Equally curious, but foreign to my present purpose, is the inquiry into the sources of the inorganic principles in plants and animals. These sources were inappreciable until a more refined chemistry appeared. Sea-water contains only the 1/12,400th of its weight of carbonate of lime, and yet this quantity suffices for the production of the essential components of the shells of myriads of crustaceans and corals. Whilst the atmosphere contains but 4/10,000ths to 6/10,000ths of its volume of carbonic acid, the amount in sea water is more by a hundred times, and yet in this medium we find another world of animal and vegetable life, which finds re-united in the ammonia and carbonic acid the same conditions which enable human beings on the surface of the solid earth (terra firma) to live and to maintain their species.
It would even seem that the essential constituents of some organs have altered in the course of ages, without affecting, or being materially affected by, the principles of life. Thus it would seem that fossil bones contain the fluate (fluorure de calcium) of calcium in much larger quantities than the bones of recent animals; and the same remark has been made in respect of the composition of the crania of men found at Pompeii. They resemble in this respect the antediluvian fossil remains.
Thus, imperceptibly, as it were, proceed the grand operations of nature, and if accidentally any vast collection of excreta should happen to be found, as in the guano islands of the dry regions of America, they seem not to affect the life or health of those animals which repose on them. It is the same in the dry regions of Southern Africa, where sheep and cattle, in order to protect them from wild animals, must, on the approach of evening, be collected into a fold or kraal, surrounded by a strong fence of the mimosa, and carefully shut in. On this surface, of no great extent, sheep and oxen stand or rest for the evening: their excreta accumulate, but do not putrefy, for the air on the kraal is pure comparatively, and never injurious to the sheep or cattle; the surface of the kraal is, moreover, generally dry, even when the soil may be accidentally inundated by rain, which, when it falls, as it does occasionally, descends in torrents. From the African soil is thus withdrawn by man the excreta of all the domestic animals; the semi-barbarous Boer never returns it to the soil, and thus the loss is permanent; but it would seem that this loss, caused by man’s interference, in no shape, as far as can be observed, affects the fertility of the soil, called on to reproduce only the native pasture, or the wild herbs natural to it. It is otherwise when man demands from the soil heavier exhausting crops of wheat and hemp, tobacco, &c.: his interference with nature’s balance must be gone into, or soon his hopes of a harvest would be in vain. Then comes the theory of manures, a theory beset with difficulties, and which, besides involving man in much labour and expense, is productive, or presumed to be on sufficiently probable grounds the cause, of some, if not of many, of the diseases which afflict humanity. However this may be, whatever be the extent to which a dense population and a neglect of the so-called sanitary regulations subject man to infirmity and disease, one thing is certain—he has interfered with nature’s balance, and must take on himself the whole task. If he shuts up a harbour mouth, refusing entrance to the tide, confining within the harbour a portion of that ocean water which nature intended should be constantly agitated by tides and currents, he may expect as results that the shores of that harbour will soon become uninhabitable by man. All animals instinctively shun the sick, leaving them apart; man crowds them together into close, ill-ventilated hospitals, sweeping off in hundreds those whom the battle had spared.
It were foreign to the object of this work to enter more fully into the history of that dissolution of animal structures which forms so important a part of the materials we call manure, destined to restore to the soil that which artificial crops had deprived it of. Every part of animal bodies owes its origin to vegetables or plants, no part being formed by the vital force, and thus all the remains of animals of necessity form manures.
On the management of these, man’s civilization depends; without agriculture there can be no dense population; without the dense population there can be no civilization. On these points many remarkably erroneous opinions have been, and still perhaps are, maintained even by practical men, who nevertheless are often in error—merely, it is true, as to the theory on which they fancy they act, more rarely as to the practice they have from experience adopted.
In calmly considering this important question—the right management of manures composed of the excreta or the remains of animal and vegetable life, it becomes evident that several problems, atmospheric as well as terrestrial, remain yet to be solved. The surface of the soil, as modified by man’s labour, presents itself under a very different aspect to what nature intended it to be. A lake may be drained with much advantage to a country, but the surface so exposed cannot be too soon cultivated, to prevent the spread of fevers sure to arise from the decaying, fermenting, and putrefying of the lower forms of animal and vegetable life thus brought into existence, especially when aided by those epidemic constitutions of the atmosphere striking directly at man’s existence on the earth.
For civilized man there is, there can be, no repose. There are forces in nature against which, with all his industry, he may never be able to prevail. The tropical forest returns upon him the instant, as it were, that he ceases to hew it down, obliterating in an incredibly short time all traces of human labour. The lands of Western France can scarcely be secured from the inroads of the sands driven by western gales towards the interior; the bog is checked only by constant labour, and the hill where once the heath grew spontaneously, can only be retained in a green and grassy condition by the constant watchfulness and labour of men. Twenty years of neglect suffice to restore the heath, and to sweep away all vestiges of human culture.[33]