Thus during a long journey our early companions drop off one after the other, until at last we see ourselves surrounded by a crowd of new associates, who were strangers to us at the beginning of our pilgrimage.
We may cross the earth from pole to pole, or follow the sun in his diurnal course; in all directions, from north to south and from east to west, Nature will be found to change her garments as we proceed, and never to resume again those she has once cast off. The plants and animals of the temperate and cold regions of the north are different from those of the analogous regions in the southern hemisphere; and in the tropical zone each part of the world nourishes its peculiar inhabitants.
Similar changes meet our eye on ascending from the plains to the summits of high mountains. At the foot of Etna flourishes the luxuriant vegetation of a warmer sky, the palmetto (Chamærops humilis) and the pomegranate, even the cotton shrub and the sugar-cane; higher up, the cool shade of magnificent chestnut woods refreshes our path; then follows the stately oak; until finally we attain the dreary height where all vegetation ceases in the dreadful cold of an eternal winter. With every thousand feet we rise above the level of the sea, we seem to have advanced nearer and nearer to the pole.
This wonderful change of form, which decorates the various regions of the earth with such an endless variety of organised existence, alike prevails in the realms of ocean. Here we find every larger sea-basin nourishing its peculiar inhabitants, and discover at various vertical distances beneath the surface of the sea, changes in organic nature similar to those we observed at different distances above its level.
Thousands of extinct animal and vegetable forms, which have successively flourished and disappeared, teach us the important lesson, that all created beings are made but for a season. It is only during a determined epoch of planetary life that each genus or species finds that combination of outward circumstances, under which it is able to attain its highest perfection. But imperceptibly, in the course of ages, the external world modifies its nature; families once flourishing in a different atmosphere decline and wither; they are no longer able to maintain themselves against new forms of life starting up in all the vigour of youth, and disappear from the scene, supplanted by races which must one day vanish in their turn.
Organic life is no less dependent on place than it is on time. Of the numberless animal and vegetable forms that people the earth, each finds in only one spot the scene of its greatest size and its greatest profusion. Some endowed with a more pliable or energetic nature occupy a large space upon the surface of the globe; we find them in the enjoyment of healthy existence scattered far and wide over whole hemispheres, while others are obliged to content themselves with the narrowest birthplace, and are not seldom confined to a single bay, or a single mountain side.
A great part of the magic charm of nature is owing no doubt to this deep and mysterious connexion between the soil and its productions. Here all is harmony; we feel it in our hearts; and our eye delights in the consonance of forms and colours, as our ear in the concord of sweet sounds. And where is the mortal artist whose paintings could rival the ever-changing panorama which the Master of all worlds unfolds through all zones, from pole to pole? His pictures constantly fade away; but they are perpetually succeeded by new creations of equal beauty. Happy the man whose eye is open to their charms! Every ramble through the woods and fields is to him a banquet of pure and inexhaustible delight.
The causes which confine the life of animals and plants to circumscribed localities are in many cases easily to be traced. The warmth or coldness of the sea, resulting from currents, geographical position, and depth; tranquil or disturbed, pure or troubled waters; abundance or scarcity of food, solidity or softness of the ground, sufficiently explain why many species of marine animals appear in some places in considerable numbers, while in others they are totally wanting. A superficial view of their organisation often shows us at once the physical properties their habitat must necessarily possess. By looking at a fucus we immediately see whether it requires the protection of tranquil waters, or is able to bid defiance to the floods; whether it is made to anchor upon the rock, or to sink its roots into a more yielding soil.
In many cases, however, the causes which regulate the distribution of the sea-animals are still enveloped in darkness, and we no more know why the tropical seas bring forth in some places numerous coral-reefs, and none at all in other to all appearance just as favourably situated localities, than we do why the tea-plant is confined to a small corner of Asia, or the Peruvian cinchonas to a narrow girdle on the Andes.
Evidently, besides the influences known to us, there are many other hidden ones at work, whose conflicting powers draw round every living creature a mysterious circle, whose bounds it is unable to transgress. Their discovery belongs to the future, and certainly forms one of the most interesting subjects for the naturalist's inquiries.