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 From the African soil is thus withdrawn by man the excreta of all the domestic animals; the semi-barbarous Boer never returns it to the soil, and thus the loss is permanent; but it would seem that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.