My dear Farrer

Since I wrote this morning to you a horrid fear has crossed my mind.² It is possible, though not probable, that the rounded particles of brick in the castings, may have been rolled about by wind & rain, & afterwards been swallowed by the worms.—³ This possible source of error would be avoided if you could send me some castings from a gravel-walk with underlying brick rubbish; for the rubbish will thus have been protected from being rolled or rounded.

For Heaven sake forgive me if you can & believe me | yours very sincerely | Ch. Darwin