciduous leaves, to Coniferæ, Rosacea, and Amentaceæ ; rich meadows appear, abounding with tender herbs; the epiphytal Orchideæ disappear, and are replaced by terrestrial fleshy-rooted species; mosses clothe the trunks of aged trees; decayed vegetables are covered with parasitical fungi; and the waters abound with Confervæ. Approaching the poles trees wholly disappear; dicotyledonous plants of all kinds become comparatively rare; and grasses and cryptogamic plants constitute the chief features of vegetation. To what cause, except that of temperature, and perhaps light, these effects are to be ascribed, is unknown. They are found to exist equally towards either pole; and it is evident, from the uniform manner in which the influence of the controlling cause, whatever it may be, is exercised, that the laws under which the geographical distribution of plants is determined, are as certain and immutable as any of those with the nature of which we are acquainted. It is probable that temperature