Dear Sir

I am very much obliged to you for your kindness in having sent me your valuable memoir on the embryology of the extinct cephalopods.² The work must have been one of immense labour & the results are extremely interesting.— Permit me to take this opportunity to express my sincere regret at having committed two grave errors in the last edition of my Origin of Species, in my allusion to your & Prof. Cope’s views on the acceleration & retardation of development.³ I had thought that Prof. Cope had preceded you; but I now well remember having formerly read with lively interest & marked a paper by you (somewhere in my Library) on fossil Cephalopods with remarks on the subject.⁴ It seems, also, that I have quite misrepresented your joint view.⁵ This has vexed me much. I confess that I have never been able fully to grasp what you wish to show, & I presume this must be owing to some dullness on my part.— I assumed, though I had no right to make any such assumption, that the kind of explanation which I have given, was what you intended. As the case stands, the law of acceleration & retardation seems to me to be a simple statement of facts; but the statement, if fully established, would no doubt be an important step in our knowledge. But I had better say nothing more on the subject, otherwise I shall perhaps blunder again.

I assure you that I regret much that I have fallen into two such grave errors, & with much respect, I remain Dear Sir | Yours faithfully | Ch. Darwin