To C. G. Semper   26 November [1878]¹

My dear Professor Semper

When I published the 6^th. Edit. of the Origin I thought a good deal on the subject to which you refer, & the opinion therein expressed was my deliberate conviction.— I went as far as I could, perhaps too far, in agreement with Wagner.² Since that time I have seen no reason to change my mind, but then I must add that my [every] attention has been absorbed on other subjects.— There are two different classes of cases, as it appears to me, viz those in which a species becomes slowly modified in the same country (of which I cannot doubt there are innumerable instances) & those cases in which a species splits into 2 or 3 or more new species, & in these latter cases, I sh^d. think nearly perfect separation would greatly aid in their “specification,”—to coin a new word.—

I am very glad that you are taking up this subject, for you will be sure to throw much light on it.—³ I remember well long ago oscillating much; when I thought of the Fauna & Flora of the Galapagos Isl^d, I was all for isolation,— when I thought of S. America I doubted much.—⁴

Pray believe me | Yours very sincerely | Ch. Darwin

P.S. I hope that this letter will not be quite illegible, but I have no amanuensis at present.