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though this departure is not quite peculiar to it. If we take off the hard, shelly skin of a chrysalis, not very near its time of change, we find what appears to be nearly a homogeneous mass of white matter in a semifluid state, without any semblance of limbs, members, or organs. Yet all the parts of the future fly are there, perfectly separate and distinct, though not yet fully developed. In the outer skin, however, which has acquired consistency by exposure, the shape of the limbs and external organs is definitely marked. On the front of a chrysalis, we usually perceive in the centre, running from the head, half-way down the body, a double line, which covers the tongue; on each side of this are ranged three other folds, marking the positions of the three pairs of legs: these folds are broadest at the head, and taper to a point; then come the antennæ, long and slender, one on each side the legs; in some moths, however, they are very wide and short; and outside them, the fore-wings folded down on the breast, small of course, but still displaying the future form, and even the nervures; the hind-wings cannot be seen, because they are folded directly beneath the others. Now, as I said, the tongue usually lies straight down the middle of the breast, but in some of the larger Sphinxes this organ is destined to be in the future moth of unusual length and size; and therefore, as in this species, it is not folded down with the other members, and covered only by the common skin of all, but has a separate skin or sheath, projecting from the head, with the tip (in some instances recurved) resting on the breast, looking very much like the trunk of an elephant. The tongue or sucker, when the perfect insect is evolved, is an organ well worth a moment's examination, as a beautiful instance of the modification of a part to adapt it to altered circumstances. Look here! I will unfold the apparatus, nearly two inches long, yet when rolled up in this beautiful spiral, curl within curl, scarcely larger than the head of the pin with which I am opening it. It is tubular throughout its whole length; and what, is singular, it is composed of two parts perfectly separable, you see, each part being a half cylinder, yet, when placed side by side, meeting so exactly as to form a tube quite air-tight. This is proved by the very purpose for which it is used; for if it were in any part pervious to air, a vacuum could not be formed in it, and consequently the honey of the flowers could not be sucked through it. Now, in this long cylinder, who could detect the slightest analogy to the hard-toothed jaws of a beetle? yet, in fact, the two halves of the cylinder are neither more nor less than the two jaws, altered and modified to suit the
necessities of the insect ; for a Sphinx placed at the outside of a tubular flower, furnished only with a pair of short, hard jaws, would be in somewhat the same condition as the fox whom the stork invited to dinner ; but as it is, who does not see the hand of a God in all this?
[To be continued.]
PROGRESS OF VEGETATION ON OLD BUILDINGS.
SEEDS, to our eyes invisible, will find
When considering plants with reference to geography, it is not enough to say that such a family grows in a climate of this or that description, or under such a parallel of latitude. This view would be very imperfect, and would in fact scarcely lead to any definite results. A Flora, or descriptive catalogue of all the plants growing in one country, would prove a most unsafe guide to a person examining the botanical productions of another country situated at the same distance from the equator. There are other influences besides mere geographical position which have great weight in determining the diffusion of plants, none of which must be omitted from the account.
The great agents in promoting vegetation are heat, light, and moisture, to which must be added air and constitution of soil. Now, were the earth of uniform structure, level, and distributed into proportions of land and water everywhere the same, there would most likely ensue a regularity of climate governed by fixed and applicable laws; and, in that case, probably, vegetables would be arranged in zones, each having its defined limits, which it would rarely pass. But this is far from being the case : until we have examined all the features of any particular country, it is impossible to say what is its climate, and what are its vegetable productions. The existence of a mountain influences not only the vegetation of the mountain itself, but in a measure that of the surrounding country. Abundance of springs and rivers, not only irrigate the earth, but lower the temperature by evaporation. Extensive plains of sand occasion drought in the adjoining countries ; for the parched winds which traverse them are, during their transit, exhausted of their moisture, and recruit themselves by absorbing a new supply from the moister countries at which they arrive. On the other hand, winds which have traversed the ocean, reach the shore overcharged with moisture, which they precipitate in rain, or deposit in dew. Again, as the water of the ocean varies much less in temperature than the atmosphere or the dry land, maritime countries are in general subject to cooler summers and warmer winters than inland countries ; and since the density of water is diminished by heat as well as by excessive cold, a current of the ocean setting from a hot climate may occasion a high temperature in some land, which, from its geographical position, ought to be cold; or a current drifting along masses of ice may chill the air and retard the vegetation of a country, which we might expect to find clothed with perpetual verdure.
The general rule, then, that the abundance and luxuriance of botanical productions varies with the latitude, is modified by the causes I have just enumerated. Subject, however, to these limitations, it may be laid down as a principle, that the number of species goes on progressively increasing, from the poles to the equator, and that in the torrid zone it reaches its maximum. The size of vegetables too, as well as the rate of growth, goes on increasing from the poles to the equator, and the number of woody species, as well as their proportion to herbaceous species, follows the same rule.
It is a law also of universal application, that temperature diminishes in a ratio, bearing a certain proportion to the elevation of any place above the earth's surface, that ratio being such as to give a difference of one degree, for from three to five hundred feet of elevation. If we suppose, for instance, a range of mountains to occupy the site of London: at an elevation of 1,800 feet, we should find the temperature of Edinburgh ; at an elevation of 3,200 feet we should encounter that of the Orkneys; and at an elevation of 10,800 feet we should be assailed by the frosts of Spitzbergen. The plains of Sicily produce palms and the sugar-cane, but 100 feet above the level of the sea these plants cease; the belt above, extending for 1,000 feet, is occupied by oranges, olives, and rice; still higher up, rice, wheat, and maize cease, and are succeeded by oaks and chestnuts, which in their turn give place to fir, beech, and birch, and these, dwindling to shrubs, yield to leafless lichens, which last, at an elevation of 10,000 feet, finally disappear, and all vegetation is at an end.
The Tropics present yet greater contrasts : the forests of South America, to the elevation of 3,000 feet, are thick with leafy evergreen trees, palms, and arborescent ferns; and, as if the ground did not supply sufficient surface for the exuberant vegetation, the branches of the trees are laden with parasitic plants of exquisite structure, while the grasses, not as with us forming a verdant tapestry, grow in gigantic tangled masses. elevation of 3,250 feet, the palms disappear, but the tree-ferns continue for 2,000 feet more. Belt succeeds belt, each approaching more and more nearly the characters of the vegetation of colder regions, until at last even the Alpine plants disappear, and at an elevation of 15,600 feet, the limits of perpetual snow are attained.
Besides the variation which proceeds from decrease of temperature, there is little doubt that the degree of humidity in the air exercises an important influence ; but so little is at present known of the laws which regulate the condition of the atmosphere in this respect, that we can only cursorily allude to this part of the subject. That temperature, however, is the main agent in promoting or retarding vegetation, is evident from the fact that the