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]