Tuesday, May 18, 2010

The Share Worms Have Taken

The share which worms have taken in the formation of the layer of vegetable mould, which covers the whole surface of the land in every moderately humid country, is the subject of the present  volume. This mould is generally of a blackish colour and a few inches in thickness. In different districts it differs but little in appearance, although it may rest on various subsoils. The uniform fineness of the particles of which it is composed is one of its chief characteristic features; and this may be well observed in any gravelly country, where a recently-ploughed field immediately  adjoins one which has long remained undisturbed for pasture, and  where the vegetable mould is exposed on the sides of a ditch or  hole. The subject may appear an insignificant one, but we shall see that it possesses some interest; and the maxim "de minimis non  curat lex," does not apply to science. Even Elie de Beaumont, who  generally undervalues small agencies and their accumulated effects,  remarks: {1} "La couche tres-mince de la terre vegetale est un  monument d'une haute antiquite, et, par le fait de sa permanence,  un objet digne d'occuper le geologue, et capable de lui fournir des  remarques interessantes." Although the superficial layer of  vegetable mould as a whole no doubt is of the highest antiquity,  yet in regard to its permanence, we shall hereafter see reason to  believe that its component particles are in most cases removed at  not a very slow rate, and are replaced by others due to the  disintegration of the underlying materials.  As I was led to keep in my study during many months worms in pots filled with earth, I became interested in them, and wished to learn how far they acted consciously, and how much mental power they displayed. I was the more desirous to learn something on this  head, as few observations of this kind have been made, as far as I  know, on animals so low in the scale of organization and so poorly  provided with sense-organs, as are earth-worms.  In the year 1837, a short paper was read by me before the Geological Society of London, {2} "On the Formation of Mould," in which it was shown that small fragments of burnt marl, cinders, etc., which had been thickly strewed over the surface of several  meadows, were found after a few years lying at the depth of some  inches beneath the turf, but still forming a layer. This apparent  sinking of superficial bodies is due, as was first suggested to me  by Mr. Wedgwood of Maer Hall in Staffordshire, to the large  quantity of fine earth continually brought up to the surface by  worms in the form of castings. These castings are sooner or later spread out and cover up any object left on the surface. I was thus led to conclude that all the vegetable mould over the whole country has passed many times through, and will again pass many times through, the intestinal canals of worms. Hence the term "animal mould" would be in some respects more appropriate than that commonly used of "vegetable mould."  Ten years after the publication of my paper, M. D'Archiac, evidently influenced by the doctrines of Elie de Beaumont, wrote  about my "singuliere theorie," and objected that it could apply  only to "les prairies basses et humides;" and that "les terres  labourees, les bois, les prairies elevees, n'apportent aucune  preuve a l'appui de cette maniere de voir." {3} But M. D'Archiac  must have thus argued from inner consciousness and not from  observation, for worms abound to an extraordinary degree in kitchen  gardens where the soil is continually worked, though in such loose  soil they generally deposit their castings in any open cavities or  within their old burrows instead of on the surface. Hensen  estimates that there are about twice as many worms in gardens as in  corn-fields. {4} With respect to "prairies elevees," I do not know  how it may be in France, but nowhere in England have I seen the  ground so thickly covered with castings as on commons, at a height  of several hundred feet above the sea. In woods again, if the loose leaves in autumn are removed, the whole surface will be found strewed with castings. Dr. King, the superintendent of the Botanic Garden in Calcutta, to whose kindness I am indebted for many observations on earth-worms, informs me that he found, near Nancy in France, the bottom of the State forests covered over many acres with a spongy layer, composed of dead leaves and innumerable worm- castings. He there heard the Professor of "Amenagement des Forets"  lecturing to his pupils, and pointing out this case as a "beautiful  example of the natural cultivation of the soil; for year after year  the thrown-up castings cover the dead leaves; the result being a  rich humus of great thickness."  

The Formation of Vegetable Mould Through the Action of Worms with Observations on Their Habits, Charles Darwin

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