More Letters of Charles Darwin — Volume 2

(FIGURE 6.)

Your expectation of the metamorphic schists sending veins into neighbouring rocks is quite new to me; but I much doubt whether you have any right to assume fluidity from almost any amount of molecular change. I have seen in fine volcanic sandstone clear evidence of all the calcareous matter travelling at least 4 1/2 feet in distance to concretions on either hand (page 113 of "S. America") (541/2. "Some of these concretions (flattened spherical concretions composed of hard calcareous sandstone, containing a few shells, occurring in a bed of sandstone) were 4 feet in diameter, and in a horizontal line 9 feet apart, showing that the calcareous matter must have been drawn to the centres of attraction from a distance of four feet and a half on both sides" ("Geological Observations on S. America," page 113).) I have not examined carefully, from not soon enough seeing all the difficulties; but I believe, from what I have seen, that the folia in the metamorphic schists (I do not here refer to the so-called beds) are not of great length, but thin out, and are succeeded by others; and the notion I have of the molecular movements is shown in the indistinct sketch herewith sent {Figure 6}. The quartz of the strata might here move into the position of the folia without much more movement of molecules than in the formation of concretions. I further suspect in such cases as this, when there is a great original abundance of quartz, that great branching contemporaneous veins of segregation (as sometimes called) of quartz would be formed. I can only thus understand the relation which exists between the distorted foliation (not appearing due to injection) and the presence of such great veins.

I believe some gneiss, as the gneiss-granite of Humboldt, has been as fluid as granite, but I do not believe that this is usually the case, from the frequent alternations of glossy clay and chlorite slates, which we cannot suppose to have been melted.

I am far from wishing to doubt that true sedimentary strata have been converted into metamorphic schists: all I can say is, that in the three or four great regions, where I could ascertain the relations of the metamorphic schists to the neighbouring cleaved rocks, it was impossible (as it appeared to me) to admit that the foliation was due to aqueous deposition. Now that you intend agitating the subject, it will soon be cleared up.

LETTER 542. TO C. LYELL. 27, York Place, Baker Street {1855}.

I have received your letter from Down, and I have been studying my S. American book.

I ought to have stated {it} more clearly, but undoubtedly in W. Tierra del Fuego, where clay-slate passes by alternation into a grand district of mica-schist, and in the Chonos Islands and La Plata, where glossy slates occur within the metamorphic schists, the foliation is parallel to the cleavage — i.e. parallel in strike and dip; but here comes, I am sorry and ashamed to say, a great hiatus in my reasoning. I have assumed that the cleavage in these neighbouring or intercalated beds was (as in more distant parts) distinct from stratification. If you choose to say that here the cleavage was or might be parallel to true bedding, I cannot gainsay it, but can only appeal to apparent similarity to the great areas of uniformity of strike and high angle — all certainly unlike, as far as my experience goes, to true stratification. I have long known how easily I overlook flaws in my own reasoning, and this is a flagrant case. I have been amused to find, for I had quite forgotten, how distinctly I give a suspicion (top of page 155) to the idea, before Sharpe, of cleavage (not foliation) being due to the laminae forming parts of great curves. (542/1. "I suspect that the varying and opposite dips (of the cleavage-planes) may possibly be accounted for by the cleavage-laminae...being parts of large abrupt curves, with their summits cut off and worn down" ("Geological Observations on S. America," page 155). I well remember the fine section at the end of a region where the cleavage (certainly cleavage) had been most uniform in strike and most variable in dip.

I made with really great care (and in MS. in detail) observations on a case which I believe is new, and bears on your view of metamorphosis (page 149, at bottom). (Ibid., page 149.)

(FIGURE 7.)

In a clay-slate porphyry region, where certain thin sedimentary layers of tuff had by self-attraction shortened themselves into little curling pieces, and then again into crystals of feldspar of large size, and which consequently were all strictly parallel, the series was perfect and beautiful. Apparently also the rounded grains of quartz had in other parts aggregated themselves into crystalline nodules of quartz. {Figure 7.}

I have not been able to get Sorby yet, but shall not probably have anything to write on it. I am delighted you have taken up the subject, even if I am utterly floored.

P.S. — I have a presentiment it will turn out that when clay-slate has been metamorphosed the foliation in the resultant schist has been due generally (if not, as I think, always) to the cleavage, and this to a certain degree will "save my bacon" (please look at my saving clause, page 167) (542/2. "As in some cases it appears that where a fissile rock has been exposed to partial metamorphic action (for instance, from the irruption of granite) the foliation has supervened on the already existing cleavage-planes; so, perhaps in some instances, the foliation of a rock may have been determined by the original planes of deposition or of oblique current laminae. I have, however, myself never seen such a case, and I must maintain that in most extensive metamorphic areas the foliation is the extreme result of that process, of which cleavage is the first effect" (Ibid., page 167).), but {with} other rocks than that, stratification has been the ruling agent, the strike, but not the dip, being in such cases parallel to any adjoining clay-slate. If this be so, pre-existing planes of division, we must suppose on my view of the cause, determining the lines of crystallisation and segregation, and not planes of division produced for the first time during the act of crystallisation, as in volcanic rocks. If this should ever be proved, I shall not look back with utter shame at my work.

LETTER 543. TO J.D. HOOKER. Down, September 8th {1856}.

I got your letter of the 1st this morning, and a real good man you have been to write. Of all the things I ever heard, Mrs. Hooker's pedestrian feats beat them. My brother is quite right in his comparison of "as strong as a woman," as a type of strength. Your letter, after what you have seen in the Himalayas, etc., gives me a wonderful idea of the beauty of the Alps. How I wish I was one-half or one-quarter as strong as Mrs. Hooker: but that is a vain hope. You must have had some very interesting work with glaciers, etc. When will the glacier structure and motion ever be settled! When reading Tyndall's paper it seemed to me that movement in the particles must come into play in his own doctrine of pressure; for he expressly states that if there be pressure on all sides, there is no lamination. I suppose I cannot have understood him, for I should have inferred from this that there must have been movement parallel to planes of pressure. (543/1. Prof. Tyndall had published papers "On Glaciers," and "On some Physical Properties of Ice" ("Proc. R. Inst." 1854-58) before the date of this letter. In 1856 he wrote a paper entitled "Observations on 'The Theory of the Origin of Slaty Cleavage,' by H.C. Sorby." "Phil. Mag." XII., 1856, page 129.)

Sorby read a paper to the Brit. Assoc., and he comes to the conclusion that gneiss, etc., may be metamorphosed cleavage or strata; and I think he admits much chemical segregation along the planes of division. (543/2. "On the Microscopical Structure of Mica-schist: " "Brit. Ass. Rep." 1856, page 78. See also Letters 540-542.) I quite subscribe to this view, and should have been sorry to have been so utterly wrong, as I should have been if foliation was identical with stratification.

I have been nowhere and seen no one, and really have no news of any kind to tell you. I have been working away as usual, floating plants in salt water inter alia, and confound them, they all sink pretty soon, but at very different rates. Working hard at pigeons, etc., etc. By the way, I have been astonished at the differences in the skeletons of domestic rabbits. I showed some of the points to Waterhouse, and asked him whether he could pretend that they were not as great as between species, and he answered, "They are a great deal more." How very odd that no zoologist should ever have thought it worth while to look to the real structure of varieties...

2. IX.VI. AGE OF THE WORLD, 1868-1877.

LETTER 544. TO J. CROLL. Down, September 19th, 1868.

I hope that you will allow me to thank you for sending me your papers in the "Phil. Magazine." (544/1. Croll published several papers in the "Philosophical Magazine" between 1864 and the date of this letter (1868).) I have never, I think, in my life been so deeply interested by any geological discussion. I now first begin to see what a million means, and I feel quite ashamed of myself at the silly way in which I have spoken of millions of years. I was formerly a great believer in the power of the sea in denudation, and this was perhaps natural, as most of my geological work was done near sea-coasts and on islands. But it is a consolation to me to reflect that as soon as I read Mr. Whitaker's paper (544/2. "On Subaerial Denudation," and "On Cliffs and Escarpments of the Chalk and Lower Tertiary Beds," "Geol. Mag." Volume IV., page 447, 1867.) on the escarpments of England, and Ramsay (544/3. "Quart. Journ. Geol. Soc." Volume XVIII., page 185, 1862. "On the Glacial Origin of certain Lakes in Switzerland, the Black Forest, Great Britain, Sweden, North America, and elsewhere.') and Jukes' papers (544/4. "Quart. Journ. Geol. Soc." Volume XVIII., page 378, 1862. "On the Mode of Formation of some River-Valleys in the South of Ireland."), I gave up in my own mind the case; but I never fully realised the truth until reading your papers just received. How often I have speculated in vain on the origin of the valleys in the chalk platform round this place, but now all is clear. I thank you cordially for having cleared so much mist from before my eyes.

LETTER 545. TO T. MELLARD READE. Down, February 9th, 1877.

I am much obliged for your kind note, and the present of your essay. I have read it with great interest, and the results are certainly most surprising. (545/1. Presidential Address delivered by T. Mellard Reade before the Liverpool Geological Society ("Proc. Liverpool Geol. Soc." Volume III., pt. iii., page 211, 1877). See also "Examination of a Calculation of the Age of the Earth, based upon the hypothesis of the Permanence of Oceans and Continents." "Geol. Mag." Volume X., page 309, 1883.) It appears to me almost monstrous that Professor Tait should say that the duration of the world has not exceeded ten million years. (545/2. "Lecture on Some Recent Advances in Physical Science," by P.G. Tait, London, 1876.) The argument which seems the most weighty in favour of the belief that no great number of millions of years have elapsed since the world was inhabited by living creatures is the rate at which the temperature of the crust increases, and I wish that I could see this argument answered.

LETTER 546. TO J. CROLL. Down, August 9th, 1877.

I am much obliged for your essay, which I have read with the greatest interest. With respect to the geological part, I have long wished to see the evidence collected on the time required for denudation, and you have done it admirably. (546/1. In a paper "On the Tidal Retardation Argument for the Age of the Earth" ("Brit. Assoc. Report," 1876, page 88), Croll reverts to the influence of subaerial denudation in altering the form of the earth as an objection to the argument from tidal retardation. He had previously dealt with this subject in "Climate and Time," Chapter XX., London, 1875.) I wish some one would in a like spirit compare the thickness of sedimentary rocks with the quickest estimated rate of deposition by a large river, and other such evidence. Your main argument with respect to the sun seems to me very striking.

My son George desires me to thank you for his copy, and to say how much he has been interested by it.

2. IX.VII. GEOLOGICAL ACTION OF EARTHWORMS, 1880-1882.

"My whole soul is absorbed with worms just at present." (From a letter to Sir W. Thistleton-Dyer, November 26th, 1880.)

LETTER 547. TO T.H. FARRER (Lord Farrer).

(547/1. The five following letters, written shortly before and after the publication of "The Formation of Vegetable Mould through the Action of Worms," 1881, deal with questions connected with Mr. Darwin's work on the habits and geological action of earthworms.)

Down, October 20th, 1880.

What a man you are to do thoroughly whatever you undertake to do! The supply of specimens has been magnificent, and I have worked at them for a day and a half. I find a very few well-rounded grains of brick in the castings from over the gravel walk, and plenty over the hole in the field, and over the Roman floor. (547/2. See "The Formation of Vegetable Mould," 1881, pages 178 et seq. The Roman remains formed part of a villa discovered at Abinger, Surrey. Excavations were carried out, under Lord Farrer's direction, in a field adjoining the ground in which the Roman villa was first found, and extended observations were made by Lord Farrer, which led Mr. Darwin to conclude that a large part of the fine vegetable mould covering the floor of the villa had been brought up from below by worms.) You have done me the greatest possible service by making me more cautious than I should otherwise have been — viz., by sending me the rubbish from the road itself; in this rubbish I find very many particles, rounded (I suppose) by having been crushed, angles knocked off, and somewhat rolled about. But not a few of the particles may have passed through the bodies of worms during the years since the road was laid down. I still think that the fragments are ground in the gizzards of worms, which always contain bits of stone; but I must try and get more evidence. I have to-day started a pot with worms in very fine soil, with sharp fragments of hard tiles laid on the surface, and hope to see in the course of time whether any of those become rounded. I do not think that more specimens from Abinger would aid me...

LETTER 548. TO G.J. ROMANES. Down, March 7th.

I was quite mistaken about the "Gardeners' Chronicle;" in my index there are only the few enclosed and quite insignificant references having any relation to the minds of animals. When I returned to my work, I found that I had nearly completed my statement of facts about worms plugging up their burrows with leaves (548/1. Chapter II., of "The Formation of Vegetable Mould through the Action of Worms," 1881, contains a discussion on the intelligence shown by worms in the manner of plugging up their burrows with leaves (pages 78 et seq.).), etc., etc., so I waited until I had naturally to draw up a few concluding remarks. I hope that it will not bore you to read the few accompanying pages, and in the middle you will find a few sentences with a sort of definition of, or rather discussion on, intelligence. I am altogether dissatisfied with it. I tried to observe what passed in my own mind when I did the work of a worm. If I come across a professed metaphysician, I will ask him to give me a more technical definition, with a few big words about the abstract, the concrete, the absolute, and the infinite; but seriously, I should be grateful for any suggestions, for it will hardly do to assume that every fool knows what "intelligent" means. (548/2. "Mr. Romanes, who has specially studied the minds of animals, believes that we can safely infer intelligence only when we see an individual profiting by its own experience...Now, if worms try to drag objects into their burrows, first in one way and then in another, until they at last succeed, they profit, at least in each particular instance, by experience" ("The Formation of Vegetable Mould," 1881, page 95).) You will understand that the MS. is only the first rough copy, and will need much correction. Please return it, for I have no other copy — only a few memoranda. When I think how it has bothered me to know what I mean by "intelligent," I am sorry for you in your great work on the minds of animals.

I daresay that I shall have to alter wholly the MS.

LETTER 549. TO FRANCIS GALTON. Down, March 8th {1881}.

Very many thanks for your note. I have been observing the {worm} tracks on my walks for several months, and they occur (or can be seen) only after heavy rain. As I know that worms which are going to die (generally from the parasitic larva of a fly) always come out of their burrows, I have looked out during these months, and have usually found in the morning only from one to three or four along the whole length of my walks. On the other hand, I remember having in former years seen scores or hundreds of dead worms after heavy rain. (549/1. "After heavy rain succeeding dry weather, an astonishing number of dead worms may sometimes be seen lying on the ground. Mr. Galton informs me that on one occasion (March, 1881), the dead worms averaged one for every two-and-a-half paces in length on a walk in Hyde Park, four paces in width" (loc. cit., page 14).) I cannot possibly believe that worms are drowned in the course of even three or four days' immersion; and I am inclined to conclude that the death of sickly (probably with parasites) worms is thus hastened. I will add a few words to what I have said about these tracks. Occasionally worms suffer from epidemics (of what nature I know not) and die by the million on the surface of the ground. Your ruby paper answers capitally, but I suspect that it is only for dimming the light, and I know not how to illuminate worms by the same intensity of light, and yet of a colour which permits the actinic rays to pass. I have tried drawing triangles of damp paper through a small cylindrical hole, as you suggested, and I can discover no source of error. (549/2. Triangles of paper were used in experiments to test the intelligence of worms (loc. cit., page 83).) Nevertheless, I am becoming more doubtful about the intelligence of worms. The worst job is that they will do their work in a slovenly manner when kept in pots (549/3. Loc. cit., page 75.), and I am beyond measure perplexed to judge how far such observations are trustworthy.

LETTER 550. TO E. RAY LANKESTER.

(550/1. Mr. Lankester had written October 11th, 1881, to thank Mr. Darwin for the present of the Earthworm book. He asks whether Darwin knows of "any experiments on the influence of sea-water on earthworms. I have assumed that it is fatal to them. But there is a littoral species (Pontodrilus of Perrier) found at Marseilles." Lankester adds, "It is a great pleasure and source of pride to me to see my drawing of the earthworm's alimentary canal figuring in your pages."

Down, October 13th {1881}.

I have been much pleased and interested by your note. I never actually tried sea-water, but I was very fond of angling when a boy, and as I could not bear to see the worms wriggling on the hook, I dipped them always first in salt water, and this killed them very quickly. I remember, though not very distinctly, seeing several earthworms dead on the beach close to where a little brook entered, and I assumed that they had been brought down by the brook, killed by the sea-water, and cast on shore. With your skill and great knowledge, I have no doubt that you will make out much new about the anatomy of worms, whenever you take up the subject again.

LETTER 551. TO J.H. GILBERT. Down, January, 12th, 1882.

I have been much interested by your letter, for which I thank you heartily. There was not the least cause for you to apologise for not having written sooner, for I attributed it to the right cause, i.e. your hands being full of work.

Your statement about the quantity of nitrogen in the collected castings is most curious, and much exceeds what I should have expected. In lately reading one of your and Mr. Lawes' great papers in the "Philosophical Transactions" (551/1. The first Report on "Agricultural, Botanical, and Chemical Results of Experiments on the Mixed Herbage of Permanent Grassland, conducted for many years in succession on the same land," was published in the "Philosophical Transactions of the Royal Society" in 1880, the second paper appeared in the "Phil. Trans." for 1882, and the third in the "Phil. Trans." of 1900, Volume 192, page 139.) (the value and importance of which cannot, in my opinion, be exaggerated) I was struck with the similarity of your soil with that near here; and anything observed here would apply to your land. Unfortunately I have never made deep sections in this neighbourhood, so as to see how deep the worms burrow, except in one spot, and here there had been left on the surface of the chalk a little very fine ferruginous sand, probably of Tertiary age; into this the worms had burrowed to a depth of 55 and 61 inches. I have never seen here red castings on the surface, but it seems possible (from what I have observed with reddish sand) that much of the red colour of the underlying clay would be discharged in passing through the intestinal canal.

Worms usually work near the surface, but I have noticed that at certain seasons pale-coloured earth is brought up from beneath the outlying blackish mould on my lawn; but from what depth I cannot say. That some must be brought up from a depth of four or five or six feet is certain, as the worms retire to this depth during very dry and very cold weather. As worms devour greedily raw flesh and dead worms, they could devour dead larvae, eggs, etc., etc., in the soil, and thus they might locally add to the amount of nitrogen in the soil, though not of course if the whole country is considered. I saw in your paper something about the difference in the amount of nitrogen at different depths in the superficial mould, and here worms may have played a part. I wish that the problem had been before me when observing, as possibly I might have thrown some little light on it, which would have pleased me greatly.

2. IX.VIII. MISCELLANEOUS, 1846-1878.

(552/1. The following four letters refer to questions connected with the origin of coal.)

LETTER 552. TO J.D. HOOKER. Down, May {1846}.

I am delighted that you are in the field, geologising or palaeontologising. I beg you to read the two Rogers' account of the Coal-fields of N. America; in my opinion they are eminently instructive and suggestive. (552/1. "On the Physical Structure of the Appalachian Chain," by W.B. and H.D. Rogers. Boston, 1843. See also "Geology of Pennsylvania," by H.D. Rogers. 4 volumes. London and Philadelphia, 1843.) I can lend you their resume of their own labours, and, indeed, I do not know that their work is yet published in full. L. Horner gives a capital balance of difficulties on the Coal-theory in his last Anniversary Address, which, if you have not read, will, I think, interest you. (552/2. "Quart. Journ. Geol. Soc." Volume II., 1846, page 170.) In a paper just read an author (552/3. "On the Remarkable Fossil Trees lately discovered near St. Helen's." By E.W. Binney. "Phil. Mag." Volume XXIV., page 165, 1844. On page 173 the author writes: "The Stigmaria or Sigillaria, whichever name is to be retained... was a tree that undoubtedly grew in water.") throws out the idea that the Sigillaria was an aquatic plant (552/4. See "Life and Letters," I., pages 356 et seq.) — I suppose a Cycad-Conifer with the habits of the mangrove. From simple geological reasoning I have for some time been led to suspect that the great (and great and difficult it is) problem of the Coal would be solved on the theory of the upright plants having been aquatic. But even on such, I presume improbable notion, there are, as it strikes me, immense difficulties, and none greater than the width of the coal-fields. On what kind of coast or land could the plants have lived? It is a grand problem, and I trust you will grapple with it. I shall like much to have some discussion with you. When will you come here again? I am very sorry to infer from your letter that your sister has been ill.

LETTER 553. TO J.D. HOOKER. {June 2nd, 1847.}

I received your letter the other day, full of curious facts, almost all new to me, on the coal-question. (553/1. Sir Joseph Hooker deals with the formation of coal in his classical paper "On the Vegetation of the Carboniferous Period, as compared with that of the Present Day." "Mem. Geol. Surv. Great Britain," Volume II., pt. ii., 1848.) I will bring your note to Oxford (553/2. The British Association met at Oxford in 1847.), and then we will talk it over. I feel pretty sure that some of your purely geological difficulties are easily solvable, and I can, I think, throw a very little light on the shell difficulty. Pray put no stress in your mind about the alternate, neatly divided, strata of sandstone and shale, etc. I feel the same sort of interest in the coal question as a man does watching two good players at play, he knowing little or nothing of the game. I confess your last letter (and this you will think very strange) has almost raised Binney's notion (an old, growing hobby-horse of mine) to the dignity of an hypothesis (553/3. Binney suggested that the Coal-plants grew in salt water. (See Letters 102, 552.) Recent investigations have shown that several of the plants of the Coal period possessed certain anatomical peculiarities, which indicate xerophytic characteristics, and lend support to the view that some at least of the plants grew in seashore swamps.), though very far yet below the promotion of being properly called a theory.

I will bring the remainder of my species-sketch to Oxford to go over your remarks. I have lately been getting a good many rich facts. I saw the poor old Dean of Manchester (553/4. Dean Herbert.) on Friday, and he received me very kindly. He looked dreadfully ill, and about an hour afterwards died! I am most sincerely sorry for it.