[Footnote 15: "The same earth nourishes both wholesome and noxious plants, and the nettle is often next the rose."] [Footnote 16: Professor Owen.] CHAPTER VI. "All things are but altered, nothing dies, And here or there the unbodied spirit flies." DRYDEN. The most universal character impressed on all created things that sense allows us to recognize, or philosophical inquiry to demonstrate, is "change." While nothing is more certain, few things pass less observed; or, when first announced, more stagger conviction. An old man sees the yew-tree of his boyish days apparently the same. Gilpin tells us "eight hundred years is no great age for an oak[17]!" The cliff which we left "beetling" seems to beetle still; mountains appear to be everlasting; yet, were seas and rivers to disclose even a small part of their mission, the Danube or the Volga might tell of millions of tons of soil carried from higher levels to the Black Sea and the Caspian. Animals, too, are mighty agents in recording the mutability of the matter of the universe. Coral Reefs, never spoken of in smaller terms than miles and fathoms, are the vast ocean structures of countless millions of animalcules, which serve, as it were, to link together the two great kingdoms of organic nature—the animal and vegetable creation. The microscopic geologist informs us of whole strata, well-nigh entirely composed of the silicified skeletons of insects. Sir Charles Lyell further impresses on us the reality of continual change, by referring (and, as it would appear, with increasing probability) even the stupendous changes demonstrated by geology to the agency of causes still in operation. Animals, however, besides the curious structures which they combine to contribute, are individually undergoing constant change. Man is not only no exception, but he is a "glaring" example. The whole human race are in hourly progress of mutation. "In the midst of life we are in death," is a truth to which physiology yields its tribute of illustration. Every moment we are having the old particles of our bodies silently taken away, and new materials as silently laid down. Surrounding influences, as air, moisture, temperature, &c. which, during life, are necessary to existence—the moment the breath leaves us, proceed to resolve the body into the elements of which it was composed. In all cases, change may be regarded as the combined result of two forces: the force acting, and the body acted on—that is to say, of certain external agents and certain forces inherent in the thing changed. Animals are no exceptions to this view, and diseases are amongst a multitude of other exemplifications of it; but, in order to distinguish these more clearly, it is desirable that we should be familiar with those more ordinary changes in the body which are constantly going on; and to some of these were Abernethy's early investigations directed.