My dear D^r. Gilbert.

I have been much interested by your letter, for which I thank you heartily.¹ There was not the least cause for you to apologize 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, & much exceeds what I sh^d. have expected.² In lately reading one of your & M^r Lawe’s great papers in Phil. Trans. (the value & importance of which cannot in my opinion be exaggerated)³ I was struck with the similarity of your soil with that near here; & 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 & 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 & 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 w^d 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 in large quantities from beneath the overlying brackish mould on my lawn; but from what depth I cannot say. That some must be brought up from a depth of 4 to 5 or 6 ft is certain, as the worms retire to this depth during very dry & very cold weather. As worms devour greedily raw flesh & dead worms, they w^d devour dead larvæ eggs &c &c in the soil, & 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,⁴ & here worms may have played a part.— I wish that this problem had been before me when observing, as possibly I might have thrown some little light on it, which would have pleased me greatly.

Believe me dear D^r Gilbert | Yours sincerely Ch. Darwin