This world would not be habitable were it not for the humus with which its bare rocks are clothed. The humus is the living covering of the skeleton, and its formation has taken ages. The primitive bare rock which has been 'weathered' by the changing seasons gets clothed with a growth of lichen. This thin but rough covering entangles stray particles, and thus by its own decay affords a nidus for a stronger growth. This stronger growth, by chemical action and physical force, works further into the rock, on which the soaking rains and rending frosts have an increasing effect, and thus, partly by the disruption of the inorganic rock, and partly by the increase in ever-growing quantities of vegetable decay, the humus rises, as it were, 'on stepping-stones of its dead self' until it is able to afford footing and nourishment for the stately forest tree, and its fertility finally becomes sufficient to attract the attention of the husbandman. This humus, the loose, mainly organic covering of the rocks, is formed, as we have seen, by crumbling rocks from below and by the constant additions of dead organic matter which are deposited upon the surface. These additions of organic matter, be they in the form of dead animals, dead leaves, dung, or what not, become humified, and thus the stock of humus tends steadily to increase. The greater the stock of humus the greater the fertility, and the greater the fertility the greater will be the amount of dead organic matter to increase the stock of humus. The conversion of the dead organic matter into humus is a biological process, and is caused by the animals which live in the humus, and is perfected by the growth of fungi. On this account I ventured some years ago to speak of the humus as the 'Living Earth,' and I take it that no more important addition has ever been made to the stock of human knowledge than the recognition that the humus teems with life, and that its fertility and healthiness depend entirely upon biological processes. If the humus be sterilised, either by heat or antiseptics, it becomes absolutely barren. It was at one time supposed that the fertility of the soil depended mainly upon the process of nitrification, whereby nitrogenous organic matter is converted into soluble nitrates which are absorbed by the roots of plants, and there can be no doubt that these nitrifying organisms are most important. The causes of the fertility of the soil are probably far more complex than we suppose, and I think it may be said that we are as yet only upon the threshold of our knowledge with regard to them.