| OCR Text |
Show 174 'l'TIICKNESS OF TilE MOULD CHAP. III. summary 15 tons as the weight of the castings annua1ly thrown up on an acre of land, each worm must annually eject 20 ounces. .A. fullsized casting at the mouth of a single burrow often exceeds, as we have seen, an ounce in weight ; and it is probable that worms eject more than 20 full-sized castings during a year. If they eject annua1ly more than 20 ounces, we may infer that the worms which live in an acre of pasture land must be less than 26,836 in number. Worms live chiefly in the superficial mould, which is usually from 4 or 5 to 10 and even 12 inches in thickness; and it is this mould which passes over and over again through their bodies and is brought to the surface. But worms occasionally burrow into the subsoil to a much greater depth, and on such occasions they bring up earth from this greater depth; and this process has gone on for countless ages. Therefore the superficial layer of mould would ultimately attain, though at a slower and slower rate, a thickness equal to the depth to which worms ever burrow, were there not other opposing agencies at work which carry away to a CrrAP. III. ANNUALLY ACCUMULATED. 175 lower level smne of the finest earth which is continually being brought to the surface by worms. How great a thickness vegetable mould ever attains, I have not had good opportunities for observing; but in the next chapter, when we consider the burial of ancient buildings, some facts will be given on this head. In tho two last chapters we shall see that the soil is actually increased, though only to a small degree, through the agency of worms; but their chief · work iR to sift the finer from the coarser particles, to mingle the whole with vegetable debris, and to saturate it with their intestinal secretions. Finally, no one who considers the facts given in this chapter-on the burying of small objects and on the sinking of great stones left on the surface-on the vast number of worms which live within a moderate extent of ground-on the weight of the castings ejected from the mouth of the same burrow-on the weight of all tho castings ejected within a known time on a measured space-will hereafter, as I believe, doubt that worms play an important part in nature. |
