[page] 530

The Formation of Mould by Worms.—As Mr. Fish asks me in so obliging a manner whether I continue of the same opinion as formerly in regard to the efficiency of worms in bringing up within their intestines fine soil from below, I must answer in the affirmative.¹ I have made no more actual measurements, but I have watched during the last 25 years the gradual, and at last complete, disappearance of innumerable large flints on the surface of a field with very poor soil after it had been laid down as pasture. I have also purposely covered a few yards square of a grass-field with fine chalk, so as to observe the worms burrowing up through it, and leaving their castings on the surface, which were soon spread out by the rain. The Regent's Park in early autumn is a capital place to observe the wonderful amount of work effected under favourable circumstances by worms, even in the course of a week or two. My observations in Staffordshire were chiefly made on poor, sandy grass-land, and I think that Mr. Fish will find that the proportion by weight or measure of carbon in poor soil is but small, and that the decay of the Grass will account for but a small proportion of the matter successively deposited on the surface. Except when peat or peaty soil is forming, the carbon compounds seem soon to be decomposed and disappear. Judging from the quick rate at which I have proved that the surface becomes covered with fine soil, if the mere decay of the Grass were as effective as Mr. Fish thinks, many feet in thickness would be formed in the course of a few centuries—a result which would be as surprising as delightful to the dwellers on poor land, or indeed on any land, which is never overflowed by mud-bearing water. In ordinary soils the worms do not burrow down to great depths, consequently fine vegetable soil is not accumulated to any inordinate thickness. Charles Darwin, May 9.

¹ David Taylor Fish (1824-1901), London gardener and botanical author writing in Gardeners' Chronicle (17 April) 1869, p. 418. Fish was referring to Darwin's views expressed in Darwin 1838 and 1840. After receiving a letter from Darwin, Fish wrote again in Gardeners' Chronicle, 1869, p. 501:

8 May, 1869 Since writing my chapter on worms, I am indebted to the courtesy of the author, for the paper read by Charles Darwin, Esq., before the Geological Society, and published in their Transactions for 1837, p. 505. Its perusal has scarcely modified the views I have already expressed, but it reveals a marvellous coincidence, inasmuch as this careful observer notices the two other modes of elevation of surface adverted to in my chapter, and dismisses them as inefficient, while he clearly and emphatically credits the worms with the whole of the work. And yet it would appear, from pages 506 and 507, that the soil found above the marl, &c, was not of the same quality as that found below it. I presume Mr. Darwin would attribute the difference to the fact of the earth having passed through the stomachs of the worms. In fact, this is obviously his theory, for he adds, that the surface soil would be more properly called animal than vegetable mould. I, however, would accept the fact of the difference in quality as a strong proof that the bottom soil had not been brought up by the worms, but brought down rather by the plants. The greater part of it has an atmospheric origin. Plants incarnate the sun himself in a visible body; this body is nurtured and sustained mainly by invisible food, produced by the atmosphere, and when it perishes part of it goes back to the air; but a large portion of it is spread out over the surface of the earth in the form of vegetable mould. And doubtless in this emptying of the invisible air down, as it were, in a solid mass of decomposing carbon annually, and hourly, in fact, over the surface of the world, through the instrumentality of every living plant, we have agents of sufficient energy, force, and persistency to cover the earth with vegetable mould without the assistance of manure. The amount of carbon utilised and converted from gases into solids in a single year is well nigh incalculable, and would account for all the elevations of surface so graphically described in the admirable paper of Mr. Darwin. It would be gratifying to many of your readers to know if this distinguished geologist still adheres to his original estimate of the usefulness and power of worms as earth-lifters and surface-elevators. Of this he may be assured, that I had no idea of running counter to any theory of so eminent and careful a naturalist when I began my chapter on worms. Even now I would gladly retreat, if I could consistently with the two rules that I have laid down for myself as a writer:—Is it true? then it ought to be spoken. Is it right? then it ought to be done. But what will Mrs. Grundy say? Well, what matters? No great man will ever respect a little one the less for speaking his mind freely, even though it may be in opposition to him. And truth has been said by some to have fallen from heaven down into the gutter, and the humblest—those closest to the gutter—sometimes find it first. D. T. Fish.

Darwin later referred to the objection in Earthworms (1881) p. 6:

In the year 1869, Mr. Fish rejected my conclusions with respect to the part which worms have played in the formation of vegetable mould, merely on account of their assumed incapacity to do so much work. He remarks that "considering their weakness and their size, the work they are represented to have accomplished is stupendous." Here we have an instance of that inability to sum up the effects of a continually recurrent cause, which has often retarded the progress of science, as formerly in the case of geology, and more recently in that of the principle of evolution.

See Correspondence vol. 17, p. 222.